Extended Defect States in CdTe/ZnTe Photojunction

The paper concerns the influence of defects states on the open-circuit voltage (V-oc) of a semiconductor photocell illuminated with different light intensity. The measured V-oc versus the number of photons (illumination intensity) curve is compared with the analogous dependence described by the theoretical model applied. Illumination of a semiconductor leads to a shift of the energy position of the quasi Fermi level relative to the Fermi level in thermal equilibrium. As the illumination intensity increases, the energy of the quasi Fermi level of electrons and holes shifts and influences directly the V-oc value. The defects existing inside the band gap contribute adequately to the structure of V-oc versus illumination intensity dependence due to the exchange interaction between free carriers and defect states, which appears in the case of energy coincidence. Comparison of V-oc measured and predicted by the proposed model for a specific illumination intensity allows the determination of the energy of defect states in the band gap. The extending defects damping of V-oc were studied for the p-ZnTe/n-CdTe photocell, and the results were compared with those obtained for the Si-based photocell. Value of open-circuit voltage was measured for a sample illuminated by a controlled number of laser photons that appear in bunches. The structure of V-oc versus illumination intensity curve for the ZnTe/CdTe photocell reveals four discrete defects states related to dislocations and located at the energies 26.6, 43.4, 57.0 and 67.4 meV below the Fermi level at thermal equilibrium.