Characterization of the defect in CIGSe solar cell by admittance spectroscopy

In this work, we used admittance spectroscopy to accurately characterize the defect energy distribution of defects in the absorption layer of a CuIn1−xGaxSe2 (CIGSe) solar cell. We found that the dark current of the CIGSe cell decreased by an order of magnitude, and its ideality factor dropped from 2.16 to 1.85 after annealing. The capacitance of the CIGSe solar cell before annealing was higher than that after annealing under reverse bias. Moreover, a higher free carrier concentration was found in the absorption layer of the cell before annealing. The results of our tests show that the built-in electric voltages of the CIGSe cell before and after annealing were 0.52 and 0.64 V, respectively. After annealing, the activation energy of defects in the absorption layer decreased, and their concentration remained almost constant, as measured by admittance spectroscopy. The decrease of the defect activation energy indicated a corresponding decrease in the Shockley–Read–Hall recombination probability of defects in the present cell; hence, the increase of the open circuit voltage and parallel resistance of the CIGSe solar cells after annealing resulted in improved performance.