Revealing the Impact of Na-Ag Single-Site Co-Doping in CZTSSe

Abstract

Cation doping/substitution is a widely used strategy to overcome the serious opencircuit voltage deficit caused by unfavorable defects and defect clusters in kesterite materials such as CZTSSe. Single cation substitution has proved effective in improving PV performance. However, single element doping of CZTSSe has its own limitations because dopants can reduce carrier concentrations and introduce unfavorable lattice distortion. Co-doping offers broader possibilities for regulating the optoelectronic properties of CZTSSe materials but has seldom been explored due to the enhanced complexity in such systems. Herein, a single-site co-doping strategy is proposed by using Na and Ag ions, which both target the same copper lattice site in CZTSSe. The Na + ions passivate Cu-related defects, but the introduction of Ag + reduces the lattice distortion that Na ions create. Conversely, Na ions help to compensate for the loss of carrier concentration caused by Ag + doping. This study demonstrates that the synergistic effects of Na and Ag co-doping can reduce the open-circuit voltage (V OC ) deficit without loss of carrier concentration. After optimization, we achieved a Voc of 559 mV and a fill factor (FF) of 67.3% and an overall power conversion efficiency of 13.2%.