Analysis of CuIn_xGa_1-xSe₂ solar cells using admittance spectroscopy under light bias

We report on the application of a new approach to admittance spectroscopy conducted with simultaneous white light bias to the analysis of several CuIn_xGa_1-xSe₂ solar cells fabricated at the University of Delaware. Admittance spectroscopy in the dark shows the typical behavior for most CIGS devices including constant capacitance at lower temperatures across a wide range of measurement frequencies from 225 to 71100 Hz. Then, as the temperature increases, an admittance step is observed, consistent with carrier capture and emission involving a ∼250 meV deep acceptor state within in the depletion region. A significant change in behavior is observed upon exposure of the device to light during such measurements; namely, the capacitance is greatly increased at low temperatures but this decreases linearly with increasing temperature back to the dark values. The admittance step appears unaffected except to the extent that it is masked by the photo-induced capacitance change at low temperatures. Higher light levels result in a greater increase in capacitance at low temperatures. The observed photo-induced enhancement of capacitance is also frequency dependent with the greatest changes occurring at the lowest frequencies. We interpret the results in terms of a population of photogenerated carriers trapped in band tail states at the lower temperatures, which escape if given a sufficient time at a given temperature. At low light levels the rise in photocapacitance is limited by the available photocarriers and so the photo-induced behavior is reduced. The results offer some new insights for understanding the operation of solar cells.