Effects of dangling bonds on the recombination function in amorphous semiconductors

A closed-form expression for the recombination function in steady-state illuminated hydrogenated amorphous silicon (a-Si: H) is given for the case that recombination occurs mainly via the dangling-bond states. Based on the three charge conditions (positive, neutral or negative) for the dangling bonds, the three corresponding occupation functions are derived; an expression for the recombination function R DB is thereby obtained. The latter differs considerably from the commonly used Shockley-Read and Hall function RSRH. As an illustration, the limiting carrier in an a-Si: H p-i-n solar cell under reverse voltage is shown to be either one with a longer drift length, using KSRH, or one with a shorter drift length, using R DB. Therefore one can conclude that the use of the proper recombination function is critical for a discussion of the relevant physical parameters involved in the description of p-i-n devices. Interpretation for ambipolar diffusion length and photoconductivity are also mentioned as cases where the use of the appropriate occupation and recombination functions are of decisive importance.