Effect of the Impurity Incorporation on the Performance of Cu(In,Ga)Se2 Semiconductor Solar Cells

Chalcopyrite Cu(In,Ga)Se-2 (CIGS) semiconductor films are important as next generation solar cell materials and have great potential for use in device applications, especially for fabricating high-efficiency solar cells with practically no degradation of the absorber layer. In order to understand the physical properties of the CIGS films, the CIGS absorber layers have been characterized by many research groups so that the solar cell efficiencies could be further improved and theoretical efficiencies could be approached as far as practicable. To improve the efficiency of CIGS solar cells, a quantitative and depth-resolved elemental analysis of photovoltaic thin films is strongly required. In this work, using a three-stage evaporation process, different CIGS thin films were prepared on molybdenum back contacts deposited on soda-lime glass substrates. Surface analyses via XPS and SIMS were performed to characterize the CIGS thin films and compare their compositional distribution according to the depth. The average concentrations of the matrix elements, Cu, In, Ga, and Se were compared with the quantitative results of ICP-AES and EPMA. To identify the impurities in the CIGS layer, distributions of trace elements were also observed, according to the depth, by SIMS. Using SEM and TEM, we carried out detailed microstructural studies in cross-sections of CIGS thin film solar cells that had different cell efficiencies. The elemental composition, impurity distribution, and microstructure of the CIGS thin film solar cells were investigated to understand their effects on the solar cell conversion efficiency.