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 atomiclike 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.