Excited State Charge Transfer in Dyads of ZnO Nanocrystals and Organic or Transition Metal Dyes
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Excited State Charge Transfer in Dyads of ZnO Nanocrystals and Organic or Transition Metal Dyes Julia E. Saunders, Adam S. Huss, Jon Bohnsack, Kent R. Mann, David A. Blank, and Wayne L. Gladfelter Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455, U.S.A. ABSTRACT To better understand the specific charge transfer events that occur within a dye-sensitized solar cell (DSSC), we synthesized well-defined ZnO:dye dyads. The ZnO nanocrystals were synthesized following literature procedures from zinc acetate and a hydroxide source in ethanol. The absorption onset of the ZnO nanocrystals was observed using UV-vis measurements, from which estimated nanocrystal diameters were determined. At room temperature, the synthesis yielded nanocrystals ranging in diameter from 2-4 nm. Dispersions of ZnO nanocrystals in ethanol were mixed with solutions containing 5΄΄-phenyl-3΄,4΄-di(nbutyl)-[2,2΄:5΄,2΄΄] terthiophene-5-carboxylic acid. Using FT-IR and fluorescence spectroscopy, it was verified that the dye molecules were adsorbed to the ZnO surface via their carboxylate groups while the number of dye molecules adsorbed to the surface was quantified using a combination of techniques. Adsorption isotherms were employed to probe surface coverage of the dye onto the nanocrystals to yield an adsorption equilibrium constant of 1.5 ± 0.2 x 105 M-1. The ability of ZnO nanocrystals to quench the emission of the dye by an electron transfer mechanism was observed and elucidated using ultra-fast laser spectroscopy where the time-scale for electron injection from the dye to the ZnO was determined to be 5.5 ps. INTRODUCTION With the emergence of dye-sensitized solar cells (DSSCs) on semiconducting films, the past several years have seen an increased interest in exploring the adsorption of various dyes to semiconducting, nanocrystalline surfaces [1]. The most efficient dye-sensitized solar cells (DSSCs) use nanocrystal TiO2 films to which are attached these ruthenium complexes such as N3 and N719 [2,3]. Numerous studies have provided valuable insight into the dynamics of these and analogous photosystems [4,5], but the lack of site homogeneity in binding dye molecules to TiO2 films and nanocrystals (NCs) is a significant impediment to extracting details about the electron transfer across the interface [4,6-8]. Although zinc oxide is emerging as a potential semiconducting component in DSSCs [9-12], there is less known about the factors controlling charge separation across the dye/ZnO interface. Zinc oxide crystallizes in the wurtzite lattice and has a band gap of 3.37 eV. One of the features that makes ZnO especially attractive is the remarkable ability to control the morphology of the films. Using solution deposition processes, one can prepare NCs, nanorods and nanowires having a variety of shapes and dimensions [13-23]. The aim of this research has been to understand complexation of 5΄΄-phenyl-3΄,4΄-di(nbutyl)[2,2΄:5΄,2΄΄]terthiophene-5-carboxylic acid, 1 (Figure 1), to the surface of dispersible
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