Free carrier ambipolar diffusion length in amorphous semiconductors

An expression for the ambipolar diffusion length (L-amb) in amorphous semiconductors involving explicitly the densities of free carriers and their band mobilities is derived. Thus, L-amb and the photoconductivity sigma(photo) can be related to the same set of transport parameters (band mobilities and carrier lifetimes). The formal equivalence between the expression derived here for L-amb and the one previously used in the literature (and involving drift mobilities and 'total carrier lifetimes') is shown. The expression derived here for L-amb served together with that for sigma(photo) as a basis for (1) determining to what extent the characteristic electronic transport behaviour of a given film corresponds to that of a 'truly intrinsic' film, or not and (2) deducing a parameter mu(0) tau(0) that characterizes intrinsic layer quality for use in p-i-n solar cells.