TABLE OF CONTENTS For many solar cell absorber films the minority carrier diffusion length (L) is an important parameter. It depends on the density of the effective non-radiative recombination centers. We will consider the relationship between these two parameters below. In an hydrogenated amorphous silicon p-type thin film positively charged dangling bonds that are homogeneously distributed are the main candidate for these defects although recombination also should occur at neutral dangling bonds. In a poly-Si p-type thin film again positively charged dangling bonds, but ones that are located at grain boundaries, stacking faults, twins and dislocations, are the defects believed to provide the limiting recombination centers for charge carriers. (In many cases impurities act to provide these centers, but we are focussing in this section primarily on intrinsic defects.) In the following sections we provide simplified derivations of minority carrier diffusion lengths for each of several types of thin film materials, not including monocrystalline layers. The latter do not differ from bulk monocrystalline materials and derivations of the corresponding DL can be found in many texts on the subject (e.g. A.H. Fahrenbruch and R.H. Bube, in Fundamentals of Solar Cells, Academic Press, New York, 1983).
We make the assumption that the major contribution to the recombination rate of carriers at room temperature arises from recombination at dangling bonds. Recombination can occur both at charged dangling bonds and at neutral dangling bonds. In the absence of an electric field the diffusion length L can...
