Understanding Aniline Surface Treatment of CdTe
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Understanding Aniline Surface Treatment of CdTe Kevin D. Dobson, Stephanie A. Einstein, Daniel D. Sadowsky, Brian E. McCandless and Robert W. Birkmire Institute of Energy Conversion, University of Delaware, Newark, DE 19716, USA ABSTRACT Surface etching of CdTe under illumination in aqueous aniline-based solutions has been investigated. Analysis of treated samples shows the formation of crystalline Te on the CdTe surface. Analysis of bath and treatment conditions indicated the presence of aniline, Cl- ions, dissolved O2, consistent illumination and careful control of pH as critical for successful surface etching. A photocatalytic mechanism is proposed for the reaction, where CdTe is etched by H2O2 or similar, generated through reduction of dissolved O2 by conduction band electrons and complemented by oxidation of aniline. The generated Te is stable in the bath and the treatment provides reproducible surfaces. CdTe/CdS devices of 12% conversion efficiency were processed with 30 min aniline treatments. The toxicity of aniline is a concern and criteria for possible substitutes are discussed. INTRODUCTION Surface preparation of CdTe remains a critical aspect of CdTe/CdS photovoltaic device processing. It is generally accepted that treatment of the surface prior to back contacting is carried out to remove contaminants and to produce a Te-rich layer to allow easier reaction with metal impurities, generally Cu, added to improve back contact properties [1,2]. CdTe treatments generally involve oxidizing wet chemical etches, for example Br2/methanol (BM) and HNO3/H3PO4 (NP), but these often tend to be reactive, corrosive and toxic, and care is required to prevent over-etching of the CdTe surface. Recently, a new surface treatment was reported by Roussillon et al. [3], where CdTe/CdS structures where treated under illumination in red wine and in aqueous solutions based on aniline (see Figure 1a for structure). Following contacting, devices treated with these solutions showed significant improvements in performance, exceeding 11% conversion efficiency, compared to devices that had not received treatment. It was proposed that the observed improvements in initial device performance were due to clogging of pinholes and neutralization of electrical micro-nonuniformities through the adsorption of charged colloids. In the case of the aniline-based treatment, the formation of charged polyaniline colloids was expected via electropolymerization of aniline on the illuminated CdTe surface. In this paper, the chemistry of aniline-based treatments of CdTe is investigated. Particular emphasis is placed on evaluating the nature of the treated surface, identifying and understanding the chemical mechanisms involved and optimizing the treatment conditions for device processing. Device results highlight this treatment to be very promising for processing of good quality CdTe/CdS devices. The toxicity of aniline, however, is a concern and, with understanding of the chemical mechanisms involved, possible substitutes are being sought. EXPERIM
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