A 9% efficiency of flexible Mo-foil-based Cu2ZnSn(S, Se)4 solar cells by improving CdS buffer layer and heterojunction interface* *Project supported by the National Natural Science Foundation of China (Grant Nos. 62074037, 61574038, 51961165108, and 51972332), the Natural Science Foundation of Fujian Province, China (Grant No. 2017J01503), the Education and Scientific Research Project of Fujian Province, China (Grant No. JAT190010), the

Abstract

Flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cells show great potential applications due to low-cost, nontoxicity, and stability. The device performances under an especial open circuit voltage (V OC) are limited by the defect recombination of CZTSSe/CdS heterojunction interface. We improve the deposition technique to obtain compact CdS layers without any pinholes for flexible CZTSSe solar cells on Mo foils. The efficiency of the device is improved from 5.7% to 6.86% by highquality junction interface. Furthermore, aiming at the S loss of CdS film, the S source concentration in deposition process is investigated to passivate the defects and improve the CdS film quality. The flexible Mo-foil-based CZTSSe solar cells are obtained to possess a 9.05% efficiency with a V OC of 0.44 V at an optimized S source concentration of 0.68 mol/L. Systematic physical measurements indicate that the S source control can effectively suppress the interface recombination and reduce the V OC deficit. For the CZTSSe device bending characteristics, the device efficiency is almost constant after 1000 bends, manifesting that the CZTSSe device has an excellent mechanical flexibility. The effective improvement strategy of CdS deposition is expected to provide a new perspective for promoting the conversion efficiency of CZTSSe solar cells.