Evaluation of junction quality of buffer-free Zn(O,S):Al/Cu(In,Ga)Se2 thin-film solar cells

Cu(In,Ga)Se2 (CIGS) solar cells without buffer layers, namely, buffer-free solar cells, and those with conventional CdS buffer layers were fabricated. Zn(O,S):Al (AZOS) was used as a transparent conductive oxide layer. The AZOS/CIGS interface in the buffer-free solar cells and the AZOS/CdS/CIGS interface in the CdS-buffered solar cells were investigated by bright-field scanning transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and fast Fourier transform. It was found that the open-circuit voltage and fill factor of the buffer-free solar cells are lower than those of the CdS-buffered solar cells, which is attributed to the nonepitaxial growth of AZOS on the CIGS layer, forming defects and deteriorating the junction quality of the buffer-free solar cells.