Macroscopic and microscopic electrical properties of Cu(In,Ga)Se₂ thin-film solar cells with various Ga/(In + Ga) contents

CuIn_1-xGa_xSe₂ (CIGS) thin-films were deposited by a three-stage co-evaporation process. We obtained an optimum value for the Ga/(In + Ga) ratio of CIGS solar cells of 0.29, which exhibits a band-gap of 1.14 eV and has the highest conversion efficiency. The Ga/(In + Ga) ratio in CIGS solar cells is one of main characteristics that can improve efficiency, but the optimum value is still uncertain. In this study, we investigated the local electrical properties, which are closely related to the device properties, of CIGS according to the Ga/(In + Ga) ratio. We measured the local current of the films using conductive atomic force microscopy. The local current indicates relatively small values for the current ratio and the average current on the film surface, which has a high shunt resistance and a low series resistance in high-efficiency CIGS thin-films. However, low efficiency CIGS exhibits the opposite electrical behavior. Thus, the macroscopic and microscopic electrical behaviors are closely correlated with the conversion efficiency and with the device factors of CIGS thin-film solar cells with a varying Ga/(In + Ga) ratio. These results suggest that the control of carrier transport over the grains will improve the conversion efficiency of CIGS thin-film solar cells.