Dielectric spectroscopy of relaxation processes in Cu(In,Ga)Se2 solar cells

Within a systematic study, the authors investigate the frequency, temperature and bias voltage dependence of the complex admittance of Cu(In,Ga)Se₂ solar cells prepared by rapid thermal processing. Measured admittance spectra performed in a frequency range between 100 Hz and 13 MHz and a temperature range from 5 K to 300 K uncover distinct loss peaks related to a series of impurity states in the cell material. The temperature dependence of the peak frequency shows thermal activation. Four different activation energies could be identified with values of about 260 meV, 195 meV, 44 meV, and 22 meV corresponding to the energetic position of four distinct trap levels within the Cu(In,Ga)Se₂ material. From the dependence of the admittance on the voltage bias, it is possible to distinguish between majority carrier and minority carrier traps. The authors also demonstrate that the defect structure of the Cu(In,Ga)Se₂ cells displays metastable behavior