Probing Interfacial Defects and Stability Challenges in Metal Halide Perovskites

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 26%. However, highly defective interfaces with charge extraction layers, the low hurdle for ionic migration, and the structural flexibility of the perovskite structure still pose both opportunities and challenges to their commercialization in light-harvesting applications. Combinatorial characterization approaches are vital for probing and analysing such instabilities.<br/><br/>We discuss the use of combinatorial techniques^[1-3] to probe the effect of halide segregation in mixed iodide-bromide lead perovskites with desirable electronic band gaps near 1.8eV attractive for tandem photovoltaic cells. We report recent insights from in-situ photoluminescence and X-ray diffraction techniques, and further demonstrate a temperature-dependent reversal of halide segregation at temperatures above ambient that may prove highly beneficial for solar cells under operating conditions^[3]and is attributed to the trade-off between the temperature activation of segregation, for example through enhanced ionic migration, and its inhibition by entropic factors.<br/><br/>In addition, we demonstrate the vital role of absorption spectroscopy in determining the suitability of processing techniques for FAPbI₃ films, which have recently been shown to exhibit a peculiar effect of “intrinsic quantum confinement” (QC)^[4,5] We reveal that such QC features are clearly detrimental to photovoltaic operation.^[6] A meta-analysis of literature reports, covering 244 articles and 825 photovoltaic devices incorporating FAPbI₃ films reveals that PCEs rarely exceed a 20% threshold when such absorption features are present.^[6] We discuss how such effects can be mitigated through different approaches to FAPbI₃ film growth.<br/><br/>We further demonstrate a combined modelling and experimental approach^[7]towards exploring the origins of energy-level alignment at the interface between wide-bandgap mixed-halide perovskites and charge-extraction layers, which still causes significant losses in solar-cell performance, focusing on FA_0.83Cs_0.17Pb(I_1-xBr_x)₃ with bromide content x ranging from 0 to 1, and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine| (PTAA). Through a combination of time-resolved photoluminescence spectroscopy and numerical modeling of charge-carrier dynamics^[7]we reveal that open-circuit voltage losses associated with a rising energy-level misalignment derive from increasing accumulation of holes in the HOMO of PTAA, which then subsequently recombine non-radiatively across the interface via interfacial defects. These findings highlight the urgent need for tailored charge-extraction materials exhibiting improved energy-level alignment with wide-bandgap mixed-halide perovskites.<br/><br/>[1] A. J. Knight, J. Borchert, R. D. J. Oliver, J. B. Patel, P. G. Radaelli, H. J. Snaith, M. B. Johnston, and L. M. Herz, ACS Energy Letters <b>6</b>, 799 (2021).<br/>[2] V. J.-Y. Lim, A. M. Ulatowski, C. Kamaraki, M. T. Klug, L. Miranda Perez, M. B. Johnston, and L. M. Herz,<br/>Adv. En. Mater. <b>12</b>, 2200847 (2022).<br/>[3] A. D. Wright, J. B. Patel, M. B. Johnston, L. M. Herz, Advanced Materials <b>35</b>, 2210834 (2023).<br/>[4] A. D. Wright, G. Volonakis, J. Borchert, C. L. Davies, F. Giustino, M. B. Johnston, and L. M. Herz,<br/>Nature Materials <b>19</b>, 1201 (2020).<br/>[5] K. A. Elmestekawy, A. D. Wright, K. B. Lohmann, J. Borchert, M. B. Johnston, and L. M. Herz, ACS Nano <b>16</b>, 6940 (2022).<br/>[6] K. A. Elmestekawy, B. M. Gallant, A. D. Wright, P. Holzhey, N. K. Noel, M. B. Johnston, H. J. Snaith, L. M. Herz, ACS Energy Letters <b>8</b>, 2543 (2023).<br/><i>[7] J. E. Lee, S. G. Motti, R. D. J. Oliver, S. Yan, H. J. Snaith, M. B. Johnston, and L. M. Herz</i><i>, </i><br/>Advanced Functional Materials <b>34</b>, 2401052 (2024).