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We report on efficient polymeric white organic light-emitting diodes with unprecedented stability. The investigated devices are based on an electroluminescent copolymer of electron and hole-transporting units and red-, green-, and blue-emitting chromophores. We find that the glass transition of the polymer (Tg=182.5 °C) is the process determining the relation between thermal annealing during fabrication and device lifetime. For devices annealed below Tg, the device lifetime significantly increases with increasing annealing temperature. For annealing temperatures above Tg, however, the current density in the devices rapidly increases while their lifetime slightly decreases. Insight into the underlying processes is provided by atomic force microscopy phase imaging and by UV/visible and fluorescence spectroscopy. We also investigated the influence of the operating temperature of the device: besides the commonly known fact that elevated operating temperatures reduce the lifetime, we discovered that the acceleration coefficient, which determines the scaling of the device lifetime with applied current density, was reduced. At the glass transition, the device lifetime no longer depended on the current density. The device lifetime was improved even further by introducing an additional cross-linkable hole-transport layer. Optimized devices achieve a half-luminance lifetime of 1860 h when operated at room temperature and at an initial luminance of 500 cd m-2. As a result of the relatively balanced stability of the three chromophores, the emission spectrum remains virtually unchanged over the entire device lifetime. Finally, to reduce the time required for the lifetime measurements, we propose to analyze the voltage increase over the first 10–50 h of the lifetime test and find that this allows precisely estimating the lifetime of our devices. [ABSTRACT FROM AUTHOR]

The authors study the role of the conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulphonate) (PEDOT:PSS) in determining the transient and steady-state operation of a blue-emitting polymer light-emitting diode. Combining the results from photoemission spectroscopy, time-of-flight photocurrent measurements, and studies on hole-only devices reveals a significant barrier for the injection of holes into the polymer. Simulations with a numerical drift-diffusion model, however, show that the injection currents determined from single-carrier devices cannot account for the rapid transient luminance onset and the efficient steady-state luminance output of the corresponding bipolar light-emitting devices. It is shown that the transient electroluminescence traces measured at different external bias can be well reproduced when assuming the presence of a weak barrier for electron extraction at this interface, which is attributed to electron accumulation at a thin phase-segregated PSS-rich layer at the surface of PEDOT:PSS. In addition, interface conditioning, presumably due to electron trapping near PEDOT:PSS, renders the anode-polymer interface nearly Ohmic. This conditioning, however, occurs on intermediate time scales normally not addressed by either transient or steady-state measurements. [ABSTRACT FROM AUTHOR]

Nondispersive hole transport in two polyspirobifluorene copolymers containing either 10% anthracene or 10% carbazole was studied in detail by the charge-generation layer time-of-flight (TOF) technique over a wide range of electric fields and temperatures. The TOF transients of both polymers showed a clear plateau indicating nondispersive transport of charge carriers. Zero-field mobilities were found to be in the order of 10-6 cm2/V s at room temperature. Results were analyzed within the framework of the Gaussian disorder model to extract the parameters of the charge-carrier transport. The width of the transport density of states was determined to be 83 meV for the polyspirobifluorene-anthracene copolymer and 89 meV for the polyspirobifluorene-carbazole copolymer. At lower temperatures a change of slope in the temperature dependence of the zero-field mobility was observed. At higher temperatures the TOF transients were modified by a cusp. Both phenomena can be explained within the framework of the Gaussian disorder model. [ABSTRACT FROM AUTHOR]

Hole transport in a polyspirobifluorene homopolymer and a statistical polyspirobifluorene-triarylamine copolymer has been studied in detail employing the charge-generation layer time-of-flight (TOF) technique over a wide range of electric fields and temperatures. Both materials exhibit nondispersive TOF signals after injection of a sheet of charge carriers from a thin (10 nm) perylene-diimide charge-carrier generation layer into a relatively thick (d>1 μm) polymer film. Results were analyzed within the framework of the Gaussian disorder model and the charge transport parameters were extracted for both polymers. The zero-field hole mobility of the spirohomopolymer was found to be on the order of 10-6 cm2/V s, whereas the copolymer showed a considerably lower hole mobility of 6×10-8 cm2/V s. The width of the density of states σ was determined to be 86 meV for the homopolymer and 107 meV for the copolymer. The latter polymer also showed an increased positional disorder due to the statistically incorporated triarylamine units. [ABSTRACT FROM AUTHOR]