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

• ZnO–TiO2 core–shell NW arrays were synthesized by a LBLAR method.
• The thickness of TiO2 shells can be controlled by the number of LBLAR cycles.
• Device with a certain TiO2 shell thickness shows much better performance.
• The energy barrier between ZnO and TiO2 contributes to the better performance.

TiO2 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–TiO2 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–TiO2 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 TiO2 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 TiO2 shell are believed to contribute to the improved performance of the solar cells based on the ZnO–TiO2 core–shell NW arrays. In addition, effects of the TiO2 shell thickness on the performance of the as-fabricated devices are also discussed in detail.