The CdS layers for CdS/CdTe heterojunction solar cell application have been prepared by screen printing and sintering. The slurry for screen printing is a mixture of CdS powder, $CdCl_2$ powder and propylene grycol.
Electrical properties of sintered CdS layers have been investigated by the measurements of electrical resistance, Hall effect and temperature dependence of resistance.
The Microstructures and electrical resistivity of sintered CdS layers were closely related to the preparation condition, such as the amount of $CdCl_2$ added to the slurry, the heating rate and the sintering temperature and time.
The grain size of CdS increased with the larger $CdCl_2$ addition, the higher sintering temperature and the longer sintering time. And the CdS grains grew to a maximum size fo about 15 ㎛ m$ in the specimen with 10wt.% $CdCl_2$ initially, sintered at 650℃ for 30 min. It was found that the more $CdCl_2$ was added to the slurry, the more Cl was left in the sintered CdS layers. These phenomena were explained on the basis of enhanced sintering and evaporation of the initially added $CdCl_2$.
From Hall effect and resistivity of the sintered layers, measured at room temperature, it was observed that the carrier concentrations of the layers increased as sintering temperature and the amount of added $CdCl_2$ (up to 6 wt.%) increased, and that the carrier mobility of the layers increased up to a certain range of $CdCl_2$ added and sintering temperature and time. These results were discussed in terms of donor dopings for CdS grains and potential barriers due to the grain boundary states.
In the sintered layers obtained by this experiment, photosensitivity was weak because of high carrier concentrations of $~10^{17}cm^{-3}$ and low potential barriers.
The sintered CdS layers were annealed in $H_2$, $N_2$ and Ar. The layers of high resistivity (about 10Ω·cm) showed the sharp drop in resistivity in Ar, $H_2$ (more shap in $H_2$) but those of low resistivity (about $10^{-1} Ω·cm) showed a decrease only in $H_2$ atmosphere. These were explained in terms of evaporation of $CdCl_2$ left at grain boundaries and the increased donor densities due to $Cd^{ex}$.
