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Chapter 12
Quantum Dot Sensitized Solar Cells
Karan Surana and R.M. Mehra
Abstract Quantum Dot Sensitized Solar Cells (QDSSCs) are currently a field of
intense research across the globe as they provide a promising cost-effective alternative for efficient energy conversion. The wide acceptance of QDs is due to their
exceptional optical properties, size-tunable electro- and photoluminescence,
multiple exciton generation (MEG), and broad absorption spectra. However, the
progress of QDSSCs is confronted with many challenges. The basic strategies of
enhanced photovoltaic characteristics depend on factors like—suppressed charge
carrier recombination at the interfaces, improved photon absorption, and construction of tandem structures. Exploiting further nanoscience and nanotechnology
will be essential in overcoming these hurdles for achieving better functional
quantum dot sensitized solar cells. The current research work focuses on how the
prevalent challenges are being overcome in order to develop efficient functioning
As we strive for better technology, the urge to develop, cost effective, easily
manufactured, and efficient alternative energy sources increases significantly across
the globe. Among the alternative energy sources such as wind, water, biomass,
solar, geothermal, significant research focus has gone into the conversion of solar
energy into electrical energy using solar cells. A solar cell is a solid-state electronic
device that converts the solar energy directly into electricity by photovoltaic
(PV effect). Ever since the French scientist Alexandre Edmund Becquerel discovered the PV effect in 1839, the research in the field of solar cells has continuously
progressed; the ongoing research has led to the fourth generation of solar cells,
which are based on low-cost and easily processable materials like quantum dots
K. Surana R.M. Mehra (&)
Material Research Laboratory, School of Basic Sciences & Research,
Sharda University, Greater Noida, UP 201310, India
© Springer Nature Singapore Pte Ltd. 2018
Z.H. Khan (ed.), Nanomaterials and Their Applications,
Advanced Structured Materials 84,
K. Surana and R.M. Mehra
(QDs), natural dyes, and perovskite. Among these, quantum dot sensitized solar
cells (QDSSCs) have emerged as a strong PV candidate for future clean energy
QDSSCs are a promising low-cost alternative to existing PV technologies such
as crystalline silicon (Si) and thin inorganic films. A QDSSC makes use of quantum
dots (QDs) as light absorbing material. QDs are semiconducting nanoparticles
whose excitons (bound pairs of conduction band electrons and valence band holes)
are confined in all three spatial directions thus showing excellent quantum confinement effect. The wide acceptance of QDs is due to their exceptional
size-depended optical and electronic properties, size-tunable electro- and photoluminescence, multiple exciton generation (MEG), and broad absorption spectra. By
tailoring the size of the QDs, their absorption spectrum can be tuned, thus, different
sized QDs having absorption maxima peaks in the entire visible spectrum can be
used in QDSSCs to harness solar energy efficiently. This could easily raise the
power conversion efficiency above the Shockley–Queisser limit of 31% for conventional Si-based solar cells.
QDSSC Structure
QDSSCs comprise of four main components as shown in Fig. 12.1: (i) a 2–20 lm
thick, mesoporous, wide bandgap semiconductor film (typically TiO2 or ZnO)
composed of crystalline nanoparticles abutting one another on transparent conducting oxide (TCO) glass (ii) QDs adsorbed onto the mesoporous layer (iii) a hole
conductor electrolyte interpenetrating the nanocrystalline semiconductor network
and (iv) a counter electrode. The interfacial properties of the QDs should be such
that it gets properly adsorbed onto the wide bandgap semiconductor surface while
efficiently absorbing radiation from the entire UV–Vis–IR spectrum. However, the
progress of QDSSCs is confronted with many challenges including—finding
appropriate hole-carriers for each type of QDs; establishing suitable redox electrolyte which is non-corrosive to the specific QDs; ensuring no path for recombination of the excited photons; determining the proper combination of QDs in
Fig. 12.1 Schematic diagram of Quantum Dot Sensitized Solar Cell [1]
Quantum Dot Sensitized Solar Cells (QDSSCs)
multi-layered structure for optimum photovoltaic characteristics, and few others.
Here on, the progress made in the performance of QDSSCs is depicted on the basis
of power conversion efficiency, and emphasis is given on the importance of energy
level alignment of the system in order to increase the light to electric power conversion efficiency.
Working Principle
First, let’s have a general understanding of how QDSSC works. The device when
irradiated under sunlight, electron–hole pairs are generated owing to photoexcitation of QDs. The excited electron is transported from the valence band (VB) or the
HOMO to the conduction band (CB) or the LUMO of the QD and is swiftly
injected into the LUMO of the wide bandgap semiconductor, leaving a hole in the
valence band (VB) of the QD. The oxidized QD is subsequently restored by
electron donation from the redox couple. Simultaneously, the injected electron in
the CB of semiconductor travels through the porous structure into the conducting
glass and reaches the counter electrode via the external load where it is intercepted
by the redox couple. The generated voltage corresponds to the difference between
the conduction band energy level of the (wide bandgap) semiconductor and the
redox potential of the electrolyte. Consequently, without any perpetual chemical
alteration, electrical power is generated. The schematic diagram of the above
mechanism is shown in Fig. 12.2.
Fig. 12.2 Schematic diagram of the working mechanism of a QDSSC [1]
K. Surana and R.M. Mehra
The QDs of groups III–V, II–VI, or IV–VI, particularly CdS, CdTe, CdSe,
core-shell structures like CdSe/CdS, CdS/ZnS, CdSe/ZnS, CdSeTe/ZnS have been
widely exploited for QDSSC. The highest efficiency has not yet reached an
admirable figure, meaning it is still far from the one obtained with DSSC but the
hope is still there as wide modifications are possible in the arena of QDSSC. With
desired permutations and combinations of the various components, a sustainable
efficiency can be achieved in QDSSC for commercial application.
The fabrication and study of solar cells based on CdS and CdSe QDs assembled
onto 1-D TiO2 nanofibers were presented by Sudhagar et al. [2]. In this study, TiO2
nanofibers were electrospun on FTO substrates. The nanofibrous TiO2 electrodes
were sensitized with CdS QDs via chemical bath deposition (CBD) technique for
2–5 cycles and followed by further sensitization with CdSe QDs for half day.
Finally, polysulfide electrolyte and platinum counter electrode were used to
assemble the QDSSC. The coupled CdS:CdSe QDSSC showed a significantly high
IPCE of 80% for the TiO2:CdS (4 cycles):CdSe. The corresponding values of Voc,
Jsc, FF, and η were 0.64 V, 9.74 mA/cm2, 42.3, and 2.69%, respectively. The
improved photovoltaic characteristics were credited to the variation of particle size
in CdS QDs, which lead to the quasi-Fermi-level alignment and consequently
resulted in cascade energy level structure in the order of TiO2 < CdS < CdSe.
Hence the introduction of a CdS layer between TiO2 and CdSe elevated the conduction band edge of CdSe, giving a higher driving force for the injection of excited
electrons out of the CdSe layer.
An efficiency of *1% was published by Chen et al. [3] using oleic acid
(OA) capped CdSe QDs. The QDSSC consisted of a 5 µm thick TiO2 layer spread
using doctor blade method on FTO glass. The TiO2 surface was modified with
mercaptopropionic acid (MPA) linker prior to immersion in CdSe QD solution.
Compared to TOPO capped CdSe QDs, the amount of OA-capped QDs adsorbed
on MPA modified TiO2 electrode was found to be about 1.7 times greater thereby
confirming improvement in the QD loading and light-harvesting capability.
A platinum counter electrode was used, and the QDSSC performance was compared using two types of electrolytes, viz. iodide/triiodide and polysulfide. Iodide
electrolyte showed an efficiency of 0.97% with a higher Voc of 0.65 V, while the
QDSSC with polysulfide electrolyte gave higher values of Jsc and FF.
Chen et al. [4] prepared solar cells using hydrothermally grown ZnO nanowires
as photoanode. Chemical bath deposition method was used to deposit successive
layers of CdS and CdSe QDs. Platinum-based counter electrode and polysulfide
electrolyte formed the final solar cells. The hierarchical arrangement of
ZnO < CdS < CdSe is necessary for the better transfer of electrons from the conduction band of CdSe QDs to ZnO. The parameters acquired from the
I–V characteristics were Jsc = 5.19 mA/cm2, Voc = 0.66 V, FF = 0.415, and
η = 1.42%. The solar cells with arrangement ZnO/CdS/CdSe showed better performance compared with that of ZnO/CdS. This was attributed to broader light
absorption capability and effective charge injection kinetics in the ZnO-/CdS-/
CdSe-based solar cell. Additionally, the obtained electron lifetime was 13.8 ms for
Quantum Dot Sensitized Solar Cells (QDSSCs)
Table 12.1 Comparative study of TiO2, CdSe, and CdS as presented by Osada et al. [5]
Jsc (mA/cm2)
Voc (V)
η (%)
TiO2 only
the former solar cell while only 6.2 ms for the latter device suggesting lowering in
interface charge recombination rate by CdSe QDs sensitization.
A thorough comparative study on various arrangements of TiO2, CdSe, and CdS,
was presented by Osada et al. [5] in 2014 using polysulfide electrolyte and the Cu2S
counter electrode. The obtained results are compiled in Table 12.1. The results
clearly confirm that the appropriate band alignment for the photoanode is TiO2/
CdS/CdSe, thereby suggesting that the injected electrons were swiftly transferred to
TiO2 and the reverse electron transfer did not matter for the efficiency.
Recently few studies demonstrated the positive effect of doping ions into sulfide
QDs, such as Hg2+ into PbS [6], and Mn2+ into CdS [7], which increased the current
density thereby enhancing the efficiency of the solar cells. Kim et al. [8] fabricated
TiO2 electrodes sensitized with CdS, PbS/CdS, Mn-PbS/CdS, and PbS/Mn-CdS
using SILAR technique followed by a coating of two cycles of ZnS passivation
layer over the sensitized electrodes. Further, polysulfide electrolyte and CuS
counter electrode were used. Mn-doped films showed a substantial increase in JSC
as compared to the corresponding photoanodes without dopants. PbS/Mn-CdS film
gave an efficiency of 3.55% with a Jsc of 12.9 mA/cm2 and Voc of 0.56 V. The
reason for the enhanced photovoltaic properties was attributed to the impact of Mn
impurities on the host material, which created new energy states, thus delaying the
exciton recombination time and allowing better charge separation.
Tian et al. [9] reported a chemical passivation strategy method for synthesis of
mesoporous ZnO photoelectrode, which enhanced the apertures in the photoelectrode for harvesting more QDs. Also, a thin layer of TiO2 nanoparticles was
introduced on the surface of ZnO so as to reduce the surface charge recombination.
The alignment under study was CdS > CdSe > ZnO > TiO2. Under passivated
condition, the obtained efficiency was 4.68% compared to 2.4% obtained under
non-passivated condition. Employment of the passivation layer significantly
increased the amount of harvested QDs while improving their distribution. Further,
it may have triggered the enhancement in charge recombination resistance while
prolonging the electron lifetime, which contributed in improving the overall efficiency and characteristics of QDSSC.
Chuang et al. [10] in 2014 reported a certified high efficiency in QDSC.
Patterned ITO substrate was used to spin coat ZnO nanoparticles. Few layers of
PbS-TBAI (Tetrabutylammonium iodide) and PbS-EDT (1,2-ethanedithiol) were
spin coated successively. Coating of MoO3, Al, and Au were done on the electrodes
K. Surana and R.M. Mehra
by thermally evaporating corresponding material on the substrate, and the results
were compared. The final device architecture was ITO/ZnO/PbS‐TBAI/PbS-EDT/
MoO3/Al or Au. The obtained values of Voc, Jsc, FF, and η were 0.55 V,
24.2 mA/cm2, 63.8, and 8.55%, respectively. These characteristics were attained
due to the tailoring of band alignment at the interfaces of QD/QD and QD/anode.
Hossain et al. [11] in their article established that CdSe-sensitized TiO2 SCs
show noticeably slower charge recombination at the TiO2/CdSe interface with the
superior light harvesting of long wavelength photons giving rise to enhanced
overall device performance of the cascaded CdS/CdSe solar cells. This resulted in
the inference that while the CdS buffer layer might actually be indispensable in few
types of SCs, yet it actually is redundant and even causes a loss in efficiency in
CdSe-sensitized TiO2 SCs. The photovoltaic performance recorded for the QDSSC
utilizing a scattering layer over TiO2 with 9 layers of SILAR grown CdSe QDs,
polysulfide electrolyte, and Cu2S counter electrode was Voc = 0.579 V,
Jsc = 15.77 mA/cm2, FF = 56.95%, and η = 5.21%.
A highly efficient QDSC based on Cd1 − xMnxSe quantum dots was reported by
Tian et al. [12]. Modification of TiO2 with CdS was adopted to improve the
adsorption of Cd1 − xMnxSe to form a photoanode with the structure of TiO2/
CdS/Cd1 − xMnxSe. Maximum external quantum efficiency (EQE) of 74% at
580 nm was obtained with Cd0.8Mn0.2Se QDs. The improved quantum efficiency
was ascribed to the rise in light-harvesting, charge-transfer, and charge-collection
efficiencies. An efficiency of 6.33% with a Jsc of 19.15 mA/cm2 and Voc of 0.58 V
was achieved.
Recently Wei et al. [13] presented a high efficiency of 11.23% using
CdSexTe1 − x QDs decorated TiO2 electrode passivated with ZnS layer.
A polysulfide electrolyte and Cu2S counter electrode were used. The reason for
such efficiency was attributed to the SiO2 fumed electrolyte; the existence of SiO2
nanoparticles in the electrolyte created an energy barrier for the recombination
between photo-generated electrons from the QDs as well as the recombination
between the electrolyte and the injected electrons from TiO2.
Bhattacharya et al. [14] fabricated a solar cell without the use of a mesoporous
layer. The QD layer played the dual role of trapping photons as well as transporting
them to the FTO layer. The efficiency obtained was low, but a decent fill factor of
66.08% and a Voc of 1.41 V were obtained, which suggests superior band alignment
and faster electron transfer.
In order to achieve a superior performing solar cell, an appropriate arrangement of
QDs of different sizes has to be identified so as to harvest the maximum region of
the electromagnetic spectrum, precisely the UV, Visible, and IR region. Further
establishing a suitable band alignment between the stacked QDs and the mesoporous semiconductor is an unavoidable requirement, achieving which would lead
Quantum Dot Sensitized Solar Cells (QDSSCs)
to a faster charge transfer. This also entails proper loading of the QDs on the
semiconductor surface. The focus needs to be laid down on the impedance analysis
of the different junctions of the solar cell, so that the configurations can be altered
suitably. Such studies can identify the appropriate requirement of Fermi level of the
counter electrode, thereby a suitable alternative can be chosen. As a final thought,
the basic strategies of enhanced photovoltaic properties depend on the reduced
carrier recombination at the various interfaces, improved light absorption by photon
management, and construction of tandem structures. Exploiting further nanoscience
and nanotechnology will be an essential in overcoming these hurdles for achieving
better functional quantum dot sensitized solar cells.
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