The development of kesterite Cu2ZnSn(S,Se)4 thin-film solar cells is currently hindered by the large deficit of open-circuit voltage (Voc), which results from the easy formation of CuZn antisite acceptor defects. Suppressing the formation of CuZn 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 CuZn or CuCd acceptor defects have high concentration and are the dominant defects in Cu2ZnSn(S,Se)4 or Cu2CdSnS4, but the AgZn 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 Voc 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)2ZnSn(S,Se)4 alloys should be ideal light-absorber materials for achieving higher efficiency kesterite solar cells. A new strategy is proposed to overcome the Voc bottleneck and increase the efficiency of the kesterite solar cells. This is achieved by forming composition-graded (Cu1-xAgx)2ZnSn(S,Se)4 alloys as the absorber layer.
Bibliographical noteFunding Information:
The work at Fudan was supported by the Special Funds for Major State Basic Research, National Natural Science Foundation of China (NSFC), International collaboration project, Program for Professor of Special Appointment (Eastern Scholar), and Fok Ying Tung Education Foundation. S.C. is supported by NSFC under Grant No. 91233121 and Shanghai Rising-Star Program (14QA1401500). The work at Bath is supported by the Royal Society and the EPSRC (Grant nos. EP/K016288/1 and EP/L017792/1). The work at NREL was funded by the U.S. Department of Energy (DOE), under Contract No. DE-AC36-08GO28308.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
- electrical conductivity
- kesterite semiconductors
- thin-film solar cells