Synthesis and Characterization of Cyanoacrylic Carboxyl Group-Containing Conjugated Molecules and Their Application in P3HT/TiO2 Layered Heterojunction as Interface Modifiers
Date Issued
2008
Date
2008
Author(s)
Weng, Wei-Hsiang
Abstract
In this study, a series of fully-conjugated molecules were synthesized and used as interface modifier (IM) to enhance the compatibility of PTs and TiO2. In order to prevent the charge separation and transport between the interface was blocked by IM, the LUMO level of IMs should between the LUMO of PTs and the conduction band of TiO2. For this reason, the bandgap of IMs were decreased by either increasing the number of thiophene rings or establishing a donor-acceptor structure. In addition, cyanoacrylic carboxyl group was introduced to the molecule structure of IMs. The electron-withdrawing group effectively decreases the LUMO and carboxyl moiety provides anchoring sites to binding with TiO2. The synthesis of IMs involves the synthesis of thiophene oligomers and thiophene derivate of bezothiadiazole by Kumada and Suzuki coupling. Then, Vilsmeier-Haack reaction was adopted to perform one-side formylation of conjugated molecules. Finally, a cyanoacrylic carboxyl group was introduced into the molecular structure by Knoevenagel condensation. The as-synthesized IMs were characterized by NMR and mass spectrometry. The UV/Vis absorption of IMs showed a visible red-shift with increasing thiophene rings, implying the decrease of bandgap. The HOMO level of IMs were determined by cyclic voltammetry and the LUMO level of these IMs were then calculated from the summation of HOMO and bandgap. The results indicated the LUMO of W-2, W-3 and W-4 were lower than that of P3HT. Then, the IMs were used as functional molecules to form self-assembled monolayers on a flat titania thin film. The flat dense-TiO2 thin film was made using spray-pyrolysis method of titanium precursor, followed by calcination at 450oC. Crystallization and morphology of the dense-TiO2 thin film was examined by XRD and scanning electron microscopy. The properties of SAMs were characterized using EDX and contact angle measurements. After being modified by IMs, the contact angle increased from approximate 0o to 55o, indicating the TiO2 surface became hydrophobic. Subsequently, poly(3-hexylthiophene) (P3HT) was spin-coated on top of the IM-covered TiO2 substrate to form a bilayer heterojunction. The PL spectra and quenching efficiency measurements of the thus-prepared heterojunction indicated the decrease of bandgap and LUMO level of cyanoacrylic carboxyl group-containing molecules enhances the photo-induced charge transfer efficiency between polymer and titania.
Subjects
self-assembled monolayer
interface modifier
titanium oxide
bilayer heterojunction
Type
thesis
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