Temperature Dependence of the Internal Quantum Efficiency of Cu(In,Ga)Se2-Based Solar Cells

We measured the temperature-dependent internal quantum efficiency (IQE) of Cu(In,Ga)Se 2 -based (CIGS) solar cells. The largest differences in IQE spectra measured between 100 and 300 K were observed in the wavelength range, corresponding to the light absorbed exclusively in the CIGS layer. Absorbers in the investigated cells were grown using a one-stage process. Since all elements are supplied at a constant rate, the obtained layers are free of the band gap grading, therefore collection in the bulk of the layer is not affected by a quasi-electrical field. This allows us to discuss temperature changes in collection solely in terms of recombination. We associate the change in IQE with recombination via defects present in the bulk of absorber. The two cases of donor and acceptor defects are discussed. Using SCAPS software and basic handbook formulas that describe the emission and capture rates of carriers, we estimate a range of basic parameters of the possible bulk defects in CIGS that are responsible for the temperature change of IQE spectra. Our results suggest that IQE may be controlled by shallow defects of ionization energy of 45 and 60 meV for the donor and acceptor cases, respectively. We calculate IQE spectra at different temperatures. The temperature change of simulated spectra reproduces the same tendency as experimental characteristics.