Nanoscale Electronic Characterization of CBD-CdS/CZTSSe Interfaces: Surface Potential, Defects, and Carrier Separation

Abstract

Cu₂ZnSn(S,Se)₄ (CZTSSe) is a promising earth-abundant absorber for next-generation thin-film photovoltaics; however, interface-related defects and electronic inhomogeneity at the CdS/CZTSSe junction can significantly limit carrier collection and device performance. In this work, the nanoscale electronic properties of chemically bath-deposited (CBD) CdS/CZTSSe heterojunctions are investigated with particular emphasis on surface-potential variations, defect-associated electronic states, and interfacial carrier separation. CZTSSe absorber layers are prepared and subsequently coated with CBD-CdS to form a heterojunction, followed by comprehensive structural, morphological, optical, and electrical characterization. Kelvin probe force microscopy (KPFM) is employed to spatially resolve the surface-potential distribution across individual grains, grain boundaries, and the CdS/CZTSSe interface under dark and illuminated conditions. The resulting nanoscale potential maps provide insights into local band bending, charge redistribution, and photovoltage generation, while variations in contact potential difference are correlated with defect-rich regions and grain-boundary characteristics. Complementary spectroscopic and optoelectronic measurements are used to establish the relationship between interfacial chemistry, defect states, and charge-transfer behavior. Particular attention is given to identifying potential recombination-active regions that contribute to local potential fluctuations and reduced carrier extraction. The study establishes a direct nanoscale correlation between microstructural features and electronic behavior at the CBD-CdS/CZTSSe interface. These findings provide a deeper understanding of interfacial carrier dynamics and demonstrate the potential of KPFM-assisted analysis for optimizing CdS deposition and defect passivation strategies in CZTSSe-based thin-film solar cells. The approach offers a pathway toward rational interface engineering for improved charge separation, reduced recombination, and enhanced photovoltaic performance.

Keywords: CdS buffer layer; CZTSSe heterojunction; Surface Potential