Photoluminescence study of polycrystalline CdS/CdTe thin film solar cells

To achieve high efficiencies in CdS/CdTe thin film solar cells, it is known that a CdCl2 activation process is necessary. In addition, the presence of oxygen during this treatment can have a beneficial effect. The mechanisms involved in this process are not fully understood. In this work, we present a photoluminescence (PL) study of two sample types: cells that received their CdCl2 activation in an air ambient and cells that were treated in vacuum. The luminescence at the front (through the glass substrate) and at the back surfaces (after the partial removal of the back contact by ion sputtering) is investigated. Three main luminescence bands are observed: an excitonic emission around 1.59 eV, an emission band at 1.55 eV and a broad band at 1.4 eV, related to cadmium vacancy–chlorine defect complexes. The 1.55 eV emission is shown to be related to oxygen. Using measurements at very low excitation intensity, we were able to resolve two components: a donor–acceptor pair (DAP) transition and a free electron–acceptor transition. The acceptor level involved is located at approximately 47 meV above the valence band. The 1.4 eV band, related to a cadmium vacancy–chlorine complex, is broader at the junction than at the free surface and shifted to lower energy, possibly related to sulfur interdiffusion at the interface.