Solvothermal Preparation, Processing, and Characterization of Nanocrystalline CuIn 1-x Al x Se 2 Materials
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1165-M05-03
Solvothermal Preparation, Processing, and Characterization of Nanocrystalline CuIn1-xAlxSe2 Materials Christopher L. Exstrom1, JiĜt2OHMQtþHN1, Scott A. Darveau1, Anatole Mirasano1, David S. Paprocki1, Megan L. Schliefert1, Matt A. Ingersoll1, Laura E. Slaymaker1, R.J. Soukup2, N.J. Ianno2, C.A. Kamler2 1
Department of Chemistry, University of Nebraska at Kearney, Kearney, NE 68849-1150 Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 685880511 2
ABSTRACT We report solvothermal preparations of nanocrystalline CuIn1-xAlxSe2 materials prepared from the reaction of Se, CuX2 (X = Cl- or stearate), InCl3, and Al(oleate)3 in refluxing oleylamine for 30 minutes to 3 hours. Scanning electron microscopy (SEM) images reveal morphologies consisting of hexagonal plates (100-400 nm diameter) with smaller isomorphic nodules. Micro-Raman spectroscopy, x-ray diffraction, and optical bandgap data are consistent with Al3+ incorporation into the chalcopyrite structure. For aluminum-containing reactions, product Al/(In+Al) ratios are estimated to be between 0.15 and 0.35 regardless of the indiumaluminum stoichiometry employed in the reaction. When Se is added to the reaction last, the reaction pathway involves an early-formed Cu2-xSe(s) intermediate that appears to react with Inand Al-containing species simultaneously. This intermediate is avoided when heating InCl3, Al(oleate)3, and Se together prior to Cu addition, but the final product includes Se contamination that must be removed or reacted by annealing. INTRODUCTION The potential utility of CuInSe2 as a thin-film solar cell absorber material has been limited by its low bandgap relative to the 1.37 eV value that has been determined to be an optimal match for terrestrial solar irradiance [1]. Partial substitution of In3+ ions with smaller Group III ions serves to increase the bandgap, and the highest absorber layer efficiency (19.5%) has been observed for a CuIn1-xGaxSe2 (x = 0.26) thin film with a bandgap of 1.14 eV [1]. Device performance deteriorates with higher Ga3+ concentrations [2,3]. There exists the potential for broad bandgap tunability through the substitution of In3+ with relatively small amounts of Al3+. Using reasonable bowing factor estimates, the optimum 1.37 eV bandgap is predicted to be achieved when x = 0.25 in CuIn1-xAlxSe2 [4]. Thin-film CuIn1-xAlxSe2 has been prepared via co-evaporation [5-7], RF magnetron sputtering [8], DC sputtering followed by selenization [9-13], and chemical bath deposition (CBD) [14]. An unexplored avenue is the preparation and study of nanocrystalline CuIn1-xAlxSe2. Recent reports of solvothermal preparations of nanocrystalline CuIn1-xGaxSe2 indicate that the use of surfactant solvents such as oleylamine and trioctylphosphine stabilize nanoparticles early in their growth stages and facilitate rapid element mixing [15]. The solvothermal reaction method is attractive because it is scalable, cost-effective, and no vacuum is employed. In this paper, we report the solvothermal preparation and
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