Simulation Study on the All-Inorganic CsSn_<i>x</i>Ge_1–<i>x</i>I₃-Based Perovskite Solar Cells Using Isotypic Perovskites as Hole Transport Layers

All-inorganic Sn–Ge-based perovskite solar cells (PSCs) have made great progress in recent years. Furthermore, they can be used as promising lead-free absorbers for PSCs, and p-type-doped CsSnI3, CsGeI3, and CsSn0.5Ge0.5I3 could also be used as good hole transport layers (HTLs). In this simulation work, CsSnI3, CsGeI3, and CsSn0.5Ge0.5I3 are used as both absorbers and HTLs. The effects of the dopant concentration of HTLs, the thickness of absorbers, and HTLs on the photovoltaic performance of PSCs were studied to optimize the device structures. The maximum efficiencies from high to low are 28.35%, 26.35%, 25.84%, 25.23%, 18.83%, 17.49%, and 11.79% for the TiO2/i-CsSnI3/p-CsSnI3, TiO2/i-CsSn0.5Ge0.5I3/p-CsSn0.5Ge0.5I3, TiO2/i-CsSn0.5Ge0.5I3/p-CsSnI3, TiO2/i-CsSnI3/p-CsGeI3, TiO2/i-CsSn0.5Ge0.5I3/p-CsGeI3, TiO2/i-CsGeI3/p-CsGeI3, and TiO2/i-CsGeI3/p-CsSnI3, respectively. The TiO2/i-CsGeI3/p-CsSnI3 cell exhibits the lowest efficiency of 11.79% in all of the simulated PSCs due to the spike-like band offset at the i-CsGeI3/p-CsSnI3 interface and high recombination rate in the p-CsSnI3 region. It is found that the n-p structures could have better photovoltaic performance (thickness of i-film approaching zero) than the conventional n-i-p structures for the TiO2/i-CsSnI3/p-CsSnI3, TiO2/i-CsGeI3/p-CsGeI3, and TiO2/i-CsSn0.5Ge0.5I3/p-CsSn0.5Ge0.5I3 PSCs if the defects in HTLs created by high doping can be effectively controlled. The efficiencies of PSCs are sensitive to the defect density and defect level position, and the influence of defect density on the PV performance is larger than that of the defect level position. The solar cells could maintain high power conversion efficiency for defect density below about 5 × 1017 cm–3. Furthermore, the increase of the interface trap density is found to reduce the photovoltaic performance of PSCs. Our study provides insight into the optimal design of CsSnxGe1–xI3-based PSCs.