Revealing the Role of Potassium Treatment in CZTSSe Thin Film Solar Cells

Potassium (K) post-treatment on CIGSSe has been shown to yield the highest efficiency reported to date. However, very little is known on the effect of K doping in CZTSSe and the mechanism behind the efficiency improvement. Here we reveal the mechanism by which K enhances the charge separation in CZTSSe. We show that K accumulates at the CdS/CZTSSe, passivating the recombination at the front interface and improving carrier collection. K is also found to accumulate at the CZTSSe/Mo interface and facilitates the diffusion of Cd into the absorber which affects the morphology and grain growth of CZTSSe. As revealed by the C-V, external quantum efficiency, and color J-V test, K doping significantly increases the carrier density, improves carrier collection, and passivates the front interface and grain boundaries, leading to the enhancement of V_oc and J_sc. The average power conversion efficiency has been promoted from 5% to above 7%, and the best 7.78% efficiency has been achieved for the 1.5 mol % K-doped CZTSSe device. This work offers some new insights into the K doping effects on CZTSSe via solution-based approach and demonstrates the potential of facile control of K doping for further improvement of CZTSSe thin film solar cells.