Design and Verification of a PECVD fabricated Multi-Layer Nano- Scale Photovoltaic Device Shima Hajimirza The University of Texas at Austin Dept. of Mechanical Engineering Austin, Texas, USA Email: Shima@ices.utexas.edu John R. Howell The University of Texas at Austin Dept. of Mechanical Engineering Austin, Texas, USA Email: jhowell@mail.utexas.edu Milo Holt The University of Texas at Austin Dept. of Electrical and Computer Engineering Austin, Texas, USA Sayan Saha The University of Texas at Austin Dept. of Electrical and Computer Engineering Austin, Texas, USA Deji Akinwande The University of Texas at Austin Dept. of Electrical and Computer Engineering Austin, Texas, USA Sanjay Banerjee The University of Texas at Austin Dept. of Computer and Computer Engineering Austin, Texas, USA Abstract This paper summarizes the results of computational and experimental studies of an enhanced thin film solar structure. The cell structure consists of a reflective aluminum layer beneath an 80nm absorbing layer of amorphous silicon, coated with a top layer of transparent and conductive indium tin oxide (ITO). The structure is mounted on a glass substrate. We first use constrained optimization techniques along with numerical solvers of the electromagnetic equations to specify the layer thicknesses of the design for maximized efficiency. Numerical analysis suggests that solar absorptivity in the thin film silicon can be enhanced by a factor of 2. The proposed design is then fabricated using Plasma Enhanced Chemical Vapor Deposition techniques, along with a control sample of bare silicon absorber for comparison. AFM imaging and spectrophotometry experiments are applied to estimate the realized thin film dimensions, deposition error, unwanted oxidation volume and the resulting reflectivity spectra. Comparisons of the measured and simulated reflectivity spectra of the fabricated cells, as well as Monte Carlo simulations based on incorporating random geometry errors in the numerical simulations suggest that the measured spectra are in accordance with the expected curves from simulations. 1. Introduction Amorphous silicon (a-Si) is an inexpensive choice of active layer material for thin film photovoltaic devices. Compared to crystalline silicon, a-Si has fewer constraints (e.g. temperature, substrate choice, etc.), can be deposited with smaller dimensions, and has a significantly higher solar absorption at the same thickness [1]. In addition, compared with other existing semiconductors, silicon is an abundant and well- studied material for photo-electronic uses. In spite of those benefits, in thin film solar cells made of a-Si, the very small (< 100nm) thickness of silicon, and the less desirable electric properties of a-Si (compared to crystalline silicon) significantly hinder photonic absorption and photon-to-electricity conversion of the device, respectively. Mechanical adjustments (a.k.a. light trapping) to the thin film structure can modify the optical deficiencies of silicon to a large extent. Most commonly, light trapping is done by depositing extra layers of coating, cladding or grating on either sides of the thin film silicon. These surfaces modify the effective path length of light in the absorbing material by the same mechanisms as in the thick film cells, as well as effects that only appear in sub-wavelength dimensions. More specifically, thin and transparent antireflective coatings (such as transparent conductive oxide layers) introduce a gradual change in the effective refractive index of the absorbing layer to reduce surface reflectivity [2]. Furthermore, coatings designed to change the refractive index on the rear side of a device can reflect energy back through the silicon for an additional round of absorption [3-6]. Sub-wavelength plasmonic surfaces and metallic particles can produce forward scattering and localize light below the diffraction limit, effects which both lead to enhanced absorption. Metallic gratings and particles of even Proceedings of the ASME 2013 Heat Transfer Summer Conference HT2013 July 14-19, 2013, Minneapolis, MN, USA HT2013-17271 1 Copyright © 2013 by ASME Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 10/02/2014 Terms of Use: http://asme.org/terms