The effect of impurities on the doping and VOC of CdTe/CdS thin film solar cells

Abstract

The effect of introducing impurities in CdTe, namely antimony (Sb) and oxygen (O), on the net carrier concentration in CdS/CdTe solar cells and on their open-circuit voltage ( V OC) has been investigated. Oxygen was introduced in the CdTe films during the deposition of this layer by the close-spaced sublimation process. The total pressure was held constant at 1330 Pa (N 2 and O 2). The amount of oxygen was varied by varying its partial pressure. Antimony was introduced into CdTe using a post-deposition diffusion process. Following the deposition of CdTe a thin film (a few nm) of Sb was deposited onto the CdTe surface and subsequently heat-treated to cause in-diffusion of Sb. The temperature and time during the diffusion process were varied in the range of 300–525 °C and 20–160 min respectively. In both instances it was possible to vary (increase) the doping concentration in CdTe. The increase in doping was accompanied by an increase in V OC. However, in all instances the doping in CdTe reached a maximum value, beyond which further increases were not possible leading to saturation in V OC. The highest V OC measured was similar to state-of-the-art values in the range of 800–830 mV, and the highest doping concentration measured was in the 10 16 cm − 3 range.