Highly efficient polaritonic light-emitting diodes with angle-independent narrowband emission

Authors acknowledge funding by the Volkswagen Foundation (no. 93404; M.C.G.), the Leverhulme Trust (RPG-2017-213; M.C.G), the European Research Council under the European Union Horizon 2020 Framework Programme (FP/2014-2020)/ERC grant agreement no. 640012 (ABLASE; M.C.G) and the Alexander von Humboldt Foundation (Humboldt Professorship; M.C.G.). A.M. acknowledges funding through an individual fellowship of the Deutsche Forschungsgemeinschaft (404587082; A.M.) and from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement no. 101023743 (PolDev; A.M.).Angle-independent narrowband emission is required for many optoelectronic devices, ranging from high-definition displays to sensors. However, emerging materials for electroluminescent devices, such as organics and perovskites, show spectrally broad emission due to intrinsic disorder. Coupling this emission to an optical resonance reduces the linewidth, but at the cost of inheriting the severe angular dispersion of the resonator. Strongly coupling a dispersionless exciton state to a narrowband optical microcavity could overcome this issue; however, electrically pumped emission from the resulting polaritons is typically hampered by poor efficiencies. Here we present a universal concept for polariton-based emission from organic light-emitting diodes by introducing an assistant strong coupling layer, thereby avoiding quenching-induced efficiency losses. We realize red- and green-emitting, narrowband (full-width at half-maximum of less than 20 nm) and spectrally tunable polaritonic organic light-emitting diodes with up to 10% external quantum efficiency and high luminance (>20,000 cd m−2 at 5 V). By optimizing cavity detuning and coupling strength, we achieve emission with ultralow dispersion (<10 nm spectral shift at 60° tilt). These results may have wide-reaching implications for on-demand polariton emission and demonstrate the practical relevance of strong light–matter coupling for next-generation optoelectronics, particularly display technology.Publisher PDFPeer reviewe

Efficient anisotropic polariton lasing using molecular conformation and orientation in organic microcavities

F.L.R acknowledges funding from the Alexander von Humboldt Foundation through a Humboldt Fellowship. A.M. acknowledges funding from the European Union Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 101023743 (PolDev). D.D.C.B. thanks the Sumitomo Chemical Company and DIC Corporation for provision of PFO and SD1, respectively. D.D.C.B. further acknowledges support from King Abdullah University of Science and Technology, Jiangsu Industrial Technology Research Institute (JITRI) and JITRI-Oxford IMPACT Institute (R57149/CN001). This research was financially supported by the Alexander von Humboldt Foundation (Humboldt Professorship to M.C.G.) and by the European Research Council under the European Union Horizon 2020 Framework Programme (FP/2014-2020)/ERC Grant Agreement No. 640012 (ABLASE).Organic exciton-photon polariton lasers are promising candidates for efficient generation of coherent light at room temperature. While their thresholds are now comparable with conventional organic photon lasers, tuning of molecular conformation and orientation as a means to control fundamental properties of their emission and further enhance performance remains largely unexplored. Here, we first report a two-fold reduction in the threshold of a microcavity polariton laser based on an active layer of poly(9,9-dioctylfluorene) (PFO) when 15% β-phase conformation is introduced. We then take advantage of the liquid crystalline properties of PFO and use a thin photoalignment layer to induce nematic alignment of the polymer chains. The resulting transition dipole moment orientation increases the Rabi energy, bringing the system into the ultra-strong coupling regime and facilitating anisotropic polariton lasing with an eight-fold reduction in absorbed threshold, down to 1.14 pJ/0.36 μJcm-2 for the direction parallel to the orientation, with no emission along the orthogonal direction. This represents the first demonstration of anisotropic polariton lasing in conjugated polymer microcavities and a lower threshold than current organic vertical cavity surface-emitting photon and polariton lasers. Dipole orientation offers new opportunities for switchable, more efficient polaritonic devices and observation of fundamental effects at low polariton numbers.Publisher PDFPeer reviewe

A substrateless, flexible, and water-resistant organic light-emitting diode

This research was financially supported from the Leverhulme Trust (RPG-2017-231), the EPSRC NSF-CBET lead agency agreement (EP/R010595/1, 1706207), the DARPA NESD programme (N66001-17-C-4012) and the RS Macdonald Charitable Trust. 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). C.M. acknowledges funding from the European Commission through a Marie Skłodowska Curie individual fellowship (703387). A.M. acknowledges funding through an individual fellowship of the Deutsche Forschungsgemeinschaft (404587082). M.C.G. acknowledges funding from the Alexander von Humboldt Stiftung (Humboldt-Professorship).Despite widespread interest, ultrathin and highly flexible light-emitting devices that can be seamlessly integrated and used for flexible displays, wearables, and as bioimplants remain elusive. Organic light-emitting diodes (OLEDs) with µm-scale thickness and exceptional flexibility have been demonstrated but show insufficient stability in air and moist environments due to a lack of suitable encapsulation barriers. Here, we demonstrate an efficient and stable OLED with a total thickness of ≈ 12 µm that can be fully immersed in water or cell nutrient media for weeks without suffering substantial degradation. The active layers of the device are embedded between conformal barriers formed by alternating layers of parylene-C and metal oxides that are deposited through a low temperature chemical vapour process. These barriers also confer stability of the OLED to repeated bending and to extensive postprocessing, e.g. via reactive gas plasmas, organic solvents, and photolithography. This unprecedented robustness opens up a wide range of novel possibilities for ultrathin OLEDs.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

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

Wireless magnetoelectrically powered organic light-emitting diodes

This work was supported by a scholarship to J.F.B. donated by Beverly and Frank MacInnis to the University of St Andrews, the European Union Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement no. 101023743 (PolDev to A.M.), The Leverhulme Trust (RPG-2017-231), the Alexander von Humboldt Foundation (Humboldt Professorship to M.C.G.), the DFG-funded Research Training Group “Template-Designed Organic Electronics (TIDE)” (RTG2591), and the EPSRC NSF-CBET lead agency agreement (EP/R010595/1, 1706207).Compact wireless light sources are a fundamental building block for applications ranging from wireless displays to optical implants. However, their realization remains challenging because of constraints in miniaturization and the integration of power harvesting and light-emission technologies. Here, we introduce a strategy for a compact wirelessly powered light-source that consists of a magnetoelectric transducer serving as power source and substrate and an antiparallel pair of custom-designed organic light-emitting diodes. The devices operate at low-frequency ac magnetic fields (~100 kHz), which has the added benefit of allowing operation multiple centimeters deep inside watery environments. By tuning the device resonance frequency, it is possible to separately address multiple devices, e.g., to produce light of distinct colors, to address individual display pixels, or for clustered operation. By simultaneously offering small size, individual addressing, and compatibility with challenging environments, our devices pave the way for a multitude of applications in wireless displays, deep tissue treatment, sensing, and imaging.Peer reviewe

Narrowband organic light-emitting diodes for fluorescence microscopy and calcium imaging

Funding: Leverhulme Trust (RPG-2017-231), the EPSRC NSF-CBET lead agency agreement (EP/R010595/1, 1706207), the DARPA NESD program (N66001-17-C-4012) and the RS Macdonald Charitable Trust. C.M. acknowledges funding from the European Commission through a Marie Skłodowska Curie individual fellowship (703387). A.M. acknowledges funding through an individual fellowship of the Deutsche Forschungsgemeinschaft (404587082). Y.D. acknowledges support from the Chinese Scholarship Council (CSC). L.T. acknowledges studentship funding through the EPSRC CM-CDT (EP/L015110/1). M.S. acknowledges funding by the Royal Society (Dorothy Hodgkin Fellowship, DH160102).Fluorescence imaging is an indispensable tool in biology, with applications ranging from single‐cell to whole‐animal studies and with live mapping of neuronal activity currently receiving particular attention. To enable fluorescence imaging at cellular scale in freely moving animals, miniaturized microscopes and lensless imagers are developed that can be implanted in a minimally invasive fashion; but the rigidity, size, and potential toxicity of the involved light sources remain a challenge. Here, narrowband organic light‐emitting diodes (OLEDs) are developed and used for fluorescence imaging of live cells and for mapping of neuronal activity in Drosophila melanogaster via genetically encoded Ca2+ indicators. In order to avoid spectral overlap with fluorescence from the sample, distributed Bragg reflectors are integrated onto the OLEDs to block their long‐wavelength emission tail, which enables an image contrast comparable to conventional, much bulkier mercury light sources. As OLEDs can be fabricated on mechanically flexible substrates and structured into arrays of cell‐sized pixels, this work opens a new pathway for the development of implantable light sources that enable functional imaging and sensing in freely moving animals.PostprintPeer reviewe

Strong light-matter coupling for reduced photon energy losses in organic photovoltaics

Funding: Volkswagen Foundation (no.93404) (MCG), individual fellowship of the DeutscheForschungsgemeinschaft (404587082) (AM).Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photo-active layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture.Publisher PDFPeer reviewe

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