Quality factor in a-Si:H nip and pin diodes

We analyze the forward characteristics of a-Si:H nip and pin diodes. At low bias, a well-defined exponential region exists, described by a noninteger quality factor n between 1.2 and 1.7. With increasing temperature, the quality factor decreases. This behavior can be understood with a model based on electron and hole recombination in the i layer, which relates the temperature dependence of the quality factor to the distribution of localized states in the amorphous silicon. The predictions of the model are supported by numerical calculations in which the diode device equations are solved for a given distribution of localized states. The different ideality factors are due to different energy dependencies of the density of deep states in the i layer.