Influence of Cd0.6Zn0.4S buffer layer on the band alignment and the performance of CZTS thin film solar cells

Abstract

The CdS film is one of the best candidates for the n-type buffer layer in Cu 2 ZnSnS 4 (CZTS) solar cells. However, the inappropriate band alignment at CdS/CZTS heterojunction result in the high barrier for the photogenerated carriers, which decrease the open-circuit voltage and the short-circuit current. In this paper, the crystallization and the optical bandgap of the Zn-doping Cd 0.6 Zn 0.4 S film fabricated by chemical bath deposition were studied. XRD showed that the lattice constant of hexagonal Cd 0.6 Zn 0.4 S decreased a little after Zn doping because of the shorter bond length of Zn–S. Photoluminescence and transmittance spectra showed the band gap of Cd 0.6 Zn 0.4 S thin film increased from 2.59 to 2.88 eV due to Burstein-Moss shift. The conduction band offset (CBO) obviously decreased from 0.51 eV in CZTS/CdS heterojunction to 0.27 eV in CZTS/Cd 0.6 Zn 0.4 S heterojunction which result in the short-circuit current increasing from 12.25 to 15.05 mA/cm 2 . The conversion efficiency (E ff ) and fill factor (FF) of the solar cell based on CZTS/Cd 0.6 Zn 0.4 S were increased from 2.31% to 31.06 to 4.88% and 47.45, comparing to CZTS/CdS solar cell. • The effects of the Zn-doping Cd 0.6 Zn 0.4 S film fabricate of Cu 2 ZnSnS 4 thin film solar cells were studied in detail. • To our knowledge, the (CBO) decreased from 0.51 eV in CZTS/CdS heterojunction to 0.27 eV in CZTS/Cd 0.6 Zn 0.4 S heterojunction. • The best Eff. of the fabricated CZTS/Cd 0.6 Zn 0.4 S thin film solar cell is 4.88% with the V oc of 628 mV, J sc of 15.05 mA/cm 2 .