Junction formation of Cu ₃BiS ₃ investigated by Kelvin probe force microscopy and surface photovoltage measurements

Recently, the compound semiconductor Cu ₃BiS ₃ has been demonstrated to have a band gap of ~1.4 eV, well suited for photovoltaic energy harvesting. The preparation of polycrystalline thin films was successfully realized and now the junction formation to the n-type window needs to be developed. We present an investigation of the Cu ₃BiS ₃ absorber layer and the junction formation with CdS, ZnS and In ₂S ₃ buffer layers. Kelvin probe force microscopy shows the granular structure of the buffer layers with small grains of 20-100 nm, and a considerably smaller work-function distribution for In ₂S ₃ compared to that of CdS and ZnS. For In ₂S ₃ and CdS buffer layers the KPFM experiments indicate negatively charged Cu ₃BiS ₃ grain boundaries resulting from the deposition of the buffer layer. Macroscopic measurements of the surface photovoltage at variable excitation wavelength indicate the influence of defect states below the band gap on charge separation and a surface-defect passivation by the In ₂S ₃ buffer layer. Our findings indicate that Cu ₃BiS ₃ may become an interesting absorber material for thin-film solar cells; however, for photovoltaic application the band bending at the charge-selective contact has to be increased.