Effect of TiO2 shell layer prepared by wet-chemical method on the photovoltaic performance of ZnO nanowires arrays-based quantum dot sensitized solar cells

TiO 2 shell layer on the surface of ZnO nanowires (NWs) was synthesized by a layer by layer adsorption and reaction (LBLAR) method. Scanning electron microscopy image shows that the prepared ZnO–TiO 2 core–shell NW arrays have a similar morphology with the well-aligned ZnO core NW arrays, providing the LBLAR cycles is controlled within a certain number. The prepared ZnO–TiO 2 core–shell NW arrays were sensitized with CdS quantum dots to assemble quantum dots sensitized solar cells. Results indicate that a dramatic increase in open-circuit voltage and fill factor is achieved with an introduction of the TiO 2 shell layer with a thickness of about 7   nm, resulting in a substantial improvement of the overall energy conversion efficiency. A rapid electron transfer from the CdS QDs to the ZnO NWs and a reduced electron recombination rate due to the energy barrier occurred at the interface between the ZnO core and the TiO 2 shell are believed to contribute to the improved performance of the solar cells based on the ZnO–TiO 2 core–shell NW arrays. In addition, effects of the TiO 2 shell thickness on the performance of the as-fabricated devices are also discussed in detail.