Browsing by Author "Burn, PL"
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- ItemThe binding and fluorescence quenching efficiency of nitroaromatic (explosive) vapors in fluorescent carbazole dendrimer thin films(Royal Society of Chemistry, 2013-01-01) Shaw, PE; Cavaye, H; Chen, SSY; James, M; Gentle, IR; Burn, PLWe present a study on three generations of fluorescent carbazole dendrimers that exhibit strong binding with nitroaromatic compounds accompanied by photoluminescence (PL) quenching, making them attractive sensing materials for the detection of explosives such as 2,4,6-trinitrotoluene (TNT). The absorption and release of vapors of the (deuterated) TNT analogue 4-nitrotoluene (pNT) from thin films of the dendrimers were studied with a combination of time-correlated neutron reflectometry and PL spectroscopy. When saturated with pNT the PL of the films was fully quenched and could not be recovered with flowing nitrogen at room temperature but only upon heating to 40-80 [degree]C. Although the majority of the absorbed pNT could be removed with this method the recovered films were found to still contain a residual pNT concentration of [similar]0.1 molecules per cubic nanometer. However, the proportion of the PL recovered increased with generation with the third generation dendrimer exhibiting close to full recovery despite the presence of residual pNT. This result is attributed to a combination of two effects. First, the dendrimer films present a range of binding sites for nitroaromatic molecules with the stronger binding sites surviving the thermal recovery process. Second, there is a large decrease of the exciton diffusion coefficient with dendrimer generation, preventing migration of the excitation to the remaining bound pNT.© 2013, Royal Society of Chemistry
- ItemControlling hierarchy in solution-processed polymer solar cells based on crosslinked P3HT(Wiley-VCH Verlag GmbH & Co. KGaA, 2013-01-01) Tao, C; Aljada, M; Shaw, PE; Lee, KH; Cavaye, H; Balfour, MN; Borthwick, RJ; James, M; Burn, PL; Gentle, IR; Meredith, PUnderstanding and controlling the morphology of donor/acceptor blends is critical for the development of solution processable organic solar cells. By crosslinking a poly(3-n-hexylthiophene-2,5-diyl) (P3HT) film we have been able to spin-coat [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) onto the film to form a structure that is close to a bilayer, thus creating an ideal platform for investigating interdiffusion in this model system. Neutron reflectometry (NR) demonstrates that without any thermal treatment a smaller amount of PCBM percolates throughout the crosslinked P3HT when compared to a non-crosslinked P3HT film. Using time-resolved NR we also show thermal annealing increases the rate of diffusion, resulting in a near-uniform distribution of PCBM throughout the polymer film. XPS measurements confirm the presence of both P3HT and PCBM at the annealed film's surface indicating that the two components are intermixed. Photovoltaic devices fabricated using this bilayer approach and suitable annealing conditions yielded comparable power conversion efficiencies to bulk heterojunction devices made from the same materials. The crosslinking procedure has also enabled the formation of patterned P3HT films by photolithography. Pillars with feature sizes down to 2 μm were produced and after subsequent deposition of PCBM and thermal annealing devices with efficiencies of up to 1.4% were produced.© 2013, Wiley-VCH Verlag GmbH & Co. KGaA
- ItemCorrelation of diffusion and performance in sequentially processed P3HT/PCBM heterojunction films by time-resolved neutron reflectometry(Royal Society of Chemistry, 2013-01-01) Lee, KH; Zhang, YL; Burn, PL; Gentle, IR; James, M; Nelson, A; Meredith, PControl over the structure of donor/acceptor blends is essential for the development of solution processable organic solar cells (OSCs). We have used time-resolved neutron reflectometry (NR) and in situ annealing to investigate the nanoscale structure and interdiffusion of sequentially spin-coated thin films of poly(3-n-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and correlated the evolving structure with the device performance. While the as-prepared film shows a clear two-layer structure it is evident that (19 wt%) PCBM has percolated throughout the lower P3HT layer. Upon heating, analysis of time-resolved NR data shows that the diffusion process is dependent on the annealing temperature. At temperatures up to 110 [degree]C, the two-layer structure is retained and only a small amount of PCBM diffuses from the interface into the lower layer, increasing the total PCBM content throughout the P3HT layer to 26 wt%. Significantly, this small change in acceptor content leads to a profound increase in device performance; with the power conversion efficiency (PCE) of the OSCs increasing from 0.47% (unannealed, 19 wt% PCBM) to 3.23% (annealed, 26 wt% PCBM) with the latter showing a similar efficiency to devices prepared from a blend containing 50 wt% PCBM. Further annealing at 120 and 130 [degree]C sees rapid interdiffusion between the two layers, along with an overall expansion in the thickness of the bilayer film. Despite the complete intermixing of the PCBM and P3HT to form a structure resembling a bulk heterojunction, essentially no improvement in device performance was observed for annealing at temperatures above 110 [degree]C. © 2013, Royal Society of Chemistry
- ItemDetermination of fullerene scattering length density: a critical parameter for understanding the fullerene distribution in bulk heterojunction organic photovoltaic devices(American Chemical Society, 2014-01-27) Clulow, AJ; Armin, A; Lee, KH; Pandey, AK; Tao, C; Velusamy, M; James, M; Nelson, A; Burn, PL; Gentle, IR; Meredith, PFullerene derivatives are commonly used as electron acceptors in combination with (macro)molecular electron donors in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. Understanding the BHJ structure at different electron donor/acceptor ratios is critical to the continued improvement and development of OPVs. The high neutron scattering length densities (SLDs) of the fullerenes provide effective contrast for probing the distribution of the fullerene within the blend in a nondestructive way. However, recent neutron scattering studies on BHJ films have reported a wide range of SLDs ((3.6–4.4) × 10–6 Å–2) for the fullerenes 60-PCBM and 70-PCBM, leading to differing interpretations of their distribution in thin films. In this article, we describe an approach for determining more precisely the scattering length densities of the fullerenes within a polymer matrix in order to accurately quantify their distribution within the active layers of OPV devices by neutron scattering techniques.© 2014, American Chemical Society.
- ItemThe 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, IROf 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.
- ItemInvestigating morphology and stability of Fac-tris (2-phenylpyridyl)iridium(III) films for OLEDs(Wiley-Blackwell Publishing, 2011-06-21) Smith, ARG; Ruggles, JL; Cavaye, H; Shaw, PE; Darwish, TA; James, M; Gentle, IR; Burn, PLStable film morphology is critical for long-term high performance organic light-emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out-of-plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as-prepared blended films of fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)] in 4,4'-bis(N-carbazolyl) biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)(3) in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4-carbazoyl-9-ylphenyl)amine (TCTA)/Ir(ppy)(3):CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films. © 2011, Wiley-Blackwell.
- ItemMorphology of all-solution-processed “bilayer” organic solar cells(Wiley-VCH Verlag Berlin, 2011-02-08) Lee, KH; Schwenn, PE; Smith, ARG; Cavaye, H; Shaw, PE; James, M; Krueger, KB; Gentle, IR; Meredith, P; Burn, PLWe investigate the evolution of the vertical morphology in a solution-processed P3HT/PCBM “bilayer” organic solar cell using a combination of techniques, including neutron reflectometry. By correlating the device performance with the active layer morphology, we establish that the solution processed bilayer concept is a misnomer and sequential solution processing is an elegant way to make bulk heterojunction organic solar cells with high efficiency. © 2011, Wiley-VCH Verlag Berlin
- ItemRealtime 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, IROrganic 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.
- ItemSolid state dendrimer sensors: effect of dendrimer dimensionality on detection and sequestration of 2,4-dinitrotoluene(American Chemical Society, 2011-09-22) Cavaye, H; Shaw, PE; Smith, ARG; Burn, PL; Gentle, IR; James, M; Lo, SC; Meredith, PWe compare two dendrimers, which contain the same luminescent chromophores but differ in dimensionality, for the detection of an explosive analyte via PL quenching. Each dendrimer has first generation biphenyl dendrons with 2-ethylhexyloxy surface groups but differ in the core units. One dendrimer has a bifluorene core and hence has a â€œplanarâ€ structure, whereas the second has four bifluorene units tetrahedrally arranged around an adamantyl center and hence has a â€œthree-dimensionalâ€ structure. Solution Sternâ€“Volmer measurements have previously been reported to show that the three-dimensional dendrimer has a higher binding constant than that of the more planar compound. Films of the dendrimers rapidly detect 2,4-dinitrotoluene (DNT) with thinner films (âˆ¼25 nm) being more responsive than thicker films (âˆ¼85 nm). Neutron reflectometry measurements show that the analyte can diffuse completely through the films with the three-dimensional dendrimer absorbing more of the analyte. The rate of recovery of the PL was faster for the planar dendrimer than the three-dimensional material showing that large binding constants are not necessary for reversible detection of analytes.© 2011, American Chemical Society
- ItemSolid-state dendrimer sensors: probing the diffusion of an explosive analogue using neutron reflectometry(American Chemical Society, 2009-11-03) Cavaye, H; Smith, ARG; James, M; Nelson, A; Burn, PL; Gentle, IR; Lo, SC; Meredith, PDetermining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (230 ± 30 Å) and thick (750 ± 50 Å) films of a first-generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,9′,9′-tetra-n-propyl-2,2′-bifluorene core, can rapidly and reversibly detect p-nitrotoluene by oxidative luminescence quenching. For both the thin and thick films the photoluminescence (PL) is quenched by p-nitrotoluene by 90% in 4 s, which is much faster than that reported for luminescent polymer films. Combined PL and neutron reflectometry measurements on pristine and analyte-saturated films gave important insight into the analyte adsorption process. It was found that during the adsorption process the films swelled, being on average 4% thicker for both the thin and thick dendrimer films. At the same time the PL was completely quenched. On removal of the analyte the films returned to their original thickness and scattering length density, and the PL was restored, showing that the sensing process was fully reversible. © 2009, American Chemical Society
- ItemTime-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, PWe 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.