The Effect of the Dielectric Constant of a Solution of CdSe Quantum Dots on Electrophoretic Deposition of the Dots
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The Effect of the Dielectric Constant of a Solution of CdSe Quantum Dots on Electrophoretic Deposition of the Dots Nathanael J. Smith Department of Physics and Astronomy, Middle Tennessee State University, 1301 East Main St., Murfreesboro, TN 37132 ABSTRACT The effect on film quality of the dielectric constant εb of the bath solution used for electrophoretic deposition of CdSe quantum dots (QDs) was investigated. Various combinations of solvents yielding different εb were tested, with the best films produced by a bath consisting of hexanes and acetone. Two different types of film were observed, depending on the volume fraction of acetone in the bath: thin, but strongly adhered films for εb < 12 and thick but loose films for εb > 12. This behavior is explained in terms of the rate at which QDs arrive at the surface of the electrode, which depends on the electrophoretic mobility of the QDs and therefore also the dielectric constant of the bath. INTRODUCTION The fabrication of quantum dot sensitized solar cells (QDSSC) relies on the formation of a layer of semiconductor quantum dots (QD) on a high band gap substrate material such as TiO2 or ZnO [1]. Typically the QDs are either grown in situ [2] or deposited from colloidal suspensions, in which the QDs are tethered to the substrate by means of a linking molecule [3]. Electrophoretic deposition (EPD) is an alternative method of film deposition that has been used for many years to deposit a wide variety of different materials (for example, see the review article by Van der Biest and Vandeperre [4]). EPD is a simple, fast and scalable technique. It works by using an electric field to move charged particles in a colloidal suspension to an electrode of opposite polarity. The particles stick to the electrode, forming a film the thickness of which can be controlled by varying deposition parameters such as time, voltage, and particle concentration. Herman has shown that EPD can be used to reliably and quickly deposit films of CdSe QDs [5– 7], and EPD has already been used in the fabrication of QDSSCs[8–11]. In Herman’s work, CdSe QD films of equal thickness grew on both the positive and negative electrodes. This was shown to be the result of unequal numbers of positively and negatively charged QDs initially present in the deposition bath [7]. Film growth continues on both electrodes until the charged QDs with the smallest initial concentration are depleted, whereupon film growth stops. In this work quite different behavior of film growth from a suspension of CdSe QDs is reported. For QDs suspended in hexanes, no film was deposited on either electrode. Upon addition of acetone to the suspension, film growth was observed on the positive electrode only. Two qualitatively different types of film were observed, depending on the volume fraction of acetone in the bath: thin, well adhered films, or thick, but poorly adhered films. The behavior of the film growth can be explained in terms of the speed with which the QDs arrive at the substrate surface, which in turn depends on their electrophoretic mobi
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