Determining and Controlling the Stoichiometry of Cu₂ZnSnS₄ Photovoltaics: The Physics and Its Implications

Cu₂ZnSnS₄ (CZTS) is an environmentally friendly photovoltaic material with promising applications in thin-film solar cells. Although CZTS's efficiency is currently too low, stoichiometry/defect engineering presents a strategy for further improvement. As-grown CZTS is typically disordered and therefore prone to form secondary phases, making the final product stoichiometry difficult to determine and even harder to control. We use first-principles quantum mechanics in combination with Monte Carlo simulations to determine CZTS stoichiometry under various experimental conditions. We develop an approach to predicting the optimal CZTS stoichiometry, explaining the physical origin of Zn-enrichment observed in experiments. We further propose practical ways to introduce more free carriers into CZTS in order to screen observed local potential fluctuations, increase conductivity, and ultimately improve the efficiency of CZTS.