Electrophoretic Deposition of CdSe Nanocrystals for Photovoltaic Applications
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Electrophoretic Deposition of CdSe Nanocrystals for Photovoltaic Applications Nathanael J Smith1, Kevin J Emmett2, and Sandra J Rosenthal1 1 Department of Chemistry, Vanderbilt University, Nashville, TN, 37240 2 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, 37240 ABSTRACT CdSe nanocrystals chemically linked to nanocrystalline titanium dioxide substrates form a promising material for nanostructured photovoltaic devices. The usual method for attaching nanocrystals to the titanium dioxide substrate is by means of a linking molecule (such as mercaptopropionic acid) or in-situ growth. In this paper, we report the use of an alternative technique, electrophoretic deposition (EPD), to directly deposit already formed CdSe nanocrystals onto the substrate. In EPD, a voltage is established between two electrodes that are immersed in a solution of nanocrystals. At room temperature, a fraction of the nanocrystals are thermally charged, and these charged nanocrystals migrate to the electrodes and adhere to the surface. A significant advantage of EPD over current techniques is the speed with which the nanocrystals are deposited: EPD takes only a few minutes, compared to the several hours required for the alternative techniques. As a proof of principle, we have fabricated initial photovoltaic devices based on electrophoretically deposited CdSe nanocrystals on a planar TiO2 thin film. INTRODUCTION Interest in semiconductor nanocrystals for use in photovoltaic cells has arisen primarily for two reasons: (1) They have easily tunable band gaps[1], allowing devices based on them to be engineered to have excellent overlap with the solar spectrum, and (2) they offer the possibility of utilizing photon energy in excess of the band gap that is currently wasted as heat [2]. The recent reports of multi-exciton generation in single semiconductor nanocrystals [3,4] has generated extra interest in the second point at the present time. One of the many ways in which quantum dot based solar cells can be realized is to use the dots to sensitize a high bandgap semiconductor, such as TiO2, in a direct analogy to dye-sensitized solar cells [5]. The nanocrystals can be attached using a chemical linking molecule, or more commonly they are grown in-situ. These techniques are time consuming. For example, to chemically link CdSe nanocrystals to TiO2 films using mercaptopropionic acid as the linking molecule required a total of 16 hours [5]. CdS nanocrystals grown in-situ require multiple immersions in solutions of Cd and S precursors [6,7], while growth of CdSe nanocrystals by chemical solution deposition takes several hours [8,9]. We present here an alternative method for attaching nanocrystals to TiO2, that of electrophoretic deposition (EPD). As well as taking only a few minutes to perform, EPD can give precise control over film thicknesses. Electrophoretic deposition is a simple thin film deposition technique. The process of EPD occurs when charged particles in solution are subject to an applied electric field. T
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