Characterization of Cu(InGa)Se2 Solar Cells using Etched Absorber Layers

An aqueous Br-etch to smooth the surface of Cu(InGa)Se₂ thin films and reduce their thickness is used to enhance characterization of Cu(InGa)Se₂ solar cells. Two applications of this etch will be presented. First, the etch is used to obtain the smooth surface necessary for precise optical characterization by spectroscopic ellipsometry. Optical constants of etched Cu(InGa)Se₂ match those of films peeled from the substrate, as has previously been used to provide the smooth surface for characterization. The optical properties of CdS grown on the etched Cu(InGa)Se₂ have been determined and are compared to those of single crystal CdS. Second, the etch is used to controllably reduce the Cu(InGa)Se₂ thickness for characterizing the effect of absorber layer thickness. Devices have been fabricated using Cu(InGa)Se₂ layers with thicknesses from 0.4 to 1.8 mum with fill factor greater than 74% over the entire range. The main loss in efficiency with absorber layers less than 1mum is from lower short circuit current due partly to incomplete optical absorption. Solar cell losses with the thin specular absorber layers obtained by etching are compared to those with rougher deposited films