Hierarchical growth of TiO₂ nanosheets on anodic ZnO nanowires for high efficiency dye-sensitized solar cells

We present a novel route to hierarchical core-shell structures consisting of an anodic ZnO nanowire core surrounded by a shell of TiO₂ nanosheets (ZNW@TNS). This material combines the beneficial properties of enhanced electron transport, provided by the nanowire core, with the high surface area and chemical stability of the TiO₂ shell. Quasi-solid-state dye-sensitized solar cells (qssDSSCs) are prepared using different quantities of either the bare ZnO nanowires or the hierarchical nanowire structures and the effect on cell performance is examined. It is found that whilst the addition of the bare ZnO nanowires results in a decrease in cell performance, significant improvements can be achieved with the addition of small quantities of the hierarchical structures. Power conversion efficiencies of up to 7.5% are achieved under 1 Sun, AM 1.5 simulated sunlight, with a ∼30% increase compared to non-hierarchical mesoporous TiO₂ films. A solid-state DSSC (ssDSSC) with a single component solid polymer also exhibits excellent efficiency of 7.2%. The improvement in cell performance is related to the improved light scattering, surface area and electron transport properties via the use of reflectance spectroscopy, BET surface area measurements and electrochemical impedance spectroscopy.