Synthesis and Application of Electronic Oxides for Solar Energy
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Synthesis and Application of Electronic Oxides for Solar Energy James M. Kestner, Anna Chorney, Joshua J. Robbins, Yen-jung Huang, Tyrone L. Vincent1, and Colin A. Wolden Department of Chemical Engineering and Division of Engineering 1 Colorado School of Mines, Golden, CO 80401-1887, USA. Lawrence M. Woods ITN Energy Systems, Inc., Littleton, CO 80545, USA ABSTRACT Plasma-enhanced chemical vapor deposition (PECVD) is being developed as a flexible coating technology for a variety of oxides. In this paper we discuss the synthesis of transparent conducting oxides (TCOs), insulating oxides and electrochromic oxides. Tin oxide was synthesized using mixtures of SnCl4 and O2. By proper control of processing conditions the resistivity of this material may be varied from 10-3 < ρ < 105 Ω-cm. Films of varying resistivity were employed as buffer layers in CdS/CdTe solar cells. Preliminary device results have demonstrated that integration of a tin oxide buffer layer was very beneficial for cell performance. In addition, we demonstrate the PECVD synthesis of WO3 from WF6/O2/H2/Ar mixtures. The plasma process space that yielded adherent, transparent tungsten oxide was established. The deposited films were both amorphous and reversibly electrochromic. High temperature annealing above 400 ºC converted the films into a polycrystalline state.
INTRODUCTION Plasma-enhanced chemical vapor deposition (PECVD) is a flexible coating technology that has been employed for the fabrication of a variety of thin films. In this paper we discuss the synthesis of transparent conducting oxides (TCOs), insulating oxides and electrochromic oxides. TCOs form the transparent electrodes in photovoltaic (PV) devices, light emitting diodes (LEDs), and flat panel displays. Insulating oxides find application both as buffer layers and as protective coatings. Electrochromic materials may be reversibly converted from transparent to opaque to modulate sunlight in architectural applications. To date these materials have been fabricated primarily by physical vapor deposition techniques such as sputtering or laser ablation. In such cases the ceramic target sets the composition of the deposited film. The fabrication of multi-layer structures requires multiple targets, and it is difficult to make fine adjustments to film composition. Other drawbacks of PVD techniques include low target utilization and the production of asperities that compromise film smoothness. We have been developing PECVD as an alternative oxide coating technology. Potential advantages include the ability to finely tailor film properties and produce functionally graded films by manipulation of process parameters. In addition multi-layer structures could be synthesized by simply altering the gas composition. In previous work we have discussed the use of PECVD to produce conducting tin oxide [1-2]. In this paper we discuss the application of these materials to the solar energy devices shown in Figure 1. The first device is the CdTe/CdS heterojunction solar cell shown in Figure 1(a). Cadmium
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