Novel two-step CdS deposition strategy to improve the performance of Cu2ZnSn(S,Se)4 solar cell

Abstract

Abstract Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have drawn worldwide attention for their promising photovoltaics performance and earth-abundant element composition, yet the record efficiency of this type of device is still far lower than its theoretical conversion efficiency. Undesirable band alignment and severe non-radiative recombination at CZTSSe/CdS heterojunction interfaces are the major causes limiting the current/voltage output and overall device performance. Herein, we propose a novel two-step CdS deposition strategy to improve the quality of CZTSSe/CdS heterojunction interface and thereby improve the performance of CZTSSe solar cell. The two-step strategy includes firstly pre-deposits CdS thin layer on CZTSSe absorber layer by chemical bath deposition (CBD), followed with a mild heat treatment to facilitate element inter-diffusion, and secondly deposits an appropriate thickness of CdS layer by CBD to cover the whole surface of pre-deposited CdS and CZTSSe layers. The solar energy conversion efficiency of CZTSSe solar cells with two-step deposited CdS layer approaches to 8.76% (with an active area of about 0.19 cm2), which shows an encouraging improvement of over 87.98% or 30.16% compared to the devices with traditional CBD-deposited CdS layer without and with the mild annealing process, respectively. The performance enhancement by the two-step CdS deposition is attributed to the formation of more favorable band alignment at CZTSSe/CdS interface as well as the effective decrease in interfacial recombination paths on the basis of material and device characterizations. The two-step CdS deposition strategy is simple but effective, and should have large room to improve the quality of CZTSSe/CdS heterojunction interface and further lift up the conversion efficiency of CZTSSe solar cells.