Device Physics Study Of CdTe Solar Cell

As one of the thin film solar cells with high efficiency, low cost and long lifetime, CdTe polycrystalline thin film solar cells have attracted focus in the photovoltaic field. The device physics study is an effective method to develop high efficiency CdTe solar cells. This thesis has investigated the effect of ZnTe:Cu back contact preparation process on the structure and properties, and discussed the action of Cu atom in ZnTe:Cu thin films. The CdTe solar cells have been prepared to investigate the effect of ZnTe:Cu back contact preparation process on the device characteristics. The main factors to limit conversion efficiency of CdTe solar cells have been analyzed after the device characteristics of CdTe polycrystalline thin film solar cells were simulated based on material parameters from the experiment measurements and the literature reported.The effects of copper concentration and annealing temperature on the structure and the optical, electrical properties of ZnTe:Cu films have been investigated in detail. The results show that the phase transformition between hexagonal and cubic exists in ZnTe:Cu films with CuXTe phase formation. The coordination number of Cu atom does not vary with copper concentration and annealing temperature. The optical energy gap of ZnTe:Cu films increases to 2.15eV from 2.07eV , and the mobility ranges from 0.03 cm2/V·Sec to 0.39 cm2/V·Sec, when the annealing temperature increased. The action of Cu atom in ZnTe:Cu films and the abnormal conductivity-temperature curves of ZnTe:Cu films have been studied. The effects of cell structure, illumination intensity, copper concentration and annealing temperature of ZnTe:Cu films, temperature and frequency on the device characteristics of CdTe polycrystalline thin film solar cells have been investigated systematically. The results indicate that the Voc increases with increasing annealing temperature of ZnTe:Cu. The Isc decreased when the annealing temperature is below than 180℃or is higher than 200℃. The back contact layer improved the performance of CdTe solar cells, reduced RS. The carrier concentration decreases to 1014 cm-3 from 1015 cm-3 with the increase of anneal temperature. The A increases with the decrease of temperature, while J0 varies in oppositive direction. The cell capacitance varies with illumination intensity is related to deep level in energy gap. The capacitance decreases with the increase of temperature when the positive voltage is high. The number of peaks in C-V curves varies with frequency is related to interface states between the films in device. The peak intensities decreases seriously when the frequency reduces, it is explained with the diffusion capacitance theory of p-n junction on alternating voltage.The I-V and C-V characteristics dependences on the device structure, back contact barriers, the carrier concentration of CdS and CdTe, temperature and frequency have been simulated. The results show that the roll-over phenomena in I-V curves has been abated by the application of ZnTe:Cu back contact. The roll-over phenomena appeares in J-V curves when the back contact barrier increases to 0.47 eV, and disappeares when temperature increases to 380K. The double peak phenomena appeares in C-V curves when the back contact barrier increases to 0.42 eV, and disappeares when temperature increases to 380K. The CdTe effective doping concentration increases with the increase of the absorber doping concentration. A model of double-diode is used to explain the results. The main factors to limit conversion efficiency of CdTe solar cells have been analyzed. So increasing the absorber doping concentration and improving the back contact is a effective way for developing high efficiency CdTe solar cells.