Highly efficient, ultrathin, Cd-free kesterite solar cells in superstrate configuration enabled by band level tuning via Ag incorporation

Kesterite, or Cu₂ZnSn(S,Se)₄ (CZTS) is promising in developing sustainable PV technology due to its earth-abundant, non-toxic composition. However, issues including instability of interface, high density of defects that fails to allow the short charge-collection length to meet its light absorption needs, use of Cd that fails to comply with the restriction of hazardous substances (RoSH), are promoting the development of alternative, eco-friendly device structure. Here, this study reports an important progress on this subject by adopting the superstrate configuration to kesterite, thus to realize advantageous light management and high defect tolerance in an ultrathin device. By incorporating Ag in kesterite to overcome the detrimental alignment at the pristine interface, a solar cell with PCE of 8.1% has been fabricated with ~ 200 nm absorber and ~ 15 nm TiO₂ buffer, representing a PCE improvement of nearly three-fold from the baseline Cu₂ZnSn(S,Se)₄ device and breaking the 5% PCE limit for superstrate kesterite cells to date. Moreover, this enables the sole use of TiO₂ as novel buffer material free of toxic Cd. Further analysis reveals the critical role of Ag in synergistically tailoring band offset and bandgap, along with largely reduced density of defects, leading to this substantial performance improvement.