MULTIPLE EFFECTS OF THE CdCl_2 ACTIVATION TREATMENT ON THE DEVICE PROPERTIES OF CdTe/CdS SOLAR CELLS BY MONITORING THE ANNEALING TEMPERATURE

摘要

In our laboratory we fabricate high efficiency CdTe solar cells, exceeding 15 % efficiency, with a low temperature process, where all the layers are deposited by vacuum evaporation with temperature below 450 °C. The CdCl_2 treatment is typically prepared by putting drops of a methanol solution with a specific concentration of CdCl_2. Starting from a previous work where we studied the effect of the different amount of CdCl_2 by changing the methanol concentration, we have this time analyzed the recrystallization treatment by changing the annealing temperature from 310 °C up to 410 °C in air. By doing so we address the multiple effects that this process has on the material and device properties for CdTe solar cells. Activated CdTe layers have been analyzed by means of X-Ray diffraction spectroscopy and atomic force microscopy. Finished devices with efficiencies from 8% for the low temperature annealing up to more than 14% for the high temperature ones, have been analyzed by current-voltage, capacitance-voltage and drive level capacitance profiling techniques, showing that net carrier concentration is dependent with temperature. A detailed analysis of the formation of intermixed CdS/CdTe layer with temperature is also reported by external quantum efficiency.