<p>The rising demand for environmentally sustainable energy solutions has driven significant interest in lead-free inorganic perovskite solar cells as alternatives to the toxic lead-based counterparts. Despite promising advances, challenges remain in optimizing the efficiency of lead-free PSCs for practical applications. This study investigates cesium germanium tri-iodide (CsGeI₃) as a lead-free perovskite absorber layer, with four zinc-based electron transport layers and five kesterite quaternary-based hole transport layers. Using the SCAPS-1D simulation software, 20 unique device structures were modeled, systematically varying the charge transport layers to identify optimal configurations. Zinc-based electron transport layers were chosen for their large band gaps and high optical transmittance, while kesterite-based hole transport layers were selected due to their high absorption coefficients, tunable band gaps, abundance, and non-toxicity. A systematic methodology is adopted to analyze the effect of the charge transport materials on the absorption, quantum efficiency, energy band alignment, electric field intensity, recombination rate, carrier density, thickness, doping concentration, temperature, reflection and interface defect densities of the PSC in detail. Notably, the CMTS/CsGeI₃/CdZnS structure achieved the highest power conversion efficiency of 25.78%, with a short-circuit current density of 24.49&#xa0;mA/cm², a fill factor of 86.39%, and an open-circuit voltage of 1.22&#xa0;V. These findings suggest that carefully selected charge transport layer combinations can significantly enhance the efficiency of lead-free CsGeI₃-based PSCs.</p> Graphical abstract <p></p>