Doping effects of dimethyl-tin-dichloride on material properties of CdS films and on formation of CdS∕CdTe heterostructures

A series of systematic investigations has been conducted to examine the doping effects of dimethyl-tin-dichloride (DMTC) on the structural, electrical, and optical properties of CdS films used for photovoltaic applications. After the DMTC doping, Sn and Cl concentrations in CdS are found to increase to 6×1017 and 4.5×1018atoms∕cm3, respectively; the CdS grain size also tends to increase. A slight increase in the electrical conductivity is discussed in conjunction with the increase in the grain size and the Cl donor concentration. In addition, the optical-absorption spectra reveal almost no change in the band gap of CdS due to the doping, while the spectra indicate a deterioration of the crystallinity of the heavily doped CdS. The influence of the DMTC doping into the CdS films on the formation of the CdS∕CdTe heterostructure is also discussed. With an increase in the grain size of the CdS underlayer due to the DMTC doping, the degree of the (111) preferential orientation of CdTe increases, which generally corresponds to an improvement in the photovoltaic performance. On the other hand, a fairly small reduction is observed in the sulfur fraction in the CdTe1−xSx mixed-crystal interlayer, which indicates no marked deterioration at the CdS∕CdTe interface. The DMTC doping is also effective in reducing the optical reflectance in almost the entire active spectral region of the CdS∕CdTe solar cell.