Effects of solution-grown CdS on Cu(InGa)Se2 grain boundaries

Complete Cu(In,Ga)Se2 (CIGS) solar cells in which the collecting heterojunctions were formed by chemical bath deposition of CdS were analyzed by transmission electron microscopy and energy dispersive spectroscopy. The CIGS was produced at 400 or 580 °C as either single layers or in a two-step bilayer process. The compositions of grain boundaries were found to be the same as the grains before CdS deposition except in the low temperature-deposited bilayer but after CdS was formed the grain boundaries were found to be anion deficient with some evidence of Cu loss from the CIGS and residual Cu found in the CdS in most cases. The results are consistent with n-type doping of the surface of the CIGS and wrapping of the junction around the grains. In bilayer films the grain boundaries were found to be more open containing many voids and this facilitated penetration of the CdS into the boundaries.