Sensitization of Titanium Dioxide Photoanodes with Cadmium Selenide Quantum Dots Prepared by SILAR: Photoelectrochemical and Carrier Dynamics Studies

Quantum dot sensitized solar cells have the potentiality to emerge as viable third-generation photovoltaic devices. In these cells, the photoanode is constituted by a nanostructured oxide layer sensitized to the visible by suitable quantum dots (QDs). Here, we report on some aspects of the sensitization of titanium dioxide nanoparticulate electrodes with CdSe QDs deposited by the so-called successive ionic layer adsorption and reaction (SILAR) method. Parallel SILAR experiments with nanoparticulate and rutile single crystal electrodes indicate that an almost full coverage of the titanium dioxide surface is achieved from the initial stages of the process. However, photoelectrochemical experiments done with both SILAR and colloidal CdSe QD sensitized electrodes indicate a faster recombination in the former case. Furthermore, a study of the ultrafast carrier dynamics of CdSe QDs by means of the near-field heterodyne detection transient grating method reveals that the rate of the photogenerated electron injection from the QD to the oxide significantly decreases with the number of deposition cycles. This parallels the performance of the photoanode as well as the overall behavior of the photovoltaic cell, which tends to saturate as the number of SILAR cycles increases. The intrinsic limitations of the photoanodes prepared by SILAR are attributed to both the existence of a high concentration of QD/QD interfaces and the absence of effective passivation of the TiO2 and CdSe surfaces.