ABSTRACT

Compositional engineering technology has played a crucial role in the significant advancement of perovskite solar cells to produce high-quality and improved solar performance. Germanium is one of the potential replacements for the toxic lead active layer in perovskite solar cells due to its similar optical and electronic properties. However, germanium-based materials recorded the lowest efficiency of 8.6% compared to tin-based perovskites (< 13%) and lead-based materials (< 25%). To address this challenge, the influence of mixed A-cations (Methylammonium [MA], Formamidinium [FA], Ethylammonium [EA], Caesium [C_s]) with varied composition (0.25, 0.5, 0.75) on all possible combinations was investigated to estimate the structural and optoelectronic properties compatibility. This study used the SCAPS 1-D simulation tool to optimise the photoexcited charge carrier generation, collection and extraction. The findings show that a high doping concentration of caesium (> 50%) in germanium-based perovskite produced a perfect cubic-like structure with a tolerance factor ranging from 0.99 to 1.11. In addition, the optimum estimated bandgap for enhanced exciton generation was between 1.6 eV and 1.8 eV. Further optimisation revealed photovoltaic (PV) performance was optimised at an absorber thickness of 600 nm and maximum defect density of 10¹⁶ cm⁻³ (open-circuit voltage [V_oc] = 0.64 V; short-circuit current density [J_sc] = 16.55 mA/cm²; fill factor [FF] = 76.17% and power conversion efficiency (PCE) of 8.02%). The minimum carrier lifetime was calculated to be 10 ns at a diffusion length of 0.72 nm and further increased as the absorber quality improved. In addition, an optimal conduction band offset (CBO) range exists between −0.58 eV and −0.38 eV, with PCE above 10% and the materials with valence band offset (VBO) ranging from 0.67 eV to −0.23 eV will further improve the PV performance of mixed germaniumbased perovskite solar cells. It is believed that this study can be helpful in developing and designing germanium-based solar cells for further improvement of the PV performance and increasing their potential to be a competitive material in lead-free variants.

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