Nano-scale sulfurization of the Cu2ZnSnSe4 crystal surface for photovoltaic applications

Abstract

The objective of this study was to find an effective method to improve V_OC without J_SC loss for Cu₂ZnSnSe₄ (CZTSe) monograin layer solar cells. Sulfurization of the surface of the kesterite absorber layer may lead to enhanced device efficiency via band gap widening at the surface. Surface sulfurization was carried out in two steps: a CdS layer was first deposited onto the CZTSe crystals by a chemical solution deposition method, and then the CdS-coated CZTSe was annealed at elevated temperature in evacuated quartz ampoules. The thickness of the sulfurized surface of CZTSe crystals was varied by adjusting the thickness of the CdS layer (from 100 nm to 200 nm) and by modifying the temperature of the annealing process from 400 to 700 °C for 60 min. SEM, EDX and Raman analysis showed that the CdS layer still existed on the surface of CZTSe crystals after annealing at 400 °C. After annealing at higher temperatures, the CdS layer disappeared and a new surface layer was formed. Annealing at 570 °C resulted in secondary phases on the surface, which are probably caused by decomposition reactions on the CZTSe crystal surface. Annealing at 700 °C formed a well-crystallized Cu₂ZnSn(S, Se)₄ thin layer on the surface of the CZTSe crystals, which was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The Raman peak located at 331 cm⁻¹ provides strong evidence that a wider band gap Cu₂ZnSn(S, Se)₄ surface layer is formed after the sulfurization while the CdS peak at 308 cm⁻¹ has disappeared. Compositional profiles of EDX and XPS showed that S is located in the surface layer, but Cd has diffused into the bulk of the crystal and acts as a dopant. The content of S in Cu₂ZnSn(S, Se)₄ depended on the CdS layer thickness.