Prof Henry Snaith FRS

Abstract:

Mesoporous ceramics and semiconductors enable low-cost solar power, solar fuel, (photo)catalyst and electrical energy storage technologies. State-of-the-art, printable high-surface-area electrodes are fabricated from thermally sintered pre-formed nanocrystals. Mesoporosity provides the desired highly accessible surfaces but many applications also demand long-range electronic connectivity and structural coherence. 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.

Abstract:

Solid-state dye-sensitized solar cells offer the possibility of high-power conversion efficiencies due to theoretically lower fundamental losses in dye regeneration. Despite continuous progress, limitations in charge diffusion through the mesoporous photoanode still constrain the device thickness and hence result in reduced light absorption with the most common sensitizers. Here we examine block copolymer-assembled photoanodes with similar surface area and morphology but a large variation in crystal size. We observe that the crystal size has a profound effect on the electron transport, which is not explicable by considering solely the ratio between free and trapped electrons. Our results are consistent with the long-range mobility of conduction band electrons being strongly influenced by grain boundaries. Therefore, maximizing the crystal size while maintaining high enough surface area will be an important route forward. © 2013 American Chemical Society.

Abstract:

Lithium salts have been shown to dramatically increase the conductivity in a broad range of polymeric and small molecule organic semiconductors (OSs). Here we demonstrate and identify the mechanism by which Li(+) p-dopes OSs in the presence of oxygen. After we established the lithium doping mechanism, we re-evaluate the role of lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) in 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-Spirobifluorene (Spiro-OMeTAD) based solid-state dye-sensitized solar cells (ss-DSSCs). The doping mechanism consumes Li(+) during the device operation, which poses a problem, since the lithium salt is required at the dye-sensitized heterojunction to enhance charge generation. This compromise highlights that new additives are required to maximize the performance and the long-term stability of ss-DSSCs.

Abstract:

We present an investigation on the optimisation of solid-state dye sensitized solar cells (SDSCs) comprising mesoporous tin oxide photoanodes infiltrated with poly(3-hexylthiophene-2,5-diyl) (P3HT) hole conductor and sensitized with an organic dye. We chose both the SnO(2) and P3HT for their high charge carrier mobilities and conductivities, but as a result preclude conventional device configurations because of high leakage current and low shunt-resistance. To minimize the "hole leakage current" through the FTO anode, we employed a double compact layer structure, and to minimize "electron leakage current" at the silver cathode, we developed a protocol for depositing an optimal P3HT "capping layer". After optimisation of cell fabrication, the electron lifetime is increased considerably and the solar cells exhibited simulated AM1.5 full sun solar power conversion efficiencies in excess of 1%.