Influence of the a-Si:H structural defects studied by positron annihilation on the solar cells characteristics

Abstract

The influence of the power density, PD, on the density and structure of defects of undoped a-Si:H thin films, deposited by r.f.-PECVD, is studied by the constant photocurrent method, (CPM), and by slow positron beam spectroscopy, respectively. Deep defect density, NDD, remains approximately constant at 1016 cm-3, typical of device quality material, for PD in the range 7-20 mW·cm-3, as calculated from CPM. Out of this range, NDD increases roughly one order of magnitude for both lower and higher power densities. Positron annihilation spectroscopy reveals the predominance of two kinds of vacancy type defects in the films: large vacancy clusters or voids for PD∼7 mW·cm-3 and small vacancy type defects as PD increases to ∼30 mW·cm-3. The behaviour of solar cells, prepared with a-Si:H intrinsic layers with similar deep defect densities, varies with the S parameter of the a-Si:H film, Sfilm: as Sfilm increases, which can be related to a lower material density, the solar cell characteristics degrades. © 2002 Elsevier Science B.V. All rights reserved.