Photovoltage Decay Measurements by Photo-Assisted Kelvin Probe Force Microscopy on Cu(In,Ga)Se2 Solar Cells

Photovoltage measurements are performed on Cu(In,Ga)Se
₂
(CIGS) solar cells with various Ga composition ratios [Ga/(Ga + In)] using photo-assisted Kelvin probe force microscopy (P-KFM). From the temporally-averaged photovoltages acquired by P-KFM under intermittent light illumination at various frequencies, photovoltage decay phenomena resulting from different carrier recombination processes are investigated. The sample with high Ga composition in CIGS exhibits an elongated time constant for the slow recombination process and an increased contribution ratio of the fast recombination process in the entire photovoltage decay processes. The slow process is attributed to the recombination of spatially separated electrons and holes in the surface n-type layer and CIGS bulk layer, respectively, or to the existence of the electron trap near the CdS/CIGS interface. On the other hand, the fast process is attributed to direct recombination of electrons and holes in the CIGS bulk. The influence of the carrier separation effect on photo-carrier recombination around grain boundaries in the CIGS solar cells is also discussed.