Browsing by Author "McEwan, JA"

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    The influence of glass transitions on diffusion in OLED stacks
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) McEwan, JA; Clulow, AJ; Shaw, PE; Nelson, A; Yepuri, NR; Darwish, TA; Burn, PL; Gentle, IR
    Of all of the organic electronic devices thus far conceived, organic light emitting diodes (OLEDs) have been the most successfully applied in a commercial setting. With OLED displays now available in the television and portable device markets, the appetite for their continued development continues to garner considerable research interest. Optimised OLED device architectures typically comprise a number of organic layers with thicknesses between 10 nm and 100 nm sandwiched between inorganic electrodes. Each of the organic layers used in the device is sequentially deposited in an order that optimises charge transport and capture, and light emission from the devices. The fidelity and stability of these multilayer organic stacks is therefore of paramount importance in determining the efficiencies and operational lifetimes of OLED devices. Neutron reflectometry is a powerful technique for probing the layered structures found within OLEDs by utilising selective deuteration to provide contrast between or within the layers.1–3 Modelling the changes in the neutron reflectivity profiles of the OLED stacks deposited onto smooth substrates allows for the visualisation of changes in the layered structure in a non destructive manner. In this talk we will outline our recent efforts to relate the thermal properties of the organic materials used in OLED devices with their diffusion behaviour under thermal stress. Our collaboration with the National Deuteration Facility has led to the synthesis of a number of previously unobtainable deuterated analogues of semiconducting molecules typically used in OLEDs and that have a range of thermal characteristics.3–5 These molecules were used in time-resolved reflectometry experiments that have allowed us to systematically build up an understanding of the importance of glass transitions for the stability of OLED stacks.
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    Realtime structural characterisation of thin film OLED stacks during thermally induced diffusion - the importance of glass transitions
    (International Conference on Neutron Scattering, 2017-07-12) Nelson, A; McEwan, JA; Clulow, AJ; Shaw, PE; Darwish, TA; Yepuri, NR; Burn, PL; Gentle, IR
    Organic Light Emitting Diode (OLED) devices are ubiquitous in the displays of many modern electronic devices, including televisions and mobile phones. High efficiency OLEDs are typically made as a sequentially deposited multilayer, with different organic semiconductor layers being required for hole/electron injection/transport, as well as light emission. The operational lifetime, efficiency and fidelity of these OLEDs depends on the structural stability of this multilayer stack, and understanding the factors that affect this stability are crucial in developing new devices. We have systematically characterised the kinetics of structural evolution in these systems as they experience thermally induced diffusion. These leading edge experiments are performed on the Platypus neutron reflectometer, which has been pioneering event mode acquisition techniques to capture quickly changing reflection signals as the multilayers inter-diffuse. Selective deuteration of these semiconductor materials is also critical in providing neutron contrast between each of the layers, without which the experiment could not be carried out. Our investigations reveal the importance of glass transition temperatures on the stability of these OLED systems and provide clear guidelines for material choices when designing new devices. Indeed, with knowledge of each of the T \'s one can predict the way in which diffusion occurs. For example, use of a high Tg emissive layer does not necessarily prevent diffusion from taking place.
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    Time-resolved neutron reflectometry and photovoltaic device studies on sequentially deposited PCDTBT-fullerenel layers
    (ACS Publications, 2014-09) Clulow, AJ; Tao, C; Lee, KH; Velusamy, M; McEwan, JA; Shaw, PE; Yamada, NL; James, M; Burn, PL; Gentle, IR; Meredith, P
    We have used steady-state and time-resolved neutron reflectometry to study the diffusion of fullerene derivatives into the narrow optical gap polymer poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) to explore the sequential processing of the donor and acceptor for the preparation of efficient organic solar cells. It was found that when [6,6]-phenyl-C61-butyric-acid-methyl-ester (60-PCBM) was deposited onto a thin film of PCDTBT from dichloromethane (DCM), a three-layer structure was formed that was stable below the glass-transition temperature of the polymer. When good solvents for the polymer were used in conjunction with DCM, both 60-PCBM and [6,6]-phenyl-C71-butyric-acid-methyl-ester (70-PCBM) were seen to form films that had a thick fullerene layer containing little polymer and a PCDTBT-rich layer near the interface with the substrate. Devices composed of films prepared by sequential deposition of the polymer and fullerene had efficiencies of up to 5.3%, with those based on 60-PCBM close to optimized bulk heterojunction (BHJ) cells processed in the conventional manner. Sequential deposition of pure components to form the active layer is attractive for large-area device fabrication, and the results demonstrate that this processing method can give efficient solar cells. © 2014, American Chemical Society.