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Angewandte
Eine Zeitschrift der Gesellschaft Deutscher Chemiker
Chemie
www.angewandte.de
Akzeptierter Artikel
Titel: High Pressure Band Gap Engineering in Lead-Free Cs₂AgBiBr₆
Double Perovskite
Autoren: Qian Li, Yonggang Wang, Weicheng Pan, Wenge Yang, Bo
Zou, Jiang Tang, and Zewei Quan
Dieser Beitrag wurde nach Begutachtung und Überarbeitung sofort als
"akzeptierter Artikel" (Accepted Article; AA) publiziert und kann unter
Angabe der unten stehenden Digitalobjekt-Identifizierungsnummer
(DOI) zitiert werden. Die deutsche Übersetzung wird gemeinsam mit der
endgültigen englischen Fassung erscheinen. Die endgültige englische
Fassung (Version of Record) wird ehestmöglich nach dem Redigieren
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Für die AA-Fassung trägt der Autor die alleinige Verantwortung.
Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.201708684
Angew. Chem. 10.1002/ange.201708684
Link zur VoR: http://dx.doi.org/10.1002/anie.201708684
http://dx.doi.org/10.1002/ange.201708684
10.1002/ange.201708684
Angewandte Chemie
COMMUNICATION
High Pressure Band Gap Engineering in Lead-Free Cs2AgBiBr6
Double Perovskite
Abstract: Novel inorganic lead-free double perovskites with
improved stability are regarded as alternatives to state-of-art hybrid
lead halide perovskites in photovoltaic devices. The recently
discovered Cs2AgBiBr6 double perovskite exhibits attractive optical
and electronic features, making it promising for various
optoelectronic applications. However, its practical performance is
hampered by the large band gap. In this work, remarkable band gap
narrowing of Cs2AgBiBr6 is, for the first time, achieved on inorganic
photovoltaic double perovskites through high pressure treatments.
Moreover, the narrowed band gap is partially retainable after
releasing pressure, promoting its optoelectronic applications. This
work not only provides novel insights into the structure-property
relationship in lead-free double perovskites, but also offers new
strategies for further development of advanced perovskite devices.
Lead hybrid perovskite (e.g. MAPbI3, MA = CH3NH3+) solar
cell has been progressing at an unprecedented rate recently,
and the highest certified power conversion efficiency now
exceeds 22%, making perovskites the first solution-processable
photovoltaic material to surpass the efficiency of dominant
crystalline silicon panels.[1] However, the deployment of hybrid
perovskite photovoltaics on a large scale still faces two main
challenges. The first one is the intrinsic instability of hybrid
perovskites upon exposure to moisture, heating, and light. This
issue has been partly addressed by replacing the organic part
with cesium ion to produce purely inorganic perovskites with
improved stability.[2] The second challenge is the toxicity of lead.
Lead plays a vital role in perovskite to achieve superior
photovoltaic performance, and thus alternatives to lead must
fulfill stringent criteria to not compromise efficiency and
stability.[3] Recently, the cation-transmutation strategy that
converts two divalent Pb2+ ions into one trivalent cation and one
monovalent cation has been exploited to form quaternary
halides with double perovskite structure.[4] In main group
[*]
Dr. Q. Li and Prof. Z. Quan
Department of Chemistry, Southern University of Science and
Technology (SUSTech), Shenzhen, Guangdong 518055, P. R.
China
E-mail: quanzw@sustc.edu.cn
Dr. Q. Li
College of Chemistry, Nankai University, Tianjin 300071, P. R.
China
Dr. Y. Wang and Prof. W. Yang
High Pressure Synergetic Consortium (HPSynC), Geophysical
Laboratory, Carnegie Institution of Washington, Argonne, Illinois
60439, United States
W. Pan and Prof. J. Tang
Wuhan National Laboratory for Optoelectronics (WNLO) and School
of Optical and Electronic Information, Huazhong University of
Science and Technology (HUST),Wuhan 430074, P. R. China
Prof. B. Zou
State Key Laboratory of Superhard Materials, Jilin University,
Changchun 130012, P. R. China
E-mail: zoubo@jlu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
elements, only trivalent Bi3+ cation possesses the same
electronic configuration with Pb2+, and Bi-based Cs2AgBiBr6
double perovskite has been successfully synthesized and
exhibited attractive features, including long carrier recombination
lifetime and excellent stability, which make it promising for
photovoltaic applications.[5] However, the large Cs2AgBiBr6 band
gap (~2.2 eV) directly hampers its device performance to some
extent.[6] It is thus highly desirable to find an efficient method to
accurately engineer Cs2AgBiBr6 band gap and then obtain indepth insight into its structure-property relationship.[7]
High pressure can continuously modulate crystal structure
and electronic configuration, revealing the underlying
transformation mechanism and searching for new materials with
improved properties.[8] As for semiconductors, high pressure
method has been successfully utilized to investigate their
chemical/physical properties, including pressure-induced
consolidation of spherical nanoparticles into 1D nanowires and
size-dependent phase transformations.[9] It is also known that
optical band gaps of semiconducting lead halide perovskites are
also pressure-dependent.[10] However, such engineered band
gap is hardly reserved to ambient conditions, resulting in limited
application significance of high pressure study. In addition, the
undesirable phase transitions at high pressure always induce
the detrimental band gap broadening.[10b] Herein, high pressure
exploration of Cs2AgBiBr6 double perovskite is performed to
engineer the band gap. We successfully narrow the band gap of
Cs2AgBiBr6 from ~2.2 eV to 1.7 eV with a considerable
percentage of 22.3%. And the narrowed band gap of Cs2AgBiBr6
is still partially retained after releasing pressure to ambient,
owing to the incomplete recrystallization process.
In order to track the band gap evolution, UV-Vis absorption
experiments are carried out on Cs2AgBiBr6 crystal under
compression. Cs2AgBiBr6 at ambient conditions displays orange
yellow color with 2.19 eV band gap energy (Figure S1). With
increasing pressure to 2.8 GPa, the absorption edge of
Cs2AgBiBr6 shows little red shifts (Figure 1a), accompanied by
the tiny narrowed band gap and nearly unchanged crystal color
(Figure 1c). At 3.2 GPa, a clear discontinuity in band gap shift is
detected, and thereafter the band gap value of Cs2AgBiBr6
remains unchanged up to 4.0 GPa, indicating the possible
structural changes in this pressure range. Beyond 4.0 GPa, the
abrupt blue shift of absorption suggests a ~2 GPa widening of
the band gap, and Cs2AgBiBr6 crystal consequently becomes
lighter in color from orange yellow to light yellow (Figure 1c).
With further compression above 6.5 GPa, Cs2AgBiBr6 exhibits
continuous redshift of absorption edge (Figure 1b) with
significant band gap narrowing from ~2.3 eV to 1.7 eV at 15 GPa,
above which the detection of optical transmission signal
becomes difficult. Meanwhile, the crystal color of Cs2AgBiBr6
also gradually deepens into black brown. To our knowledge, it is
rare for perovskites to achieve continuous band gap narrowing
with a considerable percentage of ~22.3% over such a wide
pressure range.[10] The band gap of Cs2AgBiBr6 (1.7 eV) at ~15
GPa is comparable with that of state-of-art MAPbI3 (1.6 eV).[10d]
This article is protected by copyright. All rights reserved.
Accepted Manuscript
Qian Li, Yonggang Wang, Weicheng Pan, Wenge Yang, Bo Zou*, Jiang Tang, and Zewei Quan*
10.1002/ange.201708684
Angewandte Chemie
COMMUNICATION
Structural variation under pressure is believed to be the main
origin for this intriguing band gap evolution. At ambient
conditions, Cs2AgBiBr6 crystallizes in typical double-perovskite
structure (cubic Fm-3m symmetry) with alternating AgBr6 and
BiBr6 octahedra (Figure 1e and S2).[5] To determine the in-situ
local and global structural variations of Cs2AgBiBr6, Raman
spectroscopy and angle-dispersive synchrotron X-ray diffraction
(ADXRD) experiments are conducted at high pressure. In
Raman experiments, we obtain the abnormal discontinuities of
lattice modes and octahedral bending modes of υ(F2g) between
3.5 and 4.2 GPa (Figure S3), which are indicative of the phase
transition of Cs2AgBiBr6.[10g,11] And the gradual broadening of
lattice vibrations and weakening of υ(F2g) with further
compression may involve with the structural amorphization of
high pressure phase.[12] Such amorphization is likely to be the
structural origin for the remarkable band gap narrowing above
6.5 GPa. Furthermore, although Raman pattern recovers to its
original state after releasing pressure, the obvious broadening
and weakening of vibration modes together demonstrate the
incomplete recrystallization of the quenched sample (Figure S4).
ADXRD spectra could provide straightforward information on
structural evolution. As shown in Figure 2a, all the diffraction
peaks shift to higher angles with increasing pressure, owing to
the unit cell contraction.[13] The splitting of two diffraction peaks
as marked with asterisks at 3.1 GPa is observed (enlarged in
Figure S5), confirming the occurrence of phase transition that
we inferred from Raman and UV-Vis absorption experiments.
For high pressure Cs2AgBiBr6 above 6.5 GPa, the broad
diffraction background, as well as the peak weakening, exhibits
the gradual structural amorphization. [11,13] Furthermore,
Cs2AgBiBr6 is reverted into its original phase after fully releasing
pressure (Figure S6). Similar to Raman spectra, the diffraction
peaks of recovered sample are obviously broadened and
weakened, as well as partially disappeared, illustrating the
incomplete recrystallization and the presence of residual
amorphous phase in quenched sample (Figure S7).[14]
Figure 1. a, b) Selected UV-Vis absorption spectra of Cs2AgBiBr6 crystal under compression. c) Band gap evolution of Cs2AgBiBr6
crystal at high pressure, and the representative optical micrographs showing piezochromic transitions. d) Comparison between the
UV-Vis absorption spectra before and after 15 GPa pressure treatment. e) Crystal structure and lattice fragment of ambient
Cs2AgBiBr6 structure.
This article is protected by copyright. All rights reserved.
Accepted Manuscript
The more thrilling fact is, after releasing pressure, the recovered
sample exhibits the narrowed band gap of ~2.0 eV, which is
~8.2 % reduction compared to the starting Cs 2AgBiBr6 (Figue1c
and 1d). Such partially retainable narrowing of band gap is
significantly insightful for the further applications of Cs 2AgBiBr6.
10.1002/ange.201708684
Angewandte Chemie
COMMUNICATION
illustrates the internal distortion of AgBr 6 and BiBr6 octahedra.
Octahedral tilting promotes the band gap broadening in hybrid
halide perovskites. Take MAPbI3 for example, the octahedral
tilting during the phase transition occurs in three dimensions,
and thus Pb–I–Pb bond angles dramatically decrease from 180°
down to 144.0°, accompanied by the increase of band gap
energy. However, in high pressure Cs 2AgBiBr6, the decrease of
Bi-Br-Ag bond angle only occurs in ab plane from 180°to 166.4°,
and the Bi-Br-Ag bond along c axis still keeps 180°bond angle.
Accepted Manuscript
High pressure structure of Cs2AgBiBr6 is determined by
Rietveld refinement of ADXRD patterns (Figure 2b and 2c), with
details shown in the Supporting Information. At 4.5 GPa, cubic
Cs2AgBiBr6 transfers to tetragonal phase with I4/m space
group[15] (Table S1 and S2), which is usually observed in double
perovskites.[16] During phase transition, octahedral titling occurs
in ab plane, leading to the gigantic shortening of a, b axes
(Figure 2d) and the much smaller lattice volume (Figure 2e).
Meanwhile, the splitting of Ag-Br and Bi-Br bond shifts (Figure 2f)
Figure 2. a) Representative ADXRD patterns of Cs2AgBiBr6 perovskite at selected pressures. b, c) Rietveld refinements of
ADXRD patterns collected at 0.6 GPa and 4.5 GPa, respectively. The orange lines denote the difference between the observed
(black) and the simulated (red) profiles, and the green verticals stand for the simulated peak positions. The purple dotted line
represents the amorphous background in profile. The inset figures show the corresponding crystal structure. d, e) Evolution of
lattice constants and lattice volume at high pressure. f) Lengths of Bi-Br and Ag-Br bonds as a function of pressure. The shadow
marks the phase transition region.
This article is protected by copyright. All rights reserved.
10.1002/ange.201708684
Angewandte Chemie
Such minimized octahedral tilting of Cs 2AgBiBr6 is believed to
one major factor for its continuous band gap narrowing in a wide
pressure range.
To seek the deeper interplay between structure and band
gap behaviors, calculations are performed on cubic and
tetragonal Cs2AgBiBr6, respectively. For both phases (Figure S8
and S9), the valence band is determined by Br 4p orbital with a
quite small contribution of Ag 4d orbital, and the conduction
band is predominantly comprised of Bi 6p orbital hybridized with
little Br 4p orbital. High pressure band gap evolution of
Cs2AgBiBr6 mostly depends on structural behaviors of BiBr 6 and
AgBr6 octahedra.[10h] As shown in Figure 3a and 3b,
homogeneous octahedral contraction in cubic Cs2AgBiBr6 is
accompanied by continuous band gap narrowing, which is in
accordance with the structural and band gap behaviors below 3
GPa in experiments. As for tetragonal phase, the significant
tilting and distortion of AgBr6 and BiBr6 octahedra give rise to the
abrupt broadening of band gap (Figure 3a and 3c). It is worth
noting that band gap of tetragonal phase is persistently
broadened to 15 GPa without considering structural
amorphization. Such confliction between experiment and
calculation just confirm our speculation that structural
amorphization is the underlying mechanism for sustaining band
gap narrowing above ~6.5 GPa.[10e]
Taken together, high pressure structural evolution and
optical property of Cs2AgBiBr6 are inferred as follows. Below 3
GPa, homogenous octahedral contraction and shrinkage of BiBr-Ag bonds (Figure 2) promote the overlap of elemental orbitals,
which leads to the band gap narrowing of cubic phase (Figure
1).[10h] Then, above 3 GPa, Cs2AgBiBr6 transfers to tetragonal
phase, as cubic phase contraction cannot afford the increased
free energy anymore.[17] For tetragonal Cs2AgBiBr6, tilting and
distortion of AgBr6 and BiBr6 octahedra are directly associated
with the decrease of Bi-Br1-Ag bond angles, as well as the
elongation and shortening of Ag-Br3 and Bi-Br2 bonds (Figure 2),
respectively. Meanwhile, the octahedral distortion and offaligned Bi-Br1-Ag bonds contraction result in the less coupling
between Bi 6p/Ag 4d and Br 4p orbitals, consequently leading to
the band gap widening of tetragonal Cs 2AgBiBr6.[18] It is
therefore reasonable to conclude that the unchanged band gap
value of Cs2AgBiBr6 during phase transition is derived from the
competition between the narrowing effects of cubic phase and
broadening effects of tetragonal phase. The increase in band
gap can only be observed after the complete transition from
cubic to tetragonal Cs2AgBiBr6 between 4.0 and 6.5 GPa. Above
6.5 GPa, the subsequent structural amorphization of tetragonal
Cs2AgBiBr6 successfully induces the gigantic band gap
narrowing from ~2.3 eV to 1.7 eV. Although the precise physical
and electronic structures of the amorphous phase remain to be
elucidated, we can infer that bond contraction instead of
octahedral tilting should be promoted under compression. [10e]
Compared with lead halide perovskites, Cs 2AgBiBr6 exhibits
more intriguing band gap behaviors at high pressure. We infer
such significant band gap narrowing (~22.3 %) to the structural
nature of Cs2AgBiBr6, in which the rigid Bi-Br-Ag bonds minimize
the detrimental octahedral tilting at high pressure. The presence
of residual amorphous phase in recovered Cs2AgBiBr6 leads to
the partially retained band gap narrowing after releasing
pressure.
To sum up, through high pressure treatment, we
successfully narrow the band gap of an emerging lead-free
photovoltaic perovskite Cs2AgBiBr6 with a considerable
percentage of ~22.3%. It is encouraging that the band gap value
of Cs2AgBiBr6 (~1.7 eV) at ~15 GPa is comparable with that of
classical photovoltaic perovskites (MAPbI3). More importantly,
the recovered Cs2AgBiBr6 after releasing pressure to ambient
conditions still possess a ~ 8.2 % narrowed band gap value. The
structural and band gap relationship of Cs 2AgBiBr6 prove the
possibility of band gap engineering in lead-free double
perovskites through structural modulation. This work
demonstrates a new strategy for the rational structural design of
perovskites, as well as the exploitation of lead-free inorganic
perovskites in optoelectronics including solar cell and X-ray
detector.
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (NSFC) (No. 51772142, 11604141,
21725304), Shenzhen fundamental research programs (Nos.
JCYJ20160530190717385,
JCYJ20160530190842589,
JCYJ20170412152528921), and start-up fund and Presidential
fund from SUSTech. ADXRD measurements were performed on
the HPCAT’s beamline facility of the Advanced Photon Source
at Argonne National Laboratory. HPCAT operations are
supported by DOE-NNSA under Award No. DE-NA0001974 and
DOE-BES under Award No. DE-FG02-99ER45775, with partial
instrumentation funding by NSF. APS is supported by DOE-BES,
under Contract No. DE-AC02-06CH11357.
Keywords: lead-free halide perovskite • double perovskite • high
pressure • phase transition • band gap
[1]
Figure 3. a) Calculated band gap evolutions of cubic and
tetragonal Cs2AgBiBr6 at high pressure. b) Calculated
pressure-induced contraction of Ag-Br and Bi-Br bonds in
cubic Cs2AgBiBr6. c) Calculated Ag-Br-Bi bond related
information under compression.
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This article is protected by copyright. All rights reserved.
Accepted Manuscript
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10.1002/ange.201708684
Angewandte Chemie
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Accepted Manuscript
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10.1002/ange.201708684
Angewandte Chemie
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Q. Li, Y. Wang, W. Pan, W. Yang, B.
Zou,* J. Tang and Z. Quan*
Page No. – Page No.
High Pressure Band Gap Engineering
in Lead-Free Cs2AgBiBr6 Double
Perovskite
Accepted Manuscript
High pressure is adopted to modulate
the crystal structure and engineer the
band gap of Cs2AgBiBr6 double
perovskite. The remarkable ~22.3 %
band gap narrowing is, for the first
time,
achieved
on
inorganic
photovoltaic
double
perovskites.
Moreover, the narrowed band gap is
partially retainable after releasing
pressure, promoting its optoelectronic
applications.
This article is protected by copyright. All rights reserved.
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