Spatial effects on ideality factor of amorphous silicon pin diodes

We analyze dark current-voltage and short-circuit current versus open-circuit voltage measurements of hydrogenated amorphous silicon pin and nip diodes. The ideality factor of similar to1.4 is independent of the thickness of the intrinsic layer, indicating that recombination occurs in the p/i interface region rather than in the bulk. These results can be simulated accurately when the defect-pool model is used. Lateral amorphous silicon diodes with a uniform defect density throughout the intrinsic region have ideality factors, which do depend on the size of the intrinsic region and exceed the theoretical value of 2. We present a model to explain the ideality factor and to verify the results with numerical simulations. Values smaller than 2 are caused by unequal shifts of the quasi-Fermi levels for electrons and holes; values larger than 2 arise when the recombination is spread out over a wide region. In that case the distance between the quasi-Fermi levels is smaller than the applied voltage. (C) 2001 American Institute of Physics.