Sodium Fluoride Doping Approach to CdTe Solar Cells

Sodium is a common impurity in CdTe solar cells, yet there are relatively few reports investigating its effect on complete device structures. There is the potential for uncontrolled sodium incorporation, either from impurities in the CdTe material itself or contaminants introduced during device processing, which can affect the optoelectronic properties of CdTe. Therefore, it is important to consider the impact of sodium incorporation on device performance. In this work, we show that the deposition of a thin layer of NaF at the back surface of CdS/CdTe devices prior to metallization is an effective strategy to form a highly doped back surface and improve contact quality. High temperature (similar to 300 degrees C) annealing is required to effectively incorporate sodium throughout the device and improve the bulk doping density; however, this also leads to sodium accumulation in the CdS layer and the formation of a TeO2 layer at the back surface. We also find evidence of out-diffusion of sodium from commonly used TEC glass substrates at typical CdTe processing temperatures, despite the presence of an alkali diffusion barrier layer. Understanding this prevalent sodium diffusion in this class of solar cells is vital for further improvement of CdTe structures.