Deep surface Cu depletion induced by K in high-efficiency Cu(In,Ga)Se2 solar cell absorbers

In this work, we used K-rich glass substrates to provide potassium during the coevaporation of Cu(In,Ga)Se-2 (CIGS) absorber layers. Subsequently, we applied a postdeposition treatment (PDT) using KF or RbF to some of the grown absorbers. It was found that the presence of K during the growth of the CIGS layer led to cell efficiencies beyond 17%, and the addition of a PDT pushed it beyond 18%. The major finding of this work is the observation of discontinuous 100- to 200-nm-deep Cu-depleted patches in the vicinity of the CdS buffer layer, correlated with the presence of K during the growth of the absorber layer. The PDT had no influence on the formation of these patches. A second finding concerns the composition of the Cu-depleted areas, where an anticorrelation between Cu and both In and K was measured using scanning transmission electron microscopy. Furthermore, a steeper Ga/(In+Ga) ratio gradient was measured for the absorbers grown with the presence of K, suggesting that K hinders the group III element interdiffusion. Finally, no Cd in-diffusion to the CIGS layer could be detected. This indicates that if Cd-CU substitution occurs, either their concentration is below our instrumental detection limit or its presence is contained within the first 6 nm from the CdS/CIGS interface.