Large Voc improvement and 9.2% efficient pure sulfide Cu2ZnSnS4 solar cells by heterojunction interface engineering

Voltage deficit is the major challenge that pure sulfide kesterite devices have to face to further improve the efficiency. Different from selenide kesterite, pure sulfide kesterite Cu2ZnSnS4 has been reported to form an unfavorable “cliff"-like conduction band offset (CBO) with traditional CdS buffer. This cliff-like CBO would facilitate significant heterojunction interface recombination and result in high interface recombination velocity in the presence of high interface defect density, leading to the voltage loss. If engineering the CBO to the ideal spike-like (within the optimal range of 0.1-0.4eV) and decreasing the interface defects density, it is expected that the voltage deficit can be greatly decreased according to our device simulation. Following this way, in this work, we experimentally demonstrated high efficiency (highest 9.24%) and large Voc improvement (highest of 762 mV) in pure sulfide Cu2ZnSnS4 thin film solar cells. The mechanism underlying the Voc and efficiency improvement through the heterojunction interface band alignment and post-heat treatment engineering is investigated. The use of a new Zn, Cd)(O, S) buffer with appropriate Zn/Cd ratios is found effective to tune the conduction band offset more favorable. Post-annealing treatment result in great difference in the microstructure of CZTS/buffer interface and thereby crystalline defects. This work provides an effective way to reduce the Voc deficit of pure sulfide Cu2ZnSnS4 solar cells.