Structure and Stability of Multi-Component B-Site Mixed CsBX₃ Metal Halide Perovskites

ABX₃ [A = Cs, FA, MA; B = Pb, Sn, Ge; X = Cl, Br, I] metal halide perovskites have dominated the research landscape since the first demonstration of MAPbI₃ as an absorber layer for dye-sensitized solar cells.¹ Since then, developments in this space have realized perovskites for widespread applications in LEDs, photovoltaics, sensing, radiation detection, lasing, and more.^2,3 The ubiquity of perovskites today is partly due to their facile synthesis via multiple routes, their tolerance to high defect concentrations, and highly tunable composition whereby properties can be defined. Of late, low-dimensional perovskite-based materials, alloys, or high entropy systems (with multiple elements at the same site) have brought renewed interest.⁴<br/><br/>Widespread deployment of ABX₃ perovskites has been slowed by the inclusion of the toxic heavy metal Pb, and material instability to heat and moisture. An attractive method for promoting the formation of the desired phase(s) while limiting degradation involves site-mixing. A- and X-site mixing is currently applied to materials in state-of-the-art compositions for the current best-performing prototype solar cells. Only limited examples showing mixing at the B-site have appeared and most involve Pb-Sn derivatives that only reduce Pb content in the material, without completely eliminating it. Sn-Ge mixed perovskites have received some attention, and exhibit improved stability and solar cell performance than previous Ge-based perovskites.^5,6 A detailed understanding of the phenomena leading to such improvements using a suite of bulk spectroscopic and diffraction methods can provide insight into the mechanisms at play. Specifically, NMR spectroscopy offers atomic-level precision of the local chemical environment and has been used with much success for studying challenging and interesting perovskite materials.^7-10 Together, this toolkit can provide the means for rational design of new materials for advanced solar absorbers.<br/><br/>Our work explores new systems of mixed B-site perovskites, starting with lead-free Sn-Ge solid solutions of the formula CsSn_xGe_1−xBr₃. Their optical properties and structure were explored using optical band gap measurements, XRD, SEM/EDX, and ¹³³Cs NMR spectroscopy. We then investigate the impact of halide identity on the mixed Sn-Ge system, and explore the possibility of a triple B-site mixed Pb-Sn-Ge perovskite.<br/><br/><u><b>References</b></u><br/>(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. <i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i> (17), 6050-6051. <b>DOI: </b>10.1021/ja809598r.<br/>(2) Kim, J. Y.; Lee, J. W.; Jung, H. S.; Shin, H.; Park, N. G. <i>Chem. Rev.</i> <b>2020</b>, <i>120</i> (15), 7867-7918. <b>DOI:</b> 10.1021/acs.chemrev.0c00107.<br/>(3) Jena, A. K.; Kulkarni, A.; Miyasaka, T. <i>Chem. Rev.</i> <b>2019</b>, <i>119</i> (5), 3036-3103. <b>DOI: </b>10.1021/acs.chemrev.8b00539.<br/>(4) Xiao, Z.; Song, Z.; Yan, Y. <i>Adv. Mater. </i><b>2019</b>, <b>31</b> (47), e1803792. <b>DOI:</b> 10.1002/adma.201803792.<br/>(5) Xi, J.; Loi, M. A. <i>ACS Energy Lett.</i> <b>2021</b>, <i>6</i> (5), 1803-1810. <b>DOI:</b> 10.1021/acsenergylett.1c00289.<br/>(6) Ke, W.; Stoumpos, C. C.; Kanatzidis, M. G. <i>Adv. Mater.</i> <b>2019</b>, <i>31</i> (47), e1803230. <b>DOI:</b> 10.1002/adma.201803230.<br/>(7) Hooper, R. W.; Ni, C.; Tkachuk, D. G.; He, Y.; Terskikh, V. V.; Veinot, J. G. C.; Michaelis, V. K. <i>J. Phys. Chem. Lett.</i> <b>2022</b>, <i>13</i> (7), 1687-1696. <b>DOI:</b> 10.1021/acs.jpclett.1c04033.<br/>(8) Karmakar, A.; Bhattacharya, A.; Bernard, G. M.; Mar, A.; Michaelis, V. K. <i>ACS Mat. Lett. </i><b>2021</b>, <i>3</i> (3), 261-267. <b>DOI:</b> 10.1021/acsmaterialslett.0c00596.<br/>(9) Bernard, G. M.; Karmakar, A.; Michaelis, V. K. <i>Reference Module in Chemistry, Molecular Sciences and Chemical Engineering</i>, Elsevier, <b>2021</b>. <b>DOI:</b> 10.1016/B978-0-12-823144-9.00018-2<br/>(10) Kubicki, D. J.; Stranks, S. D.; Grey, C. P.; Emsley, L. <i>Nat. Rev. Chem.</i> <b>2021</b>, <i>5 </i>(9), 624-645. <b>DOI:</b> 10.1038/s41570-021-00309-x.