The lifetime of injected carriers in amorphous silicon p–n junctions

Abstract

In the first part of the paper the problem of measuring the lifetime τ of excess carriers in amorphous silicon (a-Si) junctions is investigated. It is shown that the crystalline-junction recovery method can be adapted to measurements on a-junctions, making it possible to determine lifetimes under conditions close to those in device operation. The method is applied to a number of a-Si configurations such as p⁺–i–n⁺, p⁺–n–M, n⁺–p–M, where M denotes an Al or Au contact. It is found that τ depends strongly on the injection level, expressed in terms of the forward current density I_f. At high levels (I_f ∼ 0·3 A cm⁻²), values of τ lie between 1 and 10μs; at injection levels normally found in photovoltaic junctions, values of τ are about ten times higher and hole diffusion lengths approach 0·2 μm. In most of the junctions studied the average injected electron and hole densities far exceed the thermal equilibrium concentrations and the recombination is a bimolecular process in the higher injection regime. The interpretation of these results is discussed in some detail.