Tailoring the Hole Transport Layer and Understanding the Impact of Sn Oxidation for Different Mixed Halide Perovskite Active Layers: On a Quest for the Perfect Match

This research work aimed to identify the optimal parameters of the hole transport layer (HTL) for enhancing the power conversion efficiency (PCE) of mixed halide (FASnI3)1–x(MAPbI3)x-based perovskite solar cells (PSCs). The study analyzed the impact of the HTL properties, such as bandgap (Eg), affinity (μ), mobility (μp), and acceptor-doping density (NA), on the photovoltaic (PV) parameters of five different PSCs with x = 0, 0.2, 0.4, 0.6, and 1.0. The study revealed that the influence of χ on open-circuit voltage (VOC) decreases for the HTL with Eg ≥ 2.95 eV, and higher Eg results in higher VOC values. The study also found that the impact of μp on short-circuit current density (JSC) becomes negligible if μp is more than 2 × 10–4 cm2 V–1 s–1, while the influence of NA is negligible for NA of more than 1.5 × 1017 cm–3. Moreover, the impact of μp is negligible on VOC of the cells at the entire NA range, while with an increase in NA, VOC of the cells increases significantly. The study found that the most suitable active layer was with x = 0.2, i.e., (FASnI3)0.8(MAPbI3)0.2, which delivered the highest PCE of 23.05%. The oxidation of Sn2+ to Sn4+ can lead to inherent acceptor doping in the absorber layer, which affects device performance. Therefore, this study also investigated the impact of Sn oxidation on the performance of PSCs with Sn-based absorber layers and found that VOC and fill factor (FF) reduce monotonically as the acceptor doping increases. The reduction in efficiency as a result of Sn oxidation is significant, reducing PCE from 23.05 to 20.86% for the champion device with x = 0.2. The findings of this study contribute to advancing the understanding of mixed halide (FASnI3)1–x(MAPbI3)x PSCs and improving their performance by selecting the appropriate HTL for specific active layers.