Effects of Back Contact Treatments on Junction Photoluminescence in CdTe/CdS Solar Cells

Device performance in CdTe/CdS solar cells is critically linked to the quality of the back contact. In this paper we report on a phenomenon wherein application of the back contact significantly alters the electro-optical properties of the absorber near the junction. We have studied the photoluminescence (PL) spectrum of the near-junction CdTe region in CdTe/CdS solar cells before and aftercontact application. It is found that the elemental Tellurium layer formed on the CdTe surface by the standard nitric-phosphoric etch process results in a dramtic qualitative change in the junction PL spectrum. Prior to NP etch, the spectrum has two peaks at energies of 1.50 eV and 1.45 eV, corresponding to recombination in bulk CdTe, and in a CdTeS alloy with 9% sulfur content, respectively.After NP etch, the relative intensity of the low-energy peak is reduced, and the peak shifts towards higher energy. These changes are consistent with a model of increased band-bending at the grain boundaries near the CdTe/CdS heterojunction.