Identification of deep level defects in CdTe solar cells using transient photo-capacitance spectroscopy

The carrier lifetime in CdTe is strongly limited by the nonradiative recombination via defects. Here, deep level defects in CdTe thin-film solar cells are revealed by transient photo-capacitance (TPC) measurement. A broad defect band centered at 1.07 eV above the valance band is identified at 90 K. The defect signal is reduced with the insertion of the CdSe layer between the CdS/CdTe heterojunction. The TPC signals are rapidly quenched with increased temperature, which suggests that this deep level defect is highly possible to act as an effective recombination center. Based on the thermal quenching model, the activation energy (E a) of the defect is estimated to be ∼0.2 eV. With the configuration coordinate model, the temperature-dependent TPC signal and the corresponding electronic transition process can be well interpreted. All the observations strongly indicate that the introduction of Se atoms into CdTe is promising to suppress the formation of deep defects.