The role of alkali doping in enhancing bulk and interface properties of Cu₂CdSnS₄ solar cells

Alkali doping has been widely employed to enhance the performance of chalcopyrite and kesterite solar cells; however, its potential for Cu₂CdSnS₄ (CCTS) solar cells remains unexplored. This study systematically investigates the impact of all alkali dopants (Li, Na, K, Rb, Cs) on CCTS solar cells, providing new insights into their interaction with the CCTS structure and its subsequent impact on optoelectronic properties. Alkali doping increases carrier density by an order of magnitude without introducing detrimental recombination centres, as highlighted by the stable minority carrier lifetime. Kelvin probe force microscopy (KPFM) reveals a reduction in upward band bending at grain boundaries, minimizing majority carrier accumulation and enhancing carrier transport. Conductive-atomic force microscopy (c-AFM) further demonstrates an enhancement in intragrain conductivity, with nanoscale surface current increasing by an order of magnitude. Among the doped samples, Na-doped CCTS achieves the highest efficiency of 8.47%, attributed to its compact film morphology and improved charge transport, which collectively yield a higher fill factor and J_SC. These findings establish alkali doping as a promising strategy for optimizing CCTS solar cells, with Na emerging as the most effective dopant to enhance device performance.