Enhancing the performance of cu₂zngese₄ solar cells with metallic and transparent back electrodes via lithium doping

Crystalline silicon (c-Si) solar cells have nearly reached their theoretical efficiency limits, constrained by the intrinsic properties of silicon. To overcome these limitations, tandem solar cell devices combining a narrow-bandgap c-Si bottom cell with a wide-bandgap top cell have emerged as a promising approach. Ge-based kesterite materials (Cu₂ZnGe(SS)e₄), offer significant potential for this application due to their tunable optoelectronic properties, earth-abundant composition, and compatibility with cost-effective fabrication techniques. However, improving the structural and electronic properties of Ge-kesterite absorbers remains critical for advancing their performance. Lithium (Li) doping, previously successful in Sn-based kesterite materials (Cu₂ZnSnSSe₄), provides an approach to enhance crystallinity, defect passivation, and grain growth in Ge-kesterite materials. This study investigates the effect of lithium doping on the structural, morphological, and optoelectronic properties of Cu₂ZnGe(SS)e₄ (CZGSe) absorbers prepared using sputtered metallic precursors. Lithium was introduced via a Lithium fluoride (LiF) nano-layer, with an optimal thickness of 10 nm significantly improving absorber quality. Doped CZGSe solar cells achieved an efficiency of 6.4% on Mo substrates, demonstrating enhanced crystallinity, grain growth, and Voc compared to undoped samples. Applying this doping strategy to transparent back electrodes showed notable material improvements, but slightly lower efficiency compared to Mo-based cells. These findings underscore the potential of lithium as a promising alkali dopant for Ge-kesterite materials, paving the way for their integration into tandem and next-generation photovoltaic devices.