Carsten Rothe

Bio: Carsten Rothe is an academic researcher from Durham University. The author has contributed to research in topics: OLED & Phosphorescence. The author has an hindex of 25, co-authored 99 publications receiving 2352 citations.

Carbazole compounds as host materials for triplet emitters in organic light-emitting diodes: polymer hosts for high-efficiency light-emitting diodes.

Abstract: A carbazole homopolymer and carbazole copolymers based on 9,9′-dialkyl-[3,3′]-bicarbazolyl, 2,5-diphenyl-[1,3,4]-oxadiazole and 9,9-bis(4-[3,7-dimethyloctyloxy]phenyl)fluorene were synthesized and their electrical and photophysical properties were characterized with respect to their application as host in phosphorescent polymer light-emitting diodes. It is shown that the triplet energy of a polymer depends on the specific connections between its building blocks. Without changing the composition of the polymer, its triplet energy can be increased from 2.3 to 2.6 eV by changing the way in which the different building blocks are coupled together. For poly(9-vinylcarbazole) (PVK), a carbazole polymer often used as host for high-energy triplet emitters in polymer light-emitting diodes, a large hole-injection barrier of about 1 eV exists due to the low-lying HOMO level of PVK. For all carbazole polymers presented here, the HOMO levels are much closer to the Fermi level of a commonly used anode such as ITO and/or a commonly used hole-injection layer such as PEDOT:PSS. This makes high current densities and consequently high luminance levels possible at moderate applied voltages in polymer light-emitting diodes. A double-layer polymer light-emitting diode is constructed comprising a PEDOT:PSS layer as hole-injection layer and a carbazole-oxadiazole copolymer doped with a green triplet emitter as emissive layer that shows an efficacy of 23 cd/A independent of current density and light output.

Violation of the exponential-decay law at long times.

Abstract: First-principles quantum mechanical calculations show that the exponential-decay law for any metastable state is only an approximation and predict an asymptotically algebraic contribution to the decay for sufficiently long times. In this Letter, we measure the luminescence decays of many dissolved organic materials after pulsed laser excitation over more than 20 lifetimes and obtain the first experimental proof of the turnover into the nonexponential decay regime. As theoretically expected, the strength of the nonexponential contributions scales with the energetic width of the excited state density distribution whereas the slope indicates the broadening mechanism.

Ionic Iridium(III) Complexes with Bulky Side Groups for Use in Light Emitting Cells: Reduction of Concentration Quenching

Abstract: Here, the photophysics and performance of single-layer light emitting cells (LECs) based on a series of ionic cyclometalated Ir(III) complexes of formulae and where ppy, bpy, and phen are 2-phenylpyridine, substituted bipyridine and substituted phenanthroline ligands, respectively, are reported. Substitution at the NˆN ligand has little effect on the emitting metal-ligand to ligand charge-transfer (MLLCT) states and functionalization at this site of the complex leads to only modest changes in emission color. For the more bulky complexes the increase in intermolecular separation leads to reduced exciton migration, which in turn, by suppressing concentration quenching, significantly increases the lifetime of the excited state. On the other hand, the larger intermolecular separation induced by bulky ligands reduces the charge carrier mobility of the materials, which means that higher bias fields are needed to drive the diodes. A brightness of ca. 1000 cd m−2 at 3 V is obtained for complex 5, which demonstrates a beneficial effect of bulky substituents.

Triplet exciton migration in a conjugated polyfluorene

Abstract: The diffusion and triplet energy relaxation in amino endcapped poly [9,9-bis(2-ethylhexy1)fluorene-2,7-diyl] has been studied using photoluminescence and photoinduced absorption, both time-resolved covering ten decades of time, dependent on temperature, excitation dose, and concentration. The results are analyzed employing the concept of dispersive hopping in a Gaussian distribution of states (DOS), characterized by width \ensuremath{\delta} of 40 meV. It is found that the initial photogenerated triplet population in the polymer films decays fairly equally by triplet-triplet annihilation and intrinsic monomolecular decay, respectively. In the latter case, the triplet excitons survive the initial 100 ms either in trap states outside the DOS at ambient temperature or within the DOS at low temperature. Further, it is clearly found that triplet migration in this conjugated polymer is both an intrachain and interchain process. Interchain hopping of triplet excitons must occur, which subsequently leads to the observation of delayed fluorescence. The nature of interchain hopping of the triplets is discussed.

Chemical sensors based on amplifying fluorescent conjugated polymers.

Abstract: The field of chemical sensing is becoming ever more dependent upon novel materials. Polymers, crystals, glasses, particles, and nanostructures have made a profound impact and have endowed modern sensory systems with superior performance. Electronic polymers have emerged as one of the most important classes of transduction materials; they readily transform a chemical signal into an easily measured electrical or optical event. Although our group reviewed this field in 2000,1 the high levels of activity and the impact of these methods now justify a subsequent review as part of this special issue. In this review we restrict our discussions to purely fluorescence-based methods using conjugated polymers (CPs). We further confine our detailed coverage to articles published since our previous review and will only detail earlier research in this Introduction to illustrate fundamental concepts and terminology that underpin the recent literature.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Abstract: A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Recent advances in organic thermally activated delayed fluorescence materials.

Abstract: Organic materials that exhibit thermally activated delayed fluorescence (TADF) are an attractive class of functional materials that have witnessed a booming development in recent years. Since Adachi et al. reported high-performance TADF-OLED devices in 2012, there have been many reports regarding the design and synthesis of new TADF luminogens, which have various molecular structures and are used for different applications. In this review, we summarize and discuss the latest progress concerning this rapidly developing research field, in which the majority of the reported TADF systems are discussed, along with their derived structure–property relationships, TADF mechanisms and applications. We hope that such a review provides a clear outlook of these novel functional materials for a broad range of scientists within different disciplinary areas and attracts more researchers to devote themselves to this interesting research field.

Semiconducting Polyfluorenes—Towards Reliable Structure–Property Relationships

Abstract: Alkylsabstituted polyfluorenes have emerged as a very attractive class of conju gated polymers, especially for display applications, owing to their pure blue and efficient electroluminescence coupled with a high charge-carrier mobility and good processability. The availability of specific and highly regroselective coupling reactions provides a rich variety of taifored polyfluorene-type paly mers and copolymers. The focus of this review, therefore, is on reliable structure-property relationships regarding (i) the interplay of substitution pattern, solid state morphology, and optical/electronic properties, and (ii) the key role of degradation-induced keto defect sites in fluorerse-type polymers, and strategies for obtaining defect-free polyfluorenes.

Organic light-emitting device

Abstract: The present invention provides a heterocyclic compound and an organic light-emitting device including the heterocyclic compound. The organic light-emitting devices using the heterocyclic compounds have high-efficiency, low driving voltage, high luminance and long lifespan.