Monitoring of a-Si:H p-i-n light induced degradation by low temperature AC conductance

The authors report on AC conductance measurements versus temperature (20 K-300 K) at different frequencies performed on p-i-n amorphous silicon solar cells. The presence of two peaks in the conductance curves is observed and explained in the context of a model which includes the temperature and frequency dependence of the AC conductance (and junction capacitance) on the density of states in doped materials and thus related to the n and p layers. This technique shows a high sensitivity to the effects of light soaking and allows one to distinguish between degradation effects arising in the intrinsic and in the doped layers. In the initial stage of degradation, a decrease of the temperature, at which the peak related to the p-doped layer occurs, is observed. For long illumination time, a linear increase of the AC conductance superimposes to the peaks. The authors relate the former phenomenon to a shift of the Fermi level in the p layer due a light induced dopant activation, and the latter to an increase of deep level in the intrinsic layer, which gives rise to a hopping path through the device