Polycrystalline $Cd_{1-x}Zn_xS$ films were prepared by coating (1-x)CdS + xZnS slurry on amorphous glass substrates and sintering in nitrogen or nitrogen atmosphere containing $ZnCl_2$ vapor to produce films with properties suitable for fabricating all-polycrytalline $Cd_{1-x}Zn_xS$/CdTe heterojunction solar cells. The microstructure and optical properties of these films has been correlated with composition, $CdCl_2$ which was added as a sintering aid, sintering temperature and atmosphere. By optimizing the preparation conditions, it is possible to produce sintered $Cd_{1-x}Zn_xS$ films on glass substrate with 15% higher optical transmission than CdS films.
All-polycrystalline CdS/CdTe solar cells have been fabricated by coating CdTe slurries, which contained various amounts of $CdCl_2$, on the sintered CdS films and sintering for 1hr at 625 ℃ in nitrogen. The amount of $CdCl_2$ was varied from 1 wt% to 10 wt%. The presence of $CdCl_2$ in the CdTe layer before sintering improves the microstructures of the sintered CdTe layer and the junction. Both of these effects lead to improved solar cell efficiency. When the amount of $CdCl_2$ is large, however, it causes the reduction of the hole concentration of the CdTe layer and the formation of a thick layer of $CdS_{1-y}Te_y$ at the compositional interface that degrade the efficiency of the solar cell. A solar efficiency of 11.6% under 87 mW/㎠ of solar illumination was observed in a CdS/CdTe solar cell that was fabricated by coating a CdTe slurry, which contained 4.5 wt% $CdCl_2$, on the sintered CdS film and by sintering the CdS-CdTe composite at 625 ℃ in nitrogen.
Sintered $Cd_{1-x}Zn_xS$/CdTe solar cells have been also fabricated by coating CdTe slurries containing 1 or 4.5 wt% of $CdCl_2$, on sintered $Cd_{1-x}Zn_xS$ films of various Zn content x, sintering for 1 hr in nitrogen at various temperatures. When the amount of $CdCl_2$ in CdTe before sintering was 1 wt%, maximum efficiency was found in a solar cell that made by sintering the composite layer of glass-$Cd_{1-x}Zn_xS-CdTe$ at 650 ℃ for 1 hr. Junction formation temperatures below 625 ℃ resulted in a poor microstructures of compositional interface which increases the leakage current, causing the reduction of the open circuit voltage and fill factor. When the junction was formed above 650 ℃, however, the electrical resistance of the $Cd_{1-x}Zn_xS$ window increases and severe interdiffusion of acceptors and donors across the junction interface takes place, resulting in the reduction of the fill factor and short circuit current. Solar efficiency of the sintered $Cd_{1-x}Zn_xS$/CdTe solar cell increases with increasing Zn content x up to 0.05, and then decreases with further increase in the Zn content when the junctions were formed at 650℃. The improvement of the solar efficiency is related to the improved optical transmission and the degradation is caused by the increase in the electrical resistance of the window layer and poor microstructure of the compositional interface.
When the amount of $CdCl_2$ in CdTe before the junction formation process was 4.5 wt%, it leads to a formation of a thick $Cd_{1-x}Zn_xS_{1-y}Te_y$ layer at the compositional interface for the $Cd_{1-x}Zn_xS$/CdTe solar cells, particularly with high Zn content in the window layer, sintered at low temperature. Thus the short circuit current and solar cell efficiency decrease with increasing the Zn content in the window, although the value of the optical transmission of the $Cd_{1-x}Zn_xS$ window was higher than that of CdS.
