Luminescence properties of Ga-graded Cu(In,Ga)Se2 thin films

Abstract Cathodoluminescence (CL) has been measured at 10 K in cross-section and plan-view configuration on Cu(In,Ga)Se 2 thin films with Ga-grading as they are used in high-efficiency solar cells. Measurements on cross-section samples show the vertical profile of the emission energy to correspond to the band-gap profile of the film as calculated from measurements of the Ga-grading. Hence, the CL method is capable to directly measure the band-gap grading in semiconductor thin-films, but often the influence of the grading on the emission energy is generally ignored in recent literature. At the same time, we observe a strong drift of excited charge carriers toward the minimum of the band-gap. The transport process can be explained by the quasi-electric field induced by the Ga-grading and applied to determine transport properties of the Cu(In,Ga)Se 2 material. Implications for luminescence investigations on band-gap graded thin-films are discussed.