Engineering Cu₂ZnSnS₄grain boundaries for enhanced photovoltage generation at the Cu₂ZnSnS₄/TiO₂heterojunction: A nanoscale investigation using Kelvin probe force microscopy

Over the past several years, kesterite Cu2ZnSnS4 (CZTS) absorber has been investigated comprehensively; however, the performance is still hampered by a large open-circuit voltage deficit associated with CZTS bulk defects and interface recombination. To overcome this trend, we report a facile approach to passivate both defect prone areas, i.e., bulk of CZTS and CZTS interface with a TiO2 buffer layer, simultaneously. The existence of oxygen ambient during TiO2 deposition has modulated the electrical properties of CZTS grain boundaries (GBs) not only inside the bulk but also at the surface of CZTS. The passivation of surface GBs is favorable for CZTS/TiO2 heterojunction electronic properties, whereas passivated bulk GBs improve the carrier transport inside the CZTS absorber. To directly probe the photovoltage generation at the CZTS/TiO2 heterojunction, Kelvin probe force microscopy is conducted in surface and junction modes. The acquired photovoltage map exhibits higher values at the GBs, which reveals an increment in downward band bending after oxygen diffusion inside the bulk of CZTS. In point of fact, the enhanced diffusion of oxygen accounts for the suppression of carrier recombination and reduction in dark current. Finally, current-voltage and capacitance-voltage measurements performed on the CZTS/TiO2 heterojunction further validate our outcomes. Our findings provide critical insight into the engineering of CZTS GBs to control electronic properties of CZTS and CZTS/TiO2 heterojunctions.