Determination of the quality of a-Si:H films: “true”transport parameters

For the characterisation of a-Si:H layers, steady-state photoconductivity (SSPC) and steady-state photocarrier grating (SSPG) are currently used. But the μτ-products deduced from these measurements are a function of the prevailing dangling bond occupation in the film and, thus, are not a measure of material quality. In the present paper the authors introduce the product μ⁰τ⁰, which is independent of dangling bond charge and which monitors material quality in terms of band mobility μ⁰ and capture time τ⁰, the latter being, in its turn, inversely proportional to dangling bond density. They present a simple evaluation procedure to deduce μ⁰τ⁰ from simultaneously performed SSPC and SSPG measurements. In undoped, but slightly n-type a-Si:H, the quality of the material will typically be related to the ambipolar diffusion length rather than to photoconductivity. The application of this method to a series of undoped a-Si:H films and solar cells incorporating these materials exhibited a good correlation between film properties and cell performances.