Journal of the Korean Electrochemical Society Vol. 10, No. 4, 2007, 257-261 257 CdSe Quantum Dots Sensitized TiO 2 Electrodes for Photovoltaic Cells Jun-Ho Yum † , Sang-Hyun Choi †† , Seok-Soon Kim, Dong-Yu Kim, and Yung-Eun Sung †† * Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, S. Korea † Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland, †† School of Chemical Engineering and Research Center for Energy Conversion and Storage, Seoul National University, Seoul 151-744, S. Korea (Received October 11, 2007 : Accepted November 1, 2007) Abstract : The electronic properties of quantum dots can be tuned by changing the size of particles without any change in their chemical composition. CdSe quantum dots, the sizes of which were controlled by changing the con- centrations of Cd and Se precursors, were adsorbed on TiO 2 photoelectrodes and used as sensitizers for photovoltaic cells. For applications of CdSe quantum dot as sensitizers, CdSe/TiO 2 films on conducting glass were employed in a sandwich-type cell that incorporated a platinum-coated conductive glass and an electrolyte consisting of an I - /I 3 - redox. The fill factor (FF) and efficiency for energy conversion ( ç) of the photovoltaic cell was 62 % and 0.32 %, respectively. Keywords : Quantum dot, Quantum dot-sensitized solar cell, CdSe, Nanostructured TiO 2 Introduction The photosensitization of nanocrystalline TiO 2 by an adsorption of a dye was pioneered by O'Regan and Grätzel and investigations continued during the past three decades because the potential as a low cost alternative to conven- tional silicon solar cells. 1-3) In this system, a very large sur- face-to-volume ratio of nanocrystalline TiO 2 is used for coverage of the dye onto a TiO 2 surface. Quantum sized semiconductor materials (quantum dots) such as CdS, PbS, InP, CdSe have recently been reported for possible use as a photosensitizer absorbing photons on TiO 2 . 4-8) Considerable interest has developed regarding quantum dots because of their unique electronic and optical properties. When their size is sufficiently small (below the exciton radius), they exhibit a quantum confinement effect leading to a size- dependent separation between the valence and conduction band. Thus, their band gap can be widened as their size is increased, permitting the optical and electronic properties of materials to be tailored for specific applications. That is, the electronic properties of quantum dots can be tuned by chang- ing the size of the particles without changing their chemical composition. The use of quantum dots as sensitizers have advantages compared to organic dyes that are an adjustable band gap or band edge, effective light harvesting, and their possible stability under sun light. Most quantum dots for use as sensitizers can be produced by in situ-precipitation on the TiO 2 . 4-7) InP quantum dots as sensitizers were obtained by synthesis in a desired size following their adsorption to the surface of TiO 2 nanoparticles. 8 In this work, CdSe quantum dots having a desired size were synthesized and used as sensitizer of TiO 2 nanoparti- cles. CdSe quantum dots have been widely studied and methods of controlling their size are well established. 9-12) The final size of CdSe dots was controlled by changing the con- centration of the Cd and Se precursors. For sensitization of TiO 2 , CdSe quantum dots, the sizes of which were 5.5 0.4 nm, with a corresponding band gap of 2.08 eV were used. CdSe quantum dots/TiO 2 photoelectrodes were then applied for use in photovoltaic cells. Experimental CdSe quantum dots were synthesized by rapidly injecting a solution of dimethylcadmium (Me 2 Cd) (Strem) and selenium (99.999%, Aldrich) in trioctylphosphine (TOP) (95%, Aldrich) into trioctylphosphine oxide (TOPO) (90%, Fluka) at 350 o C with subsequent heating at 250-280 o C Complete details have been described elsewhere. 9) The final size of the CdSe dots was controlled by changing the concentration of Cd and Se precursors when the precursors were injected under condi- tions of maintaining the temperature of the TOPO. The growth of CdSe quantum dots was achieved by secondary injection of concentrated precursors into the solution to obtain the desired size (5.5 nm) with an acceptable size dis- tribution. After washing and removing the TOPO, monodis- persed CdSe quantum dots were produced, dispersed in hexane, and were used as sensitizers of TiO 2 . Nanocrystalline TiO 2 films were prepared as described elsewhere. 2,13) A slurry was prepared by mixing commercial TiO 2 powders (P25, Degussa) with Triton X-100 and acety- lacetone and the slurry was then deposited onto conducting glass (F-doped SnO 2 (FTO), Asahi glass). After air drying, the TiO 2 films were sintered at 425 o C for 1 h. For adsorption *E-mail: ysung@snu.ac.kr