Mesoporous TiO2 single crystals delivering enhanced mobility and optoelectronic device performance

A new low-temperature synthetic method of growing semiconductor mesoporous single crystals of titanium dioxide is described; the resulting films have much higher conductivities and electron mobilities than nanocrystalline titanium dioxide. Mesoporous semiconductors and ceramics have large surface areas that can be exploited in high-performance solar cells, as photocatalysts and in batteries. This paper describes a low-temperature (below 150 °C) general synthetic method of growing micrometre-sized semiconductor mesoporous single crystals of a form of titanium dioxide (TiO2) known as anatase, based on seeded nucleation and growth inside a mesoporous template immersed in a dilute reaction solution. The authors demonstrate that both isolated crystals and ensembles incorporated into films exhibit dramatically higher conductivities and electron mobilities than nanocrystalline TiO2. Dye-sensitized solar cells made from these materials demonstrate 7.3% efficiency, the highest value so far reported using low-temperature processing. The synthesis method should be generally applicable to other functional ceramics and semiconductors. Mesoporous ceramics and semiconductors enable low-cost solar power, solar fuel, (photo)catalyst and electrical energy storage technologies1. State-of-the-art, printable high-surface-area electrodes are fabricated from thermally sintered pre-formed nanocrystals2,3,4,5. Mesoporosity provides the desired highly accessible surfaces but many applications also demand long-range electronic connectivity and structural coherence6. A mesoporous single-crystal (MSC) semiconductor can meet both criteria. Here we demonstrate a general synthetic method of growing semiconductor MSCs of anatase TiO2 based on seeded nucleation and growth inside a mesoporous template immersed in a dilute reaction solution. We show that both isolated MSCs and ensembles incorporated into films have substantially higher conductivities and electron mobilities than does nanocrystalline TiO2. Conventional nanocrystals, unlike MSCs, require in-film thermal sintering to reinforce electronic contact between particles, thus increasing fabrication cost, limiting the use of flexible substrates and precluding, for instance, multijunction solar cell processing. Using MSC films processed entirely below 150 °C, we have fabricated all-solid-state, low-temperature sensitized solar cells that have 7.3 per cent efficiency, the highest efficiency yet reported. These high-surface-area anatase single crystals will find application in many different technologies, and this generic synthetic strategy extends the possibility of mesoporous single-crystal growth to a range of functional ceramics and semiconductors.