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Small Molecule Organic Semiconductors on the Move Promises for Future Solar Energy Technology.

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Angewandte
Reviews
A. Mishra and P. Buerle
DOI: 10.1002/anie.201102326
Organic Solar Cells
Small Molecule Organic Semiconductors on the Move:
Promises for Future Solar Energy Technology
Amaresh Mishra* and Peter Buerle*
Keywords:
bulk heterojunctions ·
organic semiconductors ·
organic solar cells ·
photovoltaics ·
planar heterojunctions
Dedicated to Prof. Yasuhiko Shirota
Angewandte
Chemie
2020
www.angewandte.org
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
Angewandte
Chemie
Organic Semiconductors
This article is written from an organic chemists point of view and
provides an up-to-date review about organic solar cells based on small
molecules or oligomers as absorbers and in detail deals with devices
that incorporate planar-heterojunctions (PHJ) and bulk heterojunctions (BHJ) between a donor (p-type semiconductor) and an
acceptor (n-type semiconductor) material. The article pays particular
attention to the design and development of molecular materials and
their performance in corresponding devices. In recent years, a
substantial amount of both, academic and industrial research, has been
directed towards organic solar cells, in an effort to develop new
materials and to improve their tunability, processability, power
conversion efficiency, and stability. On the eve of commercialization
of organic solar cells, this review provides an overview over efficiencies
attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade. Approaches
to enhancing the efficiency of organic solar cells are analyzed.
1. Introduction
The finding of clean and renewable energy is one of the
major scientific and technological challenges in the 21st
century. In this respect, acquiring power from the sun using
photovoltaics (PV) is an attractive alternative to address
global environmental issues. The availability of solar energy
by far exceeds any potential future energy demands. In fact,
the amount of sun energy that reaches earth per hour (1.4 1030 J) is larger than that of the energy needed by mankind per
year.
In 1839, Alexandre Edmond Becquerel for the first time
observed the emergence of a photocurrent when platinum
electrodes covered with silver halide were illuminated in
aqueous solution (electrochemical cell); this observation has
ever since been known as the photovoltaic effect.[1] It has been
the basis for various concepts of converting solar radiation
into electricity, and has opened a new domain of alternative
energy generation. In this respect, organic (excitonic)[2] solar
cells (OSC), appear to be highly promising because of their
potential for low-cost fabrication and for the exciting science
that comes with organic semiconducting materials. The search
for new materials has been greatly extended into the field of
organic (semiconducting) molecules and polymers, which
offer the advantage of wide chemical functionalities by which
their optical, electrochemical, solubility, morphological, and
electrical properties can be tuned. OSCs can be efficiently
manufactured, they have low environmental impact, and
because of their compatibility with flexible substrates, they
could be used in many low cost modules for domestic
applications.[3] Devices based on these materials are predicted
to have a theoretical efficiency that approaches 10–15 %.[4–6]
1.1. Development of Organic Semiconductors
The term organic semiconductor is used to describe
organic materials (conjugated oligomers or polymers) that
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
From the Contents
1. Introduction
2021
2. Planar- and BulkHeterojunction Solar Cells
Made by Vacuum Evaporation
2024
3. Bulk-Heterojunction Solar Cells
Made of Small Molecules by
Solution Processing
2033
4. Bulk-Heterojunction Solar Cells
Based on Star-Shaped Dyes and
Dendrimers
2047
5. Comparison of BulkHeterojunction Solar Cells
Made by Vacuum- or SolutionProcessing
2051
6. Small Molecular
Semiconductors as n-Type
Materials in OSCs
2052
7. Latest Developments
2057
8. Summary and Future Prospects 2060
possess the ability of transporting charge carriers and have
been studied since the 1950s.[7] The electronic conductivity of
these materials lies between that of metals and insulators
spanning a broad range between 107 and 103 S cm1. Holes
and electrons in p orbitals are the typical charge carriers in
organic semiconductors. Charge transport typically depends
on the ability of the charge carriers to move from one
molecule to another, which depends on the energy gap
between highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels. These
materials are either based on oligomers such as pentacene,
anthracene, rubrene, or oligothiophenes, or on polymers such
as polypyrrole, polyacetylene, poly(3-hexylthiophene)
(P3HT), or poly(p-phenylene vinylene) (PPV). Organic
semiconductors have attracted much attention because of
their fundamental scientific importance and impressive
improvements in performance in a wide variety of photonic
applications, such as organic light-emitting diodes (OLED),
organic field-effect transistors (OFET), organic solar cells
(OSC), liquid crystals, sensors, and many more.[8–12] Devices
based on organic semiconductors greatly benefited from the
[*] Dr. A. Mishra, Prof. Dr. P. Buerle
Institute of Organic Chemistry II and Advanced Materials
University of Ulm
Albert-Einstein-Allee 11, 89081 Ulm (Germany)
E-mail: amaresh.mishra@uni-ulm.de
peter.baeuerle@uni-ulm.de
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2021
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Angewandte
Reviews
A. Mishra and P. Buerle
remarkable advances in synthetic organic chemistry that have
allowed for the synthesis of a wide variety of p-conjugated
systems with attractive optoelectronic properties. Compared
to conjugated polymers, small molecular semiconductors
offer several intrinsic advantages in organic electronic
applications. They are monodisperse in nature with welldefined chemical structures and are synthetically well reproducible. We will restrict our article to OSCs, in which organic
small molecules/oligomers (neutral, charged, and metal complexes) are used as active semiconducting components. The
physical properties (such as optical, charge carrier mobility,
HOMO/LUMO energy levels, and structural ordering) of
these organic semiconductors can be tuned by various
chemical functionalizations.
Solution and solid-state characterization of the newly
developed organic semiconductors should establish good
structure-property relationships that can be exploited for
the fabrication of OSCs. Optical measurements provide
essential information about the electronic properties and
can be used as evidence for an ongoing electron-transfer
process within these materials. Optical band gaps (Egopt) were
generally estimated from the absorption onset at the lowenergy side of the absorption band. Furthermore, the
HOMO/LUMO frontier orbital energy levels can be determined by cyclic voltammetry, which is important with respect
to the electron acceptors, exciton-transport layers, and
applied electrodes in OSCs. The electrochemical band gap
(EgCV) is calculated from the difference between the HOMO
and LUMO energy levels. These methods allow for the
selection of the most promising candidates, which can be
employed for the fabrication of efficient PV devices.
1.2. Advancement in Organic Solar Cells and Basic Principles
Work within the field of OSCs started by utilization of
organic small molecules[13–19] and later on by using functional
semiconducting polymers,[10, 20–26] thus resulting in remarkable
improvements in power-conversion efficiencies (PCEs; given
in the Tables as h) over a decade from about 1 % to over 8 %.
Amaresh Mishra received his PhD in physico-organic chemistry from Sambalpur University, India in 2000, where he studied the
synthesis and photophysical characterization
of cyanine class dyes. After a postdoctoral
stay with Prof. G. R. Newkome (1999–
2001) at the University of South Florida, he
joined TIFR, Mumbai, in 2002, where he
first developed an interest in organic electronics. After an Alexander von Humboldt
Fellowship (2005–2007) in the group of
Prof. P. Buerle, University of Ulm, he
continued as a group leader of the organic
solar cells group. His current research includes the development of donor–
acceptor based dyes and metal complexes for photovoltaic applications.
2022
www.angewandte.org
In the race for efficient OSCs, two processing techniques were
established: 1) dry processing (thermal evaporation) for
planar-heterojunction (PHJ) and bulk-heterojunction (BHJ)
solar cells and 2) solution processing (spin-coating, inkjet
printing, dip-coating, spraying technique) for BHJ solar cells
(Figure 1). Currently, record efficiencies have independently
Figure 1. Typical OSC devices based on donor–acceptor heterojunction
architectures. a) PHJ configuration. b) BHJ configuration. c) Fundamental steps occuring in donor–acceptor heterojunction solar cells:
1) Photoexitation of the donor to generate an exciton (electron–hole
pair bound by Coulomb interactions). 2) Exciton diffusion to the D–A
interface. Excitons that do not reach the interface recombine and do
not contribute to the photocurrent (longer diffusion length, LD).
3) Dissociation of bound excitons at the D–A interface to form a
geminate electron–hole pair (increased interfacial charge separation
requires optimal energy offset between LUMO of the donor and
LUMO of the acceptor material). 4) Free charge carrier transport and
collection at the external electrodes (require high charge-carrier
mobility).
been reported for tandem small-molecule/oligomer solar cells
produced by controlled thermal evaporation of the various
layers (9.8 % certified, cell size 1.1 cm2, Heliatek GmbH,[27]
Dresden/Ulm, Germany) and for solution-processed polymer
solar cells (8.3 % certified, cell size 1 cm2, Konarka,[28] Lowell
MA, USA/Nrnberg, Germany).[29]
However, these numbers are still lower than efficiencies
of 10–15 % expected for commercialization. The strong
Peter Buerle received his Ph.D. in organic
chemistry from the University of Stuttgart
(Germany, 1985) working with Prof. F.
Effenberger. After a post-doctoral year at
MIT, Boston (USA, 1986), in the group of
Prof. M. S. Wrighton, he completed his
habilitation (1994) in Stuttgart. After being
Professor of Organic Chemistry in Wrzburg
(Germany), he became Director of the
Institute for Organic Chemistry II and
Advanced Materials at the University of Ulm
(Germany, since 1996). Since October 2009
he also serves as Vice President for Research
at the University of Ulm. Current interests of the research group include
development of novel organic semiconducting materials, in particular,
conjugated poly- and oligothiophenes, their structure–property relationships,
self-assembling properties, and applications in electronic devices, in particular organic solar cells. He is co-founder of Heliatek GmbH, Dresden/Ulm,
a spin-off company for the production of organic solar cells.
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
Organic Semiconductors
motivation for achieving such efficiencies is driving chemists
toward the development of novel materials and physicists as
well as engineers toward new device architectures and
fabrication technologies. Apart from the solar cell performance, it is necessary to understand the fundamental
chemical properties and physical mechanisms of the materials
in the bulk.
Typical organic solar cells comprise a donor (p-type
semiconductor) and an acceptor (n-type semiconductor) as
active layer. At the donor–acceptor (D–A) interface, charges
are separated after photoinduced charge transfer from the
electron donor to the electron acceptor. Two principle
architectures for such a D–A interface were established in
the field: 1) two successively deposited layers of donors and
acceptors to form a planar heterojunction (PHJ) (Figure 1 a)
or 2) co-deposition, which leads to a blended d–A film and a
bulk heterojunction (BHJ) structure that has a much higher
internal interface (Figure 1 b). Schematic frontier orbital
energy levels and the basic fundamental steps occurring in
OSCs are depicted in Figure 1 c.
Current–voltage (J–V) curves represent an important and
direct characterization method of a solar cell. Figure 2 depicts
a J–V curve under dark and incident-light illumination. The
Figure 2. Current–voltage (J–V) characteristics of a typical solar cell.
Essential parameters determining the cell performance are shown:
VOC = open-circuit voltage; JSC = short-circuit current density; FF = fill
factor; Vmp and Jmp are voltage and current, respectively, at which the
power output of a device reaches its maximum. The power-conversion
efficiency h is defined as the ratio of maximum power output (Pout) to
power input (Pin). JL = light-generated current.
open-circuit voltage (VOC) and short-circuit current density
(JSC) under illumination are illustrated. VOC represents the
maximum (photo)voltage measured in a solar cell, which is
found to depend mainly on the organic material and the
energetic level of their frontier orbitals, that is, the energy
difference of the HOMO level of the donor (D) and the
LUMO level of the acceptor (A). However, VOC can also be
influenced by charge recombination processes, which cannot
be completely avoided, resulting in a lower maximum VOC.[30]
JSC represents the maximum (photo)current that could be
obtained in a solar cell. This (photo)current depends on the
number of absorbed photons that can be exploited by the
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
solar cell as long as no saturation effects occur. JSC can also
depend on the surface area of the photoactive layer, the
device thickness, and absorption coverage to harvest more
photons within the solar spectrum. Charge transport properties of organic semiconductors also play an important role to
obtain a high JSC.
The fill factor (FF) describes the quality of the solar cell
and is determined by the photogenerated charge carriers and
the fraction thereof that reaches the electrodes. The maximum area within the J–V curve, that is, the ratio of the
maximum power output (Pout) of a solar cell to the product of
its VOC and JSC determines the FF. In fact, the FF depends on
the competition between charge carrier recombination and
transport processes. Furthermore, the series resistances
significantly influence the FF and should be minimized. The
molar absorption coefficient of the molecule should be high to
obtain a high external quantum efficiency (EQE) and the
HOMO/LUMO energy levels should be properly adjusted to
give increased VOC and FF. Finally, the efficiency (h) is
determined by the ratio of power that the device produces
(Pout) and the power of the incident light (Pin) and is also
represented in Figure 2. The external quantum efficiency
(EQE) is defined by the number of photogenerated charge
carriers over the number of incident monochromatic photons.
The most critical factor for a rational design of materials
and material combinations suited for efficient OSCs is that a
multiparameter problem should be mastered, which is not
only the proper combination of donor and acceptor materials,
but as well optimization of JSC, VOC, and FF by the control of
absorption properties, HOMO–LUMO energy levels, material composition, solid-state packing, transport, and, at the
end, processing conditions.[31]
In BHJ solar cells, morphology and phase separation of
the active layer also play a vital role in determining the overall
performance. The dissociation of excitons and the creation of
charges in the active bulk layer in OSCs is significantly
influenced by its morphology. To optimize the efficiency of
new materials it is important to obtain control over the
morphology of the active layer. The active layer morphology
should reach a balance between the donor and acceptor
domains and the intermolecular interactions between the
donor and acceptor molecules. Typically, the morphology
depends on various parameters such as material composition,
processability conditions, selection of solvent, annealing
conditions (solvent/thermal), and use of additives. In general,
the molecular structure of the semiconductors is the most
critical factor in determining the nature and the degree of
ordering in the solid state. However, for thin film preparation
from solution, the solvent properties greatly affect film
nucleation and growth mechanism as well as morphology,
and thus charge transport properties. A detailed description
of morphology and its effect on solar cell performance is
beyond the scope of this review. In a very recent article,
Kemerink and co-workers clearly described the role and
effect of morphology on charge transport and performance of
polymer solar cells.[32]
The wavelength of the maximum solar photo flux is
located between 600 and 800 nm. Thus, in order to harvest
most of the photons, the absorption profile of the semi-
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angewandte
Reviews
A. Mishra and P. Buerle
conductor needs to cover the visible, the red, and the near-IR
regime. Most of the p-conjugated systems only cover the
visible region of the solar spectrum. In this respect, smallband-gap oligomers/polymers recently came into focus by
implementation of electron-rich and electron-deficient units
in the conjugated backbone. Indeed, variation of their
strength allows for proper tuning of the energy levels of the
molecular frontier orbitals. A small band gap leads to a redshifted absorption, which might improve the light harvesting
property by absorbing more photons. However, finding the
ideal band gap can be tricky. Simply making the band gap as
small as possible will not necessarily make a good solar cell.
Several aspects need to be taken care of while rationally
designing small-band-gap molecules: firstly, the band gap can
be reduced either by lowering the LUMO and or raising the
HOMO energy level of the molecule. A minimum offset of
approximately 0.3–0.4 eV between the LUMO of the donor
and the LUMO of the acceptor is necessary to ensure efficient
exciton dissociation at the D/A interface.[5, 33] This minimum
offset limits lowering of the LUMO energy level of the donor
molecule. Secondly, the increase of the HOMO energy level
by the introduction of strong donor units might also lead to
smaller band gaps, but this approach will lower the VOC of the
cell.[34] Therefore, it is of great importance to optimize the
positions of the energy levels of the donor and acceptor to
allow efficient charge separation without losing the (photo)voltage.
Various standard test conditions have been defined in
order to reproducibly characterize photovoltaic devices.
These test conditions are based on various spectral distributions (Figure 3). Air mass 1.5 global/direct (AM1.5G/
diffuse radiation and “D” accounts only for the direct
radiation. The standard conditions for photovoltaic measurements are generally AM1.5 spectrum at an irradiance of
100 mW cm2 and a temperature of 25 8C. Air mass two
(AM2) spectrum represents the direct solar spectrum on the
ground when the sun is at 60.18 zenith angle, which results in
twice the path length through the atmosphere at an irradiance
of approximately 80 mW cm2.
One of the main drawbacks for organic solar cells is the
rather moderate charge carrier mobility of most organic
semiconductors taht are in the order of 100–
108 cm2 V1 s1.[8, 35–38] p Electrons and corresponding holes
are typical charge carriers in organic semiconductors. Compared to inorganic semiconductors, the moderate transport
properties of organic materials are a consequence of the weak
intermolecular interactions, hence resulting in electronic
states localized on single molecules. In organic semiconductors the photogenerated excitons are strongly bound electron–hole pairs. Because of the high exciton dissociation
energy (ca. 100 meV), the separation into free charges does
not occur spontaneously. Excitons in organic molecules are
relatively short-lived species, which recombine within a few
nanoseconds. As a result of the short exciton diffusion length
(LD 5–10 nm), only excitons generated within this distance
from the donor–acceptor interface can be converted into free
charge carriers at room temperature, while all others decay
through radiative or nonradiative pathways.[39] On the other
hand, an organic semiconductor provides high molar absorptivity, leading to a high optical density in thin films, because of
which only small layer thicknesses on the order of 50–200 nm
are required to absorb all incident photons. Since the
development of OSCs, many books and reviews on heterojunction solar cells have appeared that describe novel
material design and various cell geometries.[10, 13, 15, 21, 22, 40–49]
In the following sections we will discuss the current state-ofthe-art in the development of small molecule/oligomeric
semiconductors for vacuum and solution-processed OSCs.
2. Planar- and Bulk-Heterojunction Solar Cells
Made by Vacuum Evaporation
Figure 3. Schematic representation of the path length, in units of Air
Mass, and its dependence on the zenith angle.
AM1.5D) simulates the terrestrial/direct solar spectrum on
the ground when the sun is at 48.28 zenith angle. The air mass
(AM) represents the proportion of atmosphere that the light
must pass through before striking the Earth relative to the
shortest path length when the sun is directly overhead and is
defined as 1/cosq. The air mass calculates the reduction in the
power of light as it passes through the atmosphere caused by
scattering and absorption by air (oxygen and carbon dioxide),
dust particles and/or aerosols in the atmosphere. The number
“1.5” indicates that the path of light in the atmosphere is
1.5 times the shortest length when the sun is at the zenith. The
letter “G” stands for “global” and includes both direct and
2024
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Organic solar cells were first presented back in 1975 by
Tang and Albrecht using microcrystalline chlorophyll-a
(Chl-a, 1) sandwiched between two metal electrodes of
different work functions. The Chl-a film was prepared by
electrodeposition on a metal-coated quartz disc. In a cell
structure of Al j Chl-a j Hg or Al j Chl-a j Au a VOC in the range
of 0.2–0.5 V, a JSC of about 4 to 10 nA, and a PCE on the order
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
Organic Semiconductors
of 0.001 % were obtained.[50] Then, metal phthalocyanine (Pc)
complexes 2 and 3 were used as p-type materials because of
their absorption in the low-energy region, high molar
absorptivity, and good thermal stability. The first organic
solar cell in a single-layer structure based on these Pc
derivatives prepared by vacuum deposition generated very
low efficiencies of less than 0.01 %.[13, 15] The performance was
further improved to around 0.7 % using merocyanine dye 4 in
a single-layer structure (Al–Al2O3/merocyanine/Ag) with an
effective area of 1 cm2. The presence of a thin interfacial
oxide layer that acts as an insulator resulted in an improved
open-circuit voltage up to 1.2 V.[51, 52]
Exciton dissociation in a single organic material requires
an applied field of more than 106 V cm1 to overcome the
exciton-binding energy and to separate electron–hole pairs
bound by Coulomb interactions.[53] However, such strong
electric fields are not generated by the voltages at which
organic solar cells typically operate. Another disadvantage of
this single-layer structure is that the positive and the negative
photoexcited charges have to travel through the same
material, thereby increasing the recombination losses. These
disadvantages led to the development of bilayer planar
heterojunction (PHJ) cells by Tang, using Cu-phthalocyanine
2 as the donor and perylene-3,4,9,10-bis-benzimidazole 5
(PTCBI) as the acceptor material sandwiched between two
electrodes of different work functions.[14] The thickness used
for the donor and acceptor materials were 30 and 50 nm,
respectively. A transparent indium-tin-oxide (ITO) was used
as the anode and silver as the cathode. The PCE was increased
to 0.95 % under AM2 conditions (75 mW cm2) with an
impressive FF of 65 % (Table 1). In these cells, after light
excitation, excitons (bound electron–hole pairs) are generated at the D/A-interface and exciton dissociation occurs by
photoinduced charge transfer from the LUMO of the donor
to the LUMO of the acceptor. Then the charges are transported to the respective electrodes and are collected
(Figure 4). For efficient charge separation, it has been found
empirically that the LUMO energy level of the acceptor has
to be at least 0.3–0.4 eV lower than the LUMO energy level of
the electron donor. In addition, the maximum theoretical
(photo)voltage produced by a solar cell is given by the
difference in the energy levels between the HOMO of the
donor and the LUMO of the acceptor. Photovoltaic devices
based on phthalocyanines as donor components of the active
layer have recently been reviewed by Torres and co-workers.[54]
Forrest and Yakimov later on prepared tandem cells using
the same dyes as Tang.[55] A 0.5 nm Ag layer was applied
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
between two separate heterojunction cells that served as
charge recombination centers. At AM1.5G illumination
conditions the tandem cells showed a more than doubled
efficiency of 2.5 % with a high VOC of 0.9 V in comparison to
what was reported by Tang for a single heterojunction cell. In
a bilayer device, Forrest et al. used CuPc (2) as donor and C60
(6) as acceptor as well as 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline 12 (BCP) as an exciton-blocking layer. With
this layer sequence, efficiencies of about 3.6 % were
obtained showing that bilayer structures allow for achieving
efficient energy conversion (Table 1).[56] By applying a 1:1
blend of CuPc and C60 as mixed layer (BHJ) with pristine C60
as the acceptor layer, an efficiency of 3.5 % was obtained.
Despite the VOC of 0.5 V was relatively low, a very high JSC of
15.4 mA cm2 was measured.[57] Furthermore, in this device
series, when a mixed CuPc:C60 layer was intercalated between
homogeneous CuPc and C60 layers, efficiencies as high as 5 %
were achieved.[58] The blend layer showed good transport of
photogenerated charge carriers to their respective electrodes
by the adjacent homogeneous layers. The PCE was significantly improved to about 5.7 % by implementation of a
tandem geometry consisting of two hybrid planar/bulk (P/B)
mixed heterojunction cells stacked in series.[17] The reported
high VOC > 1 V was double that of a single cell. Thin layers of
PTCBI 5 and BCP 12 were used as excition-blocking layers
and 4,4’,4’’-tris(3-methylphenylphenylamino)triphenylamine
13 (m-MTDATA) p-doped with 5 mol % of 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane 14 (F4-TCNQ)
together with a thin Ag layer as charge recombination center.
To overcome the problem of poor transport properties, p–
i–n type solar cells, in which the active layer is sandwiched
between two doped wide-gap layers, were introduced by the
Leo research group. Co-evaporated ZnPc 3 and C60 6 were
used in the intrinsic (i) layer that was sandwiched between pdoped m-MTDATA 13 and n-doped N,N’-dimethylperylene3,4,9,10-bis(dicarboximide) 9 (MPBI). F4-TCNQ 14 and
rhodamine B 15 were used as p- and n-type dopants,
respectively. The cell generated a moderate PCE of 1.04 %
under one sun intensity, as a consequence of high series
resistance due to significant ohmic losses in the transport
layers (Table 1).[59] In a similar p-i-n structure, when p-doped
N,N,N’,N’-tetrakis(4-methoxyphenyl)-benzidine
16
(MeOTPD) was used as hole-transport and n-doped C60 as
electron-transport layer, cell efficiencies of 1.9 % were
obtained.[60] The advantage of the p-i-n structure is that they
only absorb light in the photoactive region and thus avoid
recombination losses at the contacts and make optimum use
of the light reflected at the top contact. Mnnig, Leo and coworkers prepared a tandem cell using stakes of two p-i-n cells
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A. Mishra and P. Buerle
separated by a 1 nm thick gold interlayer.[61] The optimized
single cell gave an efficiency of 1.95 % at 125 mW cm1 light
intensity. A corresponding tandem cell showed an almost
doubled VOC and a significantly higher PCE of 2.4 %
compared to the single p-i-n cell. By optimizing the thicknesses of the blend layer and the doped wide-gap transport
layer, the efficiency of the tandem cell was further increased
to 3.8 %.[62] The JSC of the tandem cell was only about half the
value of the single p-i-n device. However, this cell architecture
showed a great potential for improvements, such as incorporation of photoactive layers with different absorption properties. In an optimized configuration both individual p-i-n cells
should able to generate similar photocurrents. The Leo
research group also developed the concept of the m-i-p-type
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(metal-intrinsic-p-doped) solar cell architecture that includes
p-doped wide-gap materials as hole transport layer and a gold
top contact.[63] Using ZnPc/C60 as intrinsic layer and p-doped
m-MTDATA 13 as hole-transport layer, a PCE of 0.6 % was
achieved. This result was improved by using p-doped
MeOTPD 16 as a hole-transport layer and the efficiency
rose to 1.44 %. This difference was attributed to the higher
hole mobility (mh) of p-MeOTPD as compared to the mMTDATA derivative.
Very recently, Yan et al. used 5,5’-di(biphenyl-4-yl)-2,2’bithiophene (BP2T, 17) as hole-transport and aluminium-8hydroxyquinoline (Alq3, 24) as electron transport layer in a
ZnPc:C60 P/B-mixed HJ solar cell. Firstly, a ZnPc layer was
deposited onto the surface of a BP2T thin film on ITO/
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Table 1: Photovoltaic parameters of OSCs prepared by vacuum-evaporation techniques.
Device structure[a]
Device[b] JSC
VOC
[mA cm2] [V]
ITO/2 (30 nm)/5 (50 nm)/Ag
Al–Al2O3/4 (100 nm)/Ag
PHJ
single
layer
tandem
PHJ
BHJ
BHJ
tandem
ITO/PEDOT:PSS/2 (11 nm)/5 (11 nm)/Ag (0.5 nm)/2 (11 nm)/5 (11 nm)/Ag
ITO/2 (20 nm)/6 (40 nm)//12 (10 nm)/Ag
ITO/2:5 [1:1] (33 nm)/6 (10 nm)/12 (7.5 nm)/Ag
ITO/2 (15 nm)/2:6 (1:1) (10 nm)/6 (35 nm)/12 (10 nm)/Ag
ITO/2 (7.5 nm)/2:6 (1:1) (12.5 nm)/6 (8 nm)/5 (50 nm)/Ag (0.5 nm)/p-13 (5 nm)/2
(6 nm)/2:6 (1:1) (13 nm)/6 (16 nm)/12 (10 nm)/Ag
ITO/PEDOT:PSS/13 (50 nm)/3:6 (1:2) (50 nm)/9 (50 nm)/LiF (1 nm)/Al
ITO/p-16 (50 nm)/3:6 (1:1) (30 nm)/n-6 (30 nm)/Al
ITO/p-16 (30 nm)/3:6 (1:2) (60 nm)/n-6 (20 nm)/Al
ITO/p-16 (30 nm)/3:6 (1:2) (60 nm)/n-6 (20 nm)/Au (0.8 nm)/p-16 (30 nm)/3:6 (1:2)
(48 nm)/n-5 (30 nm)/Al
ITO/p-16 (30 nm)/3:6 (1:2) (60 nm)/n-6 (20 nm)/Au (0.5 nm)/p-16 (125 nm)/3:6 (1:2)
(50 nm)/n-6 (20 nm)/Al
ITO/3:6 (1:2) (30 nm)/p-13 (50 nm)/p-3 (10 nm)/Au
ITO/3:6 (1:2) (30 nm)/3 (10 nm)/p-16 (50 nm)/p-3 (10 nm)/Au
ITO/PEDOT:PSS/17 (8 nm)/3 (10 nm)/3:6 (1:1) (30 nm)/6 (25 nm)/24 (5 nm)/Al
ITO/3 (25 nm)/6 (30 nm)/HPBI 10 (5 nm)/12 (15 nm)/Ag
ITO/7 (80 nm)/6 (30 nm)/12 (8 nm)/Al
ITO/8 (20 nm)/6 (50 nm)/CsF (1 nm)/Al
ITO/8 (45 nm)/5 (50 nm)/12 (10 nm)/Al
ITO/PEDOT (60 nm)/8:11 (80 nm)/LiF (0.6 nm)/Al
ITO/PEDOT (60 nm)/8 (20 nm)/8:6 (alternate evaporation 2 nm 6 times)/6 (20 nm)/12
(0.6 nm)/Al:Mg
BHJ, pi-n
p-i-n
p-i-n
tandem
p-i-n
tandem
p-i-n
m-i-p
m-i-p
P/B-HJ
PHJ
PHJ
PHJ
PHJ
BHJ
P/B-HJ
2.3
1.8
Ref.
h
Light
[%] intensity
[mWcm2]
FF
0.45 0.65 1.0 75
1.2 0.25 0.7 78
[14]
[52]
6.5
11.5
15.4
18.0
9.7
0.9
0.52
0.50
0.54
1.03
0.43
0.60
0.46
0.61
0.59
100
100
100
120
100
[55]
[56]
[57]
[58]
[17]
6.3
0.5
0.33 1.0 100
[59]
9.8
13.9
6.6
0.44 0.45 1.9 100
0.45 0.39 2.0 125
0.85 0.53 2.4 125
[60]
[61]
[61]
10.8
0.99 0.47 3.8 130
[62]
3.9
6.55
9.97
6.2
7.0
6.4
15.0
5.4
8.2
0.43
0.45
0.56
0.62
0.58
0.40
0.36
0.35
0.41
[63]
[63]
[64]
[65]
[66]
[67]
[68]
[69]
[70]
0.36
0.49
0.55
0.64
0.56
0.45
0.50
0.28
0.48
2.5
3.6
3.5
5.0
5.7
0.6
1.4
3.1
2.5
2.3
1.1
2.7
0.5
1.6
100
100
100
100
100
115
100
100
100
[a] “p-” as a prefix means that the compound was used in its p-doped form, “n-” analogously stands for n doping. [b] PHJ = planar heterojunction;
BHJ = bulk heterojunciton; P/B-HJ = planar/bulk mixed heterojunction; p-i-n = p-doped/intrinsic/n-doped; m-i-p = metal/intrinsic/p-doped.
PEDOT:PSS at a substrate temperature of 155 8C (PEDOT:
poly(3,4-ethylenedioxythiophene), PSS: polystyrolsulfonate).
The bulk layer was then deposited onto the ZnPc thin film at
Figure 4. a) Fundamental processes of donor–acceptor-based bilayer
heterojunction devices. b) Typical HOMO–LUMO energy level diagram
of donor and acceptor. Theoretically, VOC is linearly related to the builtin potential (Vbi) and is determined by the difference of HOMO of the
donor (p-type semiconductor) and LUMO of the acceptor (n-type
semiconductor) molecule.
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
100 8C. Finally, C60, Alq3, and Al layers were deposited
sequentially at room temperature. The P/B-HJ is the combination of a PHJ and a BHJ layer within the same structure and
is known to combine the benefits of both concepts. Therefore,
it takes maximum advantage of the unobstructed chargecarrier-collecting properties of individual organic layers, and
the improved exciton dissociation properties of a mixture of
donor and acceptor materials. The cell generated a PCE of
3.07 %, which is a significantly higher value compared to cells
prepared at room temperature (h = 1.75 %, Table 1).[64]
Jabbour et al. reported an improvement in the PCE of
ZnPc:C60 solar cells by insertion of an N,N’-dihexylperylene3,4,9,10-bis(dicarboximide) 10 (HPBI) interface layer
between C60 and BCP 12 exciton-blocking layer.[65] The
morphology of the BCP layer was influenced by the underneath HPBI layer, which promotes the migration of the metal
cathode into the BCP layer, thereby enhancing the charge
collection efficiency. The device with the configuration ITO/
ZnPc/C60/HPBI/BCP/Ag showed a higher PCE of 2.5 %
compared to approximately 1.5 % for a device without
HPBI layer. The improvement in PCE was ascribed to the
reduced charge recombination and series resistance resulting
in an increase of JSC and FF values.
Among the small molecule organic semiconductors,
tetracene 7 and pentacene 8 are the most widely investigated
p-type conjugated materials in OFETs with high carrier
mobilities of up to 0.1 and 3 cm2 V1 s1, respectively. Owing
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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.
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Reviews
A. Mishra and P. Buerle
to their planar p-conjugated structure they have a relatively
low band gap of 1.7 eV. These materials were as well
investigated as p-type semiconductors in photovoltaics.
OSCs fabricated using tetracene/C60-heterojunctions as photoactive layer resulted in PCEs of 2.3 % under AM 1.5 solar
illumination.[66] The high efficiency was assigned to the welldefined molecular order in the D–A heterojunction. Malliaras
and co-workers prepared OSCs using the heterostructure
pentacene/C60 as active layer and demonstrated a moderate
efficiency of 0.45 % under 115 mW cm2 illumination. The
PCE was further improved to 1.1 % by thermal annealing of
the device as a result of a small increase in the built-in
potential, which was raised from 0.3 to 0.46 V, and a large
increase in photocurrent from 0.4 to 6.4 mA cm2.[67] This
improved performance was interpreted to be caused by better
molecular ordering of the organic layers. In a similar cell
configuration using 12 as exciton-blocking layer, Kippelen
and co-workers reported an enhanced JSC of approximately
15 mA cm2 resulting in an overall PCE of 2.7 % (Table 1).[68]
OSCs were fabricated by co-evaporation of pentacene 8
and N,N’-bistridecylperylene-3,4,9,10-bis(dicarboximide) 11
(TPBI) to form the photoactive layer. These molecules form
an interpenetrating network between D–A units generating a
maximum PCE of 0.54 % under AM 1.5 conditions in an inert
environment (Table 1).[69]
Very recently, a multilayer heterojunction solar cell was
fabricated by alternating evaporation of pentacene 8 and C60
6. The concept of the cell structure was similar to that of a
BHJ cell, except that the bulk active layer was deposited by
subsequent evaporation rather than by a co-evaporation of
the blend layer. In the cell structure, the active layer consisted
of in total 24 nm thick ultrathin layers (alternating 2 nm thick
layers of pentacene and C60) sandwiched between 20 nm of
bottom pentacene and of top C60 layers. The device generated
a PCE of 1.6 % (Table 1), which is higher than that of a
bilayer-HJ cell (JSC = 6.3 mA cm2, VOC = 0.31 V, FF = 0.48,
h = 0.94 %).[70] On the other hand, BHJ solar cells prepared by
co-evaporation of pentacene/C60 showed a very poor performance caused by large leakage currents. The main drawback
in these BHJ cells is that pentacene 8 easily crystallizes in
herringbone-like packing during vacuum deposition, thus
forming a large-scale phase separation in the blend film.
Oligothiophenes are among the best-studied organic
semiconducting materials because of their well-known very
good transport properties, their high polarizability, as well as
their tunable optical and electrochemical properties.[71–73] In
1989, for the first time, a-sexithiophene 26 was implemented
as an active semiconductor material in OFETs.[74] Sakai et al.
prepared an OSC as a PHJ device using a-sexithiophene (26)
as donor and C60 (6) as acceptor exhibiting a PCE close to
0.8 %.[75] The same research group further prepared a BHJ
device with the same donor/acceptor system demonstrating
that the blend morphology strongly depends on the composition of donor and acceptor. Using a donor/acceptor blend
ratio of 1:5 the device generated a PCE of 1.5 % (Table 2).
The PCE was further improved to 2 % by inserting a C60 layer
between the blend layer and the exciton-blocking layer. By
AFM and TEM measurements it was shown that the large
2028
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excess of C60 in the blend prevents sexithiophene 26 from
crystallization and forms homogeneous blend morphologies.
In collaboration with the Leo research group, our research
group has recently developed a series of 2,2’-dicyanovinylene
(DCV) acceptor-substituted oligothiophenes (A–D–A type)
as low-band-gap p-type materials for solar cell applications.[76–82] The DCV substitution provides a reduced band
gap and intense absorption in the visible region. For example,
a,a’-bis(dicyanovinyl)quinquethiophene 27 (DCV5T) is a
bright red solid with a longest wavelength absorption
maximum at 516 nm in dichloromethane solution. The optical
band gap of oligothiophene 27 was reduced from around
2.5 eV in solution to about 1.77 eV in thin films. A multilayer
OSC was constructed using 27 as the donor and C60 (6) as the
acceptor, which formed a planar heterojunction (PHJ). The
cells generated efficiencies up to 3.4 % at 118 mW cm2
simulated sunlight when embedded between a thin excitonblocking layer, 4,7-diphenyl-1,10-phenanthroline (18, BPhin),
and a p-doped hole-transport layer, N,N-bis[4-(naphth-1yl)phenylamino)biphenyl-4-yl]-N,N-diphenylbenzidine (19,
Di-NPB; Table 2).[76, 77] Owing to the low-lying HOMO
energy level of 27 (5.6 eV), a VOC of nearly 1 V was
obtained, which was approximately 0.4 V higher compared to
ZnPc 3 (EHOMO 5.2 eV)-based OSCs discussed above. The
IPCE spectrum covers the entire spectral range from 350 to
650 nm, reaching a value of nearly 50 % at 570 nm.
This DCV5T system comprising different alkyl side chains
(27: butyl, 28: ethyl) was tested in PHJ solar cells using N,N’diphenyl-N,N’-bis(1-naphthyl)-1-1’-biphenyl-4,4’’-diamine
(20, a-NPD) as hole transporter. Devices with butyl-substituted compound 27 showed a higher PCE (3.4 %) compared
to the ethyl-substituted analogue 28 (2.5 %).[80] The reduced
efficiency in the latter case was mainly due to the lower FF.
The difference in performance was explained by the difference in molecular packing, by differences in hole injection
between the hole-transport layer and the oligothiophene as
well as by a difference in hole mobility of the two oligothiophene derivatives.
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
Angewandte
Chemie
Organic Semiconductors
Table 2: Photovoltaic parameters of OSCs containing acceptor-substituted oligothiophenes.
Device structure[c]
Device[a] JSC
VOC
[mA cm2] [V]
FF
h
[%]
Ref.
Light
intensity
[mWcm2]
ITO/PEDOT:PSS (30 nm)/26 (25 nm)/6 (25 nm)/12 (6 nm)/Ag:Mg
ITO/PEDOT:PSS (30 nm)/26:6 (1:5, 50 nm)/12 (6 nm)/Ag:Mg
ITO/PEDOT:PSS (30 nm)/26:6 (1:5, 50 nm)/6 (20 nm)/BCP (6 nm)/Ag:Mg
ITO/Au (1 nm)/p-19 (30 nm)/19 (5 nm)/27 (7 nm)/6 (40 nm)/18 (6 nm)/Al
PHJ
BHJ
BHJ
p-i-n,
PHJ
p-i-n,
PHJ
p-i-n,
PHJ
p-i-n,
PHJ
p-i-n,
PHJ
p-i-n,
PHJ
p-i-n,
PHJ
p-i-n,
BHJ
p-i-n, P/
B-HJ
m-i-p,
PHJ
m-i-p,
BHJ
m-i-p,
PHJ
m-i-p,
PHJ
m-i-p,
PHJ
m-i-p,
BHJ
m-i-p,
PHJ
m-i-p,
PHJ
m-i-p,
PHJ
m-i-p,
PHJ
m-i-p,
PHJ
m-i-p,
PHJ
0.55
0.39
0.51
0.49
0.8
1.5
2.0
3.4[b]
100
100
100
118
[75]
[75]
[75]
[76]
8.9
1.00 0.50 3.4[b] 100
[80]
8.4
1.00 0.40 2.5[b] 100
[80]
11.4
1.00 0.51 4.0[b] 130
[81]
5.9
1.13 0.27 1.3[b] 130
[81]
7.7
0.93 0.46 2.3[b] 130
[81]
6.6
0.90 0.53 3.1[d] 100
[83]
6.9
0.82 0.40 2.3[d] 100
[83]
8.2
0.88 0.42 3.0[d] 100
[83]
6.5
0.90 0.64 3.8
100
[84]
10.9
0.89 0.61 4.9
119
[85]
2.9
0.97 0.42 1.2
100
[86]
5.1
0.97 0.52 2.6
106
[86]
4.8
0.91 0.64 2.8
100
[86]
11.1
0.97 0.49 5.2
102
[86]
5.6
0.86 0.54 2.6
100
[87]
3.5
0.96 0.43 1.5
100
[87]
4.0
1.17 0.33 1.6
100
[88]
4.4
1.10 0.30 1.5
100
[88]
3.1
0.98 0.57 1.7
100
[89]
4.7
1.0
100
[89]
ITO/Au (1 nm)/p-20 (10 nm)/20 (5 nm)/27 (9.8 nm)/6 (52 nm)/18 (6 nm)/Al
ITO/Au (1 nm)/p-20 (10 nm)/20 (5 nm)/28 (10 nm)/6 (52 nm)/18 (6 nm)/Al
ITO/Au (1 nm)/p-21 (30 nm)/p-20 (10 nm)/20 (5)/27 (10 nm)/6 (40 nm)/18 (6 nm)/Al
ITO/Au (1 nm)/p-21 (30 nm)/p-20 (10 nm)/20 (5)/29 (10 nm)/6 (40 nm)/18 (6 nm)/Al
ITO/Au (1 nm)/p-21 (30 nm)/p-20 (10 nm)/20 (5)/30 (10 nm)/6 (40 nm)/18 (6 nm)/Al
ITO/Au (1 nm)/p-19 (15 nm)/19 (5 nm)/30 (8.8 nm)/6 (52 nm)/18 (6 nm)/Al
ITO/Au (1 nm)/p-19 (15 nm)/19 (5 nm)/30:6 (24.9 nm)/6 (32.5 nm)/18 (6 nm)/Al
ITO/Au (1 nm)/p-19 (10 nm)/19 (5 nm)/30 (5.9 nm)/30:6 (31.4 nm)/6 (32.5 nm)/18
(6 nm)/Al
ITO/n-6 (30 nm)/6 (15 nm)/31:6 (20 nm)/22 (5 nm)/p-22 (30 nm)/p-3 (10 nm)/Au
ITO/n-6 (5 nm)/6 (15 nm)/32:6 (30 nm)/22 (5 nm)/p-22 (10 nm)/p-19 (30 nm)/NDP9
(1 nm)/Al
ITO/6 (15 nm)/33 (6 nm)/22 (5 nm)/22:NDP9 (10 wt %, 50 nm)/NDP9 (1 nm)/Au
ITO/6 (15 nm)/34 (6 nm)/22 (5 nm)/22:NDP9 (10 wt %, 50 nm)/NDP9 (1 nm)/Au
ITO/6 (15 nm)/35 (6 nm)/22 (5 nm)/22:NDP9 (10 wt %, 50 nm)/NDP9 (1 nm)/Au
ITO/6:NDN1 (2 wt %, 5 nm)/6 (15 nm)/34:6 (2:1) (40 nm)/22 (5 nm)/22:NDP9
(10 wt %, 10 nm)/23:NDP9 (10 wt %, 30 nm)/NDP9 (1 nm)/Al
ITO/6 (15 nm)/36 (10 nm)/22 (5 nm)/22:NDP9 (30 nm)/22: NDP9 (10 nm)/p-3
(10 nm)/Au
ITO/6 (15 nm)/37 (10 nm)/22 (5 nm)/22:NDP9 (30 nm)/22: NDP9 (10 nm)/p-3
(10 nm)/Au
ITO/6 (15 nm)/38 (6 nm)/22 (5 nm)/22: NDP9 (30 nm 9 wt %)/22:NDP9 (10 nm
18 wt %)/p-3 (10 nm 4 wt %)/Au
ITO/6 (15 nm)/39 (6 nm)/22 (5 nm)/22: NDP9 (30 nm 9 wt %)/22:NDP9 (10 nm
18 wt %)/p-3 (10 nm 4 wt %)/Au
ITO/6 (15 nm)/40 (10 nm)/19 (5 nm)/19:NDP9 (53 nm 5 wt %)/NDP9 (1 nm)/Au
ITO/6 (15 nm)/41 (10 nm)/19 (5 nm)/19:NDP9 (53 nm 5 wt %)/NDP9 (1 nm)/Au
3.9
5.6
5.6
10.6
0.35
0.68
0.70
0.98
0.67 3.2
[a] See footnote [b] of Table 1. [b] For the efficiency calculation, the spectral mismatch between the sun simulator and the AM1.5G sun spectrum is
taken into account. [c] NDP9 is a p-type donor compound sold by Novaled AG, NDN1 is an n-type donor compound. [d] values are not mismatch
corrected.
Furthermore, the influence of the length of the conjugated
backbone on the OSC performance was systematically
studied for DCV-substituted quater-, quinque, and sexithiophenes—DCVnT 29 (n = 4), 27 (n = 5), and 30 (n = 6)—
forming PHJs with C60.[81, 82] Cyclic voltammetry measurements showed that the HOMO energy level raises with
increasing oligomer length, while the LUMO energy remains
essentially constant because of the electron-withdrawing
DCV end groups. It was found that the OSC performance
mainly depends on the length of the oligothiophene unit. The
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
VOC gradually decreases from 1.13 V for tetramer 29 to 1.0 V
for pentamer 27 and to 0.93 V for hexamer 30, which is
ascribed to the increasing HOMO energy. However, no
systematic trend was found for the PCEs of devices, which
were lower for tetramer 29 (h = 1.3 %) and hexamer 30 (h =
2.3 %) compared to pentamer 27 (h = 4.0 %) because of their
lower JSC and FF values (Table 2).[81] The devices were
measured at 130 mW cm2 light intensity. In these devices,
p-doped 4,4’,4’’-tris(2-naphthylphenylamino) triphenylamine
21 (TNATA) and a-NPD 20 were used as hole-transporter
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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.
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A. Mishra and P. Buerle
layers. Leo and co-workers further demonstrated the use of
sexithiophene 30 as donor material in PHJ, in BHJ, and in
combined P/B-HJ solar cells.[83] The cells generated PCEs of
3.1, 2.3, and 3.0 %, respectively (Table 2). The hybrid cell
generated a high JSC (8.2 mA cm2) compared to the PHJ and
BHJ devices owing to a better charge-separation efficiency, as
the thickness of the donor layer is below the limit of the
exciton diffusion length. However, the BHJ and the hybrid
cell suffered from transport losses in the mixed layer, which
limited the FF to a very low value of approximately 0.4. The
VOC of around 0.9 V for these devices is about 0.1 V lower
compared to OSCs prepared with quinquethiophene 27,
which is ascribed to the difference in the ionization potential
(IP) of the two derivatives (DIP = 0.1 eV). After mismatch
correction from the EQE spectrum, the combined heterojunction device showed a JSC of 5.7 mA cm-2, and VOC of 0.86,
FF of 0.43, leading to a PCE of 2.1 %.
An m-i-p-type BHJ solar cell was prepared by coevaporation of ethyl substituted sexithiophene 31 and C60 6
using 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (22, BPAPF) and ZnPc 3 as hole transporter. The cell
grown at a substrate temperature of 30 8C showed a VOC of
0.86 V, a JSC of 4.2 mA cm2, a FF of 0.44, and a PCE value of
1.6 %. When the active layer was grown on a substrate at a
temperature of 90 8C the PCE of the device increased from
1.6 % to 3.8 % (Table 2). This increase in PCE was assigned to
the improved structural ordering and favorable phase separation, which improves transport of charge carriers, resulting
in improved JSC and FF values.[84]
Recently, Leo, Pfeiffer, and co-workers reported a DCVsubstituted sexithiophene 32 and investigated the influence of
substrate heating on the device performance.[85] M-i-p type
BHJ solar cells were prepared by co-evaporation of 32 and
C60. The device prepared on a heated substrate at 90 8C gave
an excellent PCE of (4.9 0.2) % at 119 mW cm2 light
intensity under AM1.5G conditions (Table 2). In comparison,
devices prepared without substrate heating showed a PCE of
only 2.1 %, a VOC of 0.88 V, a JSC of 7.3 mA cm2, and a FF of
0.42. The increased performance of devices prepared on
heated substrates was due to a large increase in JSC and FF,
which was attributed to a change in mixed-layer morphology
induced by the substrate heating. A strong phase separation is
facilitated by heating, hence leading to better charge transport within the percolation pathways of the mixed layer.
We recently prepared a series of low-band gap DCVsubstituted oligothiophenes (DCVnT), 33–35, omitting solubilizing side chains by an efficient convergent synthetic
approach.[86] Purification of these materials by gradient
sublimation led to thermally highly stable organic semiconductors. X-ray structure analysis revealed the importance
of the DCV groups in the molecular packing and intermolecular interactions. Optical absorption spectra in solution
and in thin films showed a clear red-shift, an increase in molar
absorptivity, and lowering of the band gap with an increasing
number of thiophene units in the conjugated backbone.
Increase of the HOMO energy level was observed with
increasing oligomer length, whereas the LUMO energy value
remained essentially unaffected. These compounds were
further explored as electron donors in vacuum-processed m-
2030
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i-p-type planar and p-i-n-type bulk heterojunction organic
solar cells.
M-i-p-type PHJ solar cells generated PCEs of 1.2 % for
DCV4T (33), 2.6 % for DCV5T (34), and 2.8 % for DCV6T
(35), respectively. The increase in FF from 33 (42 %) to 34
(52 %) and 35 (64 %), showed that the exciton and charge
separation efficiencies become less field-dependent with
increasing chain length. BHJ solar cells were fabricated
using a blend of DCV5T (34) and C60 (2:1) co-evaporated on
heated substrates (90 8C) with layer thicknesses varying from
20 to 40 nm. In BHJ devices, an additional p-doped spiroNPB (23) was used as hole-transport layer. A significant
increase in JSC from 6.9 to 11.1 mA cm2 was observed with
increasing blend layer thickness. At the same time, with
increasing layer thickness, a decrease in FF from 61 % to 49 %
was observed. This clearly showed the decrease in charge
collection efficiency with increasing layer thickness in the
voltage regime around the maximum power point. The device
comprising a 40 nm quinquethiophene 34:C60 active layer
gave the best PCE of 5.2 % for a 5.06 mm2 size cell under
AM1.5G illumination (Table 2). The higher efficiency in BHJ
compared to PHJ solar cells can be attributed to the much
large internal interface between donor and acceptor materials. This is one of the highest values ever reported for organic
vacuum-deposited single junction solar cells and clearly
showed the advantage of the larger D–A interface in BHJs
compared to PHJs.
A new family of DCV-substituted quinquethiophenes 36
and 37 were prepared using Friedel–Crafts-acylation and
Knoevenagel condensation reactions.[87] A change in optical
properties was observed by replacing the vinylic proton of the
DCV group by a methyl or phenyl moieties, respectively. In
comparison to DCV-derivative 28 (lmax = 506 nm), the
absorption maximum of 36 was blue-shifted by 15 nm in
solution, while no change was observed in the absorption
maximum of oligomer 37. The blue-shift for 36 was ascribed
to the electron-donating effect (+ I effect) of the methyl
group that lowers the acceptor strength of the terminal
acceptor units. In thin films, these dyes showed absorption
maxima at 535 and 547 nm, respectively, compared to 569 nm
for 28. The calculated optical band gaps of these compounds
(ca. 1.8 eV) were in good agreement with the band gaps
obtained by electrochemical measurements. M-i-p type solar
cells prepared using oligomers 36 and 37 showed PCEs of 2.6
and 1.5 %, respectively (Table 2). The lower performance for
oligomer 37 compared to 36 was attributed to the low
intermolecular p–p interactions and high disorder due to the
nonplanar structure of the molecule, which reduced the JSC of
the respective device.
For the development of low-band-gap materials, we
further prepared A–D–A–D–A type oligomers 38 and 39
that comprise bithiophene units as donor, the electrondeficient benzothiadiazole (BTDA) as core, and trifluoroacetyl (TFA) as terminal acceptor moieties.[88] In thin films,
the oligomers showed low-energy absorption bands at 533 and
466 nm and band gaps of 1.89 and 1.83 eV. The electrochemically determined HOMO and LUMO energy levels were
about 5.82 and 3.74 eV, respectively. In m-i-p type solar
cells using C60 as acceptor, mixed pentamers 38 and 39 gave
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Organic Semiconductors
end-capped with diphenylaminofluorenyl and DCV groups.[90]
Compared to their absorption spectrum in solution (lmax =
514–526 nm), these compounds showed a broadening of the
PCEs of up to 1.6 and 1.5 %, respectively (Table 2). The
devices generated very high VOCs of up to 1.17 V because of
the low-lying HOMO energies of the donors. While the FF
obtained with these derivatives are relatively low (ca. 0.3)
because of the hole-injection barrier from the hole-transport
layer into the donor.
A–D–A type oligothiophenes 40 and 41 demonstrated,
how the electronic and device properties can be improved by
replacing the benzene ring in the benzothiadiazole (BTDA)
unit by a pyridine in thiadiazolopyridine (TDAPy).[89] Compared to the benzene unit in oligomer 40, the pyridine in 41 is
p-electron deficient; therefore, an increased electron
acceptor strength and a lowering of both, the HOMO
(DEHOMO = 0.15 eV) and the LUMO (DELUMO = 0.30 eV)
energy level results. In thin films, mixed pentamer 41 showed
a broad and red-shifted absorption (lmax = 548 nm, Egopt =
2.0 eV) compared to 40 (lmax = 503 nm, Egopt = 1.87 eV). In
PHJ solar cells, oligomer 41 gave a higher JSC of 4.7 mA cm2
relative to 3.1 mA cm2 for 40. In combination with a VOC of
1.0 V and a FF of 67 %, the devices based on 41 generated a
PCE of 3.2 %. Under similar conditions 40-based devices gave
PCEs of 1.7 % (Table 2). The excellent FF obtained with
derivative 41 should be due to a higher degree of ordering in
the thin film caused by intermolecular H-bonding interactions
by the nitrogen atom of the pyridine ring.
Wong and co-workers recently prepared a series of D–Asubstituted oligothiophenes 42–44 that are asymmetrically
charge transfer (CT) band and a red-shift of the absorption
onset by approximately 50 nm in thin films (DEopt = 1.8–
1.9 eV). The HOMO and LUMO energy levels of these dyes
were around 5.15 and 3.34 eV, respectively. PHJ devices
were fabricated by spin-coating of PEDOT:PSS onto an ITOglass substrate and subsequent vacuum-deposition of the
active layers. As-prepared devices showed moderate efficiencies in the range of 0.45–0.9 %. After thermal annealing, the
device efficiencies significantly increased to 1.6 % for bithiophene 42, to 2.1 % for terthiophene 43, and to 2.7 % for
quaterthiophene 44, respectively (Table 3). This threefold
increase in device efficiency after thermal annealing was
caused by an improved film morphology leading to a twofold
increase in both JSC and FF.
The research group of Wong also prepared D–p–A dyes
45 and 46 that comprise the same diphenylaminofluorenyl
donor but stronger tricyanovinylene (TCV) acceptor groups.
Very promising dyes were obtained with maximum absorp-
Table 3: Device characterizations of small molecule OSCs prepared by vacuum-deposition techniques.
Device structure
Concept JSC
VOC
[mA cm2] [V]
FF
h
Light
Ref.
[%] intensity
[mWcm2]
ITO/PEDOT:PSS/42 (20 nm)/5 (30 nm)/12 (8 nm)/Ag
ITO/PEDOT:PSS/43 (20 nm)/5 (30 nm)/12 (8 nm)/Ag
ITO/PEDOT:PSS/44 (20 nm)/5 (30 nm)/12 (8 nm)/Ag
ITO/PEDOT:PSS/45 (20 nm)/5 (30 nm)/12 (8 nm)/Ag
ITO/PEDOT:PSS/46 (20 nm)/5 (30 nm)/12 (8 nm)/Ag
ITO/47 (6.5 nm)/6 (40 nm)/12 (10 nm)/Al
ITO/2 (10 nm)/48:6 (2:3, 150 nm)/6 (10 nm)/Al
ITO/CuPc 2 (10 nm)/49:6 (2:3, 150 nm)/6 (10 nm)/Al
ITO/2 (10 nm)/50:6 (2:3, 150 nm)/6 (10 nm)/Al
ITO/PEDOT:PSS/51/5/Al
ITO/PEDOT:PSS/51/5/LiF/Al
ITO/PEDOT:PSS (30 nm)/52 (30 nm)/C60 (40 nm)/LiF(0.1 nm)/Al
ITO/PEDOT:PSS (30 nm)/52 (30 nm)/C70 (40 nm)/LiF(0.1 nm)/Al
ITO/6:NDN1 (5 nm, 2 wt %)/6 (25 nm)/53 (12 nm)/22: NDP9 (40 nm 20 wt %)/3:NDP9
(10 nm 2.5 wt %)/Au (4 nm)/Al
ITO/PEDOT:PSS (30 nm)/54 (5 nm)/54:6 (50 nm)/6 (10 nm)/12 (6 nm)/Al
PHJ
PHJ
PHJ
PHJ
PHJ
PHJ
BHJ
BHJ
BHJ
PHJ
PHJ
PHJ
PHJ
PHJ
4.9
6.3
6.1
4.0
3.5
7.1
4.8
4.5
5.7
3.6
4.6
2.6
3.6
2.9
0.89
0.89
0.91
0.82
0.79
0.75
0.50
0.50
0.48
0.96
1.15
0.92
0.90
0.99
0.36
0.38
0.48
0.40
0.39
0.60
0.60
0.64
0.63
0.29
0.28
0.71
0.66
0.76
1.6
2.1
2.7
1.3
1.1
3.2
1.4
1.5
1.7
1.0
1.9
1.7
2.2
1.9
100
100
100
100
100
100
100
100
100
100
80
100
100
100
[90]
[90]
[90]
[91]
[91]
[92]
[93]
[93]
[93]
[94]
[95]
[96]
[96]
[97]
P/B-HJ
8.4
0.91 0.52 4.1 100
[98]
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A. Mishra and P. Buerle
tion wavelengths at around 640 nm and low optical band gaps
of approximately 1.46 eV.[91] Compared to dyes 42 and 43, a
reduction of the LUMO energy level (3.9 eV) was observed
for 45 and 46. This shift was caused by the strong electronwithdrawing effect arising from the TCV acceptor. OSCs with
D–A-dyes 45 and 46 showed rather moderate efficiencies of
1.1–1.3 % (Table 3). This observation was ascribed to the
inefficient exciton dissociation process caused by the small
LUMO energy offset of only 0.1 eV owing to the dye LUMO
energy level (3.9 eV) with respect to the LUMO of C60
(4.0 eV).
Recently, Forrest et al. prepared BHJ solar cells using
squaraine dye 47 as donor and C60 as acceptor.[92] The
squaraine dye showed a lmax at 652 nm in solution, which
broadened and red-shifted to 700 nm in the solid state
because of strong excitonic interactions between adjacent
molecules. The HOMO/LUMO energy values derived from
electrochemical measurements were 5.3 and 3.4 eV,
respectively. A solar cell using a squaraine layer in a thickness
of 6.5 nm achieved a PCE as high as 3.2 % with a VOC of
0.75 V (Table 3). A decrease in PCE was observed with
increasing layer thickness of the squaraine, which was due to
the reduction of JSC and FF caused by poor charge transport.
Chan and co-workers reported a series of rhenium(I)
complexes 48–50 that were implemented as donor material
into the active layer of BHJ solar cells.[93] The active layer
consisted of a blend of rhenium complexes and C60 that were
co-deposited by vacuum sublimation. Additional 2 and C60
layers were used as hole- and electron-collecting materials,
respectively. The rhenium complexes served the dual purpose
of providing sensitization in the green spectral region, where
the absorption of 2 and C60 is low and ensuring efficient
charge transport. With a D/A blend ratio of 2:3 the device
based on complex 48–50 gave PCEs in the range of 1.4–1.7 %
and high FF values in excess of 0.64. (Table 3).
Roncali and co-workers prepared the star-shaped D–A
dye 51 that comprises a triphenylamine core and thienyl arms
as donors and terminal DCV groups as acceptors.[94] This dye
showed a p–p* absorption maximum at 509 nm in solution
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and at 538 nm in thin films. A bilayer HJ device was prepared
by successive thermal evaporation of the donor and the
acceptor C60, resulting in the configuration ITO/PEDOT:PSS/
51/C60/Al. The cell displayed a JSC of 3.65 mA cm2, a VOC of
0.96 V, a FF of 0.29, and a PCE value of 1.02 % under white
light illumination.[94] In a recent report, by using a LiF layer
between the acceptor C60 and the Al cathode, the cell
efficiency was further increased to about 1.9 % at
80 mW cm2 irradiation (Table 3).[95] The improvement was
due to the increase in JSC and VOC.
Shirota et al. synthesized star-shaped, hole-transporting,
amorphous material 52 and used it in a BHJ solar cell as an
electron donor in combination with C60 or C70 as electron
acceptor. The cells exhibited PCEs of 1.7 and 2.2 %,
respectively, with high FF values of 0.71 and 0.66 under AM
1.5G illumination at an intensity of 100 mW cm2 (Table 3).
The obtained high VOC values of 0.90 and 0.92 V, respectively,
were due to the low-lying HOMO energy level of 52
(5.57 eV).[96]
Diindenoperylene 53 was used as donor material in p-i-n
type PHJ solar cells.[97] The compound absorbs in the spectral
range of 450–600 nm resulting in an optical band gap of 2 eV.
In conjunction with C60 as the acceptor and ZnPc as the
complimentary absorber, devices displayed an impressive FF
of 76 % and VOC of 0.99 V, reaching PCEs close to 1.9 %. The
high FF was associated with the efficient separation of
photogenerated excitons that reach the D–A interface, in
combination with high charge-carrier-collection efficiency of
electrons and holes.
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Organic Semiconductors
Brtting et al. reported a comparative study of PHJ and P/
B-mixed heterojunction devices using nonsubstituted diindenoperylene (54) as donor material in combination with C60 as
an acceptor.[98] In P/B-HJ devices, a 50 nm bulk layer was
sandwiched between the neat 5 nm donor and 10 nm acceptor
layers. PEDOT:PSS was used as hole transport and 12 as the
exciton-blocking layer. The donor films in both device
structures were deposited on substrates heated at 100 8C,
thus increasing the size of the crystalline domains of the donor
films, which was supported by AFM measurements. This led
to the formation of a phase-separated bicontinuous network
of donors and acceptors in the bulk. The P/B-HJ devices
generated a very good PCE of 4.1 % with a JSC value of
8.4 mA cm2 and a VOC of 0.91 V. The PHJ devices prepared
by keeping the donor layer thickness of 30 nm and acceptor
layer thickness of 35 nm gave a PCE of 2.7 %, a JSC of
4.8 mA cm2, and a FF of 0.60. The efficiency of PHJ cells
could be further raised to 3.9 % by increasing the donor and
acceptor layer thicknesses to 50 and 80 nm, respectively. The
improvement was due to the increase in JSC (5.7 mA cm2) and
FF (0.74) values. The high performance in both device
architectures was ascribed to the favorable film morphology
and high crystalline order, which allows for improved charge
carrier transport towards the respective electrodes and
reduced recombination losses.
The progress in organic photovoltaics in recent years has
been tremendous, in particular for vacuum-processed solar
cells with small molecules/oligomers as the electronically
active material and efficiencies of 5.2 % for single junction
and 9.8 % for tandem cells have recently been achieved.
Compared to the rather moderate performance of the initial
one-layer or bilayer cells, this success was only possible by the
advance and development of, on one hand, novel innovative
cell architectures, such as multilayer devices (p-i-n or m-i-p)
that comprise additional hole-transport, doped hole-transport, as well as exciton-blocking layers, and the creation of
bulk heterojunctions by co-evaporation of donor and
acceptor material. On the other hand, the improvement also
came from the development of a multitude of novel dyes and
p-conjugated structures. The materials have been greatly
improved owing to the progress in versatile synthesis of
organic semiconductors and consequently the tunability of
their photophysical properties.
For donors or p-type semiconducting materials it turned
out, that the number of the most efficient classes of
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
compounds is quite limited. In single junction devices, D–p–
A dyes such as squaraine 47 or merocyanine 137 (see below,
section 5) are promising materials showing efficiencies up to
3.2 % and 4.9 %, respectively.[92] Other promising materials
are phthalocyanines, which originally and extensively have
been employed as active materials in OSCs, with up to 5 %,[58]
and oligothiophenes such as DCV-substituted quinquethiophene 34 with up to 5.2 %.[86] The latter two classes also were
the most successful for multijunction devices. Here, phthalocyanines[17] have been trimmed to 5.7 % in tandem cells, which
were recently far exceeded by oligothiophenes giving a
certified record efficiency of 9.8 % in a tandem cell of
1.1 cm2.[27] Besides very high molar absorptivities and good
charge transport properties, especially oligothiophenes show
high ionization potentials, which lead to high VOCs.
With respect to acceptors or n-type semiconducting
materials, without any doubt, fullerene C60 is the champion
molecule, which has been used in most vacuum-processed
solar cells. Most probably due to their spherical structure,
fullerenes are far superior to flat 2-dimensional systems, such
as, e.g., perylene derivatives.
Wide-band-gap triarylamines in various shapes and
molecular architectures have been made and are the most
frequently used class of compounds for hole transport layers.
Their structural variability allows for the adjustment of their
ionization potential to the photoactive layer. In the case of
exciton-blocking layers, typically phenanthrolines are most
effective and are frequently used materials.
3. Bulk-Heterojunction Solar Cells Made of Small
Molecules by Solution Processing
The major disadvantage of planar bilayer devices is the
limited interfacial area between donor and acceptor layers.
The exciton diffusion length (LD) in these materials is up to
several orders of magnitude smaller than the absorption
penetration depth. Thus, only excitons generated at the
interface can be separated into free charge carriers.[99] Therefore, the thickness of the D–A layers in these cells is very
limited, typically to the regime of the exciton diffusion length
LD. This difficulty was overcome by the realization of BHJ
architectures in which a blend of donor and acceptor
molecules is used to create a composite material exhibiting
nanoscale phase separation (Figure 5).[10, 21, 23, 100, 101] BHJ solar
cells are commonly composed of a blend film of a conjugated
Figure 5. Fundamental processes (light illumination, exciton formation, charge separation, charge migration) of bulk-heterojunction solar
cells (p = donor material, n = acceptor material).
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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polymer or a small molecule/oligomer as donor and a soluble
fullerene derivative as acceptor, which are sandwiched
between an ITO anode and a low-work-function metal
cathode. The main advantage of this approach is that the
interfacial area (photocurrent generation region) between
donor and acceptor is enormously increased, thus resulting in
a more efficient dissociation of excitons into free charge
carriers. The performance of BHJ solar cells moreover
depends on the charge carrier mobility and the nanoscale
blend morphology.[102–105] The general device structure of a
BHJ solar cell is depicted in Figure 5.
In BHJ solar cells, solution-processable conjugated polymers are typically used as p-type semiconducting phase and
many examples led to highly efficient solar cells (see above).
In recent time, small molecules/oligomers have increasingly
been used as active component in BHJ solar cells. One
advantage of the oligomer approach comes from their defined
chemical structure. In contrast to most polymers, reproducible materials become available by well-controlled chemical
reactions. In most cases solubilizing side chains are required
that allow purification of the materials on one hand and
solution-processing for solar cells on the other hand. With this
recent development, a direct comparison between the two
organic solar cell production technologies, vacuum- and
solution-processing, becomes possible when the same oligomer is used (see Section 5).
For solution-processed BHJSCs, Chen et al. prepared
DCV-terminated and regioregularly octyl-substituted septithiophene 55, which in thin films showed broad absorption
extending from 400 to 800 nm with a peak at 614 nm.
HOMO–LUMO energy levels were determined to be 5.1
and 3.4 eV, respectively.[106, 107] BHJ solar cells were prepared by spin-casting dye 55 as donor and phenyl-C61-butyric
acid methyl ester (PC61BM) 160 as acceptor from a chloroform solution in a 1:1.4 ratio. The cells generated PCEs of
3.7 % with a good VOC of 0.88 V and a very high JSC of
12.4 mA cm2 (Table 4). However, the FF of these devices was
still low. The device performance was reduced by increasing
or decreasing the ratio of PC61BM. The results demonstrated
that a 1:1.4 w/w ratio was enough to produce an effective D/A
interface for exciton dissociation and to form a percolation
pathway for charge transport to the respective electrodes.
Wong and co-workers reported D–p–A type dyes 56 and
57, in which the diphenylaminofluorenyl group acts as the
donor, the oligothiophene as a p-bridge and the DCV group
as acceptor.[108] In thin films, the compounds showed quite low
optical band gaps of around 1.86 eV. Solution-processed BHJ
solar cells were fabricated using 56 or 57 as donor and 160 as
an acceptor (1:2 w/w) resulting in rather moderate PCEs of
0.97 and 0.76 %, respectively (Table 4). By using a blend ratio
of 1:4 for 56:160 the PCE was improved to 1.72 % caused by a
VOC of 0.79 V, a JSC of 5.4 mA cm1, and a FF of 0.40.
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Oligomer 58 comprising a bithiophene unit endowed with
triphenylaminevinyl groups showed an absorption maximum
at 461 nm with a band gap of 2.54 eV. A BHJ solar cell
including an active layer of 58:160 (1:4, w/w) showed PCEs of
0.34 % under simulated AM1.5G solar irradiation at
100 mW cm2 (Table 4).[109] The moderate cell performance
was due to a low absorption coverage of the solar spectrum.
Our research group reported a series of novel A–D–A
oligomers consisting of hexyl-substituted oligothiophenes
integrated between terminal perylenemonoimides.[110] Triad
59 showed a p–p* absorption band peaking at 523 nm, which
is characteristic for perylenes resulting in an optical band gap
of 2.12 eV. The HOMO/LUMO energy levels were estimated
from electrochemical measurements to be 5.5 and 3.70 eV,
respectively. Transient absorption and time-resolved fluorescence measurements revealed that charge transfer (CT)
bands appeared only in benzonitrile solution, whereas no
evidence of CT states was found in toluene.[111] BHJ solar cells
prepared using a 1:4 mixture of 59:160 showed a VOC = 0.68 V,
a JSC = 0.7 mA cm2, and a FF = 0.31, resulting in a moderate
PCE of 0.2 % under standard AM, 1.5G conditions at
100 mW cm2 (Table 4).
D–p–A dye 60 consisting of head-to-tail coupled octi(3hexylthiophene) covalently linked to perylenemonoimide
showed a broad p–p* absorption between 300 and 550 nm
with a high molar extinction coefficient of about
45 000 L mol1 cm1.[112] The optical band gap was calculated
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Organic Semiconductors
Table 4: Device characterizations of OSCs prepared by solution-processing techniques.
Device structure
JSC
[mA cm2]
VOC
[V]
FF
h
[%]
Light intensity
[mWcm2]
Ref.
ITO/PEDOT:PSS (40 nm)/55:160 (1:1.4, 110 nm)/LiF (1 nm)/Al
ITO/PEDOT:PSS/56:160 (1:2, 160–102 nm)/LiF (1 nm)/Al
ITO/PEDOT:PSS/57:160 (1:2, 160–102 nm)/LiF (1 nm)/Al
ITO/PEDOT:PSS (30 nm)/58:PC61BM (1:4, 45 nm)/BCP (5 nm)/Al
ITO/PEDOT:PSS (30 nm)/59:160 (1:4)/Al
ITO/PEDOT:PSS (30 nm)/60:160 (1:4)/Al
ITO/PEDOT:PSS/61:160 (1:2)/Al
ITO/PEDOT:PSS/62:160 (1:2)/Al
ITO/PEDOT:PSS (40 nm)/63:160 (ca. 100 nm)/Al
ITO/PEDOT:PSS (40 nm)/64:160 (ca. 100 nm)/Al
ITO/PEDOT:PSS (40 nm)/64:172 (ca. 100 nm)/Al
ITO/PEDOT:PSS/P3HT:172:64 (5:5:1)/Al
ITO/PEDOT:PSS (50 nm)/65:172 (ca. 100 nm)/Al
ITO/PEDOT:PSS (50 nm)/66:160/Al
ITO/PEDOT:PSS (50 nm)/67:160/Al
ITO/PEDOT:PSS (40 nm)/68:160 (1:1)/LiF (0.8 nm)/Al
ITO/PEDOT:PSS (50 nm)/69:160 (1:1)/LiF (0.8 nm)/Al
ITO/PEDOT:PSS/70:160 (1:4)/Ba/Al
ITO/PEDOT:PSS/71:160 (1:2)/Al
ITO/PEDOT:PSS/P3HT:172:71 (2:2:0.5)/Ca/Al
ITO/PEDOT:PSS/72:160 (1:2)/Ba/Al
ITO/PEDOT:PSS/73:160 (1:2)/Ba/Al
ITO/PEDOT:PSS/74:160 (1:1)/Ba/Al
ITO/PEDOT:PSS/75:172 (1:3)/LiF/Al
ITO/PEDOT:PSS/76:160 (1:3)/LiF/Al
ITO/PEDOT:PSS/77:160 (1:3)/Al
ITO/PEDOT:PSS/77:172 (1:3)/Al
ITO/PEDOT:PSS/78:172 (1:2)/Al
ITO/PEDOT:PSS/79:160 (1:3)/Ba/Al
ITO/PEDOT:PSS/80:160 (1:3)/Ba/Al
ITO/PEDOT:PSS/80:172 (1:3)/Ba/Al
ITO/PEDOT:PSS/81:PC71BM (1:3)/Ba/Al
ITO/PEDOT:PSS/82:160 (1:1)/Al
ITO/PEDOT:PSS/83:160 (1:1)/Al
ITO/84:160 (1:1)/Al (annealed)
ITO/85:160 (1:1)/Al (annealed)
ITO/PEDOT:PSS/86:160 (1:1)/Al (annealed)
ITO/PEDOT:PSS/87:160 (1:1)/Al
ITO/PEDOT:PSS/87:169 (1:1)/Al
ITO/PEDOT:PSS/88:160 (1:1)/Al
ITO/PEDOT:PSS/88:169 (1:1)/Al
ITO/PEDOT:PSS/89:160 (1:1)/Al (annealed)
ITO/PEDOT:PSS/90:160 (1:1)/Al (annealed)
ITO/PEDOT:PSS/89:90:160 (1:1:1)/Al (annealed)
ITO/PEDOT:PSS/91:160 (1:1)/Al (annealed)
ITO/PEDOT:PSS/92:160 (1:1)/Al (annealed)
12.4
4.97
1.96
1.9
0.7
–
5.9
0.8
8.4
8.1
9.2
8.6
10.0
6.3
2.4
3.6
3.0
1.9
4.1
10.6
1.59
0.65
0.86
4.8
4.8
1.8
3.5
5.5
4.0
2.1
5.1
5.9
3.3
2.7
5.2
5.3
7.1
3.9
6.8
3.5
5.0
6.1
5.5
7.6
6.9
8.2
0.88
0.79
0.75
0.51
0.68
0.94
0.78
0.57
0.67
0.80
0.75
0.63
0.92
0.74
0.66
0.82
0.70
0.75
0.89
0.69
0.70
0.85
0.76
0.71
0.87
0.80
0.86
0.96
0.94
0.90
0.71
0.79
0.78
0.98
0.85
0.87
0.84
0.68
0.84
0.72
0.88
0.84
0.93
0.85
0.80
0.74
0.34
0.31
0.52
0.34
0.31
–
0.31
0.27
0.45
0.45
0.44
0.59
0.48
0.38
0.36
0.40
0.37
0.34
0.46
0.61
0.22
0.30
0.33
0.38
0.40
0.39
0.41
0.37
0.40
0.41
0.38
0.44
0.47
0.49
0.53
0.54
0.54
0.46
0.48
0.42
0.46
0.51
0.41
0.56
0.49
0.52
3.7
1.0
0.8
0.3
0.2
0.5
1.4
0.1
2.3
2.9
3.0
3.2
4.4
1.7
0.6
1.2
0.8
0.5
1.7
4.5
0.22
0.16
0.26
1.3
1.7
0.6
1.2
2.0
1.5
0.8
1.4
2.1
1.2
1.3
2.3
2.5
3.2
1.2
2.7
1.0
2.0
2.6
2.1
3.6
2.7
3.2
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
85
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
[107]
[108]
[108]
[109]
[110]
[112]
[113]
[113]
[115]
[116]
[117]
[119]
[118]
[120]
[120]
[121]
[122]
[124]
[125]
[126]
[127]
[128]
[129], [130]
[131]
[131]
[132]
[132]
[133]
[135]
[134]
[136]
[136]
[137]
[138]
[139]
[139]
[140]
[141]
[141]
[141]
[141]
[142]
[142]
[142]
[143]
[143]
to 2.12 eV. Strong quenching of the fluorescence was
observed due to the occurrence of intramolecular photoinduced electron transfer processes. The fabrication of BHJ
solar cells based on a 1:4 blend ratio of 60:160 revealed a VOC
of 0.94 V and a PCE of 0.5 % under a simulated sun spectrum
(Table 4).
Ko and co-workers reported an improvement of the
device performance by annulation of triphenylamine-oligothiophene dyads to perylene bisimide.[113] Devices based on
annulated perylene derivative 61 as donor displayed a PCE of
1.4 % as compared to 0.1 % obtained with non-annulated 62based devices (Table 4). The result was ascribed to the
improved light harvesting efficiency, increased charge carriers
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
mobility, and balanced charge transport in the devices based
on 61 and 160. The performance was further correlated to the
bulk morphology determined by AFM measurements. AFM
images of 61:160 blends demonstrated randomly distributed
islands connected to each other with interpenetrating networks. In contrast, 62:160 blends displayed large segregation
of each phase.
Organic dyes based on diketopyrrolopyrrole (DPP) have
been widely investigated as organic pigments in industrial
applications such as paints, plastics, and inks.[114] Because of
the planar conjugated structure and the electron-accepting
nature of the amide group, the DPP unit can be used to
construct low-band-gap materials. The integration of a DPP
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unit into a conjugated backbone can alter the orbital energy
levels and fine-tune the absorption wavelength. To take
advantage of both, DPP and oligothiophene units, Nguyen
and co-workers developed a new class of dyes 63–65.[115–118] To
increase the solubility of the hybrid system, the nitrogen
atoms of the DPP unit were protected by tert-butyloxycarbonyl (Boc) groups. Compound 63 showed an absorption
band edge at 700 nm (1.77 eV) in solution and at 810 nm
(1.53 eV) in thin films. The HOMO energy value determined
from ultraviolet photoelectron spectroscopy (UPS) measurements was about 4.9 eV. Using the space-charge limited
current (SCLC) method, the hole mobility of 63 was
determined to 3 106 cm2 V1 s1, which was reduced to
approximately 108 cm2 V1 s1 in blended films. When
blended with 160 in a donor/acceptor ratio of 7:3, BHJ solar
cells generated PCEs of up to 2.3 % (Table 4), which was
ascribed to the good film morphology as characterized by
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AFM measurements. Annealing of the devices was not
performed because of the possible thermal cleavage of the
Boc-protecting groups, which would change chemical and
electronic properties of the dye. To improve the thermal
stability and solubility, Nguyen and co-workers replaced
the Boc groups of the DPP unit in 64 by ethylhexyl
chains.[116, 117] The HOMO energy level of 64 determined by
ultraviolet photoelectron spectroscopy (UPS) measurement was 5.2 eV. The hole mobility (mh) of DPP 64 is
two orders of magnitude larger (1 104 cm2 V1 s1) than
that of Boc derivative 63 and does not significantly change
in blended films. As-cast devices prepared from a DPP
64:160 (1:1, w/w) blend gave a JSC of 7.87 mA cm2, a VOC of
0.77, a FF of 0.36, and a PCE of 2.2 %. After thermal
annealing (100 8C for 5 min), the PCE was increased to
2.9 % caused by a significant increase of the FF from 0.36 to
0.45 (Table 4). The slight increase in VOC by 0.13 V for 64
compared to Boc derivative 63 was ascribed to the lower
HOMO energy level of 64. This change in device performance was mainly ascribed to a significant improvement in
film morphology.
Nguyen and co-workers replaced PC61BM (160) by
PC71BM (172) as electron acceptor to improve light
harvesting in the visible region. With a D–A blend ratio of
1:1, 64:172 devices exhibited a PCE of 3.0 % with a high JSC of
9.2 mA cm2 (Table 4).[117] Compound 64 was also added to a
P3HT:172 active layer as near-IR absorber, which donates
holes to P3HT and electrons to 172.[119] The device performance of the blend system P3HT:172:64 (10:10:2 mg mL1)
showed both increased JSC (7.7 to 8.6 mA cm2) and VOC (0.6
to 0.63 V) relative to the P3HT/172-only device. The PCE
thus increased from 2.9 % to 3.2 % due to the additional light
harvesting of 64 in the 700 nm regime.
DPP derivative 65 comprising benzofuran terminal groups
was prepared and showed HOMO/LUMO energy levels of
5.2 and 3.4 eV, respectively. The mh for 65 was in the order
of 1 105 cm2 V1 s1, which increased to 3 105 cm2 V1 s1
in blended films (60:40, w/w with 172). The hole mobilities did
not change significantly upon thermal annealing. When
blended with 172 in a ratio of 3:2, the as-cast devices
showed PCEs of only 0.3 % with JSC of 1.5 mA cm2 and FF of
0.24. The device efficiency was dramatically improved by
thermal annealing. After annealing at 110 8C for 10 min, the
devices exhibited an excellent PCE of 4.4 % with high JSC =
10 mA cm2 and VOC = 0.92 V (Table 4).[118] The authors
describe the influence of the blend ratios and annealing
procedure on film morphology and device performance.[115, 117, 118]
Reynolds et al. prepared D–A–D and A–D–A type
oligomers 66 and 67 using an isoindigo unit as electron
acceptor.[120] The compounds showed broad absorption bands
with maxima at 579 and 560 nm for 66 and 67, respectively. In
thin films, the absorption bands were red-shifted by about
80 nm, leading to optical band gaps of 1.67 and 1.76 eV,
respectively. The HOMO/LUMO energy levels (66: 5.6,
3.9 eV; 67: 5.6, 3.8 eV) were estimated from cyclic
voltammetry on drop-cast films on Pt button electrodes.
When dye 66 was blended with 160 (1:1, w/w), the BHJ device
achieved a PCE of 1.7 %, which is higher compared to 0.6 %
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the solution processability. In this respect, D–A–D compound
70 that consists of a thiadiazoloquinoxaline acceptor and a
diphenylaminofluorenyl donor unit was prepared showing a
charge transfer (CT) band at 698 nm in solution with an
absorption edge extended to 880 nm (DEopt = 1.41 eV). However, the high electron affinity of the thiadiazoloquinoxaline
moiety brought the LUMO energy level of 70 close to the
LUMO level of 160, thus leading to inefficient charge
separation. The device fabricated by blending 70 with 160
(1:4) showed a photoresponse up to 950 nm with a moderate
PCE of 0.5 % and JSC of 1.9 mA cm2 (Table 4).[124]
D–A–D compound 71 containing a dibenzo[f,h]thieno[3,4-b]quinoxaline core end-capped with diphenylaminofluorene was prepared exhibiting intense p–p* absorptions in the
380–475 nm region and a very weak CT band at approx-
generated by a device prepared from 67:160 (3:2, w/w;
Table 4). The better performance of 66 compared to 67 was
partly ascribed to a higher degree of order in thin films.
Demadrille and co-workers prepared a series of oligomers
that consist of an electron-accepting fluorenone as central
unit, which is symmetrically coupled to different oligothiophene donor segments.[121] The combination of D–A units
resulted in the broadening of the absorption spectral window
due to an intramolecular charge transfer (ICT) transition.
Among this series, D–A dye 68 endowed with dialkylated
quaterthiophenes showed the best solar cell performance. The
HOMO/LUMO energy levels were determined from electrochemical measurements to 5.16 and 3.25 eV, respectively.
Devices based on 68:160 (1:1, w/w) after thermal annealing
gave a PCE of 1.2 % (Table 4).
Very recently, Porzio et al. prepared fluorenone endcapped sexithiophene 69 and used it as donor material in BHJ
solar cells. Devices prepared using 69:160 (1:1, w/w) reached
maximum PCEs of about 0.8 % under AM 1.5, 100 mW cm2
illumination.[122]
Triphenylamine-based small molecules have been extensively used as amorphous hole-transporting molecular materials in various optoelectronic applications due to their high
hole mobilities.[123] It is expected that by introduction of a
triphenylamine donor moiety into the conjugated backbone
in combination with an electron-deficient unit, a D–A-based
low-band-gap material can be obtained. Furthermore, the
nonplanar structure of a triphenylamine unit could improve
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
imately 580 nm leading to an optical band gap of 1.82 eV.
The HOMO and LUMO energy levels were determined to
5.3 and 3.3 eV, respectively. BHJ solar cells prepared
from chloroform solutions of 71 and 67 wt % of 160 gave a
PCE of 1.7 % (Table 4).[125] Oligomer 71 was incorporated
as additional donor material into P3HT:172 solar cells.[126]
The HOMO and LUMO energy levels of 71 possess
suitable band edge offsets compared to P3HT and 172.
Thus, it could act as an electron acceptor relative to P3HT
and as electron donor to 172. Photovoltaic devices based on
ternary mixtures of P3HT:172:71 (2:2:0.5, w/w) showed a
higher PCE of 4.5 % compared to the device prepared with
only P3HT:172 (2:2, w/w; JSC = 9.74 mA cm2, VOC = 0.6 V,
FF = 0.67, h = 3.9 %).
Oligomers 72 and 73, including a diphenylaminofluorenethiophene donor and benzothiadiazole acceptor showed
optical band gaps of 1.89 and 1.75 eV, respectively. Due to
their low-lying HOMO energy levels (5.30 eV for 72 and
5.25 eV for 73), the devices exhibited good VOCs of 0.70 and
0.85 V, respectively. BHJ solar cells based on these oligomers
were blended with 160 (1:2, w/w) and gave rather low PCEs of
0.22 % for 72 and 0.16 % for 73, which was due to low JSC and
FF values (Table 4).[127, 128]
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thiophene units.[133] The HOMO and LUMO energy values
determined from electrochemical measurements were 5.27
and 3.1 eV, respectively. Devices prepared with active layers
of 78:172 (1:2, w/w) spin-coated from dichlorobenzene
solutions had PCEs of 1.96 % and a high VOC of 0.96 V.
D–A–D dyes 79 and 80 were prepared containing
electron-rich triphenylamines as donor and electron-deficient
2-pyran-4-ylidenemalononitrile as acceptor.[134, 135] In thin
A–D–A compound 74 comprising a triphenylamine donor
and a benzothiadiazole acceptor moiety linked by double
bonds was prepared for BHJ solar cells.[129, 130] The dye showed
two absorption bands at 398 and 544 nm, respectively, which
were attributed to p–p* and CT transitions. The HOMO and
LUMO energy levels were calculated to be 5.1 and 3.3 eV,
respectively, with a corresponding band gap of 1.8 eV. A
device based on a blended 74:160 (1:1) active layer generated
a VOC of 0.76 V and a PCE of 0.26 % at an incident light
intensity of 85 mW cm2 (Table 4). This rather poor performance could be caused by the nonplanar structure of the
triphenylamine unit and the free rotation of the double bonds.
Zhang et al. recently reported oligomers 75 and 76, which
showed two absorption bands in the range of 320–450 and
500–700 nm.[131] The HOMO energy levels determined by
UPS measurements were 5.1 and 5.2 eV, respectively. The
PCE values of solar cells fabricated from blends of these
oligomers as donor and 172 as acceptor (1:3, w/w) were 1.3 %
for 75 and 1.7 % for 76 (Table 4). Although both compounds
showed similar JSC values, the higher PCE for hexyl-substi-
tuted 76 was due to the higher VOC of 0.87 V compared to the
device based on 75 (VOC = 0.71 V). In contrast, devices
prepared from blends of analogous 77:160 (1:3, w/w) gave
PCEs of only 0.56 % with JSC values of 1.8 mA cm2.[132] The
lower PCE for 77 compared to 75 and 76 should be due to the
presence of the long dodecyl chains, which may lead to
unfavorable blend morphologies and consequently to worse
charge carrier transport. The PCE of solar cells based on 77
was improved to 1.2 % by using 172 as acceptor, giving a JSC of
3.5 mA cm2, a VOC of 0.86 V, and a FF of 0.41.
Recently, Li and co-workers prepared dye 78 including a
triphenylamine core endowed with two benzothiadiazole-
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films, both dyes showed absorption maxima at around
500 nm and optical band gaps of approximately 1.9 eV. The
HOMO and LUMO energy levels of 79 (5.28, 3.45 eV)
were lower compared to 80 (5.14, 2.76 eV). BHJ solar cells
made from 80 using 160 as acceptor (1:3, w/w) gave moderate
PCEs of 0.8 %, while the efficiency of devices made from 79
increased to 1.5 % (Table 4). This enhanced efficiency is
reflected by an improved JSC value, which is attributable to the
higher mh of 79 (1.4 105 cm2 V1 s1) compared to that of 80
(1.2 106 cm2 V1 s1) determined by the SCLC method using
the device structure ITO/PEDOT:PSS/oligomer/Au.
The replacement of internal phenyl units of oligomer 80
by thiophene units in 81 further red-shifted the absorption
maximum by 40 nm in thin films, thus lowered the optical
band gap to 1.79 eV.[136] BHJ solar cells were prepared using
81 as donor and 172 as acceptor (1:3, w/w) and showed PCEs
of 2.1 %, which is higher compared to 1.4 % for 80-based
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devices (Table 4). The results further demonstrated the
importance of thiophene units in the conjugated backbone
for OSCs.
All devices based on these triphenylamine compounds
showed lower FF values of about 0.3–0.5 caused by large
leakage currents or small shunt resistances, which was further
ascribed to the poor packing in the active layers.
A series of low-band-gap oligomers 82–86 with terminal
cyanovinylene-4-nitrophenyl acceptor units were prepared
for solution-processable BHJ solar cells. Derivatives 82 and
83 showed absorption maxima at around 640 nm in thin films
with optical band gaps of 1.67 eV. These materials can be used
as donors in BHJ solar cells with 160 as acceptor because of
their suitable HOMO and LUMO energy levels (ca. 5.25
and 3.55 eV). Devices based on 82 and 83 gave moderate
PCEs of 1.2 and 1.3 %, respectively (Table 4).[137, 138] The
mixed oligomers were further incorporated as third component in p-phenylene vinylene-based co-polymer:160 blends to
enhance the photovoltaic performance of polymer solar cells.
BHJ solar cells prepared using a blend of 82:polymer:160
after thermal annealing gave a PCE of 2.6 %, a JSC of
5.8 mA cm2, a VOC of 0.81 V, and a FF of 0.55. On the other
hand, devices comprising a 83:polymer:160 blend resulted in a
PCE of 3.16 % a JSC of 6.1 mA cm2, a VOC of 0.92 V, and a FF
of 0.54. Incorporation of the oligomers into the polymer:160
devices improved the light harvesting of the blends and
provided efficient charge transfer in copolymer and 160
phases, respectively.
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
In thin films, dyes 84 and 85 that contain thiophene and
anthracene moieties showed absorption maxima at 630 and
640 nm, respectively. The HOMO and LUMO energy levels
were determined to be 5.0 and 3.2 eV, respectively. BHJ
devices using 84 and 85 as donor and 160 as acceptor gave
PCE values of 1.4 and 1.66 % and high VOC values of 0.94 and
0.92 V, respectively.[139] The PCEs were further enhanced to
2.3 and 2.5 %, respectively, upon thermal annealing of the
blend layers. The VOC slightly decreased upon thermal
annealing, while both JSC and FF increased, thus indicating
an enhanced charge carrier transport because of an improved
packing density. Therefore, the efficiency of exciton dissociation and charge transport was improved.
Dye 86 that comprises the acenapthoquinoxaline core
showed an absorption maximum at 642 nm in thin films with
an optical band gap of 1.59 eV.[140] Thermal annealing of the
film resulted in a significant broadening of the absorption
spectra that was attributed to the interchain interactions and
to the increase in crystallinity of the materials. BHJ solar cells
prepared from as-cast and thermally annealed blend layers of
86:160 (1:1, w/w) showed overall PCEs of 2.2 and 3.2 %,
respectively (Table 4). The increased PCE for the thermally
annealed blend was attributed to the improved EQE (from
47 % to 63 %) near the absorption maximum and the
improved JSC (from 5.2 to 7.1 mA cm2).
Low-band-gap oligomers 87 and 88 comprising thienothiadiazole and benzobisthiadiazole central units showed
long-wavelength absorption maxima at 630 (Eg = 1.63 eV)
and 643 nm (Eg = 1.62 eV), respectively, in thin films.[141] The
attachment of the benzobisthiadiazole unit in 88 lowers the
HOMO/LUMO energy levels by about 0.10 eV compared to
87. In BHJ devices using 160 as acceptor, oligomer 87 gave a
PCE of 1.2 %, which is slightly higher than the PCE obtained
with 88-based devices (h = 1.0 %; Table 4). The PCEs were
further increased to 2.7 and 2.0 %, respectively, by using the
new fullerene derivative 169 as an acceptor. This improvement was attributed to the better absorption of 169 in the
visible region than that of 160 leading to higher JSC and to the
higher VOC resulting from the higher LUMO level of the
former.
Mikroyannidis et al. recently reported pyrrole bisazo dyes
89 and 90. The dyes showed broad absorption bands in thin
films with optical band gaps of 1.39 and 1.68 eV, respec-
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tively.[142] BHJ solar cells prepared using azo dyes 89:160 (1:1,
w/w) showed a PCE of 2.23 %, a VOC of 0.86 V, and JSC of
5.4 mA cm2, which was higher compared to the 90:160 device
(PCE = 1.55 %, VOC = 0.95, JSC = 4.3 mA cm2). The PCE of
the devices were further improved to 2.6 and 2.1 %, respectively, after thermal annealing of the photoactive layers
(Table 4). The higher PCE for devices prepared with 89
compared to 90 was ascribed to higher hole mobility of 89 and
efficient photoinduced charge transfer at the D/A interface.
Furthermore, BHJ devices fabricated using a ternary mixture
of 89:90:160 gave PCEs of up to 3.6 % due to the increased JSC
value of 7.6 mA cm2.
Pyrrolyl azo dye 91 showed an optical band gap of 1.54 eV
and HOMO and LUMO energy levels of 5.2 and 3.6 eV,
respectively. Complexation of 91 with BF3·Et2O formed BF2azopyrrole complex 92, which showed a broader absorption
spectrum and a lower band gap of 1.49 eV compared to 91.[143]
BF2-complex 92 was reported to be stable only in nonpolar
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solvents. BHJ solar cells with azo dyes 91 and 92 as donor and
160 as acceptor in a blend ratio of 1:1 showed PCEs of 1.76
and 1.2 %, respectively. The PCEs were increased to 2.7 and
3.15 % by thermal annealing of the device after top aluminum
deposition (Table 4). The increase in PCE for the contactannealed device was interpreted in terms of more balanced
charge transport, due to the enhanced hole mobility. The
increased photovoltaic performance of 92 was also supported
by a broad EQE spectrum with a maximum close to 60 % at
approximately 600 nm.
Marks and co-workers prepared a series of oligomers, in
which di(hexyloxy)phenylene moieties were attached to a
central anthracene (93, 94) or benzothiadiazole unit (95, 96)
by triple bonds.[144, 145] The introduction of hexyloxy groups
enhanced the solubility of these oligomers in organic solvents.
The thin-film spectra of these oligomers were red-shifted by
25 to 60 nm relative to the solution spectra, which can be
attributed to greater structural organization in the solid state.
The optical band gaps in solution were in the range of 2.2 to
2.45 eV. OFET measurements revealed that compound 94
and 96 exhibited high mobilities of 0.07 and 0.02 cm2 V1 s1,
respectively, while oligomers 93 and 95 showed significantly
lower mobilities (< 105 cm2 V1 s1). BHJ solar cells fabricated using 93 and 94/160 in a 1:1 w/w ratio gave similar JSC
values of 2.6 mA cm2 and VOC values of 0.96 and 0.93 V,
respectively. In combination with FF values of 0.45 and
0.41 %, these dyes gave PCEs of 1.2 and 1.0 %, respectively
(Table 5).[144, 145] When the D/A blend ratios were increased to
2:1, the PCE of 93 did not change, while the PCE of 94 was
increased to 1.2 %. The high VOC values obtained for these
devices were due to the low-lying HOMO energy levels of
5.5 eV. However, further increasing the content of 160 in 93
and 94 blends (1:3 w/w) led to a significant reduction in the JSC
(to 0.86 and 0.65 mA cm2, respectively) as well as in VOC (to
0.81 and 0.66 V, respectively), thus resulting in lower PCEs of
0.18 % and 0.17 %, respectively. In contrast, solar cells based
on 95 and 96/160 (1:1, w/w) gave PCEs of only 0.05 and
0.56 %, respectively.
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Table 5: Device characterizations of OSCs prepared by solution processing techniques at a light intensity of 100 mWcm2.
Device structure
JSC
VOC
[mA cm2] [V]
ITO/PEDOT:PSS (30 nm)/
93:160 (1:1, 100 nm)/LiF
(0.12 nm)/Al
ITO/PEDOT:PSS (30 nm)/
94:160 (1:1, 50 nm)/LiF
(0.12 nm)/Al
ITO/PEDOT:PSS (30 nm)/
95:160 (1:1, 50 nm)/LiF
(0.12 nm)/Al
ITO/PEDOT:PSS (30 nm)/
96:160 (1:1, 80 nm)/LiF
(0.12 nm)/Al
ITO/PEDOT:PSS (30 nm)/
97:160 (1:1, 80 nm)/LiF
(0.12 nm)/Al
ITO/PEDOT:PSS (30 nm)/
98:160 (1:1, 80 nm)/LiF
(0.12 nm)/Al
ITO/PEDOT:PSS (40 nm)/
99:160 (7:13, 100 nm)/Ca/Al
ITO/PEDOT:PSS (40 nm)/
100:160 (7:13, 100 nm)/Ca/Al
ITO/PEDOT:PSS (80 nm)/101
(70 nm)/6 (30 nm)/12
(11 nm)/Ag
ITO/PEDOT:PSS (60 nm)/
102:160 (7:3)/CsF (1 nm)/Al
ITO/PEDOT:PSS (60 nm)/
103:160 (7:3)/CsF (1 nm)/Al
ITO/PEDOT:PSS (60 nm)/
104:160 (7:3)/CsF (1 nm)/Al
ITO/PEDOT:PSS(40 nm)/
105:160 (1:4, 100 nm)/LiF
(0.6 nm)/Al
ITO/PEDOT:PSS(40 nm)/
106:160 (1:4, 100 nm)/LiF
(0.6 nm)/Al
ITO/TiO2 (150 nm)/108:107
(3:2, 120 nm)/PEDOT:PSS/Ag
ITO/TiO2 (150 nm)/109:107
(3:2, 120 nm)/PEDOT:PSS/Ag
ITO/PEDOT:PSS (150 nm)/
110:160 (1:2, 60–70 nm)/TiOx
(ca. 10 nm)/Al
ITO/PEDOT:PSS (150 nm)/
111:160 (1:2, 60–70 nm)/TiOx
(ca. 10 nm)/Al
ITO/PEDOT:PSS (150 nm)/
112:160 (1:2, 60–70 nm)/TiOx
(ca. 10 nm)/Al
ITO/PEDOT:PSS (40 nm)/113
(90 nm)/6 (40 nm)/Al
2.6
0.96 0.45 1.2
[144, 145]
2.6
0.93 0.41 1.0
[144, 145]
0.3
0.66 0.27 0.05 [145]
2.9
0.89 0.21 0.6
0.3
0.79 0.20 0.04 [144]
1.2
0.88 0.32 0.3
0.26
0.78 0.25 0.05 [146]
3.4
0.98 0.31 1.0
[146]
1.9
0.47 0.52 0.5
[147]
3.0
0.84 0.4
1.0
[148]
6.6
0.83 0.41 2.2
[149]
5.2
0.91 0.47 2.2
[149]
0.4
0.97 0.37 0.14 [150]
4.5
0.78 0.40 1.4
[150]
0.8
0.69 0.39 0.2
[154]
0.6
0.54 0.39 0.1
[154]
0.2
0.61 0.32 0.03 [155]
2.0
0.87 0.60 1.1
[155]
2.7
0.90 0.61 1.5
[155]
6.7
0.60 0.47 1.9
[156]
FF
h
[%]
Ref.
[145]
[144]
AFM images of 2:1 w/w blended films of 93 and 94/160
exhibited higher degrees of ordering than those of 95 and 96/
160 films having the same blend composition. The results
revealed that the anthracene-based oligomers possess greater
structural symmetry than the benzothiazole-based compounds, which may enhance supramolecular organization in
BHJ blends. Thermal annealing of devices based on 93 and 94
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
did not significantly affect the overall performance, whereas a
significant decrease was observed for compounds 95 and 96
upon thermal annealing. It is important to note that the
substitution of acetylenic by olefinic p spacers in anthracenebased conjugated semiconductors 97 and 98 led to substantial
decreases in the PCE (< 0.3 %) of the resulting BHJ solar cell.
Furthermore, with a similar blend composition of 2:1 w/w
ratio, solid films of 93 and 94/160 exhibited higher degrees of
order than that of 97 and 98/160 films.
Colella et al. studied the photovoltaic performance of
compounds 99 and 100 comprising 2-trimethylsilyl-bithienyl
segments coupled either to an electron-withdrawing benzothiadiazole or an electron-rich anthracene core by triple
bonds.[146] The compounds showed broad absorption spectra
with maxima around 500 nm and optical band gaps of 2.3 eV.
Devices based on 100:160 (35:65, w/w) gave a PCE of 1.0 %
and a high VOC of 0.98 V under simulated AM1.5D white light
at 100 mW cm2 (Table 5). In contrast, devices prepared from
compound 99 showed a poor PCE of only 0.05 %. The lower
JSC of 0.26 mA cm2 for device 99 compared to 3.4 mA cm2
for device 100 was mainly ascribed to the poor charge
collection efficiency, which was further reflected in the lower
EQE value of 1.7 % at 467 nm for 99 compared to 32.5 % for
100-based devices.
Pentacene derivatives rapidly undergo Diels–Alder reactions with fullerene derivatives. Therefore, Malliaras and coworkers prepared a bilayer device by spin-coating of triisopropylsilyl-ethynyl-substituted (TIPS = triisopropylsilyl) pentacene 101 followed by vacuum-deposition of the C60 layer
(TIPS = triisopropylsilyl). To improve the photovoltaic performance lithium triflate ions were incorporated as mobile
ions into the TIPS-pentacene solution during spin-coating.
Mobile ions have shown to facilitate charge injection at the
electrode/oligomer interface and to increase the FF and JSC.
Using 12 as an exciton-blocking layer, the optimized device
generated a PCE of 0.5 % (Table 5).[147] To enhance the
stability of pentacene derivatives, Malliaras and co-workers
synthesized heterocyclic analogues 102 fused with thiophene
rings. Compound 102 was synthesized as a mixture of syn- and
anti-isomers and was used for the fabrication of BHJ devices
by solution processing. The cell efficiencies were increased to
1 % by using 102 as donor and 160 as acceptor.[148] In this case,
the device performance significantly depended on the solvent
vapor annealing of the blended films. Solvent vapor annealing
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caused a reorganization of the blends into spherulites, which
consisted of a network of anthradithiophene crystallites
dispersed in an amorphous matrix composed of the fullerene.
Recently, Watkins et al. reported the photovoltaic performance of dibenzo[b.def]chrysenes 103 and 104. When
blended with 160 from chloroform solutions, the devices
reached PCEs of up to 2.2 % (Table 5).[149] The main
advantage of this class of acenes is that, unlike TIPSpentacene 101, they are nonreactive towards fullerene
derivatives, thus offering more facile device fabrication.
TIPS-ethynyl-substituted anthracene derivatives 105 and
106 substituted with naphthalene and bithiophene units were
used as electron-donor materials for organic solar cells. The
anthracene derivatives do not undergo Diels–Alder reactions
with 160 when processed from solution. BHJ solar cells that
comprise anthracene derivatives 105 or 106 (in blends with
160, 1:4, w/w) achieved PCEs of 0.14 and 1.4 %, respectively
(Table 5).[150]
It is well known that the morphology of the photoactive
layers can be controlled by the utilization of self-organizing
liquid crystalline materials. Mllen and co-workers implemented this approach using hexabenzocoronene (HBC)
derivatives which, as discotic liquid crystals, were known for
their high hole mobilities as well as for their tendency to form
self-assembled stacks.[151] BHJ solar cells were fabricated by
blending HBC derivatives as donor and N,N’-bis(1-ethylpropyl)perylene-3,4,9,10-bis(dicarboximide) (107, EPBI) as
acceptor.[152–154] Due to intermolecular and mesoscopic ordering of donor and acceptor moieties, separate pathways for
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electrons and holes, respectively, were achieved. Devices
prepared using 108 and 107 in a blend ratio of 3:2 showed a
PCE of 1.95 % under monochromatic 490 nm illumination at
0.47 mW cm2. Under these conditions, the JSC, VOC, and FF
values were 33.5 mA cm2, 0.69 V, and 0.40, respectively.[152]
The EQE reached a maximum value of 35 % between 470 and
500 nm. The incident light power was very low because a
saturation of JSC was observed at light intensities above
1 mW cm2.
Recently, Mllen and co-workers prepared solar cells
based on blends of 108 and 109 using 107 as an acceptor.[154] A
slight red-shift of the absorption spectrum was observed for
109 containing triple bonds as spacers, showing an optical
band gap of 2.9 compared to 3.1 eV for 108. BHJ solar cells
were constructed using inverted structures with electroncollecting TiO2 bottom and Ag top electrodes. This approach
allowed for the fabrication of devices with improved air
stabilities. The devices based on a 108:107 (3:2, w/w) blend
reached PCEs of 0.2 % upon solar illumination at
100 mW cm2 (Table 5). In contrast, devices using 109 as
donor material showed a decrease in PCE to 0.1 %. Compared to 108, this lower performance of 109 could be due to
differences in packing and morphologies of the blends. An
EQE of about 20–25 % was observed for these HBCs in a
wide spectral coverage of 350 to 500 nm.
To study the effect of structural organizations on device
performance, Wong et al. synthesized HBC derivatives 110–
112 containing dioctylfluorenyl moieties.[155] The intermolecular association of compound 110 was very weak due to the
presence of 6 steric dioctylfluorenyl groups as characterized
by 1H NMR spectroscopy and two-dimensional wide-angle
X-ray scattering (2D WAXS) measurements. While compounds 111 and 112 showed strong self-assembling properties
in solution, they formed ordered hexagonal columns in the
solid state. Photovoltaic devices based on 110:160 (1:2, w/w)
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showed quite poor performance with PCEs of only 0.03 %
even after thermal annealing. In contrast, thermal annealing
of devices prepared from disubstituted compounds 111 and
112:160 in a 1:2 w/w ratio, gave PCEs of 1.1 and 1.5 %,
respectively (Table 5). The higher performance for 112
compared to 111 was attributed to the greater degree of
ordering in the blended films and balanced hole and electron
mobilities (1:2 blends of oligomer and 160) of 2.8 104 and
1.2 104 cm2 V1 s1, respectively, determined in OFETs.
Nuckolls and co-workers recently reported a new device structure using molecular self-assembly
processses based on dibenzotetrathienocoronene donor 113.[156] The
devices were prepared by spin-casting of 113 onto the ITO/PEDOT:PSS
layer followed by annealing at
150 8C. Then 6 was thermally evaporated and the device structure was
completed by deposition of an Al
cathode. It has been mentioned that
compound 113 stacks into columnar superstructures in thin
films, which on thermal annealing form a supramolecular
three-dimensional network of cables. This network further
directed the assembly of 6, thus forming an interpenetrated
nanostructured p–n bulk heterojunction. Consequently, photovoltaic devices with this active layer gave a PCE of 1.9 %
with an EQE of 65 % at around 420 nm (Table 5).
BODIPY dyes have gained great interest in recent years
because of their high absorption coefficients, high fluorescence quantum yields, delocalized molecular orbitals as well
as excellent chemical and photostability.[157] In this respect,
Roncali and co-workers prepared BODIPY dyes 114 and 115
and used them as donor materials in BHJ solar cells.
Compared to the absorption spectra in solution (114: lmax =
572 nm; 115: lmax = 646 nm), in thin films, a red-shift of
approximately 20–30 nm was observed for the low-energy
absorption bands. Photovoltaic devices prepared using 114
and 115 in blends with 160 (1:2, w/w) generated efficiencies of
1.2 and 1.3 %, respectively (Table 6).[158] Due to their complementary absorption behavior, these dyes were further
implemented as mixed donor layer in BHJ solar cells with 160
in a ratio of 1:1:2. The device generated a PCE of 1.7 %, which
was about 30 % higher compared to cells containing the
individual dyes.[159] This result demonstrated that the use of
multiple donors with appropriate energy levels and complementary light-harvesting properties could be useful to obtain
efficient OSCs.
The Roncali research group attached a hexylbithiophene
unit to the axial phenyl ring of the BODIPY dye to give
derivative 116.[160] The substitution of a bithiophene unit had a
negligible effect on the electronic properties. For example, a
similar optical band gap of 1.7 eV is determined for both dyes
115 and 116. BODIPY dye 116 showed an absorption
maximum at 649 nm that was red-shifted to 672 nm in thin
films. While these BODIPY dyes exhibited similar electronic
properties, a large difference was observed in the solar cell
performance. In BHJ devices, using 160 as acceptor, BODIPY
116 showed a PCE of 2.2 % caused by a quite high JSC value of
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
Table 6: Characterizations of oligomer OSCs prepared by solutionprocessing at a light intensity of 100 mWcm2.
Device structure
JSC
VOC
(mA cm2) (V)
ITO/PEDOT:PSS/114:160 (1:2)/Al
ITO/PEDOT:PSS/115:160 (1:2)/Al
ITO/PEDOT:PSS/114:115:160
(1:1:2)/Al
ITO/PEDOT:PSS/116:160 (1:2)/Al
ITO/PEDOT:PSS/117 (20 nm)/6
(32.5 nm)/12 (10 nm)/Ag
ITO/117 (13 nm)/6 (40 nm)/12
(10 nm)/Al
ITO/118 (13 nm)/6 (33 nm)/12
(10 nm)/Al
ITO/PEDOT:PSS/119 (20 nm)/6
(32 nm)/12 (10 nm)/Ag
ITO/PEDOT:PSS/120 (20 nm)/6
(32 nm)/12 (10 nm)/Ag
ITO/PEDOT:PSS (30 nm)/121:160
(1:4, 70 nm)/Al
ITO/PEDOT:PSS (30 nm)/121:172
(1:4, 70 nm)/Al
ITO/PEDOT:PSS/123/123:167/
167/NBphen/Al
ITO/PEDOT:PSS/123/123:160/
160/NBphen/Al
4.4
4.1
4.7
0.79 0.34 1.2
0.75 0.44 1.3
0.87 0.42 1.7
[158]
[158]
[159]
7.0
5.6
0.75 0.38 2.2
0.55 0.49 1.5
[160]
[161]
6.5
0.79 0.49 2.5
[162]
5.4
0.92 0.61 3.0
[162]
3.7
0.73 0.51 1.4
[163]
3.9
0.70 0.50 1.4
[163]
5.4
0.78 0.39 1.7
[164]
8.4
0.82 0.43 3.0
[164]
10.5
0.75 0.65 5.2
[166]
7.0
0.55 0.51 2.0
[166]
FF
h
Ref.
(%)
7 mA cm2 (Table 6). The higher JSC for 116 compared to 115
was ascribed to the higher hole mobility of the former (9.7 105 cm2 V1 s1 for 116; 5.1 105 cm2 V1 s1 for 115).
Frchet and co-workers prepared PHJ solar cells using
subnaphthalocyanine (SubNc) dye 117 as donor and C60 as
acceptor. In thin films, the SubNc showed a broad absorption
band with a maximum at 688 nm. PHJ solar cells were
prepared by solution-processing of 117 on PEDOT-precoated
ITO substrates followed by thermal evaporation of C60, 12,
and the silver cathode. The device generated a PCE of 1.5 %
after thermal annealing at 120 8C (Table 6).[161] In similar cell
structures using SubNc 117 or SubPc 118 as donor and C60 as
acceptor, Torres and co-workers reported a PCE of 2.5 and
3.0 %, respectively.[162] Although the SubNc-based devices
gave higher JSC as compared to SubPc devices, the lower
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performance was mainly due to the reduced VOC and FF
values.
Oligothiophene–boron(SubPc) dyads (119, 120), in which
oligothiophenes were axially attached to the SubPc by
phenoxy and alkynyl groups were used as donor material in
SMOSCs.[163] The attached oligothiophene units contributed
to increase the absorption range of the dyads and also provide
a driving force for self-assembly. PHJ solar cells were
prepared using solution-casting of these dyads as the donor
and vacuum-processed C60 as the acceptor. The cells generated similar PCEs of 1.4 % with a reasonable FF of 0.5 and a
VOC in excess of 0.7 V (Table 6).
Aiming at broad absorption and high molar extinction
coefficients, Frchet and co-workers synthesized a series of
platinum acetylide complexes containing thienyl-benzothiadiazole cores and oligothiophene terminal units.[164] The
d orbitals of the central platinum atom can overlap with the
p orbitals of the alkyne units, leading to an enhancement of
the p-electron delocalization along the molecular backbone.
Because of this electronic structure, excitation with light led
to an efficient intersystem crossing caused by strong spin–
orbital coupling, thus facilitating the formation of triplet
excited states with lifetimes on the order of microseconds,
hence allowing extended exciton diffusion lengths.[165] Oligomer 121 showed a p–p* absorption band peaking at 430 nm
and an intramolecular charge transfer (ICT) transition at
570 nm resulting in an optical band gap of 1.9 eV. BHJ solar
cells made of Pt-complex 121 showed a PCE of 1.7 % when
blended with 160 in a ratio of 1:4 w/w (Table 6). Using 172 as
an electron acceptor, the PCE was further increased to 3 %
with a maximum external quantum efficiency approaching
50 %.
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Nakamura and co-workers reported a new solutionprocess fabrication protocol using soluble porphyrin precursor 122 and bis(dimethylphenylsilylmethyl)[60]fullerene (169,
SIMEF), which created three-layered p-i-n photovoltaic
devices.[166] The devices were fabricated by spin-coating of
the soluble porphyrin precursor, which was thermally converted into highly insoluble, crystalline tetrabenzoporphyrin
123 (p-layer) at 180 8C. Then the mixed i-layer was deposited
by spin-coating of 122:169 (3:7, w/w) followed by heating at
180 8C. The i-layer possessed a well-defined interdigitated
BHJ structure in which columnar crystals of 123 grow
vertically from the bottom p-layer. Subsequent spin-coating
of 169 in toluene onto the i-layer and heating at approximately 150 8C (for crystallization) furnished the p-i-n structure. In this device structure PEDOT:PSS was used as holetransport layer and 2,9-bis(naphthalen-2-yl)-4,7-diphenyl1,10-phenanthroline (Nbphen) was used as electron-transport
layer. The resulting devices gave a very high PCE of 5.2 % and
a JSC of 10.5 mA cm2. The use of 160 instead of 169 reduced
the PCE of the device to about 2.0 % (Table 6). The lower
performance of the 160-based device was attributed to the
irregular growth of the 123 crystals, which resulted in an
undesirable i-layer morphology as studied by scanning
electron micrograph (SEM). The difference in VOC was
ascribed to the lower LUMO level of PCBM (by ca. 0.1 V)
compared to 169.
Ionic dyes have attracted considerable interest in recent
years for use in solar cells due to their tunable absorption
properties in the visible to near-IR region and high molar
absorptivities. Marks and co-workers studied the photovoltaic
properties of a series of squaraine dyes 124–127.[167, 168] In
solution, all squaraines showed similar absorption behavior
with maxima at around 730 nm. In thin films, a strong spectral
broadening was reported for these dyes spanning the range
from 550 to 900 nm. HOMO and LUMO energy values for all
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Table 7: Device characterizations of ionic dyes prepared by solution-processing techniques.
Device structure
JSC
[mA cm2]
VOC
[V]
FF
h
[%]
Light intensity
[mWcm2]
Ref.
ITO/PEDOT:PSS/124:160 (1:3) (30 nm)/LiF/Al
ITO/PEDOT:PSS/125:160 (1:3) (30 nm)/LiF/Al
ITO/PEDOT:PSS (75 nm)/126:160 (1:3) (35 nm)/LiF/Al
ITO/PEDOT:PSS (75 nm)/127:160 (1:3) (35 nm)/LiF/Al
ITO/PEDOT:PSS (75 nm)/126:172 (1:3) (35 nm)/LiF/Al
ITO/PEDOT:PSS (75 nm)/127:172 (1:3) (35 nm)/LiF/Al
ITO/PEDOT:PSS/128:160/Ca/Al
ITO/PEDOT:PSS/129:160/Ca/Al
ITO/PEDOT:PSS/130:160/Ca/Al
ITO/PEDOT:PSS/131 (20 nm)/6 (40 nm)/24 (2.5 nm)/Al
ITO/PEDOT:PSS/132 (20 nm)/6 (40 nm)/24 (2.5 nm)/Al
ITO/PEDOT:PSS (30 nm)/133:160 (1:4) (60 nm)/LiF/Al
ITO/MoO3 (8 nm)/47:172 (1:6) (76 nm)/Al
ITO/MoO3 (8 nm)/47:172 (1:6) (78 nm)/6 (4 nm)/12 (1 nm)/LiF (0.8 nm)/Al
ITO/MoO3 (8 nm)/47 (6.2 nm)/6 (40 nm)/12 (10 nm)/Al
ITO/PEDOT:PSS (40 nm)/134:160 (50 nm)/Al
ITO/PEDOT:PSS (40 nm)/135:160 (50 nm)/Al
ITO/PEDOT:PSS (40 nm)/136:160 (50 nm)/Al
ITO/PEDOT:PSS (40 nm)/137:160 (50 nm)/Al
ITO/PEDOT:PSS (40 nm)/137:6 (1:1)/BPhen/Ag
ITO/PEDOT:PSS (40 nm)/138:160 (50 nm)/Al
ITO/PEDOT:PSS (40 nm)/138:172 (50 nm)/Al
ITO/PEDOT:PSS (60 nm)/139 (X = PF6) (30 nm)/6 (40 nm)/AlQ3 (2.5 nm)/Al
ITO/PANI:DBS (30 nm)/139 (X = PF6) (30 nm)/C60 (40 nm)/24 (2.5 nm)/Al
ITO/PEDOT:PSS (40 nm)/139 (X = ClO4)/6 (40 nm)/24 (2 nm)/Al
5.7
4.7
4.1
5.1
7.2
9.3
3.5
12.6
1.0
5.0
6.8
1.7
8.9
12.0
10.2
4.0
5.3
6.3
8.2
11.5
3.3
4.8
5.9
6.9
8.3
0.62
0.59
0.54
0.56
0.55
0.57
0.66
0.31
0.22
0.38
0.44
0.59
0.89
0.92
0.76
0.77
0.90
0.76
0.94
0.80
0.64
0.66
0.44
0.72
0.72
0.35
0.32
0.33
0.37
0.37
0.37
0.37
0.47
0.30
0.31
0.33
0.28
0.35
0.50
0.60
0.29
0.32
0.36
0.34
0.47
0.31
0.31
0.25
0.61
0.34
1.2
0.9
0.8
1.1
1.4
2.0
0.8
1.8
0.1
0.6
1.0
0.3
2.7
5.2
4.6
0.9
1.5
1.7
2.6
4.9
0.7
1.0
0.7
3.0
2.0
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
88
100
100
100
100
100
[167]
[167]
[168]
[168]
[168]
[168]
[169]
[169]
[169]
[170]
[170]
[171]
[172]
[173]
[174]
[175]
[175]
[175]
[176]
[181][a]
[177]
[177]
[179]
[179]
[180]
[a] Device prepared by vacuum deposition technique (see section 5).
derivatives, estimated by cyclic voltammetry, were 3.3 and
5.0 eV, respectively, demonstrating that the alkyl chains
have negligible effects on the redox properties. The best
photovoltaic devices were achieved with blended films of
124–127 and 160 (1:3, w/w) spin-coated from chloroform
solution and showed PCEs of 0.8–1.2 %, respectively
(Table 7).[167, 168] Although the structural variations were
minimal between 126 and 127, the crystal structure analysis
demonstrated that squaraine 127 containing n-hexenyl chains
resulted in a much more compact solid-state structure,
dramatically affecting the charge transport in thin films. The
OFET hole mobility of 127 (mh = 1.2 104 cm2 V1 s1) was
5 times higher than that of 126 (mh = 2.7 105 cm2 V1 s1),
which was also reflected in the higher PCE for 127 compared
to 126. Devices based on annealed films of 126/127 and 160
(1:3, w/w) gave PCEs of 1.4 and 2.0 %, respectively.[168]
Wrthner and co-workers prepared novel squaraine dyes
128 and 129 by introducing a dicyanovinyl group at the central
squaric acid.[169] The oligomers showed strong absorption at
683 nm (128) and 701 nm (129), respectively. BHJ solar cells
prepared from a blend of squaraines/160 (optimized ratio of
3:7, w/w for 128 and 3:2, w/w for 129) followed by thermal
annealing gave overall PCEs of 0.8 and 1.8 %, respectively
(Table 7). The higher PCE for 129 compared to 128 was
ascribed to the planar structure of the molecule, in which the
benzothiazole rings were in one plane with the squarate ring
and formed well-organized domains with 160. Dye 129
showed an unprecedented JSC value of 12.6 mA cm2 compared to 3.5 mA cm2 for 128, which was rationalized by the
more densely packed arrangement of the former. Under
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
similar conditions squaraine dye 130 gave a PCE of only 0.1 %
with a JSC of 1.0 mA cm2.
Nesch and co-workers studied the photovoltaic properties of squaraine dyes 131 and 132 in planar heterojunction
structures. The dyes were spin-coated from chloroform
solutions on top of a PEDOT:PSS layer. Then, C60 (6,
40 nm), Alq3 24 (2.5 nm), and an Al cathode were subsequently deposited by evaporation on top of the organic layers.
The cells generated PCEs of 0.6 and 1.0 %, respectively, due
to their low VOC and FF values (Table 7).[170]
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Squaraine dye 133 endowed with phthalocyanines by
pyridine–ruthenium coordinate bonds showed a broad spectral coverage from 550–850 nm in thin films caused by the
complementary absorption of both constituents. BHJ solar
cells prepared with 133:160 (1:4, w/w) resulted in a moderate
PCE of approximately 0.3 %, which was attributed to a lower
JSC of 1.65 mA cm2 (Table 7).[171]
Forrest and co-workers prepared BHJ solar cells using
squaraine 47 as the donor and 172 as the acceptor.[172] The
squaraine:172 (1:6, w/w) mixture was spin-coated from
chloroform solution onto a MoO3 buffer layer followed by
thermal annealing at 70 8C. The resulting device gave a PCE
of 2.7 % and a JSC of 8.85 mA cm2. The lower FF (0.35) for
these squaraine-based devices was attributed to a large
internal series resistance and to unbalanced charge extraction
from the low density of squaraine in the bulk. The EQE
spectrum of the device covered the region from 350 to 750 nm
with a maximum of about 48 % at 385 nm coming from 172
and about 42 % at 680 nm from dye 47. In a recent report, by
employing solvent vapor annealing and a slight modification
of the device structure, the PCE of 47:172 (1:6, w/w spin-cast
from 1,2-dichlorobenzene) devices was improved to an
excellent value of 5.2 %.[173] For the device fabrication, the
blend films were exposed to dichloromethane for 10 min
before thermal evaporation of the follow-up layers. The ascast devices displayed PCEs of 2.4 % as a result of the lower
JSC (6.9 mA cm2) and FF (0.36) values. The improved
performance was ascribed to the increase in nanostructure
scale leading to better conduction of photogenerated carriers
to the electrodes. The FF was improved to 0.50 because of the
improved molecular packing and the reduced series resistance.
The same authors also prepared bilayer devices by spincasting of squaraine 47 followed by vapor deposition of
acceptor C60. Thermal annealing of the spin-coated donor film
at 110 8C increased the squaraine surface roughness, thereby
forming an interdigitated BHJ structure with a length scale on
the order of the exciton diffusion length. The cells showed
high FF values of 0.60 and PCEs as high as 4.6 % at AM1.5G
solar illumination.[174] In contrast, the as-cast devices generated PCEs of only about 3.6 % because of lower JSC and FF
values of 8.6 mA cm2 and 0.53, respectively. The obtained
VOC values of up to 0.9 V for 47-based devices were
considerably higher compared to other squaraine-containing
(124–133) devices (Table 7).
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Wrthner and co-workers prepared a series of merocyanine dyes 134–136 with different acceptor substituents.[175] In
thin films, the merocyanines showed absorption maxima at
544 nm (134), 607 nm (135), and 649 nm (136), respectively.
Blended films of 134–136:160 (70–80 % of 160) in BHJ solar
cells generated PCEs of 0.9, 1.5, and 1.7 % respectively.
Analogous dye 137 that comprises a propylene bridging unit
when blended with 160 achieved a PCE of 2.6 % (Table 7).[176]
The higher PCE for 137 compared to 135 was ascribed to the
rigid propylene bridge, which reduced the flexibility and
ensured a planar geometry enabling an improved JSC of
8.2 mA cm2 for 137 compared to 5.3 mA cm2 for 135. These
results clearly demonstrated that dipolar molecules can be
efficiently used in solar cells, irrespective of their limitations
in charge-carrier-transport properties on the molecular level.
Merocyanine 138, that comprises a dicyanoethylenepyrrolidine acceptor unit showed broad absorption in the
near-IR region with an absorption maximum at about 771 nm
in thin films. BHJ solar cell devices were prepared using
photoactive blend layers of dye 138 and 160 or 172 exhibiting
PCEs of 0.66 and 1.0 %, respectively (Table 7).[177]
So far, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been exclusively used as buffer
layer in BHJ solar cells owing to its suitable HOMO energy
level alignment (5.0 eV), resulting in favorable hole transport to the active material. However, it has been reported that
the HOMO energy level offset of > 0.5 eV between the donor
and the PEDOT:PSS layer resulted in unfavorable holetransport kinetics.[178] Recently, Nesch and co-workers
prepared bilayer photovoltaic devices using cyanine dye 139
(X = PF6 ; EHOMO = 5.7 eV) and high work function polyaniline:dodecylbenzenesulfonic acid (PANI:DBS) or
PEDOT:PSS as the buffer layer.[179] Owing to the lower
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4. Bulk-Heterojunction Solar Cells Based on StarShaped Dyes and Dendrimers
HOMO energy level of PANI:DBS (5.4 eV), the energy
level offset at the anode/cyanine dye interface can be reduced.
BHJ solar cells based on PANI:DBS gave PCEs of 3 %
compared to 0.7 % using PEDOT:PSS as the buffer layer
(Table 7). This improvement in efficiency was caused by a
lower HOMO energy level offset at the anode/cyanine
interface, thus resulting in a balanced hole injection and
improved VOC and FF values.
Nesch and co-workers have recently achieved incident
photon to electron conversion efficiencies (IPCE) of 80 %
using cyanine dyes 139 (X = ClO4) as the electron donor and
C60 as an acceptor in bilayer OSCs.[180] The devices showed
PCEs of 2.0 % under simulated AM1.5 solar irradiation. To
increase the conductivity of the donor layer, the cyanine dye
was doped using nitrosonium tetrafluoroborate (NOBF4).
The improved device performance resulted from the insertion
of a 2 nm Alq3 buffer layer at the aluminum/C60 interface. The
device without Alq3 buffer layer showed a lower PCE of only
0.55 %. The results revealed that the large cyanine aggregates
formed by doping could be responsible for efficient exciton
transport to the charge separating heterointerface.
Shape-persistent star-shaped and dendritic maromolecules are a class of semiconductors that comprise high
molecular weight and monodispersity owing to a defined
structure. They have been successfully used as active materials in organic electronics. Our research group has reported the
synthesis of a series of star-shaped oligothiophene-perylene
D–A systems (140).[182] The dyes showed interesting photo-
physical properties, such as fast intramolecular energy[111] and
electron transfer.[183] Application of dyad 140 as the only
active material in BHJ solar cells gave only very low
efficiencies, however, by mixing with 160, moderate PCEs
of 0.25 % were obtained (Table 8).
Roncali and co-workers demonstrated a red-shift of the
absorption and emission maxima as well as a decrease in
oxidation potentials by replacing a thiophene unit in triarylamine 141 by an EDOT in 142.[184] This finding indicated a
strong electron-donating effect of the EDOT units along with
a structural rigidification. Bilayer OSCs were fabricated by
Table 8: Characterizations of branched-oligomer-based devices prepared by solution-processing techniques.
Device structure
JSC
[mA cm2]
VOC
[V]
FF
h
[%]
Light intensity
[mWcm2]
Ref.
ITO/PEDOT:PSS/140:160 (1:4)/Al
ITO/PEDOT:PSS (ca. 80 nm)/141/6 (20 nm)/Al
ITO/PEDOT:PSS (ca. 80 nm)/142/6 (20 nm)/Al
ITO/PEDOT:PSS/143:160 (1:3, 60 nm)/Al
ITO/PEDOT:PSS/143:172 (1:2, 60 nm)/Ca/Al
ITO/PEDOT:PSS (30 nm)/144:160 (1:3)/Ba/Al
ITO/PEDOT:PSS (30 nm)/145:160 (1:3)/Ba/Al
ITO/PEDOT:PSS (30 nm)/146:172 (1:3)/Mg/Al
ITO/PEDOT:PSS (30 nm)/147:172 (1:2, 80 nm)/Ca/Al
ITO/PEDOT:PSS (30 nm)/148:172 (1:2, 80 nm)/Ca/Al
ITO/PEDOT:PSS/149:160 (1:3)/Al
ITO/PEDOT:PSS/150:160 (1:4)/Al
1.4
1.7
1.5
5.9
9.5
1.5
4.8
8.6
5.2
7.8
1.1
3.35
0.60
0.67
0.32
0.86
0.87
0.93
0.81
0.85
0.84
0.88
0.85
0.94
0.29
0.30
0.30
0.46
0.52
0.43
0.39
0.33
0.31
0.44
0.24
0.40
0.25
0.3
0.1
2.3
4.3
0.6
1.3
2.4
1.4
3.0
0.3
1.3
100
100
100
100
100
100
100
100
100
100
80
100
[182]
[184]
[184]
[185]
[185]
[186]
[187]
[188]
[189]
[189]
[190]
[191]
ITO/PEDOT:PSS/151 (R = H):160 (1:2)/LiF/Al
ITO/PEDOT:PSS/151(R=SiMe3):160 (1:4)/LiF/Al
ITO/PEDOT:PSS/152 (R = SiMe3):160 (1:4)/LiF/Al
ITO/PEDOT:PSS/153:160 (1:2)/LiF/Al
ITO/PEDOT:PSS/153:172 (1:2)/LiF/Al
ITO/PEDOT:PSS/154:160 (1:3)/LiF/Al
ITO/PEDOT:PSS/155:160 (1:4)/Al
ITO/PEDOT:PSS/156:160 (1:4)/Al
ITO/PEDOT:PSS/157:160 (1:4)/LiF/Al
ITO/PEDOT:PSS/158:160 (1:4)/LiF/Al
ITO/PEDOT:PSS/159:160 (1:4)/LiF/Al
ITO/PEDOT:PSS/158:172 (1:4)/LiF/Al
ITO/PEDOT:PSS/159:172 (1:4)/LiF/Al
4.2
2.96
1.5
3.3
6.4
3.3
2.0
2.5
2.35
5.1
4.5
7.1
8.3
0.97
0.94
0.81
1.00
1.00
1.00
0.75
0.93
0.60
0.55
0.60
0.55
0.56
0.42
0.37
0.31
0.44
0.38
0.38
0.28
0.47
0.32
0.37
0.37
0.38
0.34
1.7
1.0
0.4
1.5
2.5
1.3
0.4
1.1
0.5
1.0
1.0
1.5
1.6
100
100
100
100
100
100
100
100
100
100
100
100
100
[193]
[193]
[194]
[195]
[195]
[196]
[198]
[198]
[199]
[200]
[200]
[200]
[200]
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spin-casting of 141 on a PEDOT:PSS-coated ITO
substrate followed by vacuum-deposition of a layer of
C60. Although high hole mobilities (ca. 1.1 102 cm2 V1 s1) were observed in OFET measurements,
in OSCs, these molecules showed relatively moderate
PCEs as well (141: 0.3 %, 142: 0.1 %; Table 8).
dicyanovinylene acceptors on each arm were prepared and
tested as donor materials in BHJ solar cells.[189] The thin film
absorption spectrum of oligomer 148 containing an additional
Zhan and co-workers reported a star-shaped oligomer
143, in which triarylamine was used as central branching
unit.[185] The insertion of benzothiadiazole as acceptor in 143
red-shifted the absorption maximum by 83 nm in solution
relative to that of oligomer 141, which has a similar structure
without the benzothiazole. In thin films, oligomer 143 showed
an absorption maximum at 538 nm, which was 28 nm redshifted compared to that in solution with a band gap of 1.9 eV.
Compared to 141, the HOMO energy level of 143 is about
0.35 eV lower. The hole mobility of oligomer 143 measured by
the OFET was reported to be 4.9 104 cm2 V1 s1, which was
higher than that of 141 (ca. 105 cm2 V1 s1). Photovoltaic
devices based on the blend of 143 and 160 or 172 exhibited
PCEs of 2.3 % and 4.3 %, respectively, which is one of the
highest values reported for solution-processed organic solar
cells based on branched molecules.
Star-shaped molecule 144 containing a triphenylamine
donor and a benzothiadiazole acceptor gave PCEs of 0.6 %, a
high VOC of 0.93 V and a JSC of 1.5 mA cm2 under AM1.5G
illumination at 100 mW cm2 when blended with 160 in a 1:3
ratio.[186] The PCE of devices based on star-shaped molecules
was further increased to 1.3 % by the attachment of additional
triphenylamine donor units ((!145). The higher PCE of
derivative 145 compared to 144 was mainly caused by the
increased JSC value of 4.18 mA cm2.[187] The terminal triphenylamine units were replaced by 3-hexylthiophenes in oligomer 146.[188] Devices prepared using 146:172 (1:3, w/w)
showed improved PCEs of 2.4 %, which was due to the
increased JSC values of 8.6 mA cm2 (Table 8). The enhanced
photovoltaic behavior was ascribed to better absorption and
film morphology of the blend layer.
Star-shaped D–p–A molecules 147 and 148 that comprise
a central triphenylamine donor, oligothiophene p bridges and
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vinylene group covered a broad wavelength range from 380–
750 nm, thus resulting in a red-shift of the absorption onset by
about 78 nm compared to that of 147. The optical band gaps
for 147 and 148 were 1.83 and 1.65 eV, respectively. BHJ solar
cells based on blends of these oligomers and 172 (1:2, w/w)
generated PCEs of 1.4 and 3.0 %, respectively. The improved
PCE for 148-based device was ascribed to the broad spectral
coverage due to the presence of a vinylene spacer and
resulted in a higher JSC value.
Roncali et al. prepared a tetrahedral oligothienyl silane
derivative 149, which showed a red-shifted absorption (Dl =
19 nm) compared to parent linear terthiophene.[190] When
implanted in BHJ solar cells with PC61BM 160 as acceptor in a
1:3 w/w ratio, compound 149 gave a moderate PCE of 0.3 %
under AM 1.5 simulated solar irradiation at 80 mW cm2
(Table 8).
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Organic Semiconductors
Kopidakis et al. prepared thiophene dendrimer 150 with a
phenyl core that also comprise terminal hexyl chains.[191]
Despite its relatively large optical band gap of 2.1 eV,
fabrication of BHJ solar cells based on a blend of 150 and
160 (1:4, w/w) gave a PCE of 1.3 % and a maximum EQE of
about 35 % at 405 nm under simulated AM1.5 illumination
(Table 8). Although the device generated a reasonable JSC,
the low FF and small domain morphology of the devices
indicated that carrier recombination could probably be a
limiting factor for the performance.
Our research group has recently developed an effective
approach to novel monodisperse 3D-conjugated dendritic
oligothiophenes (DOT) 151, 152 and used them in BHJ solar
cells as donor in combination with 160 as acceptor.[192–194]
Investigations of the optical properties revealed that, in
comparison to all-thiophene dendrimer 151, the introduction
of ethynyl groups in the branching units (151!152) caused a
hypsochromic shift of the low-energy absorption band. These
dendrimers showed intense and very broad absorptions
leading to optical band gaps of 2.28 and 2.4 eV, respectively.
The absorption spectra were a superimposition of multiple
chromophores, which correlate to a-conjugated oligothiophene subunits. Fluorescence measurements revealed that the
emission typically originated from the longest a-conjugated
pathway and was invariant to the excitation wavelength. This
result, together with the low fluorescence quantum yields
clearly indicated intramolecular energy transfer from shorter
chromophores to the longer ones, which then emit. The
HOMO energy levels of compounds 151 and 152 were around
5.3 and 5.6 eV, respectively. BHJ devices were prepared
using 151 and 152 as donor and 160 as acceptor. BHJ solar
cells using dendrimer 151 (R = H) in a D–A ratio of 1:2
generated a PCE of 1.7 % with a high VOC of 0.97 V (Table 8).
The study of D:A molar ratio indicated that an optimal value
of five to six thiophene units per PCBM unit is important for
higher efficiencies.
BHJ solar cells were prepared from dendrimer 152 (R =
SiMe3) as donor and 160 as acceptor in a D:A blend ration of
1:4 giving a PCE of 0.4 % and a VOC of 0.81 V.[194] Under
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
similar conditions, dendrimer 151 (R = SiMe3) generated a
higher PCE of 1.0 % because of higher JSC and VOC values. The
lower JSC for the device based on ethynylated dendrimer 152
compared to 151 could be due to unbalanced charge carrier
mobility of the former, which was also reflected in the lower
EQE value of 17 % for 152 compared to 45 % for 151. Devices
prepared from 152 (R = H) showed a lower performance with
a PCE of only 0.22 %.
Wong et al. synthesized a series of HBC derivatives
comprising dioctylfluorenyl moieties end-capped with oligothiophene dendrons and implemented as donor material in
the active layer of BHJ solar cells.[195] The HBC derivatives
showed self-association behavior into ordered structures in
solution and in the solid state. BHJ solar cells fabricated using
HBC derivative 153 as electron donor and 160 as electron
acceptor (1:2, w/w) gave PCEs of up to 1.5 % (Table 8). The
higher PCE for 153 compared to the terminal oligothiophene
dendron (9T) was ascribed to the increased JSC (from 1.42 to
3.33 mA cm2) and FF (from 0.31 to 0.44). This study clearly
demonstrated the positive effect of molecular self-organization on device performance. The PCE of devices based on 153
was further increased to 2.5 % by using 172 as acceptor caused
by an increase of the JSC value to 6.4 mA cm2.
To extend the absorption range of DOTs, Buerle and coworkers introduced an electron-accepting pyrazino-
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[2,3g]quinoxaline core into the dendritic structure.[196] Dendrimer 154 showed a broad absorption band covering 300–
700 nm and a reduced band gap of 1.7 eV compared to
thiophene dendrimer 151. Incorporation of 154 in BHJ solar
The lower VOC and FF values for 155-based devices were
assigned to the increased carrier recombination. As a
consequence of the large difference in band gap one might
expect a larger JSC for 156 in comparison to 155, however this
was not observed. This could be attributed to electron
trapping in the cyanobenzene core that impedes efficient
transfer to the acceptor 160.
Our research group recently prepared some other corefunctionalized DOTs that bear methylpyridinium acceptor
units for solar cell applications. Owing to the incorporation of
the pyridinum moiety, a bathochromic shift of the absorption
band was observed for these oligomers when compared to
nonfunctionalized DOTs, which can be attributed to an
intramolecular charge-transfer process. Dendron 157 showed
moderate PCEs close to 0.5 % in BHJ solar cells with 160 as
acceptor (Table 8).[199] The Buerle and Torres research
groups recently prepared a series of novel DOT-functionalcells as donor material along with 160 as acceptor gave
PCEs of up to 1.3 %, which is higher than for devices based
on 151 (R = SiMe3 ; Table 8). This study showed that the
longest-wavelength charge-transfer band contributes to the
EQE spectrum, which was extended up to 750 nm, and
consequently also to the photocurrent.
Kopidakis et al. prepared star-shaped molecules 155
and 156 to show, how a systematic modification of the core
unit can change the properties from molecular to device
level.[197, 198] Oligomer 155 exhibited an absorption maximum at 424 nm, which was assigned to the transition from
individual dendrons because of the meta-linkage. In
contrast, owing to the insertion of electron-poor cyano
groups (!156), an additional charge transfer band
appeared at 511 nm. In thin films, a strong red-shift and
spectral broadening was observed for 156 as compared to
155, thus indicating better ordering of the former in the
solid state. The estimated optical band gap for 156 in thin
films was 1.8 eV compared to 2.4 eV for 155. The lower
band gap for 156 was ascribed to lowering of the LUMO
energy level (DLUMO = 0.5 eV), while the HOMO energies
for both oligomers were similar. In BHJ solar cells,
oligomer 156 showed a much higher PCE of 1.1 %
compared to only 0.4 % for 155-based devices (Table 8).
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Organic Semiconductors
ized ruthenium(II) phthalocyanine (RuPc) complexes and
incorporated them as active materials in BHJ solar cells.[200]
The axially coordinated pyridine-functionalized dendritic
oligothiophenes hindered aggregation of the corresponding
RuPc and improved the light-harvesting ability by covering a
broad spectral window from 300–700 nm. BHJ solar cells
fabricated using 158:160 or 159:160 (1:2, w/w) showed PCEs
of 1.0 % under simulated solar irradiation (Table 8). When
the donor materials were blended with 172, the cell efficiencies further increased to 1.6 % with a JSC value as high as
8.3 mA cm2. These results are among the best values for
solution-processed phthalocyanine-based BHJ devices.
A large variety of p-type semiconducting small molecules/
oligomers have been tested in solution-processed BHJ solar
cells as donor material mostly blended with soluble fullerene
derivatives 160 or 172 as acceptor. Despite the efficiencies
achieved with oligomers are not yet as high as for polymers,
small molecules offer significant advantages over polymers,
such as greater ease of purification and most importantly
reproducibility in device performance due to the defined
molecular structure. Typically, the photoactive dyes are of the
type D–A or D–A–D and have been prepared in various
molecular shapes such as linear, star-shaped, or dendritic.
Squaraines and diketopyrrolopyrrol(DPP)-based dyes
reported by Forrest et al. and Nguyen et al., respectively,
belong to the most promising classes of compounds, showing
record efficiencies of 5.2 % for squaraine 47 and 4.4 % for
DPP 65.[118]
As for the vacuum-processed materials, oligothiophenes
are also among the very effcient compounds with 4.3 %[185] for
star-shaped oligothiophene 143 and 3.7 %[107] for linear
septithiophene 55. Azo-, quinoxaline-, and cyanine-based
dyes seem to be favorable structures leading to efficiencies of
3.6 % (89 und 90), 3.2 % (86), and 2.6 % (137), respectively. In
contrast to the typically symmetrical and quadrupolar dyes,
interestingly, strongly dipolar D–A merocyanine dyes
recently reported by Wrthner and co-workers gave good
efficiencies close to 2.6 % (137) because of their high molar
extinction coefficients, strong aggregation properties, and
good film morphology in blends. In this respect, acene- and
HBC-based materials, which typically show high charge
carrier mobilities because of well-ordered structures, lead to
improved JSC values and efficiencies in the range of 2 % in
organic solar cells.
5. Comparison of Bulk-Heterojunction Solar Cells
Made by Vacuum- or Solution-Processing
The Wrthner and Meerholz research groups have also
tested some of the merocyanine dyes 134–138 (see above) in
BHJ solar cells prepared by vacuum-deposition and compared the results with devices prepared by solution-processing using similar device structures.[181] The cell configuration
used
in
both
cases
was
ITO/PEDOT:PSS/
merocyanine:fullerene(1:1)/BPhen 18/Ag. The active layer
contained a mixture of the merocyanine donor and the
fullerene acceptor, which was C60 in the case of the vacuumprocessed devices and 160 for the solution-processed devices,
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
respectively. At an illumination intensity of 88 mW cm2, the
highest, excellent PCE of 4.9 % was obtained for the vacuumprocessed device based on merocyanine 137 and C60 (6),
which was nearly twice the value of 2.5 % obtained for the
solution-prepared devices. This increase in efficiency was due
to both, higher JSC (ca. 11.5 mA cm2) and FF (ca. 0.47) values
for the vacuum-processed device, while the VOC was lowered
by about 0.15 V compared to the solution-processed device
(Table 7). This lowering of the VOC was due to the lower
LUMO energy level of C60 (4.08 eV) compared to 160
(4.01 eV). The electronic energy levels can also be influenced by the degree of aggregation, which is more favorable
under vacuum-deposition, thus leading to lower VOC by
solution-processing. The increase in JSC for the vacuum
devices was further manifested by the enhanced EQE value
reaching a maximum of 73 % at 600 nm compared to 56 % for
the devices prepared by solution processing. This observation
indicates an efficient charge-carrier generation and/or reduced recombination in the solar cells with evaporated active
layers. The OFET hole mobility of the active layers prepared
by both solution and vacuum processes was found to be 2 105 cm2 V1 s1. The authors proposed that the strong dipolar
merocyanine dyes can be treated as centrosymmetric dimers,
which on the supramolecular level have a quadrupolar and
not a dipolar character favoring homoaggregation by pronounced electrostatic interaction between the molecules. This
effect could lead to efficient phase separation with fullerene
derivatives.[201] These comparative studies on vacuum- and
solution-processed BHJ solar cells using merocyanine donors
and fullerene acceptors demonstrate that with respect to
power conversion efficiency, for merocyanines, vacuum
technology is advantageous over solution techniques.
Forrest and co-workers demonstrated that following
thermal and solvent-vapor-annealing processes, the efficiency
of combined solution- and vacuum-processed devices can be
improved surpassing similar device structures made by only
vacuum technique. The vacuum-processed cell structure ITO/
squaraine 47 (6.5 nm)/C60 (40 nm)/BCP 12 (10 nm)/Al
showed PCEs of up to 3.2 %. However, bilayer devices
prepared by spin-casting of the squaraine dye, followed by
thermal annealing and subsequent vapor deposition of C60 (6)
in a cell configuration ITO/MoO3 (8 nm)/47 (6.2 nm)/C60
(40 nm)/BCP (10 nm)/Al gave higher PCEs of 4.6 %.[174] The
PCE was further improved to 5.2 % for BHJ structures of 47
and 172, in which the bulk layer was annealed using solvent
vapor (see above).[173]
It has been shown that solution-processing led to a less
favorable morphology of the active layer, while betterordered layers could be achieved by vacuum deposition or
by thermal or solvent vapor annealing of active layer(s).
However, vacuum-processing is more costly because of the
more sophisticated equipment needed. Therefore, it seems to
be worthwhile to screen materials by solution processing and
to use the most promising derivatives in vacuum-processed
solar cells to improve their performances. However and most
importantly, the materials should be vaporizable and thermally stable during the sublimation process, which restricts
the choice of molecular structures.
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6. Small Molecular Semiconductors as n-Type
Materials in OSCs
In contrast to the development of a wide range of organic
semiconductors that show effective hole-transporting (ptype) behavior, the advancement of efficient electron-transporting (n-type) materials is still rather limited. Recently, it
has been demonstrated that p-conjugated systems substituted
with electron-withdrawing moieties generally exhibit lower
LUMO energy levels and therefore facilitate electron injection, leading to an increased n-type character.
The most promising acceptor (n-type) materials used for
organic solar cells are either fullerene C60 (6) processed by
vacuum-deposition or 160/172 used for solution-processing
because of their spherical shape and high charge carrier
mobilities. The synthesis of 160 was first reported by Wudl
and co-workers in 1995.[202] The attachment of phenyl and
butyric acid methyl ester units improved the solubility of the
molecule in various organic solvent such as chloroform,
toluene and o-dichlorobenzene. The main disadvantage of
160 is its weak absorption in the visible region due to the high
degree of symmetry. Later, Janssen and co-workers developed
the corresponding C70-derivative 172, which was isolated as a
mixture of three chiral isomers in a 7:85:8 ratio.[203] 172
showed stronger absorptions in the visible region between 400
to 700 nm because of its unsymmetrical nature.
The electron mobility (me) for pure epitaxially grown films
of sublimed C60 is reported to be as high as 6 cm2 V1 s1 in
OFETs.[204] Further derivatization of C60 reduced me, as it
would be expected due to desymmetrization of the molecule.
The electron mobility of solution-processable 160 measured
by the SCLC method was reported to be 2 103 cm2 V1 s1.[37] Using OFETs later on, Anthopoulos and
co-workers reported quite high electron mobilities on the
order of 0.21 and 0.1 cm2 V1 s1, respectively, for 160 and
172.[205] Thus, these fullerene derivatives have so far been the
most widely used electron acceptor materials in OPVs due to
their high electron affinity, good charge separation ability,
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efficient electron transport property, reduced recombination,
and optimal phase separation (depending on the type of
donor material used). However, the low-lying LUMO energy
level of both PCBMs (ca. 4 eV) limits the VOC of the devices.
Some new acceptors based on fullerene derivatives 161 and
162 were developed in recent years. Implementation of these
new acceptors in BHJ solar cells in combination with pconjugated polymers as donor materials led to enhanced
performances. The replacement of the phenyl group in 160 by
thiophene in 161,[206–208] or fluorene[209] in 162 improved the
open circuit voltage by raising the LUMO energies of the
acceptors. Recent reviews by Wudl and co-workers,[46] Martn
and co-workers,[210] and Li and co-workers[211] describe the
development for fullerene-based organic semiconductors as
n-type materials in solution-processed OSCs.
Jen and co-workers reported a simple and effective
approach to improve thermal stability of BHJ solar cells by
the introduction of new amorphous fullerene derivatives as
electron-accepting materials.[212] In these structures, the
phenyl ring of 160 was replaced by a more bulky dimethylfluorene (!163) or 9,9- triphenylamine (!164), aiming at
suppressing the crystallization of the corresponding compounds in the bulk layer. BHJ solar cells based on
P3HT:PCBMs (1:0.7, w/w) were fabricated using the inverted
cell
structure
(ITO/ZnO/C60-SAM/P3HT:PCBMs/
PEDOT:PSS/Ag).[213] The devices based on 163 and 164 as
acceptors gave PCEs of 3.8 and 4.0 %, respectively, which are
comparable to those of devices with 160 (4.2 %). The slightly
lower PCE values for these new acceptors were ascribed to
the lower electron mobilities that resulted in lower JSC values
of 9.9 and 9.8 mA cm2, respectively, as compared to
10.4 mA cm2 for devices with 160. Most importantly, thermal
stability of the devices based on 163 and 164 was remarkably
enhanced by suppressing the phase segregation between the
polymer and the fullerene because of the amorphous nature
and high glass-transition temperature of the acceptor moieties.
Frchet and co-workers reported a new family of soluble
fullerene derivatives that comprise a dihydronaphthyl group.
Fullerene 165 (45 wt %), when implemented in polymer solar
cells with P3HT as donor, generated PCEs as high as 4.5 %,
which is comparable to 4.4 % obtained for devices based on
160 (40 wt %).[214]
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Fullerene bis-adducts 166–168 were recently prepared and
tested in BHJ solar cells. The LUMO energy levels of the bisadducts were about 0.1 eV higher compared to their mono-
adducts. Solar cells based on 166:P3HT showed higher PCEs
of 4.5 % compared to 3.8 % for 160:P3HT-based devices. This
higher PCE was ascribed to the enhanced VOC of 0.73 V, which
is 0.15 V higher than that of the P3HT:160 cell.[215] BHJ solar
cells prepared by using P3HT as donor and fullerene 167 as
acceptor gave a JSC of 5.9 mA cm2, a VOC of 0.72 V, a FF of
0.41, and a PCE of 1.72 %. On the other hand, devices
prepared using P3HT and mono-adduct 161 exhibited a JSC of
10.9 mA cm2, a VOC of 0.60 V, a FF of 0.61, and a PCE value
of 3.97 %.[208] The higher VOC for the bis-adduct was caused by
its higher LUMO energy level. Having a similar cell configuration, devices based on P3HT:160 gave an efficiency of
4.18 %, which is comparable to that of a P3HT:161-based cell.
The reduced JSC of the device based on the bis-adduct was
ascribed to the worse blend morphology, which generally
influences the overall photovoltaic performance.
Indene-fullerene-C60 bis-adduct 168 was prepared by Li
and co-workers. The adduct showed remarkable performance
in BHJ solar cells using thiophene derivatives as donor. The
LUMO energy level of 168 was about 0.17 eV higher
compared to 160. BHJ solar cells prepared based on
P3HT:168 (1:1, w/w) showed a higher VOC of 0.84 V and a
PCE of 5.44 %, while solar cell based on P3HT:160 displayed
a VOC of 0.58 V with an overall PCE of only 3.88 % under
similar conditions.[216] In a recent report, the PCE of the
P3HT:168 device was improved to 6.5 % by solvent-annealing
and prethermal annealing at 150 8C.[217] This improvement was
due to an increase in JSC from 9.67 to 10.6 mA cm2 and in the
FF from 0.67 to 0.73.
Fulleroisoquinolinone 170 was synthesized by palladiumcatalyzed annulation of N-alkyl benzamide with C60.[218] BHJ
devices that incorporate the fulleroisoquinolinone 170 as
acceptor and P3HT as donor exhibited a PCE of 1.0 %, a JSC of
4.3 mA cm2, VOC of 0.45 V, and FF of 0.53. The PCE was
improved to 2.3 % by thermal annealing of the active layer at
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
180 8C. The increased PCE was due to the enhancement of JSC,
VOC, and FF values to 7.0 mA cm2, 0.51 V, and 0.65,
respectively.
Mikroyannidis et al. prepared the new fullerene derivative 171 from 160 as precursor.[219] 160 was first hydrolyzed to
the corresponding carboxylic acid and then converted into the
acid chloride. The latter was then condensed with 4-nitro-4’hydroxy-a-cyanostilbene to give 171. The LUMO energy
level of 171 is about 0.2 eV higher compared to 160. When
blended with P3HT in a D:A ratio of 1:1 from chloroform
solution, BHJ solar cells generated PCEs of 4.2 % and high
VOCs of 0.86 V. Under similar conditions, P3HT:160 cells
generated a PCE of about 2.9 % and a VOC of only 0.68 V. The
PCE of devices based on P3HT:171 was further improved to
5.25 % by spin-casting the blend layer from chloroform/
acetone solvent mixture followed by thermal annealing at
120 8C. The JSC, VOC, and FF values of the corresponding
device were 10.3 mA cm2, 0.81 V, and 0.63, respectively. This
improvement was ascribed to the improved donor crystallinity and nanoscale morphology, resulting in a balanced
charge transport in the BHJ structure.
Owing to the weak absorption of 160 in the visible region,
analogous C70 derivative 172 was developed and nowadays is
one of the best acceptors in OSCs because of its stronger
absorption in the visible part of the solar spectrum, which is
ascribed to the lower symmetry of C70 compared to C60.
Devices based on low-band-gap polymers and 172 showed
very high verified efficiencies with internal quantum efficiencies close to 100 %, implying that essentially every absorbed
photon gives a separated pair of charge carriers and that all
photogenerated carriers are collected at the electrodes.[220]
The thiophene analogue of PC71BM (172), 173 was developed
showing PCEs of 3.8 % in P3HT-based solar cells.[207] Very
recently, indene-C70 bis-adduct 174 was prepared, the LUMO
energy level of which is 0.19 eV higher than that of 172. A
solar cell based on P3HT:174 gave a higher VOC of 0.84 V and
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On the other hand, the cell with 4P-TPD 25 as donor gave an
enhanced efficiency of 1.63 % with a VOC value close to 1.0 V,
a JSC value of 3.8 mA cm2, a high FF value of 0.57. The rather
low JSC value was explained by the fact that MeOTPD 16 and
4P-TPD 25 have optical band gaps around 3.0 eV (lmax
340 nm) so that only the absorption of 175 contributes to
the current. The higher JSC of 5.1 mA cm2 for ZnPc-based
devices was due to the contribution from strong absorption of
ZnPc in the near-IR region.
Camaioni et al. reported oligothiophene-S,S-dioxide 176
as an electron acceptor material in solution-processed BHJ
solar cells. Although the device based on P3HT:176 (1:1, w/w)
generated a high VOC of 0.93 V, the PCE was only 0.06 %,
which was ascribed to the less uniform blend morphologies.[224]
a higher PCE of 5.64 % in comparison to devices based on
P3HT:172 that showed a VOC of 0.58 V and a PCE of
3.96 %.[221]
Owing to their lower LUMO energy levels, functional
fullerene-based devices showed relatively low VOCs. Furthermore, the limited solubility of fullerene derivatives leads to
difficult purification steps. In this regard, it is necessary to
optimize the optical properties and energy levels by designing
new acceptor materials that have strong absorption in the
visible region and high LUMO energy levels ensuring
efficient charge transport as well as high VOC. However,
nonfullerene-based n-type materials based on small organic
molecules are nowadays rather limited.[222, 223]
Pfeiffer, Leo, Buerle and co-workers reported low-bandgap DCV-capped terthiophene 175 designed for the use in
vacuum-processed m-i-p-type bilayer heterojunction solar
cells.[78] Owing to its low-lying HOMO energy level (6.1 eV
vs. vacuum), oligothiophene 175 was used as an acceptor in
combination with different donor materials such as ZnPc 3,
MeOTPD 16, or 4,4’-bis-(N,N-diphenylamino)quaterphenyl
25 (4P-TPD). All devices showed efficiencies in the range of
1.06–1.63 %. ZnPc/175 heterojunction devices reached a VOC
value of 0.71 V, a JSC value of 5.1 mA cm2, a FF value of 0.40
with an efficiency of 1.13 % measured under 127 mW cm2
simulated sun light. Under similar conditions, the cell with
MeOTPD 16 as the donor gave a VOC value of 0.83 V, a JSC
value of 4.0 mA cm2, a FF value of 0.41 and an efficiency of
1.06 %. The relatively low FF values for both ZnPc- and
MeOTPD-based devices were caused by the s-shaped current-voltage (J–V) characteristics, which indicate a higher
hole-transport barrier at the donor/acceptor interface and
electron injection barrier between ITO and the acceptor 175.
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Sellinger et al. developed a series of nonfullerene acceptors based on 2-vinyl-4,5-dicyanoimidazoles with LUMO
energy values ranging from 2.84 to 3.5 eV.[225–227] Among
them, imidazole 177 was exploited as a suitable acceptor
blended with P3HT or poly(2,7-carbazole) (PCz) as the donor
material. Devices based on P3HT gave a VOC of 0.67 V and a
PCE of 0.45 %, while PCz-based devices gave a very high VOC
of 1.36 V with a PCE of 0.75 %. Furthermore, the good FF of
0.5 for devices comprising the PCz donor demonstrated
relatively low energy losses.
Tilley and co-workers reported a photovoltaic device
based on P3HT as donor and 2,7-bis(pentafluorophenylethynyl)hexafluorogermanofluorene 178 as acceptor (LUMO 3.5 eV). A high VOC of 0.90 V was obtained because of the
higher LUMO level of the acceptor. However, the device
showed a very low PCE of 0.035 %.[228] Tian et al. prepared D–
A type molecule 179 and used it as an acceptor blended with
poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylenevinylene]
(MEH-PPV) or P3HT as the donor. The cells generated high
VOC values of up to 1.14 V with rather low PCEs in the range
of 0.13–0.2 %.[229] These low efficiencies were attributed to the
lower electron mobility (1.15 105 cm2 V1 s1) of 179 compared to 160 (2 103 cm2 V1 s1).
Janssen and co-workers tested some diketopyrrolopyrrols
as acceptors in BHJ solar cells with P3HT as donor.[230] Cells
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based on derivatives 180 and 181 showed PCEs of 0.17 and
0.31 % and VOC values of 0.85 and 0.52 V, respectively. The
rather poor performance was the
result of rather poor FFs and photocurrents caused by poor blend morphology.
Chen and co-workers prepared
diketopyrrolopyrrols
182
and
183.[231] These compounds showed
intense absorption spanning from
300 to 700 nm. The insertion of
double bonds in 183 reduced the
band gap by 0.11 eV compared to
182. Devices using 182 as acceptor and P3HT as donor showed
a VOC of 0.81 V, a JSC of 2.36 mA cm2, and a FF of 0.52
resulting in a PCE of 1.0 % when processed from toluene. In
contrast, 183-based devices gave a PCE of only 0.58 % with a
VOC of 0.64 V, a JSC of 1.7 mA cm2, and a FF of 0.53. The
lower VOC for the latter device was ascribed to the lower
LUMO energy level of 183.
In Sections 2 and 3, various pentacene derivatives were
presented as p-type materials. Recently, Anthony and coworkers reported a series of pentacenes containing electronaccepting units such as cyano (184) or trifluoromethyl (185)
and used them as n-type material in BHJ solar cells in
combination with P3HT as donor.[232] The best solvent mixture
used for the device fabrication was toluene/dichlorobenzene
(10:3 ratio by volume). In a 1:1 w/w blend ratio of donor:acceptor, 184-based devices generated a VOC of 0.84 V, a JSC of
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
3.56 mA cm2, a FF of 0.42, and a PCE value of 1.3 %.
Similarly, a 185:P3HT-based device gave a VOC of 0.80 V, a JSC
of 3.17 mA cm2, a FF of 0.50, and a PCE value close to 1.3 %.
The authors concluded that the “sandwich herringbone”
crystal packing motif of pentacenes is one of the important
parameters for better solar cell performance.
Wudl et al. reported the potential
of 9,9’-bifluorenylidene derivative 186
as new generation acceptor material.
The reduction of the C9-C9’ bond in
186 by addition of one electron
released the steric strain along the
double bond and the system gained
14p electron aromaticity. In a device
configuration
ITO/PEDOT:PSS/
P3HT:186/Ba/Al the cell generated a
VOC of 1.1 V, a JSC close to 4 mA cm2,
a FF of 0.40, and a PCE of about 1.7 %.[233]
DCV-substituted fluorene-benzothiadiazole-based oligomer 187 was prepared and used as n-type material in OSCs.[234]
The attachment of the DCV unit increases the electron
affinity of the compound. BHJ solar cells prepared by using
P3HT:187 (1:1, w/w) showed a PCE of 0.58 % after annealing
at 65 8C. BHJ devices with optimized acceptor loading of
67 wt % (i.e. D:A ratio of 1:2 w/w) displayed an overall PCE
of 0.73 % with JSC = 2.4 mA cm2, VOC = 0.62 V, and FF = 0.49.
Due to the low glass transition temperature of oligomer 187
(Tg = 62 8C), annealing of the device at 65 8C helped to
increase the molecular order in the acceptor phase.
Wang and co-workers used dicyano-substituted quinacridone derivative 188 as n-type material in BHJ solar cells and
P3HT as donor.[235] The quinacridone showed strong absorption in the region from 550 to 700 nm, where the absorption of
P3HT and PCBM is weak. The HOMO energy level of 188
determined from UPS was 5.9 eV. The LUMO energy level
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(4.1 eV) was calculated based on the HOMO level and the
optical band gap in thin films. Owing to the poor solubility of
188 in o-dichlorobenzene, the blended film for device
fabrication was prepared from a mixture of chloroform and
o-dichlorobenzene. BHJ solar cells prepard using the structure ITO/PEDOT:PSS/P3HT:188/LiF/Al, showed a PCE of
1.57 % (JSC = 5.7 mA cm2, VOC = 0.48 V, and FF = 0.57).
In search of nonfullerene-based acceptor materials, various perylenediimides were prepared and tested in OSCs due
to their n-type character and broad absorption in the visible
region. Jabbour and co-workers fabricated solar cells by using
perylene derivatives 10 and 189 as acceptor and palladium
phthalocyanine as donor.[236] Owing to the panchromatic
absorption of the blend, the EQE spectrum of the cell covered
the region from 400 to 800 nm with a maximum of around
40 % at 600 nm. By using 10 as an acceptor, the device
achieved a PCE of 2.0 % compared to 189a and 189b, which
have only 1.0 and 1.1 %, respectively. The devices comprising
10 and 189a generate similar JSC values of 5 mA cm2, while
189b-based device showed a lower JSC of 3 mA cm2. The
lower PCE for 189a-based device was due to its poor FF
(40 %) compared to 63% for 10-based device. This difference
in photovoltaic performance could be due to larger interfacial
recombination and/or poor intermolecular packing as well as
lower carrier mobility.
Recently, Sharma and co-workers prepared the new
perylenediimides 190 and 191 and used them in BHJ devices
in combination with different small-molecule donor materials.[237, 238] Terminal cyanovinylene-4-nitrophenyl substituted
p-phenylenevinylene oligomer 84 and dithienylbenzoselenadiazole oligomer 83 were used as donors. A device with the
configuration ITO/PEDOT/84:190 (1:3.5, w/w)/Al gave a
PCE of 1.87 % with a high VOC of 0.98 V. The incorporation of
a thin ZnO layer between the blend and the Al cathode
further increased the device efficiency to 2.46 %, which was
ascribed to the enhanced light absorption by the active layer
due to the optical interference between the incident light and
the reflected light from the Al cathode.[237] The PCE was
further improved to 3.17 % upon thermal annealing showing a
JSC of 6.3 mA cm2, a VOC of 0.95 V, and a FF of 0.53. BHJ
devices based on 83:191 (1:1, w/w) gave a PCE of 1.28 %,
which was improved to 3.88 % when the blend was thermally
annealed at 100 8C for 20 min. This improvement was due to
the increase of JSC from 2.9 to 8.3 mA cm2 and of the FF from
0.43 to 0.52.[238]
Torres and co-workers used perfluorinated boron SubPc
192 as electron acceptor in vacuum-processed bilayer solar
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cells with the cell structure ITO/SubNc or SubPc/192/BCP/
Al.[162] Using SubNc 117 or SubPc 118 as donors, the devices
generated PCEs of 0.63 and 0.96 %, respectively. The lower
performance for SubNc-based devices was mainly due to their
lower VOC and FF values.
Another example of a small-molecule acceptor was
reported by Jones and co-workers, who used chlorinatedSubPc 193 as acceptor and SubPc 118 as donor.[239] The
HOMO and LUMO energy levels of 193 were reported to be
5.8 and 3.7 eV, respectively, which were 0.3 eV lower than
the energy levels of the donor SubPc 118. Vacuum-deposited
BHJ devices prepared using an ITO/MoOx/118/193/BCP/Al
configuration generated a good PCE of 2.68 % with a FF of
0.58 and a JSC of 3.53 mA cm2, thus suggesting sufficient
interfacial HOMO and LUMO offsets for efficient exciton
dissociation. Furthermore, because of the maximal interfacial
gap energy (IG = 1.8 eV), the device generated a VOC value of
1.31 V, which was quite high compared to the VOC obtained
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with fully fluorinated derivative 192 (0.94 V). This difference
was ascribed to the excessive shift in the frontier orbital
energies of 192 compared to 193. The MoOx layer in the
device was used to facilitate hole extraction at the ITO
electrode, whilst BCP was used as an exciton-blocking layer at
the Al electrode.
Among the molecular n-type semiconductors, fullerene
C60 (6) and their derivatives such as 160 or 172 are the most
widely and traditionally used acceptors in either vacuum- or
solution-processed OSCs. In the last couple of years, novel
fullerene bis-adducts, such as the indene derivative of C60, 168
and C70, 174 gave improved performances compared to the
corresponding mono-adducts due to increased VOC. Very
recently, the Holmes group reported a convenient synthesis of
PCBM and indeno-fullerene derivatives using a continuous
flow approach. By varying the feeding time, temperature, and
ratio of the reagents, significant improvements in yields and
reaction times were achieved over conventional batch processes.[240] The 3-dimensional shape and excellent electrontransport properties of the fullerene derivatives lead to a by
far superior performance in BHJ solar cells over 2D systems
such as perylenes or other 1D p-conjugated molecules with
low-lying LUMO energy levels.
7. Latest Developments
Since the acceptance of this review extremely rapid
progress has been seen in the field of small-molecule organic
solar cells including many new and exciting developments. In
this section, we provide a brief account of the recent
advancement in the field.
In the field of vacuum-processed OSCs, the Wrthner and
Meerholz research groups improved the PCE for merocyanine dye 137 from (4.5 0.4)% to (5.8 0.3)% by using
MoO3 as hole-collecting layer instead of PEDOT:PSS (Table
9).[241] The MoO3 layer was deposited by vacuum-processing.
The use of MoO3 increased the VOC to 0.96 V compared to
0.77 V obtained by using PEDOT:PSS as hole-conducting
layer. The results clearly showed the influence of the anodic
work function on the device performance (work function of
MoO3 = 5.3 eV vs. PEDOT:PSS = 5.1 eV). The low-lying
valence band of MoO3 enhances the hole collection from the
donor while the high-lying conduction band serves as an
electron barrier. Surprisingly, by using C70 instead of C60, the
PCE was significantly reduced to 1.4% due to unknown
reasons.
Table 9: Device characterizations of small molecule OSCs prepared by vacuum and solution-processing.
Device structure
Concept
JSC
VOC
[mA cm2] [V]
FF
h
Light inten[%] sity
[mWcm2]
Ref.
ITO/MoO3 (20 nm)/137:6 (1:1)/18 (6 nm)/Ag
ITO/PEDOT:PSS (30 nm)/137:6 (1:1)/18 (6 nm)/Ag
ITO/MoO3 (30 nm)/194 (3 nm)/194:6 (1:1, 35 nm)/6 (20 nm)/12 (10 nm)/Ag
ITO/MoO3 (30 nm)/194 (3 nm)/194:C70 (1:1, 35 nm)/C70 (10 nm)/12 (10 nm)/
Ag
ITO/MoO3 (30 nm)/195 (7 nm)/195:6 (1:1, 40 nm)/6 (20 nm)/12 (10 nm)/Ag
ITO/MoO3 (30 nm)/195 (7 nm)/195:C70 (1:1, 40 nm)/C70 (7 nm)/12 (10 nm)/
Ag
ITO/MoO3 (5 nm)/196 (10 nm)/6 (35 nm)/12 (10 nm)/Ag
ITO/MoO3 (5 nm)/197 (10 nm)/6 (35 nm)/12 (10 nm)/Ag
ITO/MoO3 (5 nm)/197 (7 nm)/197:6 (40 nm)/C60 (20 nm)/12 (10 nm)/Ag
ITO/MoO3 (5 nm)/197 (7 nm)/197:C70 (40 nm)/C70 (7 nm)/12 (10 nm)/Ag
ITO/6 (15 nm)/27:6 (20 nm)/22 (5 nm)/22:NDP9 (50 nm)/NDP9 (1 nm)/Au
ITO/6 (15 nm)/198:6 (20 nm)/22 (5 nm)/22:NDP9 (50 nm)/NDP9 (1 nm)/Au
ITO/C60 (15 nm)/199:6 (20 nm)/22 (5 nm)/22:NDP9 (50 nm)/NDP9 (1 nm)/
Au
ITO/6 (15 nm)/200:6 (20 nm)/22 (5 nm)/22:NDP9 (50 nm)/NDP9 (1 nm)/Au
ITO/PEDOT:PSS/201 (20 nm)/6 (50 nm)/12 (10 nm)/Al
ITO/PEDOT:PSS/52 (10 nm)/201 (10 nm)/6 (50 nm)/12 (10 nm)/Al
ITO/47 (8.5 nm)/6 (40 nm)/12 (10 nm)/Al
ITO/202 (8.5 nm)/6 (40 nm)/12 (10 nm)/Al
ITO/203 (8.5 nm)/6 (40 nm)/12 (10 nm)/Al
ITO/MoO3 (8 nm, annealed at 90 8C)/203 (20 nm)/6 (40 nm)/12 (10 nm)/Ag
ITO/PEDOT:PSS (40 nm)/204:160 (50–60 nm)/Al
ITO/PEDOT:PSS (40 nm)/205:160 (50–60 nm)/Al
ITO/MoO3 (20 nm)/205:172/Ba/Ag
ITO/HIL/206:172 (3:2, 85 nm)/Al
ITO/HIL/206:172 (3:2, 85 nm)/Al
BHJ
BHJ
P/B-HJ
P/B-HJ
12.6
12.5
6.6
9.5
0.97
0.80
0.88
0.83
0.48
0.45
0.46
0.48
5.8
4.5
2.7
3.8
100
100
100
100
[241]
[241]
[242]
[242]
P/B-HJ
P/B-HJ
11.4
14.7
0.80 0.48 4.4 100
0.79 0.50 5.8 100
[243]
[243]
PHJ
PHJ
P/B-HJ
P/B-HJ
BHJ
BHJ
BHJ
0.7
4.2
5.8
7.8
7.9
6.5
7.5
0.43
1.02
1.05
1.03
1.02
0.99
1.0
0.24
0.54
0.38
0.34
0.43
0.48
0.39
0.1
2.3
2.3
2.8
3.5
3.1
2.9
100
100
100
100
99
92
101
[244]
[244]
[244]
[244]
[245]
[245]
[245]
5.8
5.8
7.2
5.6
6.7
5.1
10.0
5.8
8.3
10.2
3.7
10.9
0.95
0.92
0.93
0.59
0.82
0.86
0.90
0.96
0.94
1.0
0.74
0.70
0.46
0.72
0.74
0.51
0.59
0.57
0.64
0.41
0.38
0.44
0.27
0.42
2.5
3.9
5.0
1.8
3.2
2.5
5.7
2.3
3.0
4.5
0.7
3.2
105
100
100
100
100
100
100
100
100
100
100
100
[245]
[246]
[246]
[247]
[247]
[247]
[248]
[249]
[249]
[249]
[250]
[250]
11.3
9.9
10.7
9.9
11.5
0.84
0.88
0.86
0.93
0.80
0.42
0.51
0.55
0.49
0.64
4.0
4.5
5.1
4.5
5.8
100
100
100
100
100
[251]
[252]
[252]
[252]
[253]
ITO/PEDOT:PSS/207:160 (1.5:1, 75 nm)/LiF (1 nm)/Al
ITO/PEDOT:PSS (40 nm)/208:160 (1:0.5)/Ca/Al
ITO/PEDOT:PSS (40 nm)/209:160 (1:0.5)/Ca/Al
ITO/PEDOT:PSS (40 nm)/210:160 (1:0.5)/Ca (20 nm)/Al
ITO/PEDOT:PSS (40 nm)/211:160 (1:0.8, 130 nm)/LiF (0.8 nm)/Al
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BHJ
PHJ
PHJ
PHJ
PHJ
PHJ
PHJ
BHJ
BHJ
BHJ
BHJ (as-cast)
BHJ
(annealed)
BHJ
BHJ
BHJ
BHJ
BHJ
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Wong and co-workers reported two D–A dyes 194 and
195, which in thin films showed absorption maxima at 542 and
684 nm, respectively.[242,243] The red-shifted absorption for dye
donor materials in vacuum-processed OSCs.[245] The replacement of thiophene unit(s) by selenophene(s) resulted in a
bathochromic shift of the longest wavelength absorption
195 was due to the strong electron-accepting character of
dicyanovinylene-substituted 2,1,3-benzothiadiazole. In a
mixed P/B-HJ architecture, oligomer 194 gave a PCE of
2.7%, which was improved to 3.8% using C70 as the acceptor
(Table 9).[242] In a similar device structure, dye 195 exhibited a
PCE of 4.4% with C60 as acceptor. The PCE of 195 was
remarkably improved to 5.8% using C70 as the electron
acceptor.[243] The similar FF for all devices was ascribed to
similar blend morphologies and charge carrier percolation
networks, while higher JSCs for devices of dye 195 were due to
the red-shifted absorption compared to dye 194.
A–D–A type dyes 196 and 197 that comprise terminal
cyano or dicyanovinylene groups connected to electronaccepting 2,1,3-benzothiadiazole units were prepared and
employed in vacuum-processed OSCs.[244] The HOMO values
of these dyes determined by UPS measurements were 5.8
and 5.4 eV, respectively. Vacuum-processed bilayer devices
band. The HOMO/LUMO energy levels were determined
from electrochemical measurements and lie in the range of
5.6 and 3.8 eV. Despite broader and more intense
absorption, vacuum-deposited BHJ solar cells fabricated
with the Se-containing co-oligomers as donor and C60 (6) as
acceptor unexpectedly displayed slightly lower performance
compared to the reference all-thiophene analogue 27 (h =
3.5%), but still in a good range of 2.5–3.1% (Table 9). It is
interesting to note that the PCEs gradually decrease with
increasing selenophene content in the co-oligomer. Photoluminescence studies of blend layers pointed toward a lower
degree of donor–acceptor phase separation for 198–200 in
comparison to reference oligomer 27.
Hirada and Adachi reported a PCE of 3.9% using
tetraphenyldiindenoperylene 201 as electron donor, C60 (6)
as electron acceptor, and PEDOT:PSS as anodic buffer layer.
PEDOT:PSS also acts as an exciton quencher and to prevent
based on 196 and 197 gave PCE of 0.1 and 2.3% (Table 9).
The lower PCE for dye 196 was mainly caused by the lowlying LUMO energy level (3.92 eV), thus resulting in an
inefficient electron transfer from the LUMO of the donor to
the LUMO of C60 (6). The use of C70 in bilayer devices with
dye 197 did not improve the device performance (h = 1.9%).
This observation is mainly due to the lower FF of 0.41 for C70
compared to 0.54 for C60-based devices and was explained by
the lower electron mobility of C70. P/B-HJ devices were also
prepared using dye 197, which gave PCEs of 2.3% with C60
and 2.8% with C70 as acceptor (Table 9). The JSC values were
well corroborated by the EQE spectra. The PCE was further
improved to 3.7% in an optimized device using higher
contents of C70 (197:C70, 1:1.5, w/w).
We have prepared a series of new DCV-substituted
quinquechalcogenophenes 198–200 and have used them as
the quenching effect, the authors introduced a 10 nm layer of
compound 52 at the donor/PEDOT:PSS interface. By combination of exciton-blocking layers at both, the anode and the
cathode, the device efficiency was increased to (5.04 0.2)%
(Table 9).[246]
Forrest and co-workers developed squaraine dyes 202 and
203 for solution-processed OSCs.[247] Both dyes showed broad
and red-shifted absorption (Dabs = 12–22 nm) relative to
squaraine 47 with N-alkyl groups. OSCs incorporating dye
202 and 203 as donor and C60 (6) as acceptor showed a
significant increase in VOC as well as improved charge-carrier
transport compared to squaraine 47. The increased VOC (0.23–
0.27 V) was consistent with the decrease in HOMO energy
level by 0.2 eV. The PCE for devices prepared using 202 was
3.2% compared to 2.5% for 203 and 1.8% for 47 (Table 9).
The device structure of dye 203 was further optimized using
an MoO3 buffer layer and silver cathode.[248] The as-cast
device achieved a JSC of 8.7 mA cm-2, VOC of 0.84 V, FF of 0.64
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Angewandte
Chemie
Organic Semiconductors
Marks and co-workers synthesized naphthodithiophenediketopyrrolopyrrole-based donor material 207, which in
combination with 160 gave PCEs of 4% when thermally
annealed at 110 oC for 10 min. The PCE of as-cast devices was
only about 1%. The improved efficiency of annealed films was
with a PCE of 4.6%. The PCE was further improved to 5.7%
by thermal annealing of the donor layer at 90 8C under a
nitrogen atmosphere for 10 min followed by deposition of C60,
BCP, and Ag cathode.
Wrthner, Meerholz, and co-workers obtained PCEs of
2.3 and 3.0% using highly dipolar D–A-based merocyanine
dyes 204 and 205.[249] The improved PCE for 205 was due to a
large increase in the photocurrent. By replacing 160 with 172
and using MoO3 as buffer layer, the PCE for device 205 was
raised to 4.5%, showing the utility of the dipolar dye in OSCs
(Table 9). Single-crystal X-ray structure analysis revealed
centrosymmetric dimeric units, resulting in an annihilation of
the dipole moments. This specific feature of supramolecular
organization explains the excellent performance of merocyanine dyes in organic solar cells. The current finding demonstrates that to avoid large energetic disorder caused by the
dipolarity of merocyanine dyes, organization into favorable
anti-parallel aggregate structures is crucial.
Bazan and co-workers synthesized a donor oligomer 206,
which has a dithienosilole core substituted with hexylbithienyl-thiadiazolopyridine at the termini.[250] The dye showed
an absorption maximum at 625 nm in solution and at 720 nm
in thin films, hence resulting in an optical band gap of 1.51 eV.
Using 172 as acceptor, the as-cast device
gave a PCE of 0.7%, which was improved
to 3.2% by thermal annealing at 110 oC
for 2 min (Table 9). Conductive and
photoconductive atomic force microscopy, dynamic secondary mass ion spectrometry (DSIMS), and grazing incident
wide angle X-ray scattering (GIWAXS)
experiments revealed that thermal
annealing led to increased molecular
ordering in the donor phase and to
improved electronic properties.
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
ascribed to the formation of necessary BHJ domains and high
optical absorbance of the molecule.[251]
Octyl-substituted septithiophenes end-capped with alkyl
cyanoacrylate groups (208–210) have been prepared and
tested in BHJ devices to investigate the effect of terminal
alkyl chains.[252] Dyes 208 and 210 that contain ethyl and
ethylhexyl chains achieved PCEs of about 4.5%, while the
PCE was increased to 5.1% for oligomer 209 with n-octyl
chains (Table 9). The result was ascribed to the better
interpenetrating network morphology, balanced charge transport, and an efficient interfacial contact of the active layer to
the Ca/Al cathode. However, using a LiF/Al cathode, the
209:160 (1:0.5, w/w) devices exhibited a lower PCE of 3.9%
with a FF of 0.49, VOC of 0.84 V, and JSC of 9.4 mA cm2, which
could be due to the interfacial contact problem between the
active layer and the electrode. The Chen research group
achieved a PCE of 5.84% using oligothiophene donor 211 that
comprises a dithienosilole core unit.[253] The optimized
211:160 ratio used was 1:0.8 by weight. The result was
ascribed to the good film quality resulting in an ideal
nanoscale interpenetrating network for charge transport to
the electrodes. All these molecules showed well-organized
structures in thin films as demonstrated by X-ray diffraction
(XRD) analysis.
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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.
Angewandte
Reviews
A. Mishra and P. Buerle
8. Summary and Future Prospects
In this review, we gave a systematic overview on the
recent progress in the design and development of small
molecule/oligomeric organic semiconductors and their utilization in organic solar cells prepared by vacuum and
solution processing and combination of both. The remarkable
performance obtained with these structurally defined materials contributed to the rapid development of OSCs. Owing to
their monodisperse nature, organic small molecules literally
offer infinite structural possibilities for improving a wide
range of properties such as molecular functionality, rigidity,
stacking, strong intermolecular (p–p) interactions, and most
importantly well-defined structure and high purity. The
chemical and physical properties of such materials can be
easily fine-tuned by varying its chemical structures and
functionalities.
As an overall summary and an overview, in Table 10 we
compile “champion” oligomers/dyes with respect to their
photovoltaic performance (PCE 5 %). The dynamic in the
field is so vibrant that 6 out of 12 examples have been
published in the last three to four months and only a year ago
very few examples existed with efficiencies over 5 %.
Although it is impossible to gain a precise molecular
structure–device performance relationship—which in particular the chemists would really like to see—we try to deduce
trends, which can be extracted as a rough guideline for the
rational development of new and better materials for
SMOSCs.
Before looking at structures, it becomes clear that the
most favorable cell design is a tandem architecture including
multiple layers, which is best constructed by vacuum techniques. Also the mixing of p-type and n-type semiconductors
in bulk heterojunctions (BHJ) shows comprehensible advantages over planar heterojunctions (PHJ), because larger p/n
interfaces are achieved and a higher chance of exciton
formation and separation is given. Concerning the processing
techniques, by either vacuum or solution, it seemed that
vacuum preparation is more favorable because of more
defined conditions (see above, Section 5). Recently, however,
both excellent solution-processed and combined solution/
vacuum-processed devices showed excellent performances.
Although solution processing seems to be advantageous
because of faster and low processing cost, vacuum evaporation offers the possibility to prepare multilayer device
structures with high structural ordering. It is possible to
prepare multilayer device structures by vacuum processing,
which is still a challenging task in the case of solutionprocessable BHJ solar cells. However, the two concepts seem
not to exclude one another, since both approaches have
already been well established. In the near future, it appears
that both technologies with their specific advantages and
drawbacks will coexist. Although efficiency and durability of
devices based on both solution and vacuum processing still
can and need to be further improved with respect to high-end
applications, these obstacles will soon be overcome, if
progress continues at its current rate.
Without taking into account the most recent, not yet
published in a scientific journal Heliatek record cell of
2060
www.angewandte.org
9.8%,[27] a similarly constructed tandem cell comprising
single-junction cells made of fluorinated F4-ZnPc 212/C60
(6) and DCV6T 32/C60 (6) prepared by the Leo research
group, Heliatek and BASF showed a certified efficiency of
(6.07 0.24)% on an area of approximately 2 cm2
(Table 10).[254] This tandem cell distinguishes itself by the
complementary absorption of the dyes leading to covering of
the whole visible range of the sun spectrum (350 to 800 nm).
In fact, in this tandem cell, the optimized 3.9 % F4-ZnPc
(212)/C60 (6) and 4.3 % DCV6T 32/C60 (6) single-junction cells
were combined. The recombination layer in the tandem
device consisted of p-doped Di-NPB 19 and n-doped C60 (6)
evaporated on top of each other. This approach resulted in
almost complete summation of the VOC of the single subcells
and a high FF in the tandem devices. The high FF obtained
with both single-junction and tandem devices indicate that the
recombination contacts at the interface between the two
subcells and below the Al electrode show an ideal ohmic
behavior. So far, other tandem cells prepared by vacuum
processing, typically used the same dye in both single
cells[61,62,255,256] or from solution by combination of two bulkheterojunction subcells that comprise wide- and smallbandgap polymers.[257,258] In this respect, Forrest and coworkers reported a CuPc (2)/C60 (6) small molecule tandem
cell in 2004. These tandem cells were prepared by stacking of
two very efficient 5 % CuPc/C60 single cells[58] and the PCE
could be improved to 5.7 %,[17] but because of the same
absorber in the individual cells, the PCE gain was not
immense.
The analysis of the general structures reveals that four
classes can be identified: Besides extended and inherently
strong absorbing p systems such as phthalocyanines (2, 3),
(benzo)porphyrin (123), and polycyclic aromatic hydrocarbons (54, 201), A–D–A, D–A–D, and D–A systems are
prominent. With respect to the basic systems, the use of the
D–A concept leads to increased absorption in the visible and
near infrared regime (and a smaller band gap). Among them,
acceptor-substituted oligothiophenes (A–D–A type, 32, 34,
209, 211) belong to the most promising systems, as well as
squarains (47), D–A-type dye 195, and polycyclic diindenoperylene 201. Rather surprisingly, the nonsymmetrical and
dipolar merocyanines (D–A type, 137) also show excellent
performance in SMOSCs.
All of the “champion” systems compiled in Table 10,
typically show excellent parameters contributing to the
overall PCE (JSC, VSC, FF), however, each system also has
some weaknesses and should give “room” for improvement.
In the case of the phthalocyanines, the rather moderate VOC is
a drawback, in most of the other systems, the FF is only
moderate. In the phthalocyanine case, a better adjustment of
the HOMO/LUMO levels with respect to the electrodes and
the fullerene acceptor could be an option to improve the
overall performance, whereas in other systems, molecular
packing in the bulk, that is, better ordering and morphology
should be the choice, which is especially hard to control in the
case of solution processing. In the case of solution processing,
morphology markedly depends on the composition of materials, type of solvent, solvent vapor, or thermal annealing
conditions, and use of additives. In contrast, in vacuum
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
663
500
600,
680
(sh)
ITO/MoO3 (30 nm)/195 (7 nm)/195:C70
(1:1, 40 nm)/C70 (7 nm)/BCP (10 nm)/Ag
ITO/PEDOT:PSS (40 nm)/211:160 (1:0.8,
130 nm)/LiF (0.8 nm)/Al
ITO/2 (7.5 nm)/2:6 (1:1) (12.5 nm)/6
(8 nm)/5 (50 nm)/Ag (0.5 nm)/p-13
(5 nm)/2 (6 nm)/2:6 (1:1) (13 nm)/6
(16 nm)/12 (10 nm)/Ag
666
578
ITO/MoO3 (20 nm)/137:6 (1:1)/Bphen
(6 nm)/Ag
ITO/MoO3 (8 nm, annealed at 908C)/203
(20 nm)/6 (40 nm)/12 (10 nm)/Ag
512
labsdonor
sol
[nm]
ITO/n-6 (4 wt %, 5 nm)/6 (15 nm)/212:6
(1:1) (40 nm)/19 (10 nm)/p-19 (5 wt %,
165 nm)/p-19 (10 %, 5 nm)/n-6 (4 wt %,
5 nm)/6 (5 nm)/32:6 (2:1, 30 nm)/22
(5 nm)/p-22 (10 %, 5 nm)/p-19 (10 %
NDP9, 40 nm)/NDP9 (1 nm)/Al
Device structure
3.60
3.88
(32)
5.45[d] 4.01[d]
5.75
5.55
(32)
5.46
(212)
HOMO LUMO
vac, P/BHJ
vac, BHJ
vac,
tandem
BHJ/BHJ
710
630,
700
5.43
3.70
vac, PHJ
vac,
5.20[b] 3.50[c] tandem
BHJ/BHJ
10.0
9.7
11.5
14.7
12.6
6.2
FF
h
Light
Compound Class
[%] intensity
p-type sc. j n-type sc.
[mWcm2]
0.90 0.64 5.7 100
1.03 0.59 5.7 100
0.80 0.64 5.8 100
0.79 0.50 5.8 100
0.97 0.48 5.8 100
1.59 0.62 6.1 100
Concept[a] JSC
VOC
[mA cm2] [V]
596,
5.25[d] 3.56[d] sol, BHJ
650(sh)
684
620
594
(32)
630,
700
(sh)
(212)
labsdonor
film
[nm]
Table 10: Photovoltaic properties of some “champion” dyes prepared by vacuum- and solution-processing showing PCEs 5 %.
[248]
[17]
[253]
[243]
[241]
[254]
Ref.
Organic Semiconductors
Angewandte
Chemie
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2061
2062
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2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
550
5.50
3.50
vac, PHJ
7.2
18.0
10.5
12.0
11.1
5.20[b] 3.50[c] vac, BHJ
sol, BHJ
sol/vac,
BHJ
vac, BHJ
10.7
3.40
3.73
FF
0.93 0.74 5.0 100
0.54 0.61 5.0 120
0.86 0.55 5.1 100
0.75 0.65 5.2 100
0.92 0.50 5.2 100
2/6
h
Light
Compound Class
[%] intensity
p-type sc. j n-type sc.
[mWcm2]
0.97 0.49 5.2 102
VOC
Concept[a] JSC
[mA cm2] [V]
5.43[d] 3.59[d] sol, BHJ
5.30
5.62
HOMO LUMO
[246]
[58]
[252]
[166]
[173]
[86]
Ref.
[a] vac = vacuum; sol = solvent; BHJ = bulk heterojunction; PHJ = planar heterojunction; P/B-HJ = planar/bulk mixed heterojunction. [b] The HOMO levels were obtained by ultraviolet electron
spectroscopy. [c] The LUMO levels were estimated from the difference oft he HOMO energy and the optical energy gap. [d] The values were determined using an Fc/Fc+ value of 5.1 eV vs. vacuum.
ITO/PEDOT:PSS/52(10 nm)/201 (10 nm)/6
–
(50 nm)/12 (10 nm)/Al
600,
680
ITO/2 (15 nm)/2:6 (1:1) (10 nm)/6
(35 nm)/12 (10 nm)/Ag
630,
700
580
492
ITO/PEDOT:PSS (40 nm)/209:160 (1:0.5)/
Ca/Al
700
670
650
ITO/MoO3 (8 nm)/47:172 (1:6) (78 nm)/6
(4 nm)/12 (1 nm)/LiF (0.8 nm)/Al
570
labsdonor
film
[nm]
ITO/PEDOT:PSS/123/123:169/169/
Nbphen/Al
530
labsdonor
sol
[nm]
ITO/6:NDN1 (2 wt %, 5 nm)/6 (15 nm)/
34:6 (2:1) (40 nm)/22 (5 nm)/22:NDP9
(10 wt %, 10 nm)/spiro-NPB:NDP9
(10 wt %, 30 nm)/NDP9 (1 nm)/Al
Device structure
Table 10: (Continued)
.
Angewandte
Reviews
A. Mishra and P. Buerle
Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067
Angewandte
Chemie
Organic Semiconductors
processing substrate temperature and the rate of deposition
play a crucial role to obtain better ordering. Despite the
described molecular systems already exhibit strong absorptions, which, for example, is an advantage over inorganic
semiconductors, it is expected that the device performance
can be further improved by designing and choosing oligomers/dyes that have a broader absorption coverage and a
lower band gap to harvest more solar energy.
In summary, different (oligomeric) materials and device
concepts that have been developed in recent years to enhance
reliability and efficiency of organic photovoltaics were
presented. Particular attention was paid to current efforts to
improve the processability and tunability of organic materials.
In this context, it is of utmost importance to reconsider the
basic principles of dye design for the improvement of the
device performance. Further exploration of structure–property correlations in the context of device efficiency as well as
durability would certainly facilitate widespread utilization of
this technology. Elucidation of the relationship between
molecular structure, intermolecular interactions, packing,
thin-film morphology, photophysical and photovoltaic properties of the dyes would be a prerequisite to overcome the
situation. Materials scientists have designed and synthesized a
great variety of appropriate semiconducting materials over
the last years. Only a few of them have been investigated
thoroughly. Possibly, the best materials are still unknown or
the potential of already available materials has yet to be
recognized. This review should be helpful to evaluate and
formulate requirements for molecular structures based on
small molecular semiconductors and to explore them in
organic solar cells. It will be exciting to see the further
development of this research field in the years to come.
We would like to thank the German Federal Ministry of
Education and Research (BMBF) in the frame of joint project
OPEG 2010 for funding our research on organic solar cell
materials. We also very much acknowledge continuous excellent collaboration with Dr. M. Pfeiffer and co-workers at
heliatek GmbH, Dresden/Ulm, as well as with Prof. Leo and
his group at IAPP, Dresden. At this end, it is also a great
pleasure to acknowledge the members of our institute, who
continuously contribute to the development of novel materials
for solar cell applications.
Received: April 4, 2011
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