Growth of a High-quality Zn(S,O,OH) thin film via chemical bath deposition for Cd‐free Cu(In,Ga)Se2 solar cells

This study focused on the characterization and optimization of a Zn(S,O,OH) thin film via chemical bath deposition (CBD) on a Cu(In,Ga)Se2 (CIGS) film in order to obtain a reproducible and high-quality Cd-free buffer layer. High-resolution images of the actual film growth during the CBD process were observed to deeply understand the growth conditions for a uniform and pinhole-free Zn(S,O,OH) film on the CIGS film. The homogeneous precipitation of Zn(S,O,OH) was suppressed through the complexation reaction in the solution, and the heterogeneous precipitation of Zn(S,O,OH) was activated on the CIGS film by increasing the NH3 concentration. At NH3 concentration of 7 M, the Zn(S,O,OH) film grown on the CIGS film completely covered the CIGS surface via the heterogeneous precipitation of Zn(S,O,OH). However, it contained many pinholes. The pinholes were eliminated by suppressing the formation of the Zn(OH)2 precipitates at a lower temperature (73 °C). At the optimized conditions of the CBD process, the buffer film grown on the CIGS film contained a large amount of oxygen in the form of Zn-O and Zn-OH bonds. By annealing the Zn(S,O,OH) film, the content of the Zn-OH bond decreased through dehydration and that of the Zn-O bond increased. The JSC of the cell performance was greatly improved by annealing the Zn(S,O,OH) film and a conversion efficiency of 14.2% with VOC=0.62 V, JSC=35.1 mA/cm2, and FF= 65.1 was achieved.