Modification of back interfacial contact with MoO3 layer in situ introduced by Na2S aqueous solution for efficient kesterite CZTSSe solar cells

As a promising absorber material for thin film solar cells, the highest reported power conversion efficiency (PCE) of Cu2ZnSn(S,Se)(4) (CZTSSe) is still far from the detailed balance limit of efficiency due to the large V-OC deficit. Back contact engineering plays an important role in promoting the CZTSSe solar cell towards realizing high PCE. In this study, a MoO3-modified layer with the highest oxidation state 6+ and better thermal stability was in situ introduced by spin-coating Na2S aqueous solution on the Mo electrode. The results demonstrate that the Na2S layer at the rear contact provides more Na to diffuse into the absorber, thus improving the crystallinity of the film and the quality of the p-n heterojunction, and the carrier collection in the neutral region and at the front interface of the devices is enhanced. The formation of the MoO3 layer can also inhibit the decomposition reaction between Mo and CZTSSe, which minimizes the secondary phases and voids, and improves the quality of the back interface. Moreover, the band alignment and interfacial barrier at the back contact were modified by the MoO3 layer, resulting in better carrier collection and the reduction of photogenerated carrier recombination. By optimizing the Na2S (MoO3) layer, the average PCE of the CZTSSe solar cell increased to 12.48% from 10.22% with the large V-OC increments of 60 mV and FF increments of 5%, and the best performance was achieved, with an active area efficiency of 12.74% without the anti-reflection coating layer.