Date of Award
Doctor of Philosophy
Dr. Ramakrishna Guda
Dr. Michael Barcelona
Dr. Sherine Obare
Dr. Clement Burns
Interfacial charge transfer, dye sensitized solar cells, two photon absorption, surface electric fields, acetylacetonate, curcumin
Interfacial charge transfer (ICT) across the molecule-TiO2 nanoparticle interface has gained enormous research attention for applications in dye sensitized solar cells (DSSC), photo-catalysis, water splitting and nonlinear optics. DSSCs are promising clean alternative energy sources. However, current DSSCs suffer from lower efficiencies and higher cost. Better understanding of the ICT processes in DSSCs can help solve these problems. We have used two strategies to understand ICT in the context of DSSCs. Firstly, we used a computationally validated anchor group, acetylacetonate (acac) to bind molecules to the semiconductor surface and facilitate charge separation. Secondly, we used natural dye sensitizers, which possess the acac anchoring group in developing cost-effective DSSCs. We monitored the ICT dynamics in acac derivatives using ultrafast luminescence and transient absorption techniques. Our results show that acac is best for sensitizing TiO2 and ICT is dependent on the mode of complexation. Also, they show that the curcuminoids with acac as anchoring group are good sensitizers for dye solar cells.
The understanding of ICT was further applied in developing materials with better two photon absorption (2PA) cross-sections. In pursuit of better 2PA materials, we used a novel idea of stationing chromophores on charge-transfer (CT) modified semiconductor nanoparticles to enhance their 2PA cross-sections. As a proof of principle, we synthesized two dye molecules with a catechol anchoring group that form (CT) complexes without injecting electrons into TiO2. Ultrafast fluorescence and transient measurements have shown that the dye molecules inefficiently inject electrons and catechol forms a CT complex with TiO2 surface. A 3-fold 2PA cross-section enhancement was observed when the dye was stationed on the TiO2 surface. The 2PA enhancement was assigned to the increased electric field on the surface of TiO2 from the catechol-TiO2 CT complex. To amplify the electric field, the TiO2 surface was modified by small molecules like catechol and salicylic acid which form CT complexes with TiO2. We have shown a 40-fold 2PA enhancement on surface modified TiO2 nanoparticles suggesting that 2PA cross-section enhancement is possible by the appropriate stationing of chromophores on surfaces via a phenomenon that can be termed as “Surface- Enhanced 2PA”.
Mghanga, Edwin, "Ultrafast Interfacial Electron Transfer across Molecule-TiO2 Nanocomposites: Towards Solar Cells and Two Photon Absorption" (2014). Dissertations. 385.