Spectral mapping of open circuit voltage of Cu(In,Ga)Se₂ thin-film solar cells via surface photo-voltage and its implications for hole carrier transport

Due to its suitable physical properties, Cu(In,Ga)Se₂ (CIGS) thin-film solar cells are commercialized and expanding its application fields. Thus, the needs for bendable CIGS solar cells are also rising. Up to date, flexible CIGS solar cells have reached power conversion efficiency (PCE) over 20%, but their performance is still limited for low open-circuit voltage. It is related to electron-hole (e-h) carrier transport. Therefore, we need to comprehend carrier behavior in the solar cells and their band structure by analyzing surface photovoltage (SPV) of CIGS thin-films on flexible substrates. By 3-step process and NaF post-deposition treatment we obtained 4 different PCE of 0, 6, 12, and 17%, altering substrate temperatures. Confocal micro-Raman spectroscopy provides phase distribution inside of the materials and the surface. Kelvin probe force microscopy (KPFM) displayed upward surface potential barrier near grain boundaries, helping e-h separation. SPV results was obtained by photo-assisted KPFM under three different wavelength lasers (640, 532, and 405 nm) with different skin depths. These optoelectrical properties from confocal micro-Raman and SPV variation induced by the phase difference can elucidate how non-uniformity affects carrier transport in the solar cell materials.