Aqueous-phase linker-assisted attachment of cysteinate(2 ^-)-capped CdSe quantum dots to TiO₂ for quantum dot-sensitized solar cells

We have synthesized water-dispersible cysteinate(2^-)-capped CdSe nanocrystals and attached them to TiO₂ using one-step linker-assisted assembly. Room-temperature syntheses yielded CdSe magic-sized clusters (MSCs) exhibiting a narrow and intense first excitonic absorption band centered at 422 nm. Syntheses at 80 C yielded regular CdSe quantum dots (RQDs) with broader and red-shifted first excitonic absorption bands. Cysteinate(2 ^-)-capped CdSe MSCs and RQDs adsorbed to bare nanocrystalline TiO₂ films from aqueous dispersions. CdSe-functionalized TiO ₂ films were incorporated into working electrodes of quantum dot-sensitized solar cells (QDSSCs). Short-circuit photocurrent action spectra of QDSSCs corresponded closely to absorptance spectra of CdSe-functionalized TiO₂ films. Power-conversion efficiencies were (0.43 ± 0.04)% for MSC-functionalized TiO₂ and (0.83 ± 0.11)% for RQD-functionalized TiO₂. Absorbed photon-to-current efficiencies under white-light illumination were approximately 0.3 for both MSC- and RQD-based QDSSCs, despite the significant differences in the electronic properties of MSCs and RQDs. Cysteinate(2^-) is an attractive capping group and ligand, as it engenders water-dispersibility of CdSe nanocrystals with a range of photophysical properties, enables facile all-aqueous linker-assisted attachment of nanocrystals to TiO₂, and promotes efficient interfacial charge transfer.