Amorphous Hydrogenated Silicon (a-Si:H) Photovoltaic Thin Films Deposited onto Ultra-Low Density Aerogels
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AMORPHOUS HYDROGENATED SILICON (a-Si:H) PHOTOVOLTAIC TH.IN FILMS DEPOSITED ONTO ULTRA-LOW DENSITY AEROGELS S.P. HOTALING AND F.P. PRONI US Air Force Rome Laboratory/OCP, Griffiss AFB, NY 13440-5700, USA
ABSTRACT Amorphous Hydrogenated Silicon (a-Si:H) photovoltaic thin films have been deposited onto ultra low density (ULD) silica aerogel substrates using plasma enhanced chemical vapor deposition (PECVD). The techniques and results of ULD aerogel fabrication, surface treatment, thin film deposition and photovoltaic (PV) characterization are discussed. DC photocurrent and transient time of flight measurements were performed for the PV characterization, while ellipsometry, transmission optical spectrophotometry, scanning electron microscopy (SEM) and low magnification optical microscopy (x64) were used to obtain qualitative and quantitative surface morphology, microstructure and film thickness data. This research indicates that ULD aerogels may be used as substrates for dielectric and metallic thin films which have electrical, optical and mechanical properties comparable to films deposited onto conventional substrates such as glass, metals or plastics. These data taken together with the lightweight of the ULD aerogels may be of interest to the space based sensor community as a possible route to large, lightweight photonic sensors or PV arrays. These results thus indicate that ULD aerogels could be used as substrate materials for other thin films of interest to the PV community, for example, we have produced ultra lightweight reflective optics using this fusion of solgel and thin film technologies [1]. I. INTRODUCTION ULD aerogels are typified by having extremely low density (and a correspondingly high porosity), hence high surface area and low weight. The obvious size/weight constraints associated with the deployment of space based electro-optical assets (large mirrors and PV sensor arrays) begs the question of the possibility of utilizing these lightweight materials as substrates for, and eventually as monolithic PV or photonic (eg. nonlinear optical) elements. This research addresses the first of these applications, while our photonic/PV monolithic elements will be treated in subsequent papers. The application of aerogels as substrate materials has received little attention due to the high porosities exhibited by the materials (eg. How does one polish and coat a cellular foam?). This high porosity yields an intrinsically rough but uniform surface topology for aerogel of density in the low hundreds of milligrams per cubic centimeter and a fractal surface geometry for lower density aerogel (densities of the order of tens of milligrams per cubic centimeter). Section II presents the details of substrate and thin film preparation and the surface quality estimation. In section III, we present the results of DC and transient photoconductivity experiments from which mobility-lifetime product (4,C) is obtained as a measure of the PV quality of the a-Si:H thin film. II. SUBSTRATE AND THIN FILM PREPARATION Aerogel Synthesis O
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