Characterization of deep defects in CdS/CdTe thin film solar cells using Deep Level Transient Spectroscopy

The presence of deep defect levels in thin film solar cells can highly affect the characteristics of the photovoltaic energy conversion. Therefore, knowledge of the origin and nature of these defects is desirable. Deep level transient spectroscopy (DLTS) was performed on a series of CdS/CdTe thin film solar cells which were activated in vacuum or air ambient. Temperature scans between 5 and 320K revealed semi-shallow and mid-gap majority traps. These mid-gap traps were also investigated using isothermal DLTS (region 250 to 330K) where the temperature is kept constant and the rate window is varied. This way the mid-gap traps can be characterized completely. Using electrical injection DLTS and optical DLTS minority traps could be detected. Electrical injection DLTS showed a single defect in the bulk of the air activated cells, while optical DLTS revealed the presence of defects close to the CdS/CdTe interface in both types of samples. The nature and origin of these defects are unknown.