Improvement of the performance of Cu2ZnSn(S,Se)4 solar cells by annealing Li-doped Cu2ZnSnS4 precursor films in air

Abstract

It is well known that the improvement of crystal quality and photoelectric properties of Cu₂ZnSn(S,Se)₄ (CZTSSe) is one of the key issues in enhancing power conversion efficiency (PCE). To resolve this problem, we selected Li₂CO₃ as a lithium source to prepare Li-doped CZTSSe by air annealing Li-doped Cu₂ZnSnS₄ (CZTS) precursor films in air, followed by selenization (denoted as CZTSSe-Li-A). It is interesting that the air annealing facilitates the incorporation of Li and O into CZTSSe as Li–O pairs. This pairing improves the crystal quality and absorption coefficient of CZTSSe-Li-A while reducing its hole concentration and resistivity compared to non-air-annealed Li-doped CZTSSe (CZTSSe-Li). By optimizing the Li doping concentration, annealing temperature and time, the highest PCE of the CZTSSe-Li-A solar cell (without the MgF₂ anti-reflection layer) reaches 11.13%, which is larger than the highest PCE of the CZTSSe-Li solar cell (10.21%). Quantitative analysis indicates that the increased PCE of the CZTSSe-Li-A solar cell compared to the CZTSSe-Li solar cell is mainly attributed to the decrease in reverse saturation current density (J₀) and series resistance (R_s) caused by the thermal effect of the air annealing, followed by an increase in photogeneration density (J_L) and shunt resistance (R_sh). The decreased J₀ stems from a reduction in interfacial defect density, the increased J_L results from an increase in light absorption (α) and the depletion region width, and the decreased R_s and increased R_sh are ascribed to improved crystal quality. This work offers a route for the improvement of PCE of CZTSSe solar cells.