The correlation of performance in CdTe photovoltaics with grain boundaries

It is assumed that the performance of polycrystalline CdTe photovoltaic thin films is correlated to microstructure, especially aspects of the microstructure related to grain boundaries. However the role of grain boundaries on the electrical performance of these films is not well understood. In an effort to gain understanding into the correlation between the performance and grain boundaries in CdTe thin films, electron backscatter diffraction (EBSD) has been used to characterize the the grain size, grain boundary structure and texture of sputtered CdTe at varying deposition pressures before and after CdCl₂ treatment. Weak axisymmetric (or fiber) textures were observed in the as-deposited films. At lower deposition pressures (111) fiber textures were observed and (110) fiber textures were observed at higher lower deposition pressures. Samples treated with CdCl₂ exhibited significant grain growth with a high fraction of twin boundaries. A much stronger correlation of performance with grain size was found when the grain size was corrected to exclude the twin boundaries. This observation confirms that the twin boundaries do indeed have different electrical properties than random high-angle boundaries.