Characterization of the Defect in CIGS Solar Cell by Admittance Spectroscopy

We used admittance spectroscopy to characterize the energy distribution of defects in CIGSe solar cells before and after annealing to investigate the mechanism of the annealing process improving battery performance. In this article, we annealed the prepared CIGSe solar cells in compressed air at 150℃ for 10 minutes. We conducted dark I-V, C-V, admittance spectroscopy, and illumination I-V tests on CIGSe solar cells before and after annealing to characterize the changes in battery performance before and after annealing. The test results of dark I-V characteristics showed that the reverse dark current of CIGSe solar cells decreased by about an order of magnitude after annealing, and the ideal factor of the cells also decreased from 2.16 before annealing to 1.85 after annealing. This means that the annealing process reduces the recombination of carriers in CIGSe solar cells. Under reverse bias, the capacitance of CIGSe solar cells is higher than that after annealing, and their C-V characteristics are linearly fitted with 1/C2 vs. V. The fitting results show that the slope of the curve increases after annealing, which means that the annealing process leads to a decrease in the free carrier concentration in the absorption layer of CIGSe solar cells, that is, a decrease in the carrier concentration contributed by defects after annealing. In addition, the built-in potentials before and after annealing of CIGSe solar cells were also obtained through fitting, which are 0.52V and 0.64V, respectively. The admittance spectrum test results of CIGSe solar cells before and after annealing showed that the defect activation energy in the absorption layer significantly decreased after annealing, but the defect concentration remained almost unchanged. The decrease in defect activation energy means that the Shockley Read Hall (SRH) recombination probability of defects in copper indium gallium selenium solar cells is reduced. In addition, the test results of the optical I-V characteristics of the battery indicate that the open circuit voltage and parallel resistance of the battery significantly increase after annealing, which is consistent with the test results of the dark I-V characteristics, C-V characteristics, and admittance spectroscopy of the solar cell. Therefore, the annealing process of CIGSe solar cells leads to a weakening of the SRH recombination of carriers in the absorption layer of the battery, thereby improving the performance of the solar cell's performances.