Passivating Grain Boundaries in Polycrystalline CdTe

Using first-principles density functional calculations, we investigate the structure and properties of three different grain boundaries (GBs) in the solar absorber material CdTe. Among the low Sigma value symmetric tilt GBs Sigma 3 (111), Sigma 3 (112), and Sigma 5 (310), we confirm that the Sigma 3 (111) is the most stable one but is relatively benign for carrier transport as it does not introduce any new states into the gap. The Sigma 3 (112) and Sigma 5 (310) GBs, however, are detrimental due to gap states induced by Te-Te and Cd-Cd dangling bonds. We systematically investigate the segregation of O, Se, Cl, Na, and Cu to the GBs and associated electronic properties. Our results show that co-doping with Cl and Na is predicted to be a viable approach passivating all gap states induced by dangling bonds in CdTe.