Back contact effects on junction photoluminescence in CdTe/CdS solar cells

Studies of junction photoluminescence in CdTe/CdS solar cells reveal that back-contact application produces a dramatic qualitative change in the junction PL spectrum. Prior to contact application, the spectrum has two peaks at energies of 1.508 eV and 1.45 eV, corresponding to recombination in bulk CdTe, and in a CdTeS alloy with 9% sulfur content, respectively. After contact application, the spectrum consists of a single broad peak at 1.48 eV. Previous studies have shown that the nitric-phosphoric (NP) etch used in the contact procedure produces a layer of elemental Tellurium (Te) on the CdTe surface. The authors' measurements utilizing Auger electron spectroscopy (AES) show that this Te layer penetrates grain boundaries down to the CdTe/CdS interface. It appears that the change in the near-junction PL spectrum is caused by a "grain boundary field effect" due to perturbations of the grain boundary conductivity and Fermi level.