Achieving Low V_OC-deficit Characteristics in Cu₂ZnSn(S,Se)₄ Solar Cells through Improved Carrier Separation

Kesterite-based thin-film solar cells (TFSCs) have recently gained significant attention in the photovoltaic (PV) sector for their elemental earth abundance and low toxicity. An inclusive study from the past reveals basic knowledge about the grain boundary (GB) and grain interior (GI) interface. However, the compositional dependency of the surface potential within GBs and GIs remains unclear. The present work provides insights into the surface potential of the bulk and GB interfaces. The tin (Sn) composition is sensitive to the absorber morphology, and therefore, it significantly impacts absorber and device properties. The absorber morphology improves with the formation of larger grains as the Sn content increases. Additionally, the presence of Sn(S,Se)2 and increased [ZnCu + VCu] A-type defect cluster density are observed, validated through Raman analysis. The secondary ion mass spectroscopy analysis reveals the altered distribution of sulfur (S) and sodium (Na) with higher near-surface accumulation. The synergistic outcome of the increased density of defects and the accumulation of S near the interface provides a larger GB and GI difference and expedites carrier separation improvement. Consequently, at an optimum compositional ratio of Cu/(Zn+Sn) = ∼0.6, the power conversion efficiency (PCE) is significantly improved from 6.42 to 11.04% with a record open-circuit voltage (VOC) deficit of 537 mV.