Electron Transfer Studies of Thiophene-Based Oligomers to Zinc Oxide Nanocrystals for Dye-Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) are an alternative to traditional solid-state solar cells which are currently used to harvest solar energy. DSSCs contain semiconductor nanomaterials and dye molecules to harvest light. Currently DSSCs are less efficient compared to solid-state cells and understanding the electron transfer rate between the dyes and nanomaterials can lead to improvements in efficieny. Oligothiophenes are model compounds for polythiophene, commonly used as light absorbers on organic bulk heterojunction solar cells, and have been studied in DSSCs. Oligothiophene-based dyes are bound to zinc oxide nanocrystals (ZnO NCs) to study the rate of electron transfer. Chapter two focuses on terthiophene dyes, an oligothiophene with three thiophenes, with either a carboxylate or phosphonate moiety at the 2-position which serves as an anchoring group to (ZnO NCs). Chapter three focuses on oligomer dyes with two to five thiophenes that were synthesized with a cyanoacrylate moiety, which serves as the anchoring group to ZnO NCs. Electronic absorption and fluorescence measurements, combined with reduction potentials, provided estimates of excited state potentials for all dyes. Static quenching of dyes’ fluorescence was observed when the dyes were bound to ZnO NCs. Ultrafast transient absorption spectroscopic experiments were used to probe the lifetimes of the dyes’ excited singlet state. In the presence of ZnO NCs, the disappearance of the singlet excited states of the dyes corresponds to the appearance of the spectroscopic signatures of the oxidized dyes. The rate of electron transfer from the dye to the nanocrystal was then found.