Effects of High Temperature Annealing on Sputtered Zn(O,S) Films for CdTe Solar Cells

Zn(O,S) is extensively employed as a promising buffer layer to replace CdS in chalcogenide and kesterite based photovoltaics, while its studies related to CdTe based solar cells are still scarce. Herein, the impacts of annealing (400 degrees C-600 degrees C) on sputtered Zn(O,S) were investigated and the performance of Zn(O-0.85,S-0.15)/CdTe solar cells were evaluated by both experimental and simulation methods. An interval state was found for sputtered Zn(O,S) to achieve low resistivity. After annealing, the carrier concentration of Zn(O-0.85,S-0.15) film was increased by up to four orders of magnitude, whereas no such increase was found for films with higher or lower S content. The elevated oxygen vacancy concentration was demonstrated to account for the increase in carrier concentration. Low-resistivity Zn(O-0.85,S-0.15) films were integrated into CdTe solar cells and the efficiency of Zn(O-0.85,S-0.15)/ CdTe devices was increased from 8.5% to 11.2% mainly attributed to the enhancements of open-circuit voltage after annealing. The typical photon absorption at wavelengths below 510 nm caused by CdS was eliminated in Zn (O-0.85,S-0.15)/CdTe devices. Combining device analysis and simulation results, the enhancement of open-circuit voltage was determined to originate from the increase in carrier concentration. These findings provide a use-ful knowledge for preparing low resistivity and wide band gap Zn(O,S) films for CdTe devices.