Band edge engineering of composite photoanodes for dye-sensitized solar cells

As dye-sensitized solar cells (DSSCs) transition from iodide/triiodide-based electrolytes to organometallic complex redox couples with higher rates of recombination with electrons in the semiconductor, there is a need for semiconductor nanostructures that can rapidly transport electrons out of the device while maintaining high surface areas for the semiconductor/dye/electrolyte interface. A previously reported composite, with TiO2 nanoparticles coating ZnO nanorods, met these criteria but suffered from a barrier to electron transfer from the TiO2 to the ZnO. Here, the band edge positions of the TiO2 and ZnO have been shifted by doping with Zr4+ and Co2+, respectively, to arrive at the desired energetic alignment. The materials were characterized using diffuse-reflectance spectroscopy and a three-electrode measurement of the open circuit photovoltage under bandgap excitation (OCV). The OCV measurement indicated that the doping moved the conduction band minimum of ZnO to a more positive potential than that of the TiO2, enabling electron transfer from dye-sensitized TiO2 nanoparticles to the underlying ZnO nanorods for efficient charge collection. However, DSSC devices fabricated with the composite nanostructures did not show improved performance. This paper details a methodology for producing and measuring band-edge shifts along with the benefits and limitations thereof.

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