Band engineering and recombination mechanisms in lead-free perovskite solar cells

All-inorganic lead-free perovskite solar cells have emerged as environmentally benign candidates; however, their device performance is still constrained by pronounced carrier recombination losses in the bulk and at interfaces. By combining energy band alignment analysis with detailed modeling of recombination mechanisms, a systematic strategy for optimizing hole transport layers is developed. The results reveal that a negative valence band offset produces a cliff-like interface, which facilitates hole extraction while also accounting for the observed variations in open-circuit voltage. Furthermore, short-circuit current losses are quantitatively attributed to different recombination pathways, modeled by incorporating radiative, Shockley–Read–Hall, Auger, and interface recombination processes. This comprehensive approach not only clarifies the correlation between energy level alignment and recombination dynamics but also highlights the competing roles of band offset and interface defects in determining device performance. The optimized device architecture, based on Ge-based lead-free perovskites, achieves a power conversion efficiency of 25.1%, with an open-circuit voltage of 1.29 V, a short-circuit current density of 22.5 mA⋅cm⁻², and a fill factor of 86.3%. These findings provide theoretical guidance for designing stable, high-performance, and environmentally friendly lead-free perovskite solar cells.