Publication date: 21st July 2025
Mixed-metal halide perovskites, with the general formula ABX3 - where A is a cation (methylammonium, formamidinium, Cs), B is a metal (Sn, Pb) and X is an anion (I, Br, Cl) – are integral to all-perovskite tandem solar cells. Although groundbreaking research has pushed efficiencies to 30.1% so far, this is still just a fraction of fundamental limits (>40%). Device parameters such as the open-circuit voltage (VOC) and fill-factor (FF) trace back directly back to the photoluminescence quantum yield.[1] This picture is complicated in Sn-containing perovskites given that the presence of Sn4+ impurities in the precursor solution leads to both the introduction of background hole dopants (serving to increase the PLQY) and non-radiative recombination centers (serving to decrease the PLQY). [3] Nevertheless, these fundamental relationships make PL microscopy a powerful technique for understanding mixed-metal halide perovskites - particularly when correlated with other compositional, structural, or topographical imaging modes.
To probe the compositional origins of photophysical heterogeneity in halide perovskites, we correlate steady-state & time-resolved, wide-field PL microscopy with nanoprobe X-ray fluorescence (n-XRF). We find that what appear to be micron sized grains in atomic force microscopy images are in fact highly heterogeneous in Sn/Pb composition. From PL microscopy, we observe that Sn-rich regions are red-shifted and higher intensity in steady-state PL images. Time-resolved photoluminescence microscopy shows that while the initial photocarrier distribution is homogeneous, these carriers funnel into more localized regions. Finally, the combination of time-resolved and steady-state photoluminescence microscopy allows us to obtain the spatial dopant density on the microscale.[3] Together, these results suggest that Sn-rich regions have a higher dopant density and play a role as recombination centres. We expect these results to be informative for the design of next-generation dopant management strategies in mixed-metal halide perovskites to supress, or even harness, self-doping.
[1] Giles E. Eperon, Maximilian T. Hörantner & Henry J. Snaith, Metal halide perovskite tandem and multiple-junction photovoltaics, Nature Reviews Chemistry, 2017, 1, 0095
[2] Kunal Datta, Junke Wang, Dong Zhang, Valerio Zardetto, Willemijn H. M. Remmerswaal, Christ H. L. Weijtens, Martijn M. Wienk, René A. J. Janssen, Monolithic All‐Perovskite Tandem Solar Cells with Minimized Optical and Energetic Losses, Advanced Materials, 2021, 34 (11), 2110053
[3] Robert J. E. Westbrook, Margherita Taddei, Rajiv Giridharagopal, Meihuizi Jiang, Shaun M. Gallagher, Kathryn N. Guye, Aaron I. Warga, Saif A. Haque & David S. Ginger, Local Background Hole Density Drives Non-Radiative Recombination in Tin Halide Perovskites, ACS Energy Lett., 2024, 9 (2), 732-739
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