Preparation and Characterization of Nickel Oxide Photocathodes Sensitized with Colloidal Cadmium Selenide Quantum Dots

Quantum dot sensitized solar cells (QDSCs) are receiving a lot of attention as promising third generation solar cells, being virtually all of them based on sensitized photoanodes. Finding efficient QD-sensitized photocathodes would pave the way toward the implementation of tandem QDSCs. In this context, NiO photocathodes have been sensitized with colloidal CdSe quantum dots directly attached to the semiconductor oxide surface. The emission spectra indicate effective hole injection from the excited state of the quantum dots to the valence band of the NiO. A maximum incident current to photon conversion efficiency of 17% at 420 nm has been achieved. For the sake of comparison, other ways to prepare and anchor the QDs have been tested. Sensitization routes based on presynthesized colloidal quantum dots show better results than in situ growth techniques such as successive ionic layer adsorption and reaction. Electrochemical impedance measurements have identified transport resistance in NiO as one of the limiting factors in the performance of the system under study. Interestingly, surface treatments based on the deposition of very thin films of either SiO2 or Al2O3 can diminish recombination at the NiO/CdSe/electrolyte interface. This work also identifies a number of possible routes for the improvement of this kind of electrodes, unveiling their potential use in tandem quantum dot solar cells.