High-brightness blue polariton organic light-emitting diodes

H2020 Marie Sklodowska-Curie Actions - 101023743; University of St Andrews; Alexander von Humboldt-Stiftung; European Research Council - 101097878; Deutsche Forschungsgemeinschaft - RTG2591Polariton organic light-emitting diodes (POLEDs) use strong light-matter coupling as an additional degree of freedom to tailor device characteristics, thus making them ideal candidates for many applications, such as room temperature laser diodes and high-color purity displays. However, achieving efficient formation of and emission from exciton-polaritons in an electrically driven device remains challenging due to the need for strong absorption, which often induces significant nonradiative recombination. Here, we investigate a novel POLED architecture to achieve polariton formation and high-brightness light emission. We utilize the blue-fluorescent emitter material 4,4′-Bis(4-(9H-carbazol-9-yl)styryl)biphenyl (BSBCz), which exhibits strong absorption and a highly horizontal transition-dipole orientation as well as a high photoluminescence quantum efficiency, even at high doping concentrations. We achieve a peak luminance of over 20,000 cd/m2 and external quantum efficiencies of more than 2%. To the best of our knowledge, these values represent the highest reported so far for electrically driven polariton emission from an organic semiconductor emitting in the blue region of the spectrum. Our work therefore paves the way for a new generation of efficient and powerful optoelectronic devices based on POLEDs.Peer reviewe

Accurate efficiency measurements of organic light-emitting diodes via angle-resolved spectroscopy

Funding: EPSRC NSF-CBET lead agency agreement (EP/R010595/1, 1706207), the Leverhulme Trust (RPG-2017-231), the Volkswagen Foundation (No. 93404), and the Alexander von Humboldt Stiftung (Humboldt-Professorship to M.C.G.). C.K. acknowledges support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1A6A3A03012331). Diese Arbeit wurde mitfinanziert durch Steuermittel auf der Grundlage des vom Sächsischen Landtag beschlossenen Haushaltes.The accurate characterization of thin-film LEDs – including organic light emitting diodes (OLEDs), perovskites and quantum dot LEDs – is crucial to our understanding of the factors that influence their efficiency and thus to the fabrication of LEDs with improved performance and stability. In addition, detailed information about the angular characteristics of LED emission is useful to assess the suitability of individual architectures, e.g. for display applications. Here, the implementation of a goniometer-based measurement system and corresponding protocol are described that allow to accurately determine the current-voltage-luminance characteristics, external quantum efficiency and luminous efficacy of OLEDs and other emerging thin-film LEDs. The system allows recording of angle-resolved electroluminescence spectra and accurate efficiency measurements for devices with both Lambertian and non-Lambertian emission characteristics. A detailed description of the setup and a protocol for assembling and aligning the required hardware are provided. Drawings of all custom parts and the open-source Python software required to perform the measurement and to analyze the data are included.Publisher PDFPeer reviewe

Orientation distributions of vacuum-deposited organic emitters revealed by single-molecule microscopy

This work was supported by the Volkswagen Foundation (No. 93404) and the DFG-funded Research Training Group “Template-Designed Organic Electronics (TIDE)”, RTG2591. M.C.G. acknowledges support from the Alexander von Humboldt Stiftung through the Humboldt-Professorship. A.M. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement No. 101023743 (PolDev).The orientation of luminescent molecules in organic light-emitting diodes strongly influences device performance. However, our understanding of the factors controlling emitter orientation is limited as current measurements only provide ensemble-averaged orientation values. Here, we use single-molecule imaging to measure the transition dipole orientation of individual emitter molecules in a state-of-the-art thermally evaporated host and thereby obtain complete orientation distributions of the hyperfluorescence-terminal emitter C545T. We achieve this by realizing ultra-low doping concentrations (10−6 wt%) of C545T and minimising background levels to reliably measure its photoluminescence. This approach yields the orientation distributions of >1000 individual emitter molecules in a system relevant to vacuum-processed devices. Analysis of solution- and vacuum-processed systems reveals that the orientation distributions strongly depend on the nanoscale environment of the emitter. This work opens the door to attaining unprecedented information on the factors that determine emitter orientation in current and future material systems for organic light-emitting devices.Publisher PDFPeer reviewe

Single molecule spectroscopy of polyfluorene chains reveals β-phase content and phase reversibility in organic solvents

Conjugated polymers are an important class of organic semiconductors that can be deposited from solution to make optoelectronic devices. Among them, poly(9,9′-dioctylfluorene) (PFO) has distinctive optical properties arising from its ability to adopt an ordered planar conformation (β phase) from a disordered glassy phase (α phase). The β phase has attractive optical properties, but the precise mechanism of its formation in solution remains unknown. Here, we have combined specifically tailored polymer synthesis and surface-passivation strategies to provide the first spectroscopic characterization of single PFO chains in solution at room temperature. By anchoring PFO molecules at one end on an anti-adherent surface, we show that isolated chains can adopt the β-phase conformation in a solvent-dependent manner. Furthermore, we find that individual PFO chains can reversibly switch multiple times between phases in response to solvent-exchange events. The methodology presented here for polymer synthesis and immobilization is widely applicable to investigate other luminescent polymers.PostprintPostprintPeer reviewe

Identification of the key parameters for horizontal transition dipole orientation in fluorescent and TADF organic light-emitting diodes

This research was financially supported by the Volkswagen Foundation (No. 93404), the European Commission under EU Horizon 2020 Grant Agreement No. 812872 (TADFlife), the European Research Council under the European Union's Horizon 2020 Framework Programme (FP/2014-2020)/ERC grant agreement no. 640012 (ABLASE), and The Leverhulme Trust (RPG-2016047). M.C.G. acknowledges support from the Alexander von Humboldt Stiftung through the Humboldt-Professorship. All computations were performed using resources kindly provided by the University of Mons (UMONS), supported by the Belgian National Fund for Scientific Research (FRS-FNRS). Computational resources were provided by the Consortium des Équipements de Calcul Intensif (CÉCI) funded by F.R.S.-FNRS under Grant 2.5020.11. Y.O. acknowledges funding from the FRS-FNRS under the grant F.4534.21 (MIS-IMAGINE).In organic light-emitting diodes (OLEDs), horizontal orientation of the emissive transition dipole moment (TDM) can improve light outcoupling efficiency by up to 50% relative to random orientation. Therefore, there have been extensive efforts to identify drivers of horizontal orientation. The aspect ratio of the emitter molecule and the glass-transition temperature (Tg) of the films are currently regarded as particularly important. However, there remains a paucity of systematic studies that establish the extent to which these and other parameters control orientation in the wide range of emitter systems relevant for state-of-the-art OLEDs. Here, recent work on molecular orientation of fluorescent and thermally activated delayed fluorescent emitters in vacuum-processed OLEDs is reviewed. Additionally, to identify parameters linked to TDM orientation, a meta-analysis of 203 published emitter systems is conducted and combined with density-functional theory calculations. Molecular weight (MW) and linearity are identified as key parameters in neat systems. In host–guest systems with low-MW emitters, orientation is mostly influenced by the host Tg, whereas the length and MW of the emitter become more relevant for systems involving higher-MW emitters. To close, a perspective of where the field must advance to establish a comprehensive model of molecular orientation is given.Publisher PDFPeer reviewe

Review: Zoran Kurelić (ed.). Violence, Art and Politics.

This work was supported by the Engineering and Physical Sciences Research Council UK (EPSRC) (project EP/N009886/1). F.T.-C. thanks EPSRC (Grant EP/L015110/1). S.F. acknowledges support from the National Science Foundation (Grant DMR 1411240). I.D.W.S. acknowledges support from a Royal Society Wolfson research merit award.Conjugated polymers (CPs) are an important class of organic semiconductors that combine novel optoelectronic properties with simple processing from organic solvents. It is important to study CP conformation in solution to understand the physics of these materials and because it affects the properties of solution-processed films. Single-molecule techniques are unique in their ability to extract information on a chain-to-chain basis; however, in the context of CPs, technical challenges have limited their general application to host matrices or semiliquid environments that constrain the conformational dynamics of the polymer. We introduce a conceptually different methodology that enables measurements in organic solvents using the single-end anchoring of polymer chains to avoid diffusion while preserving polymer flexibility. We explore the effect of organic solvents and show that, in addition to chain-to-chain conformational heterogeneity, collapsed and extended polymer segments can coexist within the same chain. The technique enables real-time solvent-exchange measurements, which show that anchored CP chains respond to sudden changes in solvent conditions on a subsecond time scale. Our results give an unprecedented glimpse into the mechanism of solvent-induced reorganization of CPs and can be expected to lead to a new range of techniques to investigate and conformationally manipulate CPs.Publisher PDFPeer reviewe

Routine based intervention in the context of group sessions with families

Suvremeni pristupi u ranoj intervenciji naglašavaju važnost podrške roditeljima i njihovu snažnu uključenost u sve oblike intervencija. Naglasak je na poštivanju razvojnih načela, koja su jednaka za svu djecu (s razvojnim poteškoćama ili bez njih). Razvojne miljokaze djeca dostižu interakcijom s odraslima te aktivnostima (rutinama), koje su dio njihove prirodne sredine. S obzirom na specifičnosti djece s razvojnim odstupanjima, roditeljima je puno izazovnije uključiti djecu u svakodnevne rutine, iskoristiti ih i razumjeti kao trenutke učenja i poticanja. S ciljem pružanja podrške roditeljima u organiziranju svakodnevnih rutina, radi poticanja sociokomunikacijskog razvoja, organizirane su radionice na kojima se roditelje poučavalo strategijama – kako iskoristiti svakodnevne aktivnosti, rutine u poticanju djetetova razvoja. Organizirano je 5 grupa roditelja (35 roditelja) djece sa sumnjom ili već utvrđenim poremećajem iz spektra autizma (21 dijete). Svaka grupa imala je 12 strukturiranih susreta, nakon kojih su roditelji ispunili evaluacijski upitnik i ocijenili zadovoljstvo programom. U slobodnom dijelu upitnika roditelji su iznimno visoko ocijenili dobivene materijale, kao i razmjenu iskustava s drugim roditeljima. Rezultati pokazuju da intervencijski pristup koji podržava roditelje da ugrađuju razvojne ciljeve u svakodnevne rutine, iako kod nas neuobičajen i nov, roditelji vrlo pozitivno ocjenjuju te su njime zadovoljni. Zaključujemo da su roditelji time dobili vrlo uspješan i snažan alat, koji im pomaže da potiču dijete u raznim vještinama. Također, poučavanje roditelja novim vještinama u grupi s njihovom djecom, pokazalo se poželjnim oblikom rada i preporučuje se češće primjenjivati taj oblik rada u svakodnevnoj praksi.Contemporary approaches to early intervention emphasize the importance of supporting parents and their strong involvement in all forms of intervention. Emphasis has been put on respecting developmental principles that are the same for all children (with or without development difficulties). Children reach developing milestones through interaction with adults and activities (routines) that are part of their natural environment. Because of the specificities of children with developmental disabilities, it is much more demanding for parents to include them in everyday routines, use them and understand them as moments of learning and encouragement. With the aim of providing support to parents in organizing daily routines for promoting socio-communication development, group sessions were organized in which parents were taught strategies for utilizing everyday activities and routines in encouraging child’s development. Five groups of parents (35 parents) of children with suspected or already diagnosed autism spectrum disorders (21 children) have been formed. Each group had 12 structured meetings, after which parents completed the evaluation questionnaire and evaluated the satisfaction with the program they were involved in together with their children. The evaluation questionnaire was highly appreciated by parents, especially regarding communication with experts, content and applicability of the program. In the open-ended part of the questionnaire, parents highly rated the obtained material, as well as the exchange of experiences with other parents. The findings suggest that an intervention approach that supports parents in incorporating developmental goals into everyday routines is a very successful and powerful tool that helps parents encourage a child in various skills. Also, teaching parents new skills together with their children proved to be a very good form of work and it is highly recommended to apply this form more frequently in everyday practice