Thermally activated delayed fluorescence (TADF)‐type compounds have great potential as emitter molecules in organic light‐emitting diodes, allowing for electrofluorescence with 100% internal quantum ...efficiency. In small molecules, TADF is achieved through the formation of intramolecular charge‐transfer states. The only design limitation is the requirement that donor and acceptor entities spatially decouple the highest occupied and lowest unoccupied molecular orbitals, respectively, to minimize exchange splitting. The development of polymeric TADF emitters, on the contrary, has seen comparably small progress and those are typically built up from monomeric units that show promising TADF properties in small molecule studies beforehand. By contrast, herein, a way to achieve TADF properties in cyclic oligomers and polymers composed of non‐TADF building blocks is shown. Due to a strongly decreased energy splitting of the polymer with respect to the individual repeating unit between the lowest singlet and triplet excited state (ΔEST) and a sufficiently high radiative decay rate kSr, a highly efficient TADF polymer with up to 71% photoluminescence quantum yield is obtained. For the first time, an encouraging method is provided for producing highly efficient TADF oligomers and polymers from solely non‐TADF units via induced conjugation, opening a new design strategy exclusive for polymers.
A thermally activated delayed fluorescence (TADF) π‐conjugated cyclic polymer composed of non‐TADF building blocks is developed. Conjugation‐induced highest occupied molecular orbital destabilization leads to a decreased singlet–triplet splitting and efficient TADF in the polymer, while the repeating unit itself shows only inefficient phosphorescence. This conjugation‐induced TADF concept represents a novel molecular design rule particularly for solution‐processable polymeric materials.
Organic light-emitting diodes (OLEDs) suffer from notorious light trapping, resulting in only moderate external quantum efficiencies. Here, we report a facile, scalable, lithography-free method to ...generate controllable nanostructures with directional randomness and dimensional order, significantly boosting the efficiency of white OLEDs. Mechanical deformations form on the surface of poly(dimethylsiloxane) in response to compressive stress release, initialized by reactive ions etching with periodicity and depth distribution ranging from dozens of nanometers to micrometers. We demonstrate the possibility of independently tuning the average depth and the dominant periodicity. Integrating these nanostructures into a two-unit tandem white organic light-emitting diode, a maximum external quantum efficiency of 76.3% and a luminous efficacy of 95.7 lm W
are achieved with extracted substrate modes. The enhancement factor of 1.53 ± 0.12 at 10,000 cd m
is obtained. An optical model is built by considering the dipole orientation, emitting wavelength, and the dipole position on the sinusoidal nanotexture.
We study thermally evaporated thin films of Ir(ppy)3 and Ir(ppy)2(acac) by means of grazing incidence X-ray diffraction (GIXRD) and grazing incidence wide-angle X-ray scattering (GIWAXS). Ir(ppy)3 ...and Ir(ppy)2(acac) are both widely used as phosphorescent green emitter molecules in organic light-emitting diodes (OLEDs) and it was previously found that differences in their average transition dipole orientation affect the light extraction efficiency in OLEDs. Here we show that in pure films both materials form crystalline grains and that these grains exhibit a preferred orientation with respect to the substrate. When doped into an amorphous host, both the orientation and formation of the crystallites remain nearly unchanged for the concentration range accessible with GIXRD and GIWAXS. This is remarkable given that the transition dipole moments have found to be oriented only for Ir(ppy)2(acac) but isotropic for Ir(ppy)3. Analysis of the crystallite size indicates that the tendency to form crystallites is stronger for Ir(ppy)3 than for Ir(ppy)2(acac). From a comparison of the thin-film diffraction data of Ir(ppy)3 to its powder pattern, we infer that Ir(ppy)3 molecules are oriented with their permanent dipole moment roughly parallel to the substrate. Our findings will guide the further understanding of the mechanisms controlling transition dipole orientation and may thus lead to further improvements in device efficiency.
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•Ir(ppy)3 and Ir(ppy)2(acac) form small crystallite grains.•Grains show a preferred orientation on the substrate.•Ir(ppy)3 molecules orient roughly parallel to the substrate.•Crystallite formation and orientation preserve when emitters are doped into host.
Organic light-emitting diodes (OLEDs) are one of the key solid-state light sources for various applications including small and large displays, automotive lighting, solid-state lighting, and signage. ...For any given commercial application, OLEDs need to perform at their best, which is judged by their device efficiency and operational stability. We present OLEDs that comprise functional layers fabricated as ultrastable glasses, which represent the thermodynamically most favorable and, thus, stable molecular conformation achievable nowadays in disordered solids. For both external quantum efficiencies and LT
lifetimes, OLEDs with four different phosphorescent emitters show >15% enhancements over their respective reference devices. The only difference to the latter is the growth condition used for ultrastable glass layers that is optimal at about 85% of the materials' glass transition temperature. These improvements are achieved through neither material refinements nor device architecture optimization, suggesting a general applicability of this concept to maximize the OLED performance, no matter which specific materials are used.
In this work, interactions between different host materials and a blue TADF polymer named P1 are systematically investigated. In photoluminescence, the host can have substantial impact on the ...photoluminescence quantum yield (PLQY) and the intensity of delayed fluorescence (
Φ
DF
), where more than three orders of magnitude difference of
Φ
DF
in various hosts is observed, resulting from a polarity effect of the host material and energy transfer. Additionally, an intermolecular charge-transfer (CT) emission with pronounced TADF characteristics is observed between P1 and 2,4,6-tris3-(diphenylphosphinyl)phenyl-1,3,5-triazine (PO-T2T), with a singlet-triplet splitting of 7 meV. It is noted that the contribution of harvested triplets in monochrome organic light-emitting diodes (OLEDs) correlates with
Φ
DF
. For devices based on intermolecular CT-emission, the harvested triplets contribute ~90% to the internal quantum efficiency. The results demonstrate the vital importance of host materials on improving the PLQY and sensitizing
Φ
DF
of TADF polymers for efficient devices. Solution-processed polychrome OLEDs with a color close to a white emission are presented, with the emission of intramolecular (P1) and intermolecular TADF (PO-T2T:P1).
Controlling the alignment of emitter molecules in the active layer of organic light-emitting diodes has become a main approach to maximize the device efficiency when emitter molecules with ...luminescence quantum yields approaching 100% are used. In order to guarantee stable device performance, the initial molecular orientation should not change over time. In this work, we study this property for a time frame of 1.5 years and storage temperatures up to 80 °C which may be reached in displays exposed to direct sun light. For the studied material systems, this temperature is close to the glass transition at which drastic morphological changes occur and a randomization of the molecule arrangement is expected. We compare two different phosphorescent emitter molecules and, additionally, investigate the impact of the substrate temperature during evaporation. Concluding this long-term study, we prove experimentally that the emitter orientation remains unchanged under those device-critical storage conditions. On the contrary, the fatal potential of heat-induced reorientation is revealed by post-annealing experiments that show a strong change of the emitter orientation at about 20 K above the glass transition temperature.
White organic light emitting diodes (WOLEDs) for general lighting applications are still mostly considered as a possibility rather than a definitive next step in solid-state lighting technology. ...Vacuum deposited WOLEDs usually run into troubles of complicated manufacturing processes, incompatible emitters, high-precision requirements for emitting layer structuring and monolithic integration. In this work, we present a simple method of achieving white emission from a single-stack OLED with three (red, green and blue) emitters utilizing a laterally structured shadow mask. Only the green emitter is deposited via this mask without any alignment requirements, allowing the decoupling of recombination zones for achieving simultaneous emission from all emitters, circumventing the need of additional charge generating layers or vacuum-breaking structuring steps. A highly tunable white emission is demonstrated, irrespective of charge transporting systems or green emitters used. The resulting WOLEDs show external quantum efficiencies and luminous efficacies up to 14% and 26 lm/W, respectively, with CIE coordinates of (0.40, 0.40) and CRI values of up to 80.
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•Engineering split recombination zones in organic light-emitting diodes through lateral emission layer structuring.•Monolithic device integration without lithography or vacuum-breaking steps.•Color mixing has been demonstrated for two individual R/G/B emitter sets achieving cold and candlelight emission.•Maximal external quantum efficiency of 14% and luminous efficacy of 26 lm/W reached.
In recent years, the organic light-emitting diode (OLED) technology has been a rapidly evolving field of research, successfully making the transition to commercial applications such as mobile phones ...and other small portable devices. OLEDs provide efficient generation of light, excellent color quality, and allow for innovative display designs, e.g., curved shapes, mechanically flexible and/or transparent devices. Especially their self emissive nature is a highly desirable feature for display applications. In this work, we demonstrate an approach for full-color OLED pixels that are fabricated by vertical stacking of a red-, green-, and blue-emitting unit. Each unit can be addressed separately which allows for efficient generation of every color that is accessible by superpositioning the spectra of the individual emission units. Here, we use a combination of time division multiplexing and pulse width modulation to achieve efficient color mixing. The presented device design requires only three independently addressable electrodes, simplifying both fabrication and electrical driving. The device is built in a top-emission geometry, which is highly desirable for display fabrication as the pixel can be directly deposited onto back-plane electronics. Despite the top-emission design and the application of three silver layers within the device, there is only a minor color shift even for large viewing angles. The color space spanned by the three emission sub-units exceeds the sRGB space, providing more saturated green/yellow/red colors. Furthermore, the electrical performance of each individual unit is on par with standard single emission unit OLEDs, showing very low leakage currents and achieving brightness levels above 1000 cd/m
at moderate voltages of around 3-4 V.
Future lighting applications will strongly benefit from transparent luminescent devices. Here, we demonstrate transparent organic light-emitting diodes (OLEDs), which provide real-time adjustment of ...the emission color. Making use of the AC/DC concept, two stacked subunits can be addressed independently via an AC signal. Combining blue and yellow emission leads to the possibility to tune the emitted color between deep blue over cold white and warm white to yellow on both emission sides. For such highly complex device architectures, the thickness of each layer needs to be adjusted carefully in order to achieve balanced and efficient emission in both directions. Therefore, optical simulations are carried out to optimize the OLED. Based on these simulations, we present transparent, indium-free OLEDs that achieve a luminous efficacy of 8.7 lm/W in bottom direction and 9.7 lm/W in top direction at a brightness level of 1000 cd/m2 for warm white emission and a peak transmission of 56%. Using an emitter combination providing red, green, and blue emission, we were able to achieve a high color-rendering index (CRI) of 84, which further expands the range of possible applications for this promising device concept.
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•A transparent organic LED is shown that consists of two independently addressable subunits.•Very balanced emission between both sides of the device has been achieved.•Wetting layer electrodes made a device completely free of indium possible.
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