Defects induced by ionizing radiations in AII-BVI polycrystalline thin films used as solar cell materials

Full text: Thin films of AII-BVI compounds are potential candidates for the manufacturing of electronic and optoelectronic devices, especially solar cells. In this paper the effects of irradiation with high-energy electrons and protons on structural, electrical and optical properties of CdS and CdSe thin films have been investigated. The films, 1-3 mm thick, were prepared by thermal-vacuum evaporation on glass substrate at a temperature of 220 deg. C. The samples were irradiated with 6-7 MeV electrons, up to a fluency of 1017 e/cm2 and with 3 MeV protons, up to a fluency of 1013 protons/cm2, respectively. XRD investigation has revealed than the films contain wurtzite-type CdS and CdSe (001) preferentially oriented in the growth direction. The defects induced by ionizing radiations have been studied by using various techniques: the space-charge-limited-current (SCLC) measurements, the thermally stimulated current spectroscopy (TSC), and the absorption and photoluminescence (PL) spectra measurements. It was found that the electrical conduction of the samples, both before and after irradiation, is controlled by different types of defect distributions, placed in the band gap of the investigated semiconducting layers. Parameters of identified defect levels were determined. A detailed discussion about the possible origin of these defects has been done, suggesting that the main defects induced by high-energy electron and proton irradiations are related with chalcogen (S or Se) vacancies. The origin of the other identified defects, with smaller ionization energies, remains unknown at this time. (authors)