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    ABSTRACT: Polymer bulk heterojunction solar cells have been constructed using a thin film molybdenum oxide (MoOx) hole extraction layer that was fabricated by thermally annealing an ammonium molybdate tetrahydrate precursor layer deposited in air by ultrasonic spray coating. Onto this layer was spray cast a PCDTBT:PC70BM film that acted as the active light-harvesting and charge-transporting layer. We optimise the processing steps used to convert the spray-cast MoOx precursor and show that the temperature at which it is annealed is critical to achieving high device efficiency as it both facilitates the removal of trapped solvent as well as driving its chemical conversion to MoOx. We demonstrate that by optimising the spray-casting and annealing process, we are able to create solar cell devices having a peak power conversion efficiency of 4.4%.
    Organic Electronics 03/2014; 15(3). DOI:10.1016/j.orgel.2013.12.028
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    ABSTRACT: We show that lateral phase separation in polymer blend thin films can proceed via the formation of a transient wetting layer which breaks up to give a laterally segregated film. We show that the growth of lateral inhomogeneities at the walls in turn causes the distortion of the interface in the transient wetting layer. By addressing the 1D phase equilibria of a polymer blend thin film confined between selectively attracting walls, we show that the breakup of a transient wetting layer is due to wall-blend interactions; there are multiple values of the volume fraction at the walls which solve equilibrium boundary conditions. This mechanism of lateral phase separation should be general.
    Physical Review Letters 09/2013; 111(12):125702. DOI:10.1103/PhysRevLett.111.125702
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    ABSTRACT: A new donor-acceptor polymer based on 9,9-dioctylfluorene is synthesized and tested in organic photovoltaic devices. Results show that the polymer exhibits good solubility in a range of organic solvents and has a high hole mobility. When blended with a PC70 BM acceptor and fabricated into a bulk heterojunction, photovoltaic devices having a maximum power conversion efficiency (PCE) of 6.2% and a peak external quantum efficiency of 74% are created. Such efficiencies are realized without any necessity for solvent additives or thermal annealing protocols.
    Macromolecular Rapid Communications 07/2013; 34(14). DOI:10.1002/marc.201300258
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    ABSTRACT: The interaction of an electronic spin with its nuclear environment, an issue known as the central spin problem, has been the subject of considerable attention due to its relevance for spin-based quantum computation using semiconductor quantum dots. Independent control of the nuclear spin bath using nuclear magnetic resonance techniques and dynamic nuclear polarization using the central spin itself offer unique possibilities for manipulating the nuclear bath with significant consequences for the coherence and controlled manipulation of the central spin. Here we review some of the recent optical and transport experiments that have explored this central spin problem using semiconductor quantum dots. We focus on the interaction between 10(4)-10(6) nuclear spins and a spin of a single electron or valence-band hole. We also review the experimental techniques as well as the key theoretical ideas and the implications for quantum information science.
    Nature Material 06/2013; 12(6):494-504. DOI:10.1038/nmat3652
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    ABSTRACT: Non-classical light sources offer a myriad of possibilities in both fundamental science and commercial applications. Single photons are the most robust carriers of quantum information and can be exploited for linear optics quantum information processing. Scale-up requires miniaturisation of the waveguide circuit and multiple single photon sources. Silicon photonics, driven by the incentive of optical interconnects is a highly promising platform for the passive optical components, but integrated light sources are limited by silicon's indirect band-gap. III-V semiconductor quantum-dots, on the other hand, are proven quantum emitters. Here we demonstrate single-photon emission from quantum-dots coupled to photonic crystal nanocavities fabricated from III-V material grown directly on silicon substrates. The high quality of the III-V material and photonic structures is emphasized by observation of the strong-coupling regime. This work opens-up the advantages of silicon photonics to the integration and scale-up of solid-state quantum optical systems.
    Scientific Reports 02/2013; 3:1239. DOI:10.1038/srep01239
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    ABSTRACT: An in-plane spin-photon interface is essential for the integration of quantum dot spins with optical circuits. The optical dipole of a quantum dot lies in the plane and the spin is optically accessed via circularly polarized selection rules. Hence, a single waveguide, which can transport only one in-plane linear polarization component, cannot communicate the spin state between two points on a chip. To overcome this issue, we introduce a spin-photon interface based on two orthogonal waveguides, where the polarization emitted by a quantum dot is mapped to a path-encoded photon. We demonstrate operation by deducing the spin using the interference of in-plane photons. A second device directly maps right and left circular polarizations to antiparallel waveguides, surprising for a nonchiral structure but consistent with an off-center dot.
    Physical Review Letters 01/2013; 110(3):037402. DOI:10.1103/PhysRevLett.110.037402
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    ABSTRACT: The characterization of morphology in blend thin-films of conjugated polymers and functionalized fullerenes is a critical aspect in organic photovoltaic (OPV) device research. Understanding the links between thin-film processing conditions, film nanostructure and photocurrent generation efficiency is necessary in order to develop this technology for commercial viability. Here, we review recent developments of experimental studies that probe sample nanostructure formation and modification during the processing steps commonly used in OPV device fabrication, potentially offering a deeper insight and more rational understating of these conditions.
    Reports on Progress in Physics 01/2013; 76(2):022501. DOI:10.1088/0034-4885/76/2/022501
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    ABSTRACT: We present broadband reflectivity spectra of a gold nanoantenna array-silicon dioxide thin film on GaAs. We show that the inclusion of the gold nanoantenna array significantly reduces the reflectivity compared with the thin dielectric film only on GaAs when an appropriate nanoantenna geometry is chosen. Our measurements reveal a highly asymmetric line-shape of the reflectivity spectra close to the nanoantenna localised surface plasmon wavelength due to an interference effect between incident and scattered light. Using detailed scattering matrix simulations we also demonstrate that the reflectivity can be further reduced over the considered wavelength range (350–1200 nm) using this approach. Our results are important for the optimised design of nanoplasmonic-dielectric structures for enhancing in-coupling of light into semiconductor (GaAs, silicon etc) solar cells.
    Solid State Communications 12/2012; 152(24):2156–2159. DOI:10.1016/j.ssc.2012.09.016
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    ABSTRACT: We report surface and interface effects in dynamics and chain conformation in the thin film of conjugated polymer PCDTBT. To probe dynamic anomalies, we measure the glass transition temperature (Tg) of PCDTBT films as a function of thickness, and find that there is a significant depression in Tg for films less than 100nm thick; a result qualitatively similar to that observed in many other polymer film systems. However, for films less than 40nm, the Tg converges to a constant value of 20K below its bulk value. Grazing incidence X-ray diffraction shows depth-dependent molecular organization that is associated with the unusual thickness-dependent dynamics. Graphical abstract
    The European Physical Journal E 12/2012; 35(12):9807. DOI:10.1140/epje/i2012-12129-3
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    ABSTRACT: We use a phase portrait approach to study the phase equilibria of a symmetric binary polymer blend confined between an attracting wall and a neutral wall. We find multiple solutions for this wall regime where only one solution exists for antisymmetric walls. We also argue that, when one wall is a free surface, roughening of the free surface upon lateral phase separation is expected, since thermodynamics alone implies that the coexisting phases should exist at different heights to minimise energy. We focus on the graphical insight provided by this phase portrait approach.
    The Journal of Chemical Physics 11/2012; 137(17):174901. DOI:10.1063/1.4764302
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