Numerical Simulation and Performance Optimization of CsGeI₃-Based Lead-Free Perovskite Solar Cells Using SCAPS-1D

Organic-inorganic lead-based halide perovskites have emerged as highly efficient photovoltaic materials, yet their toxicity and environmental instability hinder large-scale commercialization. To address these challenges, this study explores cesium germanium tri-iodide (CsGeI 3 ) as a lead-free perovskite absorber through numerical simulations using the SCAPS-1D tool. The proposed n-i-p device architecture consists of FTO/TiO 2 /CsGeI 3 /Spiro-OMeTAD/Au, where TiO 2 serves as the electron transport layer (ETL) and Spiro-OMeTAD as the hole transport layer (HTL). The influence of key parameters—including absorber thickness, ETL/HTL thickness, defect density, and temperature—on photovoltaic performance is systematically investigated. The optimized structure demonstrates a power conversion efficiency (PCE) of 27.06%, with an open-circuit voltage (V oc ) of 1.29 V, short-circuit current density ( J sc ) of 24.80 mA/cm 2 , and fill factor (FF) of 84.67%. A comparative analysis reveals that the inclusion of TiO 2 ETL significantly enhances PCE from 3.10% to 25.73%, while Spiro-OMeTAD HTL primarily improves device stability rather than efficiency. Furthermore, absorber thickness optimization shows that a 410 nm CsGeI 3 layer yields peak performance, beyond which efficiency plateaus. Temperature-dependent studies indicate optimal operation at 300 K, with performance degradation observed beyond this point. This work underscores the potential of CsGeI 3 as a viable, eco-friendly alternative to lead-based perovskites and provides critical insights into charge transport layer engineering for high-efficiency, stable solar cells.