Interface States of Heterojunction Solar Cells

The dispersion of conventional amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction solar cells shows a single time constant behavior. However, when a multicrystalline substrate is used, an additional "hump" appears in the C-f and R-f curves. A possible explanation by the introduction of a second time constant fails. As an alternative, interface states are considered as the reason of the hump. Thus, an equivalent circuit is established which is used to extract the parallel conductance and capacitance caused by the interface states. A bell-shaped conductance (normalized to frequency) and a step-shaped capacitance appear as predicted by Shockley-Hall-Read theory. The evaluation can be further improved by application of the statistical model (assumption of a fluctuating surface potential). The final data are a surface state density D_it of 2.3 10¹² V^-1cm^-2, a time constant tau_it of 1.7 ms and a standard deviation a of 0.8