Effect of copassivation of Cl and Cu on CdTe grain boundaries

Grain boundaries (GBs) and dislocations are generally viewed as detrimental to device applications, because they usually contain a high density of deep defect levels that act as recombination centers for charge carriers. Surprisingly, two leading polycrystalline thin-film solar cells based on CuInSe2 (CIS) and CdTe have produced very high efficiencies of 20\% and 16.5\%, respectively, despite that these materials contain significant amounts of GBs. Using a first-principles method, we investigate the structural and electronic properties of GBs in polycrystalline CdTe and the effects of copassivation of elements with far distinct electronegativities. Of the two types of GBs studied in this work, we find that the Cd core is less harmful to the carrier transport, but is difficult to passivate with impurities such as Cl and Cu, whereas the Te core creates a high defect density below the conduction band minimum, but all these levels can be removed by copassivation of Cl and Cu. Our analysis indicates that for most polycrystalline systems copassivation or multipassivation is required to passivate the GBs.