Next-Generation Transparent Conducting Oxides for Photovoltaic Cells: an Overview
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NEXT-GENERATION TRANSPARENT CONDUCTING OXIDES FOR PHOTOVOLTAIC CELLS: AN OVERVIEW David Ginley, Tim Coutts, John Perkins, David Young, Xiaonan Li, Phil Parilla, National Renewable Energy Laboratory, Golden, CO; Renaud Stauber, University of Colorado, Boulder, CO; Dennis Readey, Chris Duncan, Colorado School of Mines, Golden, CO.
ABSTRACT Transparent conducting oxides (TCOs) are becoming a more critical element in thin-film photovoltaic devices. In the continued drive to increase efficiency and stability while reducing cost and optimizing performance, the optical, electrical, and materials properties of TCOs gain increasing importance. TCOs can perform a variety of important functions, including contacts, antireflection coatings, and chemical barriers. In this paper, we will review some of the current advances in the field of transparent conductors and, where possible, will relate these advances to thin-film photovoltaic devices. Highlights will be on the rapidly growing collection of new nand p-type materials; the implications of these materials on PV have not been fully assessed. INTRODUCTION Transparent conducting oxides (TCOs) are becoming an increasingly important element of a variety of thin-film and flat-panel technologies. These include, but are not limited to, lowemissivity windows, electrochromic windows, flat-panel displays, and thin-film photovoltaics. Traditionally, the main set of TCOs consisted of ZnO doped with Al and F, InSnOx, and SnO2 doped with F. As the performance demands on the TCO increase, there is a growing need for new materials with improved properties. Although the basic needs of high conductivity and transparency are important, in a large number of these applications many other criteria are of equal importance. Some of these criteria are detailed in Table 1, along the lines suggested by Gordon [1]. Table 1. Key TCO Parameters [1] Key Parameters
¾Optical Performance
¾Electrical Conductivity ¾Crystallinity
Description This includes both the transparency in the visible as determined by the band edge, as well as the infrared transparency as primarily defined by the plasma edge. In addition, it may be desireable to control the reflectivity and refractive index to produce, for example, integrated antireflection coatings. This is critical, but in many systems, it is desireable to be able to controllably dope the TCO over the full range from conductive to insulating. Nominally high crystallinity is desireable to get the best mobility However many TCO H2.7.1
¾Work Function
¾Stability
¾Deposition Cost and Temperature
¾Selective Etching Chemistry ¾Hardness ¾Toxicity of the Constituents
get the best mobility. However, many TCO materials show good properties in the amorphous state. This has some real advantages in terms of process temperatures and, in some cases, interface stability. This is somewhat important for semiconductor materials, but may be critical for polymerbased devices where the work-function differences between the contacts and the quality of the interfaces provide motive for
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