Slow-muon study of quaternary solar-cell materials: single layers and p−n junctions

Abstract

Thin films and p-n junctions for solar cells based on the absorber materials Cu(In,Ga)Se2 and Cu2ZnSnS4 were investigated as a function of depth using implanted low energy muons. The most significant result is a clear decrease of the formation probability of the Mu+ state at the heterojunction interface as well as at the surface of the Cu(In,Ga)Se2 film. This reduction is attributed to a reduced bonding reaction of the muon in the absorber defect layer at its surface. In addition, the activation energies for the conversion from a muon in an atomic-like configuration to a anion bound position are determined from temperature-dependence measurements. It is concluded that the muon probe provides a measurement of the effective surface defect layer width, both at the heterojunctions and at the films. The CIGS surface defect layer is crucial for solar cell electrical performance and additional information can be used for further optimizations of the surface.