Effect of the recombination function on the collection in a p—i—n solar cell

A closed-form expression for the recombination function associated with a single type of recombination centre that can exist in three charge states is applied to the problem of bulk collection in a p—i—n solar cell of hydrogenated amorphous silicon. It is shown that a linear approximation of the corresponding recombination function can be applied to determine bulk collection, if the p—i—n diode is under reverse bias voltage, a condition that allows the assumption of a constant electric field. The effects of two kinds of recombination function on the bulk collection are compared: first, the function found for a recombination centre that can exist in two charge states (i.e. the classical Shockley—Read—Hall expression) and second, the function found for a recombination centre that can exist in three charge states (as proposed by the present authors for dangling-bond recombination). The comparison demonstrates a significant difference between the two approaches with respect to the parameters that limit collection. In the first case the longer of the carrier drift lengths is the limiting factor; in the second case it is the shorter of the two drift lengths.