Probing Strain-Induced Band Gap Modulation in 2D Hybrid Organic

3 days ago - Here we report for the first time a study of the band gap response to uniaxial tensile strain in thin 2D HOIP flakes with a general formu...
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Letter

Probing Strain-Induced Band Gap Modulation in 2D Hybrid Organic-Inorganic Perovskites Qing Tu, Ioannis Spanopoulos, Shiqiang Hao, Christopher Wolverton, Mercouri G. Kanatzidis, Gajendra Shekhawat, and Vinayak P. Dravid ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00120 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Energy Letters

Figure 1. Schematics illustrating the procedure of sample preparation and applying the strain: (A) mechanical exfoliation of 2D HOIPs onto a PI substrate; (B) spin-coating of the PMMA protection layer; (C) bending of the PI with 2D HOIPs to a cylinder; (D) cross-sectional view of the PI film bent around the cylinder with radius R. 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114

the diamond-anvil cell (DAC) up to 10s of GPa.21 For 3D HOIPs, the hydrostatic pressure will induce a series of lattice contraction and phase transitions. The band gap will usually first decrease at a low compression level and then increase as the pressure increases, which is usually attributed to the tilting of [MX6]4− octahedra and contraction of the M−X bond length.22 For 2D HOIPs, the variation in the distance between two van der Waals (VdW) layers can further contribute to the band gap change, and the band gap will monotonically decrease as the pressure increases.23−25 Recent studies on 2D lead iodide HOIPs also show a trend similar to that of 3D HOIPs under isothermal conditions, where the band gap decreases at mild pressure and then increases at high pressure.26 The specific response of the electronic properties to the hydrostatic compression depends on the chemical composition of the HOIPs.21 The observed physical properties and structural changes in HOIPs are completely reversible upon decompression.21 While aforementioned studies are of great significance in fundamental sciences, such a high hydrostatic pressure state can hardly be found in real-world devices. In contrast, tensile strain states are very common in device applications due to materials processing, device functioning, and thermal expansion. Little is known about how the mechanical strain affects the electronic properties of HOIPs, especially in the 2D form, in these strain states that can be achieved in practice. This calls for deeper understanding of the strain effects on HOIPs’ physical properties. In this work, we report the band gap engineering of thin 2D HOIP flakes through mechanical strain. We develop a simple method to apply uniaxial tensile strain to the materials and mimic the strain state the materials will confront in flexible electronics applications. We find that the band gap of 2D HOIP flakes increases as a function of the tensile strain, and this response shows up only in 2D HOIPs with a high n. This band gap modulation by mechanical strain is completely reversible. Density functional theory (DFT) simulations are performed and reveal that this strain response of the band gap is due to rotation of the inorganic [PbI6]4− octahedra and the consequential Pb−I bond stretching and increase of Pb−I−Pb angles. We quantify the magnitude of band gap response to mechanical strain that can be commonly found in real-world applications and evaluate the impact of the structural

parameter on the strain-engineered band gap response for the first time. Our study can also provide a new route to achieve controllable engineering of the optical and electronic properties of HOIPs for device applications. The observed strain−band gap relationship can be harnessed to characterize the residual strain in 2D HOIP-based devices, and simple PL measurement can thus be used to quickly evaluate the local strain level and identify the bottlenecks that limit the device performance/stability. Furthermore, our finding allows 2D HOIPs to be used in pressure0sensing applications. We synthesized a series of 2D Ruddlesden−Popper (RP) HOIP crystals with a general chemical formula of (CH3(CH2)3NH3)2(CH3−NH3)n−1PbnI3n+1 following the protocol that we developed before (see SI section II for details). 13,27 All synthesized 2D HOIP samples were characterized by powder X-ray diffraction (PXRD). The PXRD results were compared with the calculated ones from the solved crystal structures to verify the phase purity (see SI Figures S2−S5). Extensive analysis of the 2D HOIP crystals can be found elsewhere.13,27 For simplicity purpose, we will use the following notation of samples throughout the manuscript, e.g., C4n5, where the number after C indicates the number of carbon atoms in the organic spacer molecule chain and the number right after n indicates the number of inorganic [PbI6]4− layers that are sandwiched by the organic layers (see SI Figure S1 for the crystal structure schematics). Due to the presence of VdW interfaces between the organic layers, thin 2D HOIP flakes can be mechanically exfoliated28 and transferred to a flexible polyimide (PI) substrate (DuPont, 122 μm thick) using the scotch tape method (Figure 1A). The optical band gap of the resulting flakes is measured by photoluminescence (PL) (Horiba LabRAM HR Evolution Confocal Raman instrument) with a 100× lens and a 532 nm laser (for C4n3 to C4n5) or 473 nm laser (for C4n1). Due to the presence of water and oxygen, the laser can induce chemical reaction and damage the 2D HOIPs in ambient environment. To minimize the laser-induced damage, a thin layer (∼100 nm) of PMMA is spin-coated onto the flakes (Figure 1B), which can protect the flake from exposure to water and oxygen during the illumination of laser. 29 Furthermore, the laser power is kept at the lowest intensity that can be achieved in our instrument (0.01% filter, 20 GPa) due to the formation of an extremely disordered state under such high hydrostatic pressure.26 However, the stress applied here is 3 orders of magnitude lower, which is unlikely to overcome the large energy barrier to form such a disordered state. As n increases, the relative amount of inorganic [PbI6]4− layer in the 2D HOIP crystal increases. The observed dependence of the strain-engineered optical band gap shift on the n number indicates that the band gap response to the tensile strain in 2D (CH3(CH2)3NH3)2(CH3−NH3)n−1PbnI3n+1 HOIPs might be mainly due to the deformation in the inorganic layers, rather than the interlayer spacing reduction as found in some 2D HOIPs under hydrostatic pressure. To further understand the physical mechanism behind the strain effects on the band gap of 2D HOIPs, we conduct firstprinciples calculations of C4n1 and C4n4 samples based on DFT. The total energies and relaxed geometries of different samples are calculated by DFT within the generalized gradient D

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Figure 4. DFT simulation of C4n1 (A,C) and C4n4 (B,D) samples: (A,B) at 0% strain; (C,D) at 5% strain. 298 299 f4

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approximation (GGA) of the Perdew−Burke−Ernzerhof exchange−correlation functional with projector augmented wave potentials43 (see SI section IV for details). Figure 4A,B shows the density of states (DOS) in the unstrained state for C4n1 and C4n4, respectively. The band gaps are about 2.01 and 1. 43 eV for C4n1 and C4n4, respectively. The values are slightly lower than experimentally determined values, which is a known issue for DFT simulations.44,45 Here we focus more on qualitative comparison and the overall trend to unveil the underlying physical mechanism. For C4n1, the band gap does not respond to the tensile strain, remaining constant up to the 5% strain investigated in our simulations (Figure 4C), similar to the experimental results (Figure 3A). However, the band gap increases to about 1.48 eV for C4n4 at ε = 5%, which matches the trend found in experiment (Figure 3C). We perform detailed analysis of the contribution of different elemental species in the materials to the DOS (Figure 4). For both C4n1 and C4n4, only Pb and I contribute to the valence band maximum (VBM) and conduction band minimum (CBM), while C, N, and H have negligible contributions. Hence, the band gap change is mainly due to the deformation of the [PbI6]4− octahedral framework while the organic cations, including the MA+ and the alkylamine cations, have negligible effects on the response of the band gap to strain. This agrees with the fact that 2D HOIPs with a larger n number have prominent response of the band gap to strain. A close inspection of the atomic arrangement in C4n4 and C4n1 at strained and unstrained states (Figures S8) reveals that the Pb−I−Pb bond angle between two neighboring [PbI6]4− octahedra and the Pb−I bond length within a [PbI6]4− octahedron increases significantly in C4n4, which causes the change in band gap, while strained C4n1 only shows minor changes in the bond angle and bond length. The difference might arise from the fact that the structure of low-n 2D HOIP is already much more distorted compared to that of high-n HOIP,13, which renders the structure of C4n1 insensitive to mechanical strain. In conclusion, we investigated the optical band gap response of 2D RP lead iodide HOIPs to uniaxial tensile strain. We

found that the tensile strain increases the optical band gap of C4n5 at a rate of 13.3 meV/%, suggesting that mechanical strain can cause about an 80 meV band gap change in the compound’s electronic properties, before the material fractures. This strain-induced band gap modulation is fully reversible and depends on the dimensionality number n of the 2D HOIPs. For 2D HOIPs with n ≤ 3, we do not observe any straindependent band gap change, while for n ≥ 4, the band gap has a prominent dependence on the mechanical strain. DFT calculations verify closely the experimental results, revealing that band gap response to mechanical strain is mainly due to the tilting of the inorganic [PbI6]4− octahedra and the Pb−I bond stretching in the inorganic layers. Those findings provide insight into the 2D perovskites’ optical properties under uniaxial tensile strain for the first time, a behavior that needs to be addressed when it comes to real applications, such as flexible electronics, memory, and commercially viable solar cells. The strain−band gap relationship can be utilized for new strain-sensing applications based on 2D HOIPs and for residual strain characterization to improve the stability and performance of 2D HOIP-based PV devices.

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S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00120.

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Schematics of 2D to 3D HOIPs; details about synthesis and XRD characterization; AFM images of C4n5 flake; and details about DFT simulations (PDF)

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AUTHOR INFORMATION

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Corresponding Authors

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*E-mail: [email protected] (G.S.S.). *E-mail: [email protected] (V.P.D.).

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ORCID

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Qing Tu: 0000-0002-0445-6289 Ioannis Spanopoulos: 0000-0003-0861-1407

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Christopher Wolverton: 0000-0003-2248-474X Mercouri G. Kanatzidis: 0000-0003-2037-4168 Gajendra S. Shekhawat: 0000-0003-3497-288X Vinayak P. Dravid: 0000-0002-6007-3063

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The authors declare no competing financial interest.

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Importance of Electronic Dimensionality. Mater. Horiz. 2017, 4, 206−216. (15) Du, K.-z.; Tu, Q.; Zhang, X.; Han, Q.; Liu, J.; Zauscher, S.; Mitzi, D. B. Two-Dimensional Lead(II) Halide-Based Hybrid Perovskites Templated by Acene Alkylamines: Crystal Structures, Optical Properties, and Piezoelectricity. Inorg. Chem. 2017, 56, 9291− 9302. (16) Ramirez, C.; Yadavalli, S. K.; Garces, H. F.; Zhou, Y.; Padture, N. P. Thermo-Mechanical Behavior of Organic-Inorganic Halide Perovskites for Solar Cells. Scr. Mater. 2018, 150, 36−41. (17) Bush, K. A.; Rolston, N.; Gold-Parker, A.; Manzoor, S.; Hausele, J.; Yu, Z. J.; Raiford, J. A.; Cheacharoen, R.; Holman, Z. C.; Toney, M. F.; et al. Controlling Thin-Film Stress and Wrinkling during Perovskite Film Formation. ACS Energy Lett. 2018, 3 (6), 1225−1232. (18) Rolston, N.; Bush, K. A.; Printz, A. D.; Gold-Parker, A.; Ding, Y.; Toney, M. F.; McGehee, M. D.; Dauskardt, R. H. Engineering Stress in Perovskite Solar Cells to Improve Stability. Adv. Energy Mater. 2018, 8 (29), 1802139. (19) Liao, W.-Q.; Zhang, Y.; Hu, C.-L.; Mao, J.-G.; Ye, H.-Y.; Li, P.F.; Huang, S. D.; Xiong, R.-G. A Lead-Halide Perovskite Molecular Ferroelectric Semiconductor. Nat. Commun. 2015, 6, 7338. (20) Zhao, J.; Deng, Y.; Wei, H.; Zheng, X.; Yu, Z.; Shao, Y.; Shield, J. E.; Huang, J. Strained Hybrid Perovskite Thin Films and Their Impact on the Intrinsic Stability of Perovskite Solar Cells. Sci. Adv. 2017, 3 (11), No. eaao5616. (21) Jaffe, A.; Lin, Y.; Karunadasa, H. I. Halide Perovskites under Pressure: Accessing New Properties through Lattice Compression. ACS Energy Lett. 2017, 2 (7), 1549−1555. (22) Jaffe, A.; Lin, Y.; Beavers, C. M.; Voss, J.; Mao, W. L.; Karunadasa, H. I. High-Pressure Single-Crystal Structures of 3D LeadHalide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties. ACS Cent. Sci. 2016, 2 (4), 201−209. (23) Umeyama, D.; Lin, Y.; Karunadasa, H. I. Red-to-Black Piezochromism in a Compressible Pb−I−SCN Layered Perovskite. Chem. Mater. 2016, 28 (10), 3241−3244. (24) Feng, G.; Qin, Y.; Ran, C.; Ji, L.; Dong, L.; Li, W. Structural Evolution and Photoluminescence Properties of a 2D Hybrid Perovskite under Pressure. APL Mater. 2018, 6 (11), 114201. (25) Matsuishi, K.; Suzuki, T.; Onari, S.; Gregoryanz, E.; Hemley, R. J.; Mao, H. K. Excitonic States of Alkylammonium Lead-Iodide Layered Perovskite Semiconductors under Hydrostatic Pressure to 25 GPa. Phys. Status Solidi B 2001, 223 (1), 177−182. (26) Liu, G.; Gong, J.; Kong, L.; Schaller, R. D.; Hu, Q.; Liu, Z.; Yan, S.; Yang, W.; Stoumpos, C. C.; Kanatzidis, M. G.; et al. Isothermal Pressure-Derived Metastable States in 2D Hybrid Perovskites Showing Enduring Bandgap Narrowing. Proc. Natl. Acad. Sci. U. S. A. 2018, 115 (32), 8076−8081. (27) Stoumpos, C. C.; Soe, C. M. M.; Tsai, H.; Nie, W.; Blancon, J.C.; Cao, D. H.; Liu, F.; Traoré, B.; Katan, C.; Even, J.; et al. High Members of the 2D Ruddlesden-Popper Halide Perovskites: Synthesis, Optical Properties, and Solar Cells of (CH3(CH2)3NH3)2(CH3NH3)4Pb5I16. Chem. 2017, 2 (3), 427−440. (28) Tu, Q.; Spanopoulos, I.; Yasaei, P.; Stoumpos, C. C.; Kanatzidis, M. G.; Shekhawat, G. S.; Dravid, V. P. Stretching and Breaking of Ultrathin 2D Hybrid Organic−Inorganic Perovskites. ACS Nano 2018, 12 (10), 10347−10354. (29) Guo, Z.; Wu, X.; Zhu, T.; Zhu, X.; Huang, L. Electron−Phonon Scattering in Atomically Thin 2D Perovskites. ACS Nano 2016, 10 (11), 9992−9998. (30) Ugural, A. C. Mechanics of Materials; Wiley: New York, 2008. (31) Wang, Z.; Tonderys, D.; Leggett, S. E.; Williams, E. K.; Kiani, M. T.; Spitz Steinberg, R.; Qiu, Y.; Wong, I. Y.; Hurt, R. H. Wrinkled, Wavelength-Tunable Graphene-Based Surface Topographies for Directing Cell Alignment and Morphology. Carbon 2016, 97, 14−24. (32) Roldan, R.; Castellanos-Gomez, A.; Cappelluti, E.; Guinea, F. Strain Engineering in Semiconducting Two-Dimensional Crystals. J. Phys.: Condens. Matter 2015, 27 (31), 313201.

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ACKNOWLEDGMENTS The work made use of the SPID and EPIC facilities of Northwestern University’s NUANCE center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. This work was supported by the National Science Foundation IDBR Grant Award Number 1256188 and was partially supported by Air Force Research Laboratory Grant FA8650-15-2-5518. M.G.K. acknowledges support under ONR Grant N00014-17-1-2231.

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REFERENCES

(1) Green, M. A.; Hishikawa, Y.; Dunlop, E. D.; Levi, D. H.; HohlEbinger, J.; Ho-Baillie, A. W. Y. Solar Cell Efficiency Tables (Version 52). Prog. Photovoltaics 2018, 26 (7), 427−436. (2) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506. (3) NREL Best Research-Cell Efficiency Chart. https://www.nrel. gov/pv/assets/pdfs/pv-efficiency-chart.20181221.pdf (2018). (4) Choi, J.; Han, J. S.; Hong, K.; Kim, S. Y.; Jang, H. W. Organic− Inorganic Hybrid Halide Perovskites for Memories, Transistors, and Artificial Synapses. Adv. Mater. 2018, 30 (42), 1704002. (5) Rakita, Y.; Cohen, S. R.; Kedem, N. K.; Hodes, G.; Cahen, D. Mechanical Properties of APbX3 (A = Cs or CH3NH3X = I or Br) Perovskite Single Crystals. MRS Commun. 2015, 5 (4), 623−629. (6) Sun, S.; Fang, Y.; Kieslich, G.; White, T. J.; Cheetham, A. K. Mechanical Properties of Organic−Inorganic Halide Perovskites, CH3NH3PbX3 (X = I, Br and Cl), by Nanoindentation. J. Mater. Chem. A 2015, 3 (36), 18450−18455. (7) Tu, Q.; Spanopoulos, I.; Hao, S.; Wolverton, C.; Kanatzidis, M. G.; Shekhawat, G. S.; Dravid, V. P. Out-of-Plane Mechanical Properties of 2D Hybrid Organic−Inorganic Perovskites by Nanoindentation. ACS Appl. Mater. Interfaces 2018, 10 (26), 22167−22173. (8) Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 2015, 5 (20), 1500963. (9) Niu, G.; Guo, X.; Wang, L. Review of Recent Progress in Chemical Stability of Perovskite Solar Cells. J. Mater. Chem. A 2015, 3 (17), 8970−8980. (10) Rong, Y.; Hu, Y.; Mei, A.; Tan, H.; Saidaminov, M. I.; Seok, S. I.; McGehee, M. D.; Sargent, E. H.; Han, H. Challenges for Commercializing Perovskite Solar Cells. Science 2018, 361, 1214. (11) Saparov, B.; Mitzi, D. B. Organic−Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chem. Rev. 2016, 116 (7), 4558−4596. (12) Tsai, H.; Nie, W.; Blancon, J.-C.; Stoumpos, C. C.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; et al. High-Efficiency Two-Dimensional Ruddlesden− Popper Perovskite Solar Cells. Nature 2016, 536, 312. (13) Stoumpos, C. C.; Cao, D. H.; Clark, D. J.; Young, J.; Rondinelli, J. M.; Jang, J. I.; Hupp, J. T.; Kanatzidis, M. G. Ruddlesden−Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors. Chem. Mater. 2016, 28, 2852−2867. (14) Xiao, Z.; Meng, W.; Wang, J.; Mitzi, D. B.; Yan, Y. Searching for Promising New Perovskite-Based Photovoltaic Absorbers: The F

DOI: 10.1021/acsenergylett.9b00120 ACS Energy Lett. XXXX, XXX, XXX−XXX

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Letter

ACS Energy Letters 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549

(33) Si, C.; Sun, Z.; Liu, F. Strain Engineering of Graphene: a Review. Nanoscale 2016, 8 (6), 3207−3217. (34) He, K.; Poole, C.; Mak, K. F.; Shan, J. Experimental Demonstration of Continuous Electronic Structure Tuning via Strain in Atomically Thin MoS2. Nano Lett. 2013, 13 (6), 2931−2936. (35) Mohiuddin, T. M. G.; Lombardo, A.; Nair, R. R.; Bonetti, A.; Savini, G.; Jalil, R.; Bonini, N.; Basko, D. M.; Galiotis, C.; Marzari, N.; et al. Uniaxial Strain in Graphene by Raman Spectroscopy: G Peak Splitting, Gruneisen Parameters, and Sample Orientation. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79 (20), 205433. (36) Dou, L.; Wong, A. B.; Yu, Y.; Lai, M.; Kornienko, N.; Eaton, S. W.; Fu, A.; Bischak, C. G.; Ma, J.; Ding, T.; et al. Atomically Thin Two-Dimensional Organic-Inorganic Hybrid Perovskites. Science 2015, 349 (6255), 1518−1521. (37) Li, J.; Wang, J.; Zhang, Y.; Wang, H.; Lin, G.; et al. Fabrication of Single Phase 2D Homologous Perovskite Microplates by Mechanical Exfoliation. 2D Mater. 2018, 5 (2), 021001. (38) Ni, Z. H.; Yu, T.; Lu, Y. H.; Wang, Y. Y.; Feng, Y. P.; Shen, Z. X. Uniaxial Strain on Graphene: Raman Spectroscopy Study and Band-Gap Opening. ACS Nano 2008, 2 (11), 2301−2305. (39) Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8 (4), 4033−4041. (40) Tsai, H.; Asadpour, R.; Blancon, J.-C.; Stoumpos, C. C.; Durand, O.; Strzalka, J. W.; Chen, B.; Verduzco, R.; Ajayan, P. M.; Tretiak, S.; et al. Light-Induced Lattice Expansion Leads to HighEfficiency Perovskite Solar Cells. Science 2018, 360 (6384), 67−70. (41) Jones, T. W.; Osherov, A.; Alsari, M.; Sponseller, M.; Duck, B. C.; Jung, Y.-K.; Settens, C.; Niroui, F.; Brenes, R.; Stan, C. V.; et al. Lattice Strain Causes Non-Radiative Losses in Halide Perovskites. Energy Environ. Sci. 2019, 12 (2), 596−606. (42) Wang, Y.; Cong, C.; Yang, W.; Shang, J.; Peimyoo, N.; Chen, Y.; Kang, J.; Wang, J.; Huang, W.; Yu, T. Strain-Induced Direct− Indirect Bandgap Transition and Phonon Modulation in Monolayer WS2. Nano Res. 2015, 8 (8), 2562−2572. (43) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77 (18), 3865− 3868. (44) Even, J.; Pedesseau, L.; Jancu, J.-M.; Katan, C. Importance of Spin−Orbit Coupling in Hybrid Organic/Inorganic Perovskites for Photovoltaic Applications. J. Phys. Chem. Lett. 2013, 4 (17), 2999− 3005. (45) Katan, C.; Pedesseau, L.; Kepenekian, M.; Rolland, A.; Even, J. Interplay of Spin−Orbit Coupling and Lattice Distortion in Metal Substituted 3D Tri-Chloride Hybrid Perovskites. J. Mater. Chem. A 2015, 3 (17), 9232−9240.

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