Beschreibung:
A Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorber layer was formed with various atomic compositions of Cu/(Zn + Sn) to investigate the impact of Cu ratio on the p–n heterojunction under the Zn-rich, Cu-poor condition. The sputtering time of Cu increased from 2300 to 2900 sec, and the atomic ratio of Cu/(Zn + Sn) also increased from 0.69 to 0.91. The highest built-in potential (1.0 eV) was measured in the optimized condition of Cu 2700 sec ((Cu/(Zn+Sn)=0.86) through capacitance–voltage (CV) measurement due to the optimized interface between the absorber layer and the CdS buffer layer. Also, the highest photoconversion efficiency of 9.4% was achieved in the Cu 2700 sec condition. On the other hands, disk-shaped secondary phases were formed on the surface of the absorber layer in the SEM image in the case of Cu 2300 sec (Cu/(Zn + Sn) = 0.69). The disk-shaped secondary phases were analyzed as SnSSe on the surface of the absorber layer by energy-dispersive X-ray analysis (EDX). A cross-sectional transmission electron microscope (TEM) was used to observe the internal structure of the thin-film solar cells. In the EDX mapping of TEM images, the metallic precursors showed uniform dispersion when the Cu content was sufficient; when the Cu content was insufficient, the distribution of metal elements was unstable and separated from each other. Cd interdiffusion occurred more actively when the Cu content was sufficient to form a stable absorber layer because of uniform Zn dispersion. In the case of insufficient Cu content, the solar cells showed lower and unstable performances; when the Cu content was sufficient, performance and stability improvements were observed