Sintered CdS films with various thicknesses and electronic properties have been prepared by changing the coated thickness and sintering conditions. All-polycrystalline CdS/CdTe solar cells have been fabricated by coating a CdTe slurry on the sintered CdS films and sintering in an attempt to optimize the thickness and sintering conditions of CdS films whose role is to be the window as well as the front contact for the CdS/CdTe solar cell.
When 9 w/o of $CdCl_2$ is used as a sintering aid for the CdS films, the optical transmittance increases with increasing thickness of the CdS films up to 25 ㎛ and then decreases with further increase in the thickness for the CdS films that were sintered at 650℃. However, the optical transmittance increases with the decrease in the thickness for the CdS films that were sintered at 600 ℃ or 560℃. The electrical resistivity of the sintered CdS films after the sintering of CdS/CdTe composites to form n-p junction ranged from 0.8 Ω-cm to $0.2 Ω-cm.
The efficiency of the subsequently fabricated CdS/CdTe solar cells whose window layer (CdS film) is thinner than 25 ㎛ depends mainly on the optical transmittance of the window layer.
For the cells with thick CdS films the efficiency depends not only on the optical transmittance of the window layer but also on the amount of residual $CdCl_2$ in the cdS films which causes the formation of $CdS_{1-x}Te_x$ solid solution at the interface and a deep n-p junction.
Both the solid solution layer and deep n-p junction cause the sharp decrease in the spectral response of short wavelength region resulting in poor solar efficiency. Thus the highest efficiency is observed in a CdS/CdTe solar cell that was fabricated on a 12 ㎛ thick CdS film which showed the highest value of optical transmission and little residual $CdCl_2$.
To prevent the formation of a thick solid solution layer, a set of CdS films with various thicknesses and sintering conditions were heat treated to remove the residual $CdCl_2$. The efficiency of the solar cell fabricated on this heat treated CdS films are higher than those of the cells which were fabricated on the as-sintered CdS films when the thickness of the CdS films was larger than 25㎛. However, for the cells with thin CdS layer the efficiency degraded when the heat treated CdS layer used, due to the occurrence of over-sintering of the CdS films.
It was possible to prevent occurrence of over-sintering by increasing the amount of $CdCl_2$ in the CdS slurry. Thus sintered CdS films with optical transmission of greater than 65% and with electrical resistivity of less than 0.5 Ω-cm could obtained for both 10 ㎛ and 16 ㎛ thick films by adjusting the amount of $CdCl_2$ before sintering. However, the CdS/CdTe solar cells that fabricated on the 10 ㎛ thick CdS films show lower efficiency than those fabricated on the 16 ㎛ thick CdS films because that former cells have lower values of FF due to larger series resistance even though the resistivity was about the same for both the 10 ㎛ and 16 ㎛ CdS films.
The highest efficiency ($n \approx 13%$) was observed in a sintered CdS/CdTe solar cell that was fabricated on the 16 ㎛ thick CdS films which contained 13-15 w/o $CdCl_2$ before sintering and were sintered at 600℃ followed by heat treatment of 15 min at 625℃.
