XPS AND UPS investigation of NH4OH-exposed Cu(In,Ga)Se2 thin films

Photoelectron spectroscopy was used to determine the compositional and electronic changes occurring in Cu(In,Ga)Se₂ thin films as a result of immersion in aqueous ammonia solution. We find that NH₄OH-treated CIGS surfaces are preferentially etched of indium and gallium, resulting in the formation of a thin layer of a degenerate Cu-Se compound that we tentatively identify as Cu₂Se. The work function of ammonia-treated samples is found to increase by 0.6 eV relative to as-grown CIGS thin films. The uniformity of chemical bath effects (etching & deposition) was found to be improved by the addition to the bath of a non-ionic surfactant. Initial device results show that the new surfactant-based chemical bath deposition (CBD) method may lead to better and thinner CdS buffer layers.