Angewandte A Journal of the Gesellschaft Deutscher Chemiker International Edition Chemie www.angewandte.org Accepted Article Title: High Pressure Band Gap Engineering in Lead-Free Cs₂AgBiBr₆ Double Perovskite Authors: Qian Li, Yonggang Wang, Weicheng Pan, Wenge Yang, Bo Zou, Jiang Tang, and Zewei Quan This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708684 Angew. Chem. 10.1002/ange.201708684 Link to VoR: http://dx.doi.org/10.1002/anie.201708684 http://dx.doi.org/10.1002/ange.201708684 10.1002/anie.201708684 Angewandte Chemie International Edition 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. 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. 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. 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. 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: firstname.lastname@example.org 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: email@example.com 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. However, the large Cs2AgBiBr6 band gap (~2.2 eV) directly hampers its device performance to some extent. 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. High pressure can continuously modulate crystal structure and electronic configuration, revealing the underlying transformation mechanism and searching for new materials with improved properties. 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. It is also known that optical band gaps of semiconducting lead halide perovskites are also pressure-dependent. 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. 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/anie.201708684 Angewandte Chemie International Edition 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). 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. 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. 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). 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/anie.201708684 Angewandte Chemie International Edition 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 (Table S1 and S2), which is usually observed in double perovskites. 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/anie.201708684 Angewandte Chemie International Edition 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. 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. 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  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.  a) G. Hodes, Science 2013, 342, 317–318; b) M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643– 647; c) National Renewable Energy Laboratory, Best Research-Cell Efficiencies chart; www.nrel.gov/pv/assets/images/efficiency-chart.png. A. Swarnkar, A. R. Marshall, E. M. Sanehira, B. D. Chernomordik, D. T. Moore, J. A. Christians, T. Chakrabarti, J. M. 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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.