Solar cells with the structure ZnO/CdS/Cu2ZnSnSe4/Mo/glass were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 × 1015 cm–2 and then 5.4 × 1015 cm–2 of 10 MeV electrons carried out at 77 K in liquid nitrogen bath. The low temperature PL spectra before irradiation revealed two bands, a broad and asymmetrical dominant FB band at 0.94 eV from the CZTSe layer and a lower intensity band HEB at 1.3 eV, generated by defects in the CdS buffer layer. Analysis of the excitation intensity and temperature dependencies suggested that the dominant band is the recombination of free electrons with holes localised at acceptors whose energy levels are affected by potential fluctuations of the valence band due to high concentrations of randomly distributed charged defects. Irradiation did not induce any new bands in the examined spectral range (from 0.5 µm to 1.65 µm) but reduced the intensity of both bands in the PL spectra measured at 77 K without warming the cells. The higher the dose the greater was the reduction. After this the cells were warmed to 300 K and moved to a variable temperature cryostat to measure excitation intensity and temperature dependencies of the PL spectra. After irradiation the rate of red shift of the FB band with temperature rise was found to increase. Electrons displace atoms in the lattice creating primary defects: interstitials and vacancies. These defects recombine during and shortly after irradiation forming secondary defect complexes which work as deep non-radiative traps of charge carriers reducing the PL intensity and increasing the rate of the temperature red shift. Irradiation did not affect the mean depth of the band tails estimated from the shape of the low energy side of the dominant PL band.
- semiconductor compounds
- solar cells