ELECTRICAL CHARACTERIZATION OF CdTe SOLAR CELLS MADE BY A LOW TEMPERATURE FABRICATION PROCESS

In our laboratory CdTe solar cells are prepared by vacuum evaporation process (VE) with an alternative activation treatment that avoids the use of CdCl2. This process consists of flowing chlorine containing gas namely difluorochloromethane (Freon® R-22) and Argon into a quartz crystal tube at temperatures in the range of 400-440°C and subsequent air annealing. The process substitutes the typical CdCl2 activation treatment, avoiding deposition of CdCl2. Back contact is made by deposition of standard Cu/Au stacks. Different solar cells with Freon and CdCl2 treatment have been made and compared also with cells prepared by close space sublimation process (CSS) and Freon activation. Finished devices are analyzed by current-voltage and capacitance voltage at different temperatures, admittance and drive level capacitance spectroscopy have been performed and the properties are correlated with the deposition and activation parameters. Doping concentrations, defect densities, ideality factor and diode barrier height are calculated according to the different deposition and post-deposition parameters. Measurements have shown a high density of deep defects of the low temperature deposited CdTe devices compared to CSS deposited samples that result to have higher carrier concentration. Doping concentration of the CdTe and band gap profiling is strictly depending on the above mentioned parameters. A preliminary analysis on the defect identification by admittance and deep level transient spectroscopy has been performed and discussion on the possible interpretation of deep defects presence and lack of shallow defects is suggested.