Direct Correlation between Ultrafast Injection and Photoanode Performance in Quantum Dot Sensitized Solar Cells

The performance of quantum dot (QD) sensitized solar cells depends mainly on both electron injection from the QDs to the oxide matrix and recombination rates. Here we show a direct correlation between ultrafast carrier dynamics and photoanode (and complete solar cell) performance. TiO2 nanoparticulate electrodes sensitized with colloidal CdSe QDs are prepared by either direct or linker-assisted adsorption (using cysteine, p-mercaptobenzoic acid, and mercaptopropionic acid). These electrodes are examined by ultrafast carrier dynamics, photopotential, and incident photon-to-current efficiency measurements to unravel factors controlling the efficiency in a closed solar cell. Subpicosecond time-resolved measurements are carried out by means of a lens-free heterodyne transient grating technique. In general, faster electron injection is observed for QDs directly adsorbed on TiO2, which correlates with a better cell performance. Otherwise, increasingly faster electron injection is obtained as QD size decreases, regardless the mode of attachment. Photopotential measurements are performed in either sulfite or polysulfide solutions, in order to isolate different recombination pathways. The slowest recombination is reported for direct adsorption, whereas cysteine-mediated adsorption shows faster recombination. This study stresses the utility of ultrafast kinetic characterization in the development of efficient photoconverter devices.