Cadmium telluride (CdTe) alloy solar cells have seen a rise of more than 5% in the absolute conversion efficiency in the past five years. This rise is mainly credited to the development of cadmium chloride post deposition treatment (PDT). This treatment has shown to passivate the grain boundaries and aid in surface reconstruction. Though PDT has been greatly successful for single junction CdTe cells, increase in the efficiencies of magnesium and zinc alloys of CdTe – which have a higher bandgap – is yet to be realized. Also, though the macroscopic effect of the post deposition treatment is consistent and well agreed upon, the effect it has on grain boundary structure and the mechanism of surface reconstruction is still an active area of debate and research.
In this work, we have focused on understanding the effect of various temperatures used for PDT on the grain boundary potential and conductance. Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), with a tip size of less than 25nm, is used to probe the local characteristics.
The KPFM and C-AFM studies carried out confirmed that PDT affects the electronic properties of the grain boundaries. At lower temperatures, it is observed that the potential at the grain boundaries is higher than that at the grain core. As PDT temperature increases, the potential difference between the grain boundary and the grain core is observed to increase and reach a maximum value. Increasing temperature further shows an abrupt decrease in the grain boundary potential to values lower than that at the grain core. The conductance of grain boundary appears to follow an exactly inverted behavior compared to the potential variation. It is observed that the tipping point at which the potential difference at the grain boundary is the maximum, is also the temperature at which we observe maximum rise in efficiency of the solar cell.
This study creates pathways to better understand the mechanism of increase in efficiency of CdTe solar cells due to the post deposition treatment. A similar study on magnesium and zinc alloys of CdTe and attempts to engineer potential at the grain boundaries will also be presented in the final article.