Investigation of deep levels in CdTe/CdS solar cells

Polycrystalline CdTe/CdS solar cells were studied using double boxcar and correlation deep level transient spectroscopy (DLTS). The limitations of each technique as dictated by the polycrystalline nature of the CdTe/CdS heterostructure were taken into consideration. Dark and illuminated J-V and dark C-V measurements were performed in order to monitor any changes in solar cell performance during the deep level studies. In order to avoid issues associated with the metastability of certain defects, the CdTe cells were kept in the dark during the DLTS measurements. Several traps were observed in the temperature range from 90 K to 350 K. Hole traps with activation energies of 0.32, 0.45, and 0.73 eV were found in most of the samples studied to date. An electron trap with E_A≈E_c-0.14 eV was observed only in samples with low open-circuit voltages and fill factors and is believed to be a performance limiting defect. The results have been compared with published data in order to determine the chemical origin of the observed defects