Quantum Dot Surface Chemistry: Ligand Effects and Electron Transfer Reactions

With the increased interest in quantum dot sensitized solar cells (QDSCs) there comes a need to better understand how surface modification of quantum dots (QDs) can affect the excited state dynamics of QDs, electron transfer at the QD-metal oxide (MO) interface, and overall photoconversion efficiency of QDSCs. We have monitored the surface modification of solution based QDs via the steady state absorption and emission characteristics of colloidal CdSe passivated with beta-alanine (beta-Ala). The trap-remediating nature of the beta-Ala molecule, arising from the Lewis basicity of the amine group, is realized from the hypsochromic shifts seen in excitonic absorption and emission bands as well as an increase in fluorescence quantum yield. Transient absorption measurements of CdSe-TiO2 films prepared with and without beta-Ala as a linker molecule further reveal the role of the surface modifier in influencing excited state electron transfer processes. Electron transfer at this interface was found to be dependent on the method of QD deposition: CdSe-TiO2 (direct deposition, k(et) = 1.5 x 10(10) s(-1)), CdSe-linker-TiO2 (attaching linker molecule first to TiO2 so that beta-Ala interaction is minimal, k(et) = 2.4 x 10(9) s(-1)), or linker-CdSe-linker-TiO2 (linkage via full beta-Ala encapsulation in solution prior to deposition, k(et) = 6.4 x 10(8) s(-1)). These results imply that the surface chemistry of colloidal CdSe plays an important role in mediating electron transfer reactions.