Lead-free, formamidinium germanium-antimony halide (FA4GeSbCl12) double perovskite solar cells: the effects of band offsets

Abstract

Double halide perovskites have received massive attention due to their low toxicity, tunable bandgap, structural flexibility, and stability as compared to conventional 3D lead halide perovskites. Particularly, newly discovered formamidinium germanium-antimony halide (FA₄GeSbCl₁₂) double perovskites offer an excellent bandgap (∼1.3 eV) for solar cell (SC) applications. Therefore, in this study, for the first time, we have simulated FTO/TiO₂/FA₄GeSbCl₁₂/Cu₂O/Au planar SCs using SCAPS-1D, showing a maximum power conversion efficiency of 22.5% with J_sc = 34.52 mA cm⁻², V_oc = 0.76 V, and FF = 85.1%. The results show that the variation in valence and conduction band offsets (−0.4 to +0.2 eV and −0.4 to +0.57 eV) at the ETL/absorber and absorber/HTL interfaces dominate the SC performance. Also, different absorber defect densities (1 × 10¹⁴–1 × 10²⁰ cm⁻³) and thicknesses (200–3000 nm) effectively influence the PCE. Moreover, simulated impedance spectroscopy (IS) data (through SCAPS-1D) were fitted using equivalent electrical circuits to extract relevant parameters, including R_s, R_HF, and R_LF, allowing us to better discuss the physics of the device. The fitted IS results strongly revealed that enhanced SC performance is associated with higher recombination resistance and a larger recombination lifetime. Likewise, a slight variation in the R_s (0 to 2.5 Ω cm²) highly impacts the PCE (22.5% to 19.7%). Furthermore, a tandem cell is designed by combining the top cell of ethylenediammonium-FASnI₃ perovskite with the FA₄GeSbCl₁₂ bottom cell using a filtered spectrum strategy, which opens the door for multi-junction SC applications. These findings firmly reveal that the appropriate energy level alignment at interfaces with suitable material properties is the key to boosting SC performance.