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    Newman MJ, Speller EM, Barbe J, Luke J, Li M, Li Z, Wang Z-K, Jain SM, Kim J-S, Lee HKH, Tsoi WCet al., 2018,

    Photo-stability study of a solution-processed small molecule solar cell system: correlation between molecular conformation and degradation

    , Science and Technology of Advanced Materials, Vol: 19, Pages: 194-202, ISSN: 1468-6996

    Solution-processed organic small molecule solar cells (SMSCs) have achieved efficiency over 11%. However, very few studies have focused on their stability under illumination and the origin of the degradation during the so-called burn-in period. Here, we studied the burn-in period of a solution-processed SMSC using benzodithiophene terthiophene rhodamine:[6,6]-phenyl C71 butyric acid methyl ester (BTR:PC71BM) with increasing solvent vapour annealing time applied to the active layer, controlling the crystallisation of the BTR phase. We find that the burn-in behaviour is strongly correlated to the crystallinity of BTR. To look at the possible degradation mechanisms, we studied the fresh and photo-aged blend films with grazing incidence X-ray diffraction, UV–vis absorbance, Raman spectroscopy and photoluminescence (PL) spectroscopy. Although the crystallinity of BTR affects the performance drop during the burn-in period, the degradation is found not to originate from the crystallinity changes of the BTR phase, but correlates with changes in molecular conformation – rotation of the thiophene side chains, as resolved by Raman spectroscopy which could be correlated to slight photobleaching and changes in PL spectra.

    Keivanidis PE, Khan JI, Katzenmeier L, Kan Z, Limbu S, Constantinou MK, Lariou E, Constantinides G, Hayes SC, Kim J-S, Laquai Fet al., 2018,

    Impact of structural polymorphs on charge collection and non-geminate recombination in organic photovoltaic devices

    , The Journal of Physical Chemistry C, Vol: 122, Pages: 29141-29149, ISSN: 1932-7447

    The formation of different types of structural polymorphs of poly(3-hexyl-thiophene) (P3HT) affects the performance of organic photovoltaic (OPV) devices that use thermally-annealed P3HT:PCBM[60] blend films as photoactive layer. Here it is demonstrated that, when densely-packed and non-densely packed P3HT polymorphs co-exist in the P3HT:PCBM[60] layer, non-geminate charge recombination is fast; however, in a device non-geminate recombination is effectively overruled by efficient and fast charge carrier extraction. In stark contrast, when only a less-densely packed P3HT polymorph is present in the blend, non-geminate charge recombination losses are less pronounced, and the charge carrier extraction efficiency is lower. The antagonistic non-geminate charge recombination and charge carrier extraction processes in these systems are monitored by time-delayed-collection field (TDCF) and ultrafast transient absorption (TA) experiments. Furthermore, resonance Raman spectroscopy reveals that in the absence of the densely-packed P3HT polymorph, the energetic disorder present in the P3HT:PCBM[60] blend is higher. High-resolution atomic force microscopy imaging further identifies pronounced differences in the layer morphology when the polymorph distribution varies between unimodal and bimodal. These results indicate that less-densely packed P3HT polymorphs increase disorder and impede charge collection, leading to a reduction of the device fill factor.

    Lee S, Kim DB, Hamilton I, Daboczi M, Nam YS, Lee BR, Zhao B, Jang CH, Friend RH, Kim J-S, Song MHet al., 2018,

    Control of interface defects for efficient and stable quasi-2D Perovskite light-emitting diodes using nickel oxide hole injection layer

    , Advanced Science, Vol: 5, ISSN: 2198-3844

    Metal halide perovskites (MHPs) have emerged as promising materials for light‐emitting diodes owing to their narrow emission spectrum and wide range of color tunability. However, the low exciton binding energy in MHPs leads to a competition between the trap‐mediated nonradiative recombination and the bimolecular radiative recombination. Here, efficient and stable green emissive perovskite light‐emitting diodes (PeLEDs) with an external quantum efficiency of 14.6% are demonstrated through compositional, dimensional, and interfacial modulations of MHPs. The interfacial energetics and optoelectronic properties of the perovskite layer grown on a nickel oxide (NiOx) and poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate hole injection interfaces are investigated. The better interface formed between the NiOx/perovskite layers in terms of lower density of traps/defects, as well as more balanced charge carriers in the perovskite layer leading to high recombination yield of carriers are the main reasons for significantly improved device efficiency, photostability of perovskite, and operational stability of PeLEDs.

    Kim H, Lee G, Becker S, Kim J, Kim H, Hwang Bet al., 2018,

    Novel patterning of flexible and transparent Ag nanowire electrodes using oxygen plasma treatment

    , Journal of Materials Chemistry C, Vol: 6, Pages: 9394-9398, ISSN: 2050-7526

    We report a novel patterning method using oxygen plasma treatment for flexible and transparent Ag nanowire electrodes. Using a dry film photoresist as a solid-state film-type photoresist, Ag nanowires were selectively oxidized under oxygen plasma treatment. Microstructural analysis revealed that the Ag nanowires were fully oxidized after 30 s of oxygen plasma treatment, which was also reflected in the changes in the optoelectronic properties of the Ag nanowires. The fully oxidized Ag nanowires could be completely dissolved in NH3 solution (aq.), without using a toxic etchant to form sharp patterns of Ag nanowire electrodes. To further confirm the applicability of the patterning technique demonstrated here in electronic devices, MoS2 thin-film transistors (TFTs) with patterned Ag-nanowire source/drain (S/D) electrodes were fabricated and they showed similar performances to typical MoS2 TFTs with thin-film-type Ti/Au S/D electrodes.

    Das PK, Slawinska J, Vobornik I, Fujii J, Regoutz A, Kahk JM, Scanlon DO, Morgan BJ, McGuinness C, Plekhanov E, Di Sante D, Huang Y-S, Chen R-S, Rossi G, Picozzi S, Branford WR, Panaccione G, Payne DJet al., 2018,

    Role of spin-orbit coupling in the electronic structure of IrO2

    , Physical Review Materials, Vol: 2, ISSN: 2475-9953

    The delicate interplay of electronic charge, spin, and orbital degrees of freedom is in the heart of many novel phenomena across the transition metal oxide family. Here, by combining high-resolution angle-resolved photoemission spectroscopy and first principles calculations (with and without spin-orbit coupling), the electronic structure of the rutile binary iridate, IrO2, is investigated. The detailed study of electronic bands measured on a high-quality single crystalline sample and use of a wide range of photon energy provide a huge improvement over the previous studies. The excellent agreement between theory and experimental results shows that the single-particle DFT description of IrO2 band structure is adequate, without the need of invoking any treatment of correlation effects. Although many observed features point to a 3D nature of the electronic structure, clear surface effects are revealed. The discussion of the orbital character of the relevant bands crossing the Fermi level sheds light on spin-orbit-coupling-driven phenomena in this material, unveiling a spin-orbit-induced avoided crossing, a property likely to play a key role in its large spin Hall effect.

    Hamilton I, Chander N, Cheetham NJ, Suh M, Dyson M, Wang X-H, Stavrinou PN, Cass M, Bradley DDC, Kim J-Set al., 2018,

    Controlling molecular conformation for highly efficient and stable deep-blue copolymer light-emitting diodes

    , ACS Applied Materials and Interfaces, Vol: 10, Pages: 11070-11082, ISSN: 1944-8244

    We report a novel approach to the achievement of deep-blue, high-efficiency, and long-lived solution processed polymer light-emitting diodes (PLEDs) via a simple molecular-level conformation change whereby we introduce rigid β-phase segments into a 95% fluorene - 5% arylamine copolymer emission layer (EML). The arylamine moieties at low density act as efficient exciton formation sites in PLEDs whilst the conformational change alters the nature of the dominant luminescence from a broad, charge-transfer like emission to a significantly blue-shifted and highly vibronically structured, excitonic emission. As a consequence, we observe a significant improvement in Commission International de L'Eclairage (CIE) (x, y) co-ordinates from (0.149, 0.175) to (0.145, 0.123) whilst maintaining high efficiency and improving stability. We achieve peak luminous efficiency, η = 3.60 cd/A and luminous power efficiency, ηw = 2.44 lm/W; values that represent state of the art performance for single copolymer deep-blue PLEDs. These values are five-fold better than for otherwise-equivalent, β-phase poly(9,9-dioctylfluorene) (PFO) EML PLEDs (0.70 cd/A and 0.38 lm/W). This report represents the first demonstration of the use of molecular conformation as a vector to control the optoelectronic properties of a fluorene copolymer; previous examples have been confined to homopolymers.

    Gartside JC, Arroo DM, Burn DM, Bemmer VL, Moskalenko A, Cohen LF, Branford WRet al., 2017,

    Realization of ground state in artificial kagome spin ice via topological defect-driven magnetic writing.

    , Nature Nanotechnology, Vol: 13, Pages: 53-58, ISSN: 1748-3387

    Arrays of non-interacting nanomagnets are widespread in data storage and processing. As current technologies approach fundamental limits on size and thermal stability, enhancing functionality through embracing the strong interactions present at high array densities becomes attractive. In this respect, artificial spin ices are geometrically frustrated magnetic metamaterials that offer vast untapped potential due to their unique microstate landscapes, with intriguing prospects in applications from reconfigurable logic to magnonic devices or hardware neural networks. However, progress in such systems is impeded by the inability to access more than a fraction of the total microstate space. Here, we demonstrate that topological defect-driven magnetic writing-a scanning probe technique-provides access to all of the possible microstates in artificial spin ices and related arrays of nanomagnets. We create previously elusive configurations such as the spin-crystal ground state of artificial kagome dipolar spin ices and high-energy, low-entropy 'monopole-chain' states that exhibit negative effective temperatures.

    Castro-Lopez M, Gaio M, Sellers S, Gkantzounis G, Florescu M, Sapienza Ret al., 2017,

    Reciprocal space engineering with hyperuniform gold disordered surfaces

    , APL Photonics, Vol: 2, ISSN: 2378-0967

    Hyperuniform geometries feature correlated disordered topologies which followfrom a tailored k-space design. Here, we study gold plasmonic hyperuniformdisordered surfaces and, by momentum spectroscopy, we report evidence of kspaceengineering on both light scattering and light emission. Even if the structureslack a well-defined periodicity, emission and scattering are directional inring-shaped patterns. The opening of these rotational-symmetric patterns scaleswith the hyperuniform correlation length parameter as predicted via the spectralfunction method.

    Burn DM, Chadha M, Branford WR, 2017,

    Dynamic dependence to domain wall propagation through artificial spin ice

    , PHYSICAL REVIEW B, Vol: 95, ISSN: 2469-9950

    Domain wall propagation dynamics has been studied in nanostructured artificial kagome spin-ice structures. A stripline circuit has been used to provide localized pulsed magnetic fields within the artificial spin-ice (ASI) structure. This provides control of the system through electrically assisted domain wall nucleation events. Synchronization of the pulsed fields with additional global magnetic fields and the use of a focused magneto-optical Kerr effect magnetometer allows our experiments to probe the domain wall transit through an extended ASI structure. We find that the propagation distance depends on the driving field revealing field-driven properties of domain walls below their intrinsic nucleation field.

    Van DT, Caixeiro S, Fernandes FM, Sapienza Ret al., 2017,

    Microsphere Solid-State Biolasers

    , Advanced Optical Materials, Vol: 5, ISSN: 2195-1071

    Biolasers obtained from biomaterials are attracting a wealth of interest for their potential as future biosensors with enhanced sensitivity, and advanced cell tracking. Here, miniature biolasers are reported, which are formed by bovine serum albumin (BSA) protein and biosourced polysaccharides derived from land plants such as cellulose and pectin. Using a green processing route aided by simple emulsions, solid-state microspheres with diameters of 15–100 µm are fabricated and utilized as whispering gallery mode lasers with thresholds of a few µJ mm–2 and quality factors of up to 3000. Furthermore, BSA microlasers are found to be compatible with cell growth and resistant to the aqueous environment of cell culture media. This environmentally friendly and biocompatible design shows promise for future implantable biosensing devices opening a path between laser science and medicine.

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