Showing papers by "Nicolas Wyrsch published in 1993"

Role of dangling bond charge in determining μτ products for a-Si:H

Abstract: By using a model of recombination via dangling bond states, one shows that the steady-state μτ products deduced from photoconductivity and ambipolar diffusion length measurements executed on uniform a-Si:H layers, are products of the form: band mobility × recombination time . The latter are dependent on the occupation functions f+, fo, f− of dangling bond states. They are not material constants and their values differ considerably according to the specific experimental situation. The μτ products of the form band mobility × capture time are those that have a direct significance for material quality and device calculations, since f+, fo and f− do not enter into their definitions. Particulat conditions on f+, fo and f− are required so that recombination times are reduced to capture times, in other cases a connection procedure is necessary. The situation for μτ products derived from time of flight measurements is shown to be similar, except that here one has to consider the occupation functions f+, fo and f− in the dark state rather than under illumination. An illustrative example based on experimental data is given.

Amorphous silicon solar cells with graded low-level doped i-layers characterised by bifacial measurements

Abstract: Bifacial spectral response characterization of solar cells under near operating condition illumination is used in conjuncture with a novel bifacial DICE analysis to establish the collection efficiency as a function of i-layer position in p-i-n amorphous silicon solar cells. A significant portion of solar cell degradation can be explained in terms of electric field distortions which increase recombination losses. Unlike carrier lifetime reductions, the field distortions can be reduced. The numerical model is used to guide the intentional doping of the i-layer to counteract the field distortions caused by charged dangling bonds, and thus to optimize the electric field for the degraded state. Solar cells with graded low-level boron doping in the i-layer are analysed in detail. Increasing conversion efficiency during light-soaking, and enhanced stabilized n-side performance show the viability of the electric field optimization. >

The µτ Problem: New Results on Micro-Doped a-Si:H Films

Abstract: Comparative µτ-product measurements performed on a series of slightly doped (micro- doped) a-Si:H films by steady-state photocarrier grating (SSPG), steady-state photoconductivity (SSPC) and time of flight (TOF) are presented. The observed discrepancy between transient and steady-state majority carriers µτ-products is discussed within the framework of a dangling bond recombination model. Furthermore, results on the variation of µτ with doping and light-soaking are reported and ratio of capture cross-sections of charged to neutral defects are deduced.

Limits of the Constant Photocurrent Method (CPM) for the determination of the deep defect density in amorphous hydrogenated silicon (a-Si:H)

Abstract: Recent measurements on slightly phosphorous- and boron-doped a-Si:H samples have shown an unexpected discrepancy between CPM and photothermal deflection spectroscopy (PDS) on p-type samples. Here we present new systematic measurements of the photoconductivity and its power law exponent (Rose factor) in function of the light generation rate (from dark up to 1018 cm−3st-1) for different specific wavelengths typically used in CPM (1.24eV, 1.53eV and 1.8eV) and for the same series of slightly doped samples, both in the annealed and in the degraded state. The CPM technique on slightly p-type samples shows, in the low sub-bandgap range, higher absorption spectra in comparison to PDS. We show that this discrepancy does not indicate a higher defect density, but that it is the result of a supralinear, spectrally dependent Rose factor, which violates the basi condition of the CPM technique. Measuring CPM with a photocurrent much smaller than the dark current can eliminate this problem.