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 Cu2ZnSn(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 Cu2ZnSn(S,Se)(4) or Cu2CdSnS4, but the Ag Zn acceptor has only a low concentration and the dominant defects are donors in Ag2ZnSnS4. 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 Cu2ZnSn(S,Se)(4) with Ag2ZnSn(S,Se)(4), and the composition-graded (Cu,Ag)(2) ZnSn(S,Se)(4) alloys should be ideal light-absorber materials for achieving higher effi ciency kesterite solar cells.