Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu‐ and Ag‐Based Kesterite Compounds

The development of kesterite Cu 2 ZnSn(S,Se) 4 thin‐film solar cells is currently hindered by the large deficit of open‐circuit voltage ( V oc ), which results from the easy formation of Cu Zn antisite acceptor defects. Suppressing the formation of Cu Zn defects, especially near the absorber/buffer interface, is thus critical for the further improvement of kesterite solar cells. In this paper, it is shown that there is a large disparity between the defects in Cu‐ and Ag‐based kesterite semiconductors, i.e., the Cu Zn or Cu Cd acceptor defects have high concentration and are the dominant defects in Cu 2 ZnSn(S,Se) 4 or Cu 2 CdSnS 4 , but the Ag Zn acceptor has only a low concentration and the dominant defects are donors in Ag 2 ZnSnS 4 . Therefore, the Cu‐based kesterites always show p‐type conductivity, while the Ag‐based kesterites show either intrinsic or weak n‐type conductivity. Based on this defect disparity and calculated band alignment, it is proposed that the V oc limit of the kesterite solar cells can be overcome by alloying Cu 2 ZnSn(S,Se) 4 with Ag 2 ZnSn(S,Se) 4 , and the composition‐graded (Cu,Ag) 2 ZnSn(S,Se) 4 alloys should be ideal light‐absorber materials for achieving higher efficiency kesterite solar cells.