Improvement of the performance of Cu2ZnSn(S,Se)4 solar cells by annealing Li-doped Cu2ZnSnS4 precursor film 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 Li source to prepare Li-doped CZTSSe by annealing Li-doped Cu2ZnSnS4 (CZTS) precursor films in air, followed by selenization (CZTSSe-Li-A). It is interesting that the air annealing leads to the incorporation of Li and O into CZTSSe in the form of Li-O pairs, which improves the crystal quality and absorption coefficient (α) of CZTSSe-Li-A and decreases its hole concentration and resistivity compared with Li-doped CZTSSe prepared using Li-doped CZTS that is not annealed in air (CZTSSe-Li). By optimizing Li doping concentration, annealing temperature and time, the highest PCE of CZTSSe-Li-A solar cell without MgF anti-reflection layer reaches 11.13%, which is larger than the highest PCE of CZTSSe-Li solar cell (10.21%). Quantitative analysis indicates that the increased PCE of CZTSSe-Li-A solar cell compared to CZTSSe-Li solar cell is mainly attributed to the decrease in reverse saturation current density (J0) and series resistance (Rs) caused by thermal effect of the air annealing, followed by an increase in photogeneration density (JL) and shunt resistance (Rsh). The decreased J0 is due to a reduction in interfacial defect density, increased JL is due to increase in α and depletion region width, and decreased Rs and increased Rsh are due to improve crystal quality. This work offers a route for the improvement of PCE of CZTSSe solar cells.