Tuning Optical and Electronic Properties in Low-Toxicity Organic

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Tuning Optical and Electronic Properties in Low-Toxicity Organic-Inorganic Hybrid (CHNH)BiI under High Pressure 3

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Long Zhang, Chunming Liu, Yu Lin, Kai Wang, Feng Ke, Cailong Liu, Wendy L Mao, and Bo Zou J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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The Journal of Physical Chemistry Letters

Figure 1. (a) PL spectra of MA3Bi2I9 as a function of pressure along the compression and decompression path. (b) The evolution of PL intensity as a function of pressure. (c) Room temperature optical absorption spectra of MA3Bi2I9 during compression up to 16.9 GPa. (d) Band gap evolution of MA3Bi2I9 upon compression. The inset shows the Tauc plot for MA3Bi2I9 at 1 atm. (e) Optical micrographs of MA3Bi2I9 in a diamond anvil cell upon compression. The small circle at the top is a ruby sphere for pressure calibration. “R” represents images collected on pressure release.

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Figure 2. (a) Synchrotron XRD patterns of MA3Bi2I9 as a function of pressure. Black asterisks mark the appearance of new diffraction peaks. (b) Selected 2D XRD images at varying pressures. “R” represents pattern collected on pressure release.


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Figure 3. (a, b) Selected Raman and (c) Infrared spectra of MA3Bi2I9 as a function of pressure. Black asterisks mark the appearance of new peaks. The arrows trace the evolution of the modes.



Figure 4. (a)Schematic of inorganic Bi2I9 unit. (b) Calculated bridging Bi-I-Bi bond angles upon compression. (c) Selected representative Bi-I bond length changes as function of pressure. (d) Selected electronic band structure under 0.0 GPa, 8.0 GPa and 17.0 GPa, respectively.


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Figure 5. Electrical resistance of MA3Bi2I9 upon compression and decompression. Inset: temperature dependence of the resistance of MA3Bi2I9 at different pressure. At 60 GPa, MA3Bi2I9 shows increasing resistance with increasing temperature, diagnostic of metallic behavior.



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Tuning Optical and Electronic Properties in LowToxicity Organic-Inorganic Hybrid (CH3NH3)3Bi2I9 under High Pressure Long Zhang,a Chunming Liu,b Yu Lin,c Kai Wang,a,d,* Feng Ke,d Cailong Liu,a,b,* Wendy L. Mao,c,d,*, Bo Zoua,* a State

Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun

130012, China b

Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China

c

Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory,

Menlo Park, California 94025, United States d

Department of Geological Sciences, Stanford University, Stanford, California 94305, United

States Email: [email protected], [email protected], [email protected] or [email protected]


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ABSTRACT Low-toxicity, air-stable methylammonium bismuth iodide (CH3NH3)3Bi2I9 has been proposed as a candidate to replace lead-based perovskites as highly efficient light absorbers for photovoltaic devices. Here, we investigated the effect of pressure on the optoelectronic properties and crystal structure of (CH3NH3)3Bi2I9 up to 65 GPa at room temperature. We achieved impressive photoluminescence enhancement and band gap narrowing over a moderate pressure range. Dramatic piezochromism from transparent red to opaque black was observed in a single crystal sample. A structural phase transition from hexagonal P63/mmc to monoclinic P21 at 5.0 GPa and completely reversible amorphization at 29.1 GPa were determined by in situ synchrotron X-ray diffraction. Moreover, the dynamically disordered MA+ organic cations in the hexagonal phase became orientationally ordered upon entering into the monoclinic phase, followed by static disorder upon amorphization. The striking enhancement of conductivity and metallization under high pressure indicate wholly new electronic properties.

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Organic-inorganic hybrid perovskites have drawn tremendous attention for photovoltaic and optoelectronic applications owing to their low raw material cost, simple solution processing and high absorption coefficients. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) rapidly increases from the initial value of 3.8% in 2009 to above 22% within a few years.1-3 Although lead halide perovskite solar cells exhibit excellent photovoltaic properties, the toxicity of lead and intrinsic instabilities to humidity, heat, and light represent significant challenges for large-scale manufacture and implementation of these materials. In order to solve these key issues, non-toxic bismuth has emerged as a promising candidate for replacing lead, due to its isoelectronic configuration, and environmentally friendly methylammonium bismuth iodide (CH3NH3)3Bi2I9 (MA3Bi2I9) has been studied and applied to photovoltaic devices as a novel lead-free light absorption layer.4-8 PSCs employing zero-dimensional MA3Bi2I9 exhibit superior stability compared with lead-based photovoltaic devices under various environmental conditions. The lone pair of 6s2 electrons in MA3Bi2I9 can also increase the dielectric constants to induce charge separation and a polar character to facilitate charge transport. However, the wide band gap of 2.1 eV and poor carrier transport properties seriously limits the PCE of MA3Bi2I9 PSCs. Although the PCE has increased slightly with improvements in thin film technology over the last two years, it still compares poorly with lead-based materials. According to Shockley-Queisser theory, the optimal band gap value is 1.34 eV for achieving 33.7% PCE under AM1.5 solar spectrum and 1-sun illumination.9-10 At present, band gap optimization and electrical conductivity improvement have become crucial challenges for enhancing the performance of bismuth-based photoelectric devices. Therefore, a comprehensive understanding of the structureproperty relationships of those materials is crucial for guiding improvements in their performance.


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Pressure is a powerful tool for tuning the crystal structures and electronic landscape without changing chemistry.11-13 The modification of bond lengths and bond angles can dramatically improve the photovoltaic-related properties of these materials, such as mediating the photoluminescence (PL) and carrier diffusion length.14-18 In recent years, the promising response of halide perovskites under pressure has been extensively documented, including piezochromism, band gap modulation, enhanced structural stability, structural phase transitions, improved transport properties, etc.19-23 Under modest pressure, halide perovskite MAPbBr3 undergoes a phase transition and amorphization along with changes in its optical and electrical properties.13,24-26 The semiconductor-to-metal transition in MAPbI3 at ca. 60 GPa indicates dramatically altered transport properties and a unique electronic structure, which is inaccessible at ambient conditions.27 Remarkably, partially retained band gap reduction and emission enhancement of pressure-treated 2D halide perovskite materials also demonstrate that pressure engineering not only provides a new strategy for tailoring the optoelectronic properties of perovskites, but may also be applied to devices.28-29 The tendency to amorphize and undergo phase transitions in halide perovskites under mild pressure can cause band gap increase due to the distorted inorganic lattice decreasing orbital overlap, which is undesirable. Therefore, it is important to explore the effect of pressure on halide hybrids with differing dimensionality in order to realize desirable, continuous modulation of optoelectronic properties. Herein, we report the response of MA3Bi2I9 up to a pressure of 65 GPa, where we studied the optical property modification and structure evolutions of this zero-dimensional bismuth-based hybrid halide under compression and decompression. Structure-property relationships were established by performing in situ synchrotron X-ray diffraction (XRD), Raman, Infrared, PL, UV-Vis absorption, electrical resistance measurements and first-principles calculations.


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Unprecedented band gap narrowing was realized, satisfying the optimal band gap requirement of Shockley-Queisser limit along with a further narrowing trend. A structural phase transition, reversible amorphization, and dynamic transformation of the MA+ organic cations are confirmed by XRD, Raman and Infrared experiments. We observed the semiconductor-to-metal transition of MA3Bi2I9 at ca. 60 GPa. Our results not only reveal the dramatic pressure response of MA3Bi2I9 but also provide key insights into understanding structure-property relationships in MA3Bi2I9, offering direction toward developing an engineering approach to improve the photovoltaic properties of bismuth-based halide materials. We conducted in situ high-pressure PL and UV-Vis absorption experiments to study the effect of lattice compression on the optoelectronic properties of MA3Bi2I9. As shown in Figure 1a, MA3Bi2I9 exhibits a broad emission spectrum from 580 nm to 900 nm with weak PL intensity at ambient conditions, which is associated with the band-edge excitonic radiative luminescence and the large exciton binding energy.4 With increasing pressure, a gradual red shift of the PL band accompanied by about 14 fold PL enhancement was observed (Figure 1b). The PL intensity reaches a peak value at 2.5 GPa, followed by progressive weakening until complete disappearance at 9.0 GPa. Recent emission results have confirmed a dramatic enhancement in PL upon increasing the exciton binding energy by reducing the dimensionality of perovskites to achieve quantum confinement modulation.30-32 The lattice contraction and modification of well width and physical properties (dielectric screening, effective masses etc.) can increase the binding energy of excitons in semiconductor quantum wells upon compression.33-37 Therefore, the zero-dimensional, unconnected crystal structure of MA3Bi2I9 with a notable quantum confinement effect can potentially increase the exciton binding energy under modest pressure, thereby resulting in pressure-induced emission enhancement (PIEE).


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Figure 1. (a) PL spectra of MA3Bi2I9 as a function of pressure along the compression and decompression path. (b) The evolution of PL intensity as a function of pressure. (c) Room temperature optical absorption spectra of MA3Bi2I9 during compression up to 16.9 GPa. (d) Band gap evolution of MA3Bi2I9 upon compression. The inset shows the Tauc plot for MA3Bi2I9 at 1 atm. (e) Optical micrographs of MA3Bi2I9 in a diamond anvil cell upon compression. The small circle at the top is a ruby sphere for pressure calibration. “R” represents images collected on pressure release. Figure 1c shows pressure-dependent absorption spectra of MA3Bi2I9 up to 16.9 GPa. Under ambient conditions, the compound exhibits a sharp absorption edge at 583 nm, corresponding to


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the electron transition from the 1S0 to 3P1 in the Bi3+ state.38 As pressure increases, the absorption edge gradually red shifts, followed by the absorption tailing into the near-infrared above ca. 9.6 GPa. Eventually, the absorption edge is completely shifted into the near-infrared at 16.9 GPa. Upon releasing pressure, the absorption spectra of MA3Bi2I9 was restored back to its initial state, indicating a reversible process. The band gap evolution of MA3Bi2I9 was evaluated by extrapolating the linear portion of the (αdhυ)1/2 versus hυ in indirect band gap Tauc plots, where α is absorption coefficient, d is the sample thickness, and hν is photon energy (Figure 1d).39 The band gap of MA3Bi2I9 was estimated to be 2.09 eV at ambient pressure, in agreement with previous reports.5 Upon compression, we observed

a notable red shift in the band gap

accompanied by a faster narrowing rate of 76.3 meV/GPa above ca. 5 GPa compared with in the lower-pressure region of 23.5 meV/GPa, indicating an electronic structure transition. The band gap narrowed to a value of 1.35 eV at 13.2 GPa successfully approaching the Shockley-Queisser limit (1.34 eV). The band gap reduction is also reflected in optical micrographs where dramatic piezochromism in MA3Bi2I9 from transparent red at ambient pressure to opaque black at ca. 11 GPa was observed in a single crystal sample (Figure 1e). Upon decompression, the black sample returned to transparent red, again indicating a reversible change. The observed piezochromism is consistent with the evolution of the absorption spectra. Probing the structural evolution of crystals upon compression is crucial for understanding the giant band gap narrowing. Figure 2a shows the synchrotron XRD patterns upon compression to ca. 29 GPa and decompression back to ambient conditions. With increasing pressure, all Bragg diffraction peaks shift continuously toward smaller d-spacing. The emergence of new Bragg diffraction peaks at 5.0 GPa, indicates a pressure-induced structural phase transition. With further compression to ca. 16 GPa, the partial disappearance of diffraction peaks and emergence


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of broad bands suggest the onset of amorphization and loss of the long-range order of the structure.40-42 Finally, a completely amorphous phase was observed at around 29 GPa, as evidenced by two weak broad bands displayed in the raw 2D images (Figure 2b). Upon decompression, the amorphous phase recrystallized below 1 GPa, and its original crystal structure was recovered at ambient conditions (Supporting Information, Figure S1).

Figure 2. (a) Synchrotron XRD patterns of MA3Bi2I9 as a function of pressure. Black asterisks mark the appearance of new diffraction peaks. (b) Selected 2D XRD images at varying pressures. “R” represents pattern collected on pressure release. We carried out Rietveld refinement to explore the structural transition under compression. Rietveld refinement profiles for selected XRD data obtained at 1 atm and 5 GPa are shown in Figure S2 (Supporting Information). At ambient conditions, MA3Bi2I9 adopts a hexagonal structure with the space group P63/mmc (phase I), with cell parameters a = 8.542(2) Å, c = 21.721(3) Å, and V = 1372 .16(5) Å3, in good agreement with previous reports.43 At 5 GPa, our refinement suggests that the monoclinic space group P21 (phase II) with lattice constants a =


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8.01(1) Å, b = 13.94(2) Å, c =20.17(2) Å, β = 90.52(3)° and V = 2252.90 (1) Å3 is coincident with the emerging reflections for the high-pressure phase, corresponding to an enlarged unit cell (Z = 2Z0) and a lower symmetry. Within the P63/mmc phase of MA3Bi2I9, face-shared Bi2I9 clusters are separated by MA+ cations, in contrast to the corner-shared octahedra in the 3D perovskite structure (Supporting Information, Figure S3). The bismuth ion is displaced off-center in the slightly distorted BiI6 octahedra with three longer equivalent bridging Bi-I bonds and three shorter equivalent terminal Bi–I bonds along the c direction, which originates from its lone pair repulsion in a direction perpendicular to the shared octahedral face. The dynamically disordered MA+ cations can freely rotate and interlink the inorganic framework via hydrogen bonding interactions.44 In contrast with ambient phase with the much more regular BiI6 octahedra, upon compression into the P21 phase, the geometry of the BiI6 octahedron became severely distorted with the anisotropic changes of all Bi-I bond length and Bi-I-Bi bond angle. The Bi3+ cations are markedly off-centered in the ab plane. The plane formed by the three terminal or bridging Ianions is also tilted out-of-plane. The Bi3+ cations and terminal I- anions within the BiI6 octahedra do not maintain symmetry about the shared face. MA+ organic cations with a dipole moment are ordered, locked and gain a preferential orientation along the b axis in the refined monoclinic phase, which leads to severe distortion of the BiI6 octahedra and eventually induces a structural phase transition. The polar regions of MA3Bi2I9 have different alignments in the P21 phase, which are favorable for charge separation and can assist in the transport of the free charges to reduce recombination. This is also critical for photovoltaic performance.44 The Bi2I9 units can be considered as a periodic arrangement of quantum dots (QDs) surrounded by insulating MA+ organic groups under a relatively low-pressure range (less than ca. 15 GPa). As


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pressure increases to the threshold when the amorphous phase starts to emerge, continuous contraction of the lattice accompanied by a highly distorted Bi2I9 unit and static disordered MA+ organic groups could not provide enough space for maintaining long-range order of these “QDs”. Finally, the long-range order of the crystals is significantly broken with complete amorphization at ca. 29 GPa. To explore local structural changes of the inorganic framework and the dynamic behavior of MA+ organic cations under high pressure, we performed in situ Raman and Infrared experiments. As shown in Figure 3, the lattice vibrational modes in the low-frequency range are associated with the scissoring (I-Bi-I and Bi-I-Bi) and stretching (Bi-I) modes in the inorganic framework. The vibrational modes of MA+ organic cations are distributed in the fingerprint frequency range (800-3400 cm-1). The detailed peak assignment is summarized in Table S1 (Supporting Information) based on previous reports.45-47 With increasing pressure, the four Bi-I vibrational modes shift to higher frequency due to the shortening of the Bi-I bonds (Supporting Information, Figure S5a). As pressure increases to 4.4 GPa, new peaks appear along with the weakening and disappearance of several original peaks, indicating a structural phase transition and changes in the Bi2I9 inorganic framework, coinciding with the XRD results. A lower frequency C-N stretching mode (969 cm-1) disappeared accompanied by a distinct splitting of the MA rocking mode (909 cm-1), which is attributed to the MA+ organic cation reorientation in the low symmetry monoclinic phase.46,48 Simultaneously, more peaks disappear and splitting are noticed at the higher frequency range, including the symmetric N-H bending mode (1470 cm-1), symmetric N-H stretching mode (3140 cm-1) and asymmetric N-H stretching mode (3180 cm-1). Upon further compression above ca. 15 GPa, part of the peak disappeared accompanied by significantly weakened and broadened modes, suggesting the onset of breaking the long-range


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order of the crystal structure. Eventually, all vibrational modes related to the inorganic framework disappeared at ca. 24 GPa, corresponding to a completely amorphous structure, reflecting a static disorder of MA+ cations due to the reduced unit cell volume and distorted lattice.46,49 Upon decompression, the broad Raman and Infrared spectra return to their initial state.

Figure 3. (a, b) Selected Raman and (c) Infrared spectra of MA3Bi2I9 as a function of pressure. Black asterisks mark the appearance of new peaks. The arrows trace the evolution of the modes. The internal vibrational modes of MA+ organic cations show a different pressure response in Figure S5b and c (Supporting Information), which defines the interaction between the organic molecules and the inorganic framework ， as well as the evolution of hydrogen bonding. In particular, the C-N stretching mode (969 cm-1) shows the most obvious blue shift upon compression, which is ascribed to the rigid characteristics of the C-N bond. In contrast, prominent mode softening of the N-H bending mode and N-H stretching mode, suggesting that the hydrogen bonding experiences pressure-induced strengthening as the interaction between MA+ organic cations and Bi2I9 units become stronger.46 In addition, the strengthening of the


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hydrogen bonding is accompanied by hydrogen bonding network rearrangement where MA+ organic cations transition from disordered to ordered in the monoclinic phase, as evidenced by the splitting of the symmetric N-H bending modes, asymmetric N-H stretching modes and the disappearance of the symmetric N-H stretching modes. The ordering of the dumbbell-shaped MA+ organic cations can lead to asymmetric distortions of the Bi2I9 unit, and in turn, the hydrogen bonding

associated with the interaction between MA+ organic cations and the

inorganic framework which is mediated by the distortion and shrinkage of the inorganic lattice. Therefore, orientation of the MA+ cations originating from hydrogen bonding rearrangement is the driving force for the observed phase transitions. This behavior has also been observed in temperature-induced

phase

transition

by

conducting

temperature-dependent

infrared

experiments.45 According to the above analysis, an orientation change in MA+ organic cations with structural phase transitions is noted, from dynamic disorder (hexagonal phase P63/mmc) to static order (monoclinic phase P21) to finally static disorder (amorphous phase). The distinct changes in the optical properties and crystal structure imply changes in the electronic landscape, so we performed first-principles calculations to track the evolution of the electronic band structure under pressure. The results based on monoclinic MA3Bi2I9 (P21) indicate that it possesses an indirect band gap with 2.10 eV at ambient conditions and compression favors continuous band gap narrowing, in accordance with our experiments (Supporting Information, Figure S6, SI). The valence band maximum (VBM) is mainly comprised of I 5p states with a small contribution of Bi 6s states, whereas the conduction band minimum (CBM) has a strong antibonding character, which is dominated by hybridized Bi 6p states and I 5p states. Band gap evolution of MA3Bi2I9 is mostly determined by the structural behavior of Bi2I9 unit. As shown in Figure 4a, b, and c, upon compression, three bridging Bi-I-Bi


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bond angles continuously decrease accompanied by Bi-I bond contraction. As such, the I p and Bi sp orbital coupling increases the VBM and CBM electronic band dispersion (Figure 4d). Therefore, the band gap exhibits a remarkable decreas.50

Figure 4. (a)Schematic of inorganic Bi2I9 unit. (b) Calculated bridging Bi-I-Bi bond angles upon compression. (c) Selected representative Bi-I bond length changes as function of pressure. (d) Selected electronic band structure under 0.0 GPa, 8.0 GPa and 17.0 GPa, respectively. Electrical conductivity is important for evaluation of this material as a PSC or detector material. Continuous band gap narrowing suggests an increase in conductivity upon compression. As shown in Figure 5, electrical resistance measurements of MA3Bi2I9 as a function of pressure were performed up to 65 GPa. High resistance of MA3Bi2I9 shows poor electronic transport ability in the lower-pressure regime, which is much higher than the resistance of 3D metal halide perovskites.13,21 This behavior is consistent with its isolated Bi-I inorganic framework. As pressure was increased, we noticed a dramatic decrease in resistance, resulting in a minimum of 19 Ω at 65 GPa. Temperature-dependent resistance measurement under high pressure (inset in Figure 5) show that, the resistance decreases with increasing


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temperature up to ca. 56 GPa, indicating the sample is still semiconducting. Upon further compression to ca. 60 GPa, we observed an increase in resistance with increasing temperature, suggesting a metallic character.50 The metallic character was even more obvious with further compression to 65 GPa, confirming the material transformed into a metallic state. The reduction of the atomic distance and Bi-I-Bi bond angle in the inorganic framework are responsible for the metallic transition upon compression, which enhances metal halide orbital overlap and increases electronic band dispersion. Upon releasing pressure, metallic MA3Bi2I9 returned to the semiconducting state at ca. 40 GPa (Supporting Information, Figure S7). The resistance slowly increases with the release of pressure above ca. 10 GPa, and then increases rapidly upon further decompression. The pressure response of the electrical conductivity is vitally significant for practical applications and may guide in photovoltaic device design.

Figure 5. Electrical resistance of MA3Bi2I9 upon compression and decompression. Inset: temperature dependence of the resistance of MA3Bi2I9 at different pressure. At 60 GPa, MA3Bi2I9 shows increasing resistance with increasing temperature, diagnostic of metallic behavior.


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Compositional tuning of hybrid materials can profoundly affect their structural dimensionality, bond environments, octahedral framework distortion and various optoelectronic properties etc. Although the electronic band structure of A3Bi2I9 compounds (A = Cs+ or MA+) is primarily determined by the Bi-I inorganic framework, but the size, composition, orientation and stereochemistry of the A-site cation have direct influences on the surrounding cage and orientation of the Bi2I9 clusters, and in turn, leads to significant effects on electronic structure and properties.51 The compound Cs3Bi2I9 and MA3Bi2I9 are isostructural (space group P63/mmc) at ambient conditions. Under high pressure, we note that the emission enhancement (before 0.9 GPa) and metallization (28 GPa) region of Cs3Bi2I9 are obviously smaller than those of MA3Bi2I9, and no phase transition occurs in Cs3Bi2I9.34 This remarkable difference stems from the self-characteristics of Cs+ and MA+ cations. Compared with simple Cs+ cations, bulky MA+ organic cation with steric effects and hydrogen bonding, which can cause greater obstruction to reduce the gap between the Bi2I9 clusters upon compression. The hydrogen bonding network formed between organic cations and the inorganic framework is responsible for the dynamic disorder, reorientation and fixation of the organic cations. The steric effects of the organic cations have a great impact on inorganic framework structure, such as metal-halide-metal bond angle and octahedral tilting, which act as a template role for the structural evolution process.13,52 The flexibility and complexity of the MA+ cations provide an opportunity for comprehensively understanding the interaction between A-site organic molecules and inorganic skeletons, and the impacts on various optoelectronic properties in this class of organic-inorganic hybrid materials. In summary, low-dimensional bismuth-halide hybrid MA3Bi2I9 shows a remarkable pressure response in it optoelectronic properties and crystal structures upon compression. The observed PL enhancement and band gap narrowing are desirable trends for improving its photovoltaic


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performance. The pressure-driven phase transition from hexagonal to monoclinic and then reversible amorphization were observed, which could be ascribed to the MA+ organic cation orientation and the destruction of long-range order, respectively. Accordingly, MA+ organic cations transform from the initially dynamically disordered state to static order and finally to static disorder, which are confirmed by Raman and Infrared experiments. First-principles calculations identified the relationship between electronic configuration and crystal structure evolution under high pressure. The pressure-induced metallization behavior of MA3Bi2I9 implies the realization of a wholly new electronic configuration and transport properties in this fascinating photovoltaic material. Our results suggest a promising strategy to simultaneously tune the optoelectronic properties and structure of the material, and point to further exploration of the large and diverse family of low-dimensional metal halide hybrids toward improved materials-by-design.

Acknowledgments This work is supported by the National Science Foundation of China (NSFC) (Nos. 21725304, 11774120, 21673100, and 11874147), the Chang Jiang Scholars Program of China (No. T2016051), program for innovative research team (in science and technology) in university of Jilin Province, and JLU Science and Technology Innovative Research Team (No. 2017TD-01). Y. L., W. M., and F. K. contribution to data analysis and interpretation were supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (DE-AC02-76SF00515). The ADXRD measurement was performed at the 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF). Portions of this work were performed at the BL15U1 at the Shanghai Synchrotron Radiation Facility (SSRF).


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Supporting Information paragraph Experimental details, diffraction patterns upon decompression, Rietveld refinement and electrical resistance upon decompression. References (1)

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as

Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2)

Jeon, N. J.; Na, H.; Jung, E. H.; Yang, T.-Y.; Lee, Y. G.; Kim, G.; Shin, H.-W.; Il Seok,

S.; Lee, J.; Seo, J. A Fluorene-Terminated Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells. Nat. Energy 2018, 3, 682-689. (3)

Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.;

Seo, J.; Kim, E. K.; Noh, J. H. Iodide Management in Formamidinium-Lead-Halide-Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376-1379. (4)

Zhang, Z.; Li, X.; Xia, X.; Wang, Z.; Huang, Z.; Lei, B.; Gao, Y. High-Quality

(CH3NH3)3Bi2I9 Film-Based Solar Cells: Pushing Efficiency up to 1.64. J. Phys. Chem. Lett. 2017, 8, 4300-4307. (5)

Park, B. W.; Philippe, B.; Zhang, X.; Rensmo, H.; Boschloo, G.; Johansson, E. M.

Bismuth Based Hybrid Perovskites A3Bi2I9 (A: Methylammonium or Cesium) for Solar Cell Application. Adv. Mater. 2015, 27, 6806-6813. (6)

Hu, Y.; Qiu, T.; Bai, F.; Ruan, W.; Zhang, S. Highly Efficient and Stable Solar Cells with

2D MA3Bi2I9/3D MAPbI3 Heterostructured Perovskites. Adv. Energy. Mater. 2018, 8, 1703620.


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