Impact of vapor phase Ge incorporation and reaction pathways on the performance and stability of Cu2Zn(Ge,Sn)Se4 solar cells

Abstract

In this study, we investigate how variations in precursor composition and partial vapor-assisted substitution of Sn by Ge affect the material quality and device performance of CZTGSe solar cells. Precursor films with different Cu–Sn alloy ratios were selenized under controlled SnSe_2−x and GeSe_2−x vapor conditions, enabling the synthesis of Cu₂ZnSn_1−xGe_xSe₄ absorber layers with Ge/(Ge + Sn) ratios ranging from 0 to 36%. Despite the initial variations, the final absorber compositions converged due to in-process shifts driven by Sn and/or Ge incorporation and loss during the reaction. Beyond the established role of the absorber composition, we show that the reaction pathway, determined by the precursor composition, strongly influences the defect concentration, material quality, and device performance. A Cu-rich and Sn-poor (Cu/Sn > 2) starting composition that evolves into a Cu-poor Sn-rich kesterite absorber is found to be particularly beneficial. Furthermore, prolonged post deposition treatment revealed distinct stability trends: Ge-free devices degraded steadily, whereas Ge-containing devices continued to improve over ∼430 hours. We attribute this to stabilization of [V_Cu + Zn_Cu] defect clusters and re-distribution of Ge atoms. The best-performing device, with Ge/(Ge + Sn) = 9% achieved 12.5% efficiency with enhanced long-term stability, underscoring the potential of vapor-phase Ge integration for advancing physically synthesized kesterite solar cells.