Operating organic light-emitting diodes imaged by super-resolution spectroscopy

Super-resolution stimulated emission depletion (STED) microscopy is adapted here for materials characterization that would not otherwise be possible. With the example of organic light-emitting diodes (OLEDs), spectral imaging with pixel-by-pixel wavelength discrimination allows us to resolve local-chain environment encoded in the spectral response of the semi-conducting polymer, and correlate chain packing with local electroluminescence by using externally applied current as the excitation source. We observe nanoscopic defects that would be unresolvable by traditional microscopy. They are revealed in electroluminescence maps in operating OLEDs with 50 nm spatial resolution. We find that brightest emission comes from regions with more densely packed chains. Conventional microscopy of an operating OLED would lack the resolution needed to discriminate these features, while traditional methods to resolve nanoscale features generally cannot be performed when the device is operating. This points the way towards real-time analysis of materials design principles in devices as they actually operateope

Electrochemically Top Gated Graphene: Monitoring Dopants by Raman Scattering

We demonstrate electrochemical top gating of graphene by using a solid polymer electrolyte. This allows to reach much higher electron and hole doping than standard back gating. In-situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, while the 2D peak shows a different response to holes and electrons. Its position increases for hole doping, while it softens for high electron doping. The variation of G peak position is a signature of the non-adiabatic Kohn anomaly at $\Gamma$. On the other hand, for visible excitation, the variation of the 2D peak position is ruled by charge transfer. The intensity ratio of G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor charges.Comment: 7 pages, 8 figure

Vertically segregated hybrid blends for photovoltaic devices with improved efficiency

Solution-processed photovoltaic devices based on blends of conjugated polymers and inorganic semiconductor tetrapods show high efficiencies due to the good electron transport perpendicular to the plane of the film. Here, we show that by using a high-boiling-point solvent, 1,2,4-trichlorobenzene, instead of chloroform for spin-coating, we can typically obtain a threefold increase in solar power conversion efficiency in devices based on CdSe tetrapods and the poly (p -phenylenvinylene) derivative O C1 C10 -PPV. The optimized devices show AM1.5 solar power conversion efficiencies of typically 2.1% with some devices as high as 2.8%. The results can be explained by the occurrence of vertical phase separation which leads to an optimal structure for charge collection. Evidence for this structure is obtained by environmental scanning electron microscopy, photocurrent action spectra measurements, time-resolved photoluminescence, and spectroscopic measurements of exciton dissociation and charge-carrier recombination. © 2005 American Institute of Physics

Successful Treatment of Avulsion Wound in Leopard (Panthera pardus) using Pulsed Lavage – A Case Report

A leopard was referred to the Wildlife Research and Training Centre, Gorewada, Nagpur with a history of extensive wounds on the right and left shoulder. The wound was located on the shoulder bone on both the right and left shoulder, and the spine and caudal angle of the left scapula was exposed. Pulsed lavage along with other supportive therapy was used to manage the case. A 50 ml syringe fitted with an 18 gauge needle to direct the jet. The jet was intermittently pulsed cranio-caudal for 10 minutes five times a day. Pulsed lavage with 1% povidone-iodine in normal saline ensured the efficient removal of dead and necrosed tissue by utilising the pressure and at the same time removing the bacteria by cleansing and dilution. The leopard showed uneventful recovery in 18 days. The need to use antibiotics was by passed with minimal need to restrain the animal

Exciton and polaron dynamics in a step-ladder polymeric semiconductor: the influence of interchain order

We present combined results of femtosecond transient photoluminescence (PL), femtosecond transient absorption and quasi-steady-state photoinduced absorption spectroscopy on the organic semiconductor poly-6, 6', 12, 12'-tetraalkyl-2, 8- indenofluorene (PIF). By control of interchain order via the choice of the side-chain substituents, we have investigated its effect on exciton and polaron dynamics in this model, electronic material. We show that interfaces between ordered and disordered domains play a significant role in the photophysics. At high photoexcitation fluence, a high yield (similar to10%) of polarons is only observed in the ordered semiconductor. This process arises from two-step photoexcitation, first to the lowest exciton, and then to a high-energy state of opposite symmetry. In contrast, triplet exciton population is generated via sequential excitation with smaller yield (<1%) in both ordered and disordered materials. In the low fluence regime, triplet excitons are found to arise from evolution of polarons generated with low efficiency (also <1%) by diffusion-limited processes. The triplet generation yield is strongly dependent on order, with the disordered material displaying a higher yield. Polaron decay is found to be thermally activated, with a higher activation energy and lower room-temperature recombination rate in the ordered material. Furthermore, we do not find that emissive keto defects play a defining role in the PL properties of our PIF samples. Instead, absorption features of aggregate-like species, which we believe to lead to sub-gap emission, are evident in the photocurrent action spectrum of the more ordered PIF derivative

Polaron hopping mediated by nuclear tunnelling in semiconducting polymers at high carrier density

<p>The transition rate for a single hop of a charge carrier in a semiconducting polymer is assumed to be thermally activated. As the temperature approaches absolute zero, the predicted conductivity becomes infinitesimal in contrast to the measured finite conductivity. Here we present a uniform description of charge transport in semiconducting polymers, including the existence of absolute-zero ground-state oscillations that allow nuclear tunnelling through classical barriers. The resulting expression for the macroscopic current shows a power-law dependence on both temperature and voltage. To suppress the omnipresent disorder, the predictions are experimentally verified in semiconducting polymers at high carrier density using chemically doped in-plane diodes and ferroelectric field-effect transistors. The renormalized current-voltage characteristics of various polymers and devices at all temperatures collapse on a single universal curve, thereby demonstrating the relevance of nuclear tunnelling for organic electronic devices.</p>