Revealing performance limiting factors in Cu2BaSnS4 thin film solar cells

Abstract

Cation disorder in Cu2ZnSnS4 (CZTS) thin films has been a critical issue in realizing high efficiency solar cells. Recently, Ba substitution in CZTS that yields Cu2BaSnS4 (CBTS) has been suggested to improve the cation ordering and hence, the device performance. Although CBTS solar cells have shown very promising results in a short span of time, their performance is still inferior to that of the CZTS devices. In this work, the performance limiting factors in the CBTS solar cells have been systematically investigated and a pathway to improve it beyond the reported maximum efficiency has been suggested. In particular, the decisive role of defect densities at back and top interfaces (i.e., MoS2/CBTS and CBTS/CdS) and in the bulk CBTS, coefficients of various recombinations, and thickness of layers has been established. The effect of bulk defect density was found to be more profound compared to that due to the defects at the interfaces. Considerable improvement in efficiency was achieved by optimizing coefficients of Auger and radiative recombinations, carrier density of CBTS and CdS layers and facilitating current transport by tailoring the band alignment at the CBTS/CdS junction. The proposed recipe yielded the highest efficiency of ∼11%, which is significantly higher than that of the current experimental champion cell (∼6.2%).