Device Postannealing with Superficially Ag-Modified Absorber for High-Efficiency Cd-Free Cu₂ZnSnS₄ Solar Cells

Cation substitution is a promising research direction for further improving the device performance of Cu₂ZnSnS₄ (CZTS) solar cells by mitigating recombination loss at deep-level defects. Here, a superficial Ag₂ZnSnS₄ (AZTS) modification process on presulfurized cosputtered CZTS precursor is employed for fabricating a CZTS absorber film of layer-spanning grains with reduced interfacial Cu_Zn tail states of deeper level and increased minority carrier lifetime. Results show that the open-circuit voltage and short-circuit current density of the CZTS device are increased after the modification, though the FF is decreased due to a reduced carrier density of CZTS and a degraded junction electric field. With a postdevice air annealing process, the poor junction quality of the AZTS-modified CZTS can be recovered without introduction of extra interfacial deep-level defects. An encouraging efficiency of 10.50% with a remarkably high short-circuit current density of 22.9 mA cm^-2 is achieved with antireflecting coating. This study demonstrates a unique method for reducing interfacial and bulk recombination loss while improving the junction quality of CZTS, which can help in designing strategies for improving the V _OC and FF of CZTS.