Edward J. W. Crossland

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.

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.

Optical properties and limiting photocurrent of thin-film perovskite solar cells

Metal-halide perovskite light-absorbers have risen to the forefront of photovoltaics research offering the potential to combine low-cost fabrication with high power-conversion efficiency. Much of the development has been driven by empirical optimisation strategies to fully exploit the favourable electronic properties of the absorber layer. To build on this progress, a full understanding of the device operation requires a thorough optical analysis of the device stack, providing a platform for maximising the power conversion efficiency through a precise determination of parasitic losses caused by coherence and absorption in the non-photoactive layers. Here we use an optical model based on the transfer-matrix formalism for analysis of perovskite-based planar heterojunction solar cells using experimentally determined complex refractive index data. We compare the modelled properties to experimentally determined data, and obtain good agreement, revealing that the internal quantum efficiency in the solar cells approaches 100%. The modelled and experimental dependence of the photocurrent on incidence angle exhibits only a weak variation, with very low reflectivity losses at all angles, highlighting the potential for useful power generation over a full daylight cycle.

Anisotropic Charge Transport in Spherulitic Poly(3-hexylthiophene) Films

Poly(3-hexylthiophene) (P3HT) is one of the most important semiconducting polymers for organic photovoltaics and optoelectronics.[1] Yet, because of the structural complexity of semicrystalline polymers, comparatively little is known conclusively about the relationship between charge transport and microstructure in P3HT-containing semiconductors.[2–9] This lack of information arises mainly from an inability to manufacture samples with spontaneous long-range crystalline order approaching macroscopic dimensions, that allow characterisation in a similar fashion to inorganic single crystals. We have overcome this limitation and grown well-ordered P3HT spherulites, sufficiently large to measure charge transport within single oriented crystal domains, enabling measurements of the charge mobility both parallel and perpendicular to the characteristic π-stacked nanocrystalline lamellae and across spherulite boundaries. The importance of local order and molecular orientation in determining the electronic properties of polymer semiconductor films is now well established.[3] In the case of the poly(alkylthiophenes) (PAT)s an edge-on orientation of the molecules, such that the alkyl sidechains (a-axis) stand vertically and both the chain backbone (c-axis) and π-stacking direction (b-axis) lie in the plane of the film, is critical in achieving high in-plane

Block copolymer directed synthesis of mesoporous TiO2 for dye-sensitized solar cells

The morphology of TiO2 plays an important role in the operation of solid-state dye-sensitized solar cells. By using polyisoprene-block-ethyleneoxide (PI-b-PEO) copolymers as structure directing agents for a sol-gel based synthesis of mesoporous TiO2, we demonstrate a strategy for the detailed control of the semiconductor morphology on the 10 nm length scale. The careful adjustment of polymer molecular weight and titania precursor content is used to systematically vary the material structure and its influence upon solar cell performance is investigated. Furthermore, the use of a partially sp2 hybridized structure directing polymer enables the crystallization of porous TiO2 networks at high temperatures without pore collapse, improving its performance in solid-state dye-sensitized solar cells.

Monolithic route to efficient dye-sensitized solar cells employing diblock copolymers for mesoporous TiO2

We present a material and device based study on the fabrication of mesoporous TiO2 and its integration into dye-sensitized solar cells. Poly(isoprene-block-ethyleneoxide) (PI-b-PEO) copolymers were used as structure directing agents for the sol–gel based synthesis of nanoporous monolithic TiO2 which was subsequently ground down to small particles and processed into a paste. The TiO2 synthesis and the formation of tens of micrometre thick films from the paste is a scalable approach for the manufacture of dye sensitised solar cells (DSCs). In this study, we followed the self-assembly of the material through the various processing stages of DSC manufacture. Since this approach enables high annealing temperatures while maintaining porosity, excellent crystallinity was achieved. Internal TiO2 structures ranging from the nanometre to micrometre scale combine a high internal surface area with the strong scattering of light, which results in high light absorption and an excellent full-sun power conversion efficiency of up to 6.4% in a robust, 3 μm thick dye-sensitized solar cell.

Control of gyroid forming block copolymer templates: effects of an electric field and surface topography

The control over 10 nm scale porosity derived from self-assembly of copolymers is an extremely promising method for the synthesis of organic–inorganic hybrid materials applied, for example, in solar cells. Here, we report the thin film behaviour of a poly(4-fluorostyrene)-b-poly(D,L-lactide) PFS-b-PLA block copolymer which adopts the bicontinuous gyroid phase in the bulk and may be used to form a porous template suitable for patterning functional materials by selective degradation of the minority PLA domains. The response of the copolymer morphology to DC electric fields is probed at temperatures where the bulk copolymer adopts either the gyroid or the cylindrical phase. At 150 °C electric field alignment results in vertical arrays of cylinders, lamellae, and perforated lamellae while at 180 °C the gyroid phase coexists with a standing perforated lamellar phase. We show that both polymer–substrate interactions and substrate topography are critical factors determining substrate reconstruction of the gyroid phase. Spontaneous cross-film percolation of the minority network phase on a given substrate, a prerequisite for electrochemical replication, is dependent on surface topology at the scale of the gyroid unit cell. Importantly, under suitable processing conditions all these complex copolymer morphologies can be electrochemically replicated to produce highly ordered freestanding nanostructured arrays over large areas.

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.

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.

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.