Vapor-Liquid-Solid Growth of Cadmium Telluride Nanowires by Close-Space-Sublimation for Photovoltaic Applications
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Vapor-Liquid-Solid Growth of Cadmium Telluride Nanowires by Close-Space-Sublimation for Photovoltaic Applications B. L. Williams1*, B. Mendis, L. Bowen, D. P. Halliday, and K. Durose1 Department of Physics, Durham University, South Road, Durham, DH1 3LE UK 1
Now at the Stephenson Institute for Renewable Energy, University of Liverpool. L69 7ZF * Corresponding author, email: [email protected]
ABSTRACT Arrays of CdTe nanowires have been grown on conductive, flexible Mo substrates by the vapor-liquid-solid technique. A method of forming the arrays on a largely continuous CdTe film is described. For producing nanowire solar cells, this structure provides the advantage of preventing shunts. Nanowires having diameters in the range 100-500 nm and lengths up to 100 µm were generated. The influence of growth temperature, time and pressure on the morphology of deposited layers was investigated, and a mechanism for the generation of layer/nanowire combinations is postulated. Characterization by SEM, TEM and low temperature photoluminescence is presented. INTRODUCTION There has been considerable interest surrounding the fabrication of semiconductor nanowires for use in opto-electronic devices, including photovoltaic cells. Despite the large presence of CdTe in the field of photovoltaics, II-VI compounds are somewhat under-represented in the nanowire literature, compared to Si and III-V compounds. Indeed, vapor-phase based CdTe nanowire growth has rarely been reported [1]. Low dimensional structures demonstrate novel optical and electrical characteristics through quantum confinement, e.g. the conductance of nanowires becomes quantized when their diameters are comparable to the de Broglie wavelength. For photovoltaics however, the primary benefit of incorporating nanowires will be that of introducing numerous radial p-n junctions to replace the traditional planar junction. In this geometry, carriers can reach the junction region by travelling distances shorter than the minority carrier diffusion length and consequently electronhole recombination is restricted [2-4]. Semiconductor nanowires are commonly grown via the vapor-liquid-solid (VLS) process, first described by Wagner et al. [5]. The physical mechanisms of nanowire growth have been investigated in detail - for example, Wagner observed that the wire diameter was governed by the catalyst droplet diameter. The length and aspect ratio of wires has been discussed in terms of contributions from direct vapor impingement [6] and of diffusion limited migration [7]. Some examples of CdTe nanowire growth include Bi2Te3 catalyzed ‘rods’ grown by a co-evaporation method [8], template-prepared nanowires [9], and catalyst-free CdTe nanorod arrays grown on ITO [10]. It has also been observed that the use of a layer of polyvinyl alcohol dramatically improved the size uniformity of vertically aligned CdTe nanowires deposited by laser ablation [11]. Typically, the VLS method results in growth only at the catalyst sites and not directly on the substrate. Within photovoltaic cells
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