Professor Wayne Gladfelter

Professor Wayne Gladfelter
Department of Chemistry
University of Minnesota
Abstract

Metal Oxide Nanocrystals as Acceptors in Excited State Electron Transfer Reactions

In photovoltaic devices solar photons are used to separate positive and negative charges for the generation of electrical current. Solid state metal oxides play an important role as electron transport materials in most organic, polymeric, quantum dot, perovskite and dye-sensitized solar cells. In dye-sensitized solar cells the metal oxide, typically a nanocrystalline film of titanium dioxide, zinc oxide or tin dioxide, also plays a central role in the charge separation step. To minimize the complicated kinetics of the charge separation reaction in nanocrystalline metal oxide films, we use monodisperse nanocrystals as a uniform platform to serve as the electron acceptor. This has the added advantage of allowing the electron transfer kinetics to be evaluated in solution. Marcus theory teaches that the electron transfer rate constant depends on the free energy differences between the donor and acceptor states, the electronic coupling between the donor and acceptor and the reorganization energy. In the course of this study a series of fluorescent organic and metal organic dyes were synthesized. Each included an anchor, typically a carboxylic acid, that provided a means for attachment of the dye to the nanocrystal surface. The dye adsorption characteristics were evaluated using a Langmuir adsorption model which revealed the maximum number of dyes that bind to a nanocrystal as well as the binding constant. Cyclic voltammetry and spectroelectrochemical measurements established the energy and spectrum of the one- electron oxidized dye, which were critical for interpreting the spectral changes measured using ultrafast pump probe spectroscopy. Zinc oxide and indium oxide nanocrystals were used as the electron acceptors. Variation of the ZnO nanocrystal diameter allowed the evaluation of the effects of quantum confinement and estimation of the reorganization energy. Variation of the dyes’ molecular structure allowed us to map excited state electron transfer rate constant changes attributed to the difference in the electronic coupling and free energy change between the excited state of the donor and the metal oxide nanocrystals.

Wayne Gladfelter

• BS 1975 Colorado School of Mines; Advisor: Dean W. Dickerhoof
• PhD 1978 Pennsylvania State University; Advisor: Gregory L. Geoffroy
• NSF Postdoc 1978-9 Caltech; Advisor Harry B. Gray