Mesoporous TiO2 single crystals delivering enhanced mobility and optoelectronic device performance.

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.

In situ electrochemical monitoring of selective etching in ordered mesoporous block-copolymer templates.

We present a simple in situ electrochemical probe for the selective etching of the PLA component of thin film poly(4-fluorostyrene)-b-poly(D,L-lactide) (PFS-b-PLA) mesoporous block copolymer templates with a range of highly ordered microphase morphologies. Etching rates between 0.6 and 0.9 nm s⁻¹ were measured in electric-field aligned standing PLA cylinders 12 nm wide and up to 800 nm long. The etching rate within a bicontinuous gyroid network morphology is comparable to that of the hexagonally ordered cylindrical array. A microphase-separated, nonaligned but film-spanning PLA pore structure is found in cylinder forming PFS-b-PLA films immediately after spin coating that could have applications in patterning of functional nanostructured arrays. Cross-film percolation of the PLA phase is confirmed electrochemically, with an etching rate approximately half that of the highly ordered morphologies. The etching rate is independent of template thickness in all three morphologies.

Synthesis and thin film phase behaviour of functional rod-coil block copolymers based on poly(para-phenylenevinylene) and poly(lactic acid).

We report the synthesis of a series of block copolymers consisting of a rod-like semiconducting poly(2,5-di(2'-ethylhexyloxy)-1,4-phenylenevinylene) (DEH-PPV) block and a flexible poly(lactic acid) (PLA) block that can be selectively degraded under mild conditions. Such selectively degradable block copolymers are designed as self-assembling templates for bulk heterojunction donor-acceptor layers in organic solar cells. A lamellar microphase-separated domain structure was identified for block copolymers with PLA volume fractions between 29 and 79% in bulk and thin films using SAXS, TEM, and AFM. Depending on the ratio of the two blocks we find either lamellae oriented parallel or perpendicular to the substrate in thin films.

Soft-etch mesoporous hole-conducting block copolymer templates.

We present a mesoporous hole-conducting polymer film resulting from spontaneous block copolymer self-assembly based on a simple spin-coating protocol. A diblock copolymer consisting of a triphenylamine side group polymer and a poly(d,l-lactide) block (PSTPA-b-PLA) is shown to microphase separate to form ordered 13 nm cylindrical PLA microdomains embedded in the semiconducting PSTPA matrix. Partially ordered and film-spanning PLA domains could be identified in films immediately after spin coating from toluene solutions on conducting substrates. Selective mild etching of the minority PLA domains (in weak aqueous base) leads to a mesoporous hole-conducting polymer matrix. The pore structure is replicated electrochemically in platinum, demonstrating the viability of this approach to producing nano-organized heterojunction structures in thin films.

Block Copolymer Micellar Nanoreactors for the Directed Synthesis of ZnO Nanoparticles.

We report the simple one-pot synthesis of size tunable zinc oxide nanoparticles (ZnO NPs) out of an organometallic ZnO precursor using the self-assembly of solution phase polystyrene-block-poly(2-vinylpyridine) micelles. The resulting hybrid material could be deposited on various substrates in a straightforward manner with the NPs showing size-dependent absorption and photoluminescence due to the quantum-size effect. We compare the results to the assembly of preformed NPs which are selectively incorporated in the poly(2-vinylpyridine) core of the micelles due to the high affinity of ZnO to vinylpyridine.

A bicontinuous double gyroid hybrid solar cell.

We report the first successful application of an ordered bicontinuous gyroid semiconducting network in a hybrid bulk heterojunction solar cell. The freestanding gyroid network is fabricated by electrochemical deposition into the 10 nm wide voided channels of a self-assembled, selectively degradable block copolymer film. The highly ordered pore structure is ideal for uniform infiltration of an organic hole transporting material, and solid-state dye-sensitized solar cells only 400 nm thick exhibit up to 1.7% power conversion efficiency. This patterning technique can be readily extended to other promising heterojunction systems and is a major step toward realizing the full potential of self-assembly in the next generation of device technologies.

Block copolymer morphologies in dye-sensitized solar cells: probing the photovoltaic structure-function relation.

We integrate mesostructured titania arrays into dye-sensitized solar cells by replicating ordered, oriented one-dimensional (1D) columnar and three-dimensional (3D) bicontinuous gyroid block copolymer phases. The solar cell performance, charge transport, and recombination are investigated. We observe faster charge transport in 1D "wires" than through 3D gyroid arrays. However, owing to their structural instability, the surface area of the wire arrays is low, inhibiting the solar cell performance. The gyroid morphology, on the other hand, outperforms the current state-of-the-art mesoporous nanoparticle films.

Freestanding nanowire arrays from soft-etch block copolymer templates.

Nanoporous poly(4-fluorostyrene) templates on gold-coated silicon/silicon oxide substrates were prepared by the electric field alignment of poly(4-fluorostyrene)-b-poly(d,l-lactide) block copolymer thin films followed by mild degradation of the polylactide phase using dilute aqueous base. Electrochemical deposition of nanowires was accomplished using a protocol for the preparation of copper oxide. Freestanding nanowires were observed after removal of the template by either simple dissolution of the poly(4-fluorostyrene) or by treatment with UV irradiation. The annealing time, the electric field strength used to align the block copolymer films, and the template removal method are shown to influence the freestanding nanowire arrays. The "soft-etch" method described is generally useful for the preparation of templates and nanostructures that are sensitive to more aggressive template removal processes.