Na in selenized Cu(In,Ga)Se₂ on Na-containing and Na-free glasses: Distribution, grain structure, and device performances

We examined the effect of deposition of Na on Mo-coated glasses and the Na content of the substrate glass on standard production Cu(In_1-xGa_x)Se₂ (CIGS)-based solar cells fabricated by selenization of Cu-Ga-In precursor thin films. Under optimal conditions, net Na content has a larger effect on the films than does the choice of substrate glass. Device performances improved with modest amounts of added Na on borosilicate glass. Device performances on soda-lime glass were not improved by adding Na. The supply of Na appears to have been adequate from the glass itself. A peak in device performance was found as a function of integrated Na in the CIGS layer as determined by secondary ion mass spectrometry (SIMS). The Na is found primarily in the areas of decreased grain size in the selenized CIGS where Ga is also found. S, deposited with the Na does not end up in the same place as does the Na. Rather, it tends to move toward the surface and accumulate in a buried layer. This is probably due to the reaction process rather than to the microstructure. Oxygen has no apparent effect on Na behavior in the CIGS.