Abstract
The sensitive electronic environment at the quantum dot (QD)–dye interface becomes a roadblock to enhancing the energy conversion efficiency of dye-functionalized quantum dots (QDs). Energy alignments and electronic couplings are the critical factors governing the directions and rates of different charge transfer pathways at the interface, which are tunable by changing the specific linkage groups that connect a dye to the QD surface. The variation of specific anchors changes the binding configurations of a dye on the QD surface. In addition, the presence of a co-adsorbent changes the dipole–dipole and electronic interactions between a QD and a dye, resulting in different electronic environments at the interface. In the present work, we performed density functional theory (DFT)-based calculations to study the different binding configurations of N719 dye on the surface of a Cd33Se33 QD with a co-adsorbent D131 dye. The results revealed that the electronic couplings for electron transfer were greater than for hole transfer when the structure involved isocyanate groups as anchors. Such strong electronic couplings significantly stabilize the occupied states of the dye, pushing them deep inside the valence band of the QD and making hole transfer in these structures thermodynamically unfavourable. When carboxylates were involved as anchors, the electronic couplings for hole transfer were comparable to electron transfer, implying efficient charge separation at the QD–dye interface and reduced electron–hole recombination within the QD. We also found that the electronic couplings for electron transfer were larger than those for back electron transfer, suggesting efficient charge separation in photoexcited QDs. Overall, the current computational study reveals some fundamental aspects of the relationship between the interfacial charge transfer for QD@dye composites and their morphologies which benefit the design of QD-based nanomaterials for photovoltaic applications.
Highlights
Dye-sensitized photocatalytic water splitting or photovoltaic systems have stimulated intense interest in the research community because of their potential to become a substitute for non-renewable energy resources
Our results suggested that the relative positions of the dye and quantum dot (QD)’s orbitals were fairly sensitive to the adsorption geometries of the Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2 (N719) dye on the cadmium selenide (CdSe) QD surface, controlled through the specific linkage group anchoring the dye to the QD surface
We performed density functional theory (DFT)-based calculations to study the electronic and geometrical structures of a CdSe QD functionalized with N719 dye and a coadsorbent, that is, D131 dye
Summary
Dye-sensitized photocatalytic water splitting or photovoltaic systems have stimulated intense interest in the research community because of their potential to become a substitute for non-renewable energy resources. Quantum dots (QDs) possess size-tunable electronic and optical properties [2]; they can convert solar energy more efficiently than dyes. In the strong quantum confinement region, electrons and holes are treated approximately as independent particles. Their coulomb interactions are negligible compared to the quantization effect, which reduces the electron– hole recombination rates in QDs. their coulomb interactions are negligible compared to the quantization effect, which reduces the electron– hole recombination rates in QDs This allows more photogenerated charge carriers to participate in a photocatalytic reaction or photovoltaic energy conversion process
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