Transparent Conducting Oxide Sculptured Thin Films for Photovoltaic Applications
- PDF / 460,470 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 90 Downloads / 225 Views
F7.1.1
Transparent Conducting Oxide Sculptured Thin Films for Photovoltaic Applications N. J. Podraza,1 Chi Chen,1 D. Sainju,1 O. Ezekoye,2 M. W. Horn,2 C. R. Wronski,3 and R. W. Collins,1 1 Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606; 2 Department of Engineering Sciences and Mechanics, The Pennsylvania State University, University Park, PA 16802; 3 Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802. ABSTRACT Optical losses in thin film solar cells arise due to reflections at the top interfaces where dielectric discontinuities may be significant, e.g., between the glass and transparent conducting oxide (TCO) contact and between the TCO and semiconductor structure. Advanced optical engineering approaches are needed to minimize such losses. One approach is to incorporate multilayered or graded-index TCO films designed to act as broad-band anti-reflectors. Thus, it is important to be able to modulate the near-infrared/visible index of refraction of the TCO over a relatively wide range (e.g., 1.3 < n < 2.0) without increasing its extinction coefficient k or significantly degrading its electrical conductance. Here we report an investigation of SnO2 and ZnO sculptured thin films (STFs) under development for this purpose Sculptured thin films are deposited under low surface mobility conditions using stepwise or continuous variations in the polar and/or azimuthal angles of the deposition flux impinging on the surface. Deposition at a glancing polar angle leads to columnar growth, optical anisotropy, and low ordinary indices of refraction, whereas normal incidence deposition under the same conditions leads to dense isotropic films and high indices. In this study, we explore the dependence of the optical properties of SnO2 and ZnO, including index and birefringence spectra, on the polar deposition angle. Optical modeling reported here assesses the ability of the STF concept to provide tailored TCOs for advanced optical engineering of CdTe solar cell structures. INTRODUCTION An important goal in thin film photovoltaics (PV) is to maximize optical absorption in the electronically active layers of the device. This goal is often overlooked in favor of improving the electronic performance of the active layers and hence collection of the photoexcited carriers; thus, this goal presents a new set of materials challenges. One challenge to be addressed is the minimization of losses due to reflection at the interfaces that the photons encounter before the useful absorption event. Generally, then, a transparent conducting oxide (TCO) film that can serve the dual roles of optical filter and conducting contact is desired. Sculptured thin films (STFs) are an important class of materials fabricated by glancing angle deposition, optionally with simultaneous variation in polar and azimuthal angles of the incoming flux, to achieve films with tailored optical properties along three locally-defined principal axial directions [1-3]. For the challenge in PV, the ST
Data Loading...
