Defect levels in Cu2ZnSn(SxSe1-x)4 solar cells probed by current-mode deep level transient spectroscopy

Defect levels in kesterite Cu2ZnSn(S,Se)(4) (CZTSSe) solar cells have been investigated by current-mode deep level transient spectroscopy. Experiments were carried out on two CZTSSe cells with photoconversion efficiencies of 4.1% and 7.1% measured under AM 1.5 illumination. The absorber layer of the 4.1% efficiency cell was prepared by annealing evaporated ZnS/Cu/Sn stacked precursor under S/Se vapor, while the absorber of the 7.1% efficiency cell was prepared by co-evaporation of the constituent elements. The 4.1% efficiency CZTSSe cell with a S/(S+Se) ratio of 0.58 exhibited two dominant deep acceptor levels at E-v+0.12 eV, and E-v+0.32 eV identified as Cu-Zn(-/0) and Cu-Sn(2-/-) antisite defects, respectively. The 7.1% efficiency cell with purely Se composition S/(S+Se) 0 showed only one shallow level at E-v+0.03 eV corresponding to Cu-vacancy (V-Cu). Our results revealed that V-Cu is the primary defect center in the high-efficiency kesterite solar cell in contrast to the detrimental Cu-Zn and Cu-Sn antisites found in the low efficiency CZTSSe cells limiting the device performance. (C) 2014 AIP Publishing LLC.