Nanocrystalline Solutions as Precursors to The Spray Deposition of Cdte Thin Films
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ABSTRACT In this article we report the first nanoparticle-derived route to smooth, dense, phase-pure CdTe thin films. Capped CdTe nanoparticles were prepared by injection of a mixture of Cd(CH 3)2 , (n-C 8H 17 ) 3PTe and (n-C8Hi 7 )3P into (n-C 8H 17) 3PO at elevated temperatures. The resultant nanoparticles 32-45 A in diameter were characterized by x-ray diffraction, UV-Vis spectroscopy, transmission electron microscopy, thermogravimetric analysis and energy dispersive x-ray spectroscopy. CdTe thin film deposition was accomplished by dissolving CdTe nanoparticles in butanol and then spraying the solution onto Sn02-coated glass substrates at variable susceptor temperatures. Smooth and dense CdTe thin films were obtained using growth temperatures approximately 200 °C less than conventional spray pyrolysis approaches. CdTe films were characterized by x-ray diffraction, UV-Vis spectroscopy, atomic force microscopy, and Auger electron spectroscopy. An increase in crystallinity and average grain size as determined by x-ray diffraction was noted as growth temperature was increased from 240 to 300 'C. This temperature dependence of film grain size was further confirmed by atomic force microscopy with no remnant nanocrystalline morphological features detected. UV-Vis characterization of the CdTe thin films revealed a gradual decrease of the band gap (i.e., elimination of nanocrystalline CdTe phase) as the growth temperature was increased with bulk CdTe optical properties observed for films grown at 300 'C.
INTRODUCTION Interest in CdTe thin films has been greatly enhanced by the demonstration of highefficiency CdTe/CdS solar cells. Efficient cells have been prepared using various deposition methods, such as 4close-space sublimation,' electrodeposition, 2 screen printing, 3 and physical Spray pyrolysis is a low-cost and high-throughput deposition method vapor deposition. applicable to the fabrication of CdTe thin films. Aqueous solutions of CdC12 and TeO2 are commonly used for the spray pyrolysis growth of CdTe thin films with average deposition spray deposition is being pursued for temperatures between 425 and 500 'C. 5 ,6 Furthermore, 7 industrial fabrication of CdTe/CdS solar cells. In this paper we present the first nanoparticle-derived route to smooth, dense and phase100 A pure CdTe thin films. Crystalline CdTe nanoparticles in the size regime between 10 and exhibit quantum size effects due to the spatial confinement of electrons and holes. 8-12 These altered surface and grain effects, in combination with high surface area-to-volume ratio and structure, lead to significantly altered physicochemical properties. 13-20 One possible application in their use for synthesis of thin film materials. As a consequence of higher of nanoparticles lies 21 surface diffusivities and reduction in melting temperature of nanoparticle materials, ,22 nanoparticle precursor routes potentially afford lower processing temperatures; thereby offering significant advantages in the deposition of thin films. Beyond the inherent energy savings ass
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