High-efficiency stretchable light-emitting polymers from thermally activated delayed fluorescence.

Stretchable light-emitting materials are the key components for realizing skin-like displays and optical biostimulation. All the stretchable emitters reported to date, to the best of our knowledge, have been based on electroluminescent polymers that only harness singlet excitons, limiting their theoretical quantum yield to 25%. Here we present a design concept for imparting stretchability onto electroluminescent polymers that can harness all the excitons through thermally activated delayed fluorescence, thereby reaching a near-unity theoretical quantum yield. We show that our design strategy of inserting flexible, linear units into a polymer backbone can substantially increase the mechanical stretchability without affecting the underlying electroluminescent processes. As a result, our synthesized polymer achieves a stretchability of 125%, with an external quantum efficiency of 10%. Furthermore, we demonstrate a fully stretchable organic light-emitting diode, confirming that the proposed stretchable thermally activated delayed fluorescence polymers provide a path towards simultaneously achieving desirable electroluminescent and mechanical characteristics, including high efficiency, brightness, switching speed and stretchability as well as low driving voltage.

Selenium-Doped Polycyclic Aromatic Hydrocarbon Multiresonance Emitters with Fast Reverse Intersystem Crossing for Narrowband Blue Emission.

Two kinds of boron- (B), selenium- (Se), and nitrogen-doped (N) polycyclic aromatic hydrocarbon (PAH) emitters (Cz-BSeN and DCz-BSeN) with a multiresonance effect are developed for narrowband blue emission by embedding boron as an electron-deficient atom and selenium and nitrogen as electron-donating atoms into a benzo[a]naphtho[1,2,3-hi]aceanthrylene skeleton. It is found that both emitters exhibit strong spin-orbit coupling and fast reverse intersystem crossing (rate constant of 7.5-8.8 × 106 s-1) due to the heavy-atom effect of selenium, which is 2 orders of magnitude faster than its B, N-doped PAH analogue. Meanwhile, compared to parent B, Se, N-doped PAH emitter Cz-BSeN, incorporating carbazole moieties on the para position of the boron atom in DCz-BSeN not only blueshifts the emission by 7 nm without broadening its spectra but also results in an enhanced photoluminescent quantum efficiency of 93% in the doped film. The organic light-emitting diode (OLED) employing DCz-BSeN as emitter revealed narrowband blue emission at 481 nm with a small full-width at half-maximum (fwhm) of 32 nm, as well as a maximum external quantum efficiency of 22.3%, accompanied by alleviated efficiency roll-off compared to its B, N-containing counterpart.

Multiple Resonance Dendrimers Containing Boron, Oxygen, Nitrogen-Doped Polycyclic Aromatic Emitters for Narrowband Blue-Emitting Solution-Processed OLEDs.

In contrast to small-molecule multiple resonance emitters processed via vacuum evaporation technology, the design of multiple resonance dendrimers is presented by introducing the first- and second-generation carbazole dendrons in the periphery of boron, oxygen, nitrogen-doped polycyclic aromatic skeleton, for efficient narrowband blue electroluminescence by a solution process. The multiple resonance dendrimers not only keep the narrowband emission of the polycyclic aromatic skeleton, but also can suppress their intermolecular aggregation by steric carbazole dendrons, overcoming the unwanted spectral broadening in the solid state. The resultant first-generation carbazole dendrimer exhibits narrowband blue emission with promising photoluminescent quantum efficiency of 94% and delayed fluorescence with a lifetime of 139.1 µs in the solid-state film. Solution-processed organic light-emitting diodes based on the dendrimers reveal electroluminescence at 488 nm with full-width at half maximum of 39 nm, the maximum luminous efficiency of 29.2 cd A-1 , and maximum external quantum efficiency of 13.4%.

Boron-, Sulfur- and Nitrogen-Doped Polycyclic Aromatic Hydrocarbon Multiple Resonance Emitters for Narrow-Band Blue Emission.

Two boron-, sulfur- and nitrogen-doped polycyclic aromatic hydrocarbon multiple resonance thermally activated delayed fluorescence emitters with high photoluminescent quantum efficiency (88 %) and rapid reverse intersystem crossing (kRISC = 1.0×105  s-1 ) are designed and synthesized, enabling efficient narrow-band blue electroluminescence at 473 nm with full width at half maximum of 29 nm and maximum external quantum efficiency of 22.0 %, which provides an avenue to expand the structure library for multiple resonance emitters and an approach to regulate their emission properties.

Phosphonate/Phosphine Oxide Dyad Additive for Efficient Perovskite Light-Emitting Diodes.

Additives play a critical role for efficient perovskite light-emitting diodes (PeLEDs). Here, we report a novel phosphonate/phosphine oxide dyad molecular additive (PE-TPPO), with unique dual roles of passivating defects and enhancing carrier radiative recombination, to boost the device efficiency of metal-halide perovskites. In addition to the defect passivation effect of the phosphine oxide group to enhance the photoluminescence intensity and homogeneity of perovskite film, the phosphonate group with strong electron affinity can capture the injected electrons to increase local carrier concentration and accelerate the carrier radiative recombination in the electroluminescence process. Owing to their synergistic enhancement on device efficiency, quasi-two-dimensional green PeLEDs modified by this dyad additive exhibit a maximum external quantum efficiency, current efficiency, and power efficiency of 25.1 %, 100.5 cd A-1 , and 98.7 lm W-1 , respectively, which are among the reported state-of-the-art efficiencies.

Hyperfluorescent polymers enabled by through-space charge transfer polystyrene sensitizers for high-efficiency and full-color electroluminescence.

Fluorescent polymers are suffering from low electroluminescence efficiency because triplet excitons formed by electrical excitation are wasted through nonradiative pathways. Here we demonstrate the design of hyperfluorescent polymers by employing through-space charge transfer (TSCT) polystyrenes as sensitizers for triplet exciton utilization and classic fluorescent chromophores as emitters for light emission. The TSCT polystyrene sensitizers not only have high reverse intersystem crossing rates for rapid conversion of triplet excitons into singlet ones, but also possess tunable emission bands to overlap the absorption spectra of fluorescent emitters with different bandgaps, allowing efficient energy transfer from the sensitizers to emitters. The resultant hyperfluorescent polymers exhibit full-color electroluminescence with peaks expanding from 466 to 640 nm, and maximum external quantum efficiencies of 10.3-19.2%, much higher than those of control fluorescent polymers (2.0-3.6%). These findings shed light on the potential of hyperfluorescent polymers in developing high-efficiency solution-processed organic light-emitting diodes and provide new insights to overcome the electroluminescence efficiency limitation for fluorescent polymers.

Through-space charge transfer dendrimers employing oxygen-bridged triarylboron acceptors for efficient deep-blue electroluminescence.

Through-space charge transfer dendrimers consisting of dendritic triacridan donors and oxygen-bridged triarylboron acceptors are demonstrated to exhibit deep-blue thermally activated delayed fluorescence with the state-of-the-art external quantum efficiency of 14.6% for electroluminescence by a solution process.

π-Stacked Donor-Acceptor Dendrimers for Highly Efficient White Electroluminescence.

π-Stacked dendrimers consisting of cofacially aligned donors and acceptors are developed by introducing three dendritic teracridan donors with orthogonal configuration and three triazine acceptors in periphery of hexaphenylbenzene skeleton. The dendritic structure and orthogonal configuration of teracridan not only make their outer acridan segments approaching to acceptor in close distance, but also fix donor and acceptor in face-to-face alignment, leading to through-space charge transfer emission with thermally activated delayed fluorescence (TADF) effect. By regulating charge transfer strength via substituent effect of acceptor, emission color of the dendrimers can be tuned from blue to yellow/red region. Solution-processed two-color white organic light-emitting diodes (OLEDs) based on blue and yellow π-stacked donor-acceptor dendrimers exhibit the maximum external quantum efficiency of 20.6 % and maximum power efficiency of 58.9 lm W-1 , representing the state-of-the-art efficiency for all-TADF white OLEDs by solution process.

Through-Space Charge-Transfer Polynorbornenes with Fixed and Controllable Spatial Alignment of Donor and Acceptor for High-Efficiency Blue Thermally Activated Delayed Fluorescence.

Through-space charge transfer polynorbornenes with fixed and controllable spatial alignment of donor and acceptor in edge-to-face/face-to-face stacking patterns are developed for achieving high-efficiency blue thermally activated delayed fluorescence (TADF). The alignment is realized by using the cis, exo-configuration of norbornene to confine donor and acceptor in close proximity, and utilizing orthogonal and dendritic structures of donors to provide either perpendicular or parallel stacking motif relative to acceptors. Compared to edge-to-face counterparts, polynorbornenes with face-to-face aligned donor and acceptor exhibit much larger oscillator strength and higher photoluminescence quantum yield. The resulting polymers exhibit deep blue (422 nm) to sky blue (482 nm) emission and TADF effect with reverse intersystem crossing rates of 0.4-5.9×106  s-1 , giving the maximum external quantum efficiency of 18.8 % for non-doped blue organic light-emitting diodes by solution process.

Developing Through-Space Charge Transfer Polymers as a General Approach to Realize Full-Color and White Emission with Thermally Activated Delayed Fluorescence.

Through-space charge transfer polymers (TSCT polymers) that contain a non-conjugated polystyrene backbone and spatially separated donor and acceptor units for solution-processed OLEDs with full-color and white emission is reported. By tuning the charge transfer strength between donor and acceptors with different electron-accepting ability, emission color spanning from deep blue to red can be achieved. By incorporating two kinds of donor/acceptor pairs in one polymer to create duplex through-space charge-transfer channels, blue and yellow emission can be simultaneously obtained to realize white electroluminescence from a single polymer. The TSCT polymers exhibit thermally activated delayed fluorescence effect with delayed-component lifetimes in range of 0.36-1.98 µs, and unexpected aggregation-induced emission (emission intensity enhancement of up to 117 from solution to aggregation state).

Through-space charge transfer hexaarylbenzene dendrimers with thermally activated delayed fluorescence and aggregation-induced emission for efficient solution-processed OLEDs.

Through-space electron interaction plays a critical role in determining the optical and charge transport properties of functional materials featuring π-stacked architectures. However, developing efficient organic luminescent materials with such interactions has been a challenge because of the lack of well-established prototypical molecules. Here we report the design of through-space charge transfer hexaarylbenzenes (TSCT-HABs) containing circularly-arrayed electron donors (acridan/dendritic triacridan) and acceptors (triazine), which exhibit both thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) effects for high-efficiency solution-processed organic light-emitting diodes (OLEDs). Spatial separation of donors and acceptors in the TSCT-HABs induces a small singlet-triplet energy splitting of 0.04-0.08 eV, leading to delayed fluorescence with microsecond-scale lifetimes. Meanwhile, the TSCT-HABs display the AIE effect with emission intensity enhanced by 6-17 fold from solution to the aggregation state owing to their propeller-shaped configuration. Solution-processed OLEDs based on the TSCT-HABs show maximum external quantum efficiency up to 14.2%, making them among the most efficient emitters for solution-processed TADF OLEDs.

Synthesis and Electroluminescent Properties of Through-Space Charge Transfer Polymers Containing Acridan Donor and Triarylboron Acceptors.

We report the design, synthesis and electroluminescent properties of three kinds of through-space charge transfer (TSCT) polymers consisting of non-conjugated polystyrene backbone, acridan donor and triarylboron acceptors having different substituents such as hydrogen (H), fluorine (F), and trifluoromethyl (CF3). Owing to the weak electron interaction between acridan donor and triarylboron acceptor through non-conjugated connection, blue emission with peaks in range of 429-483 nm can be achieved for the polymers in solid-state film, accompanied with photoluminescence quantum yields of 26-53%. The resulting TSCT polymers exhibit small ΔEST values below 0.1 eV owing to the separated HOMO and LUMO distributions, showing thermally activated delayed fluorescence with lifetimes in range of 0.19-0.98 µs. Meanwhile, the polymers show aggregation-induced emission (AIE) effect with the emission intensity increased by up to ~33 folds from solution to aggregation state. Solution-processed organic light-emitting diodes based on the polymers containing trifluoromethyl substituent exhibit promising electroluminescent performance with maximum luminous efficiency of 20.1 cd A-1 and maximum external quantum efficiency of 7.0%, indicating that they are good candidates for development of luminescent polymers.

Realization of high-power-efficiency white electroluminescence from a single polymer by energy-level engineering.

Single white light-emitting polymers (SWPs) represent a high-fidelity system for generating white light emission from polymers without phase separation compared to polymer blend systems. However, their device performance so far has been limited because of the unwanted hole scattering caused by an energy-level mismatch between emitters and hosts, and the large injection barrier at the polymer/anode interface. Here, we report novel poly(arylene phosphine oxide)-based all-phosphorescent SWPs by using the combination of a high-HOMO-level blue phosphor and high-HOMO-level poly(arylene phosphine oxide) host to achieve a low turn-on voltage of 2.8 V, high external quantum efficiency of 18.0% and remarkable power efficiency of 52.1 lm W-1, which makes them the most efficient SWPs so far. This record power efficiency is realized by using the high-HOMO-level blue phosphor to eliminate the hole scattering effect and by using the high-HOMO-level polymer host to reduce the hole injection barrier. This result represents an important progress in SWPs to achieve efficiency surpassing that of the polymer blends currently used for solution-processed white organic light-emitting diodes (WOLEDs) and even comparable with that of the small molecules used for vacuum-deposited WOLEDs.

Blue Thermally Activated Delayed Fluorescence Polymers with Nonconjugated Backbone and Through-Space Charge Transfer Effect.

We demonstrate novel molecular design for thermally activated delayed fluorescence (TADF) polymers based on a nonconjugated polyethylene backbone with through-space charge transfer effect between pendant electron donor (D) and acceptor (A) units. Different from conventional conjugated D-A polymers with through-bond charge transfer effect, the nonconjugated architecture avoids direct conjugation between D and A units, enabling blue emission. Meanwhile, spatial π-π interaction between the physically separated D and A units results in both small singlet-triplet energy splitting (0.019 eV) and high photoluminescence quantum yield (up to 60% in film state). The resulting polymer with 5 mol % acceptor unit gives efficient blue electroluminescence with Commission Internationale de l'Eclairage coordinates of (0.176, 0.269), together with a high external quantum efficiency of 12.1% and low efficiency roll-off of 4.9% (at 1000 cd m-2), which represents the first example of blue TADF nonconjugated polymer.

Synthesis and properties of greenish-blue-emitting iridium dendrimers with N-phenylcarbazole-based polyether dendrons by a post-dendronization route.

A series of solution processible greenish-blue-emitting Ir dendrimers with polyether dendrons that consist of N-phenylcarbazole (NPC) are developed via a convenient post-dendronization method. It involves two steps: (i) the successful preparation of a reactive Ir core, namely m-HO-dfppyIr, only when the hydroxyl group is located at the meta position relative to the N atom in the C^N ligand so as to eliminate the possible resonance structure between enol and keto; and (ii) the subsequent functionalization with NPC-based polyether dendrons to afford the first, second and third generation Ir dendrimers (Ir-G1B, Ir-G2B and Ir-G3B) with ease and high yields over 60%. All these dendritic complexes possess good thermal stability with decomposition temperatures higher than 380 °C and glass transition temperatures higher than 200 °C. In addition, with the growing generation number, the intermolecular interactions between emissive Ir cores are expected to be effectively inhibited to avoid the luminescence quenching, which is confirmed from the blue-shifted emission peak and the enhanced lifetime of Ir-G3B in the solid state. As a result, on going from Ir-G1B to Ir-G3B, the maximum luminous efficiency rises upward from 4.7 to 9.2 cd A(-1) for nondoped electrophosphorescent devices. Further optimization by doping them into a dendritic H2 host leads to the improved luminous efficiencies as high as 20.0-25.2 cd A(-1).

White electroluminescence from all-phosphorescent single polymers on a fluorinated poly(arylene ether phosphine oxide) backbone simultaneously grafted with blue and yellow phosphors.

On the basis of a fluorinated poly(arylene ether phosphine oxide) backbone with both high triplet energy and appropriate HOMO/LUMO levels, highly efficient all-phosphorescent single white-emitting polymers were designed and successfully synthesized via a "two-step addition" strategy. Simultaneous blue and yellow triplet emissions were achieved to generate white electroluminescence with a promising luminous efficiency as high as 18.4 cd/A (8.5 lm/W, 7.1%) and CIE coordinates of (0.31, 0.43).

Highly efficient blue electrophosphorescent polymers with fluorinated poly(arylene ether phosphine oxide) as Backbone.

In view of the tolerance of F atoms in FIrpic to the nucleophilic aromatic substitution polymerization, an activated fluorinated poly(arylene ether phosphine oxide) backbone is used to construct novel blue electrophosphorescent polymers containing FIrpic as the blue emitter, because they can be synthesized under a milder temperature of 120 °C. Compared with the counterparts prepared at high temperature (165 °C), unexpected bathochromic shift is successfully avoided, and a state-of-art luminous efficiency as high as 19.4 cd A(-1) is achieved. The efficiency is comparable to the corresponding physical blend system, which indicates that the fluorinated poly(arylene ether phosphine oxide) has the potential to be used as the platform for the development of high-performance all-phosphorescent white polymer based on single polymer system.

Spiro-linked hyperbranched architecture in electrophosphorescent conjugated polymers for tailoring triplet energy back transfer.

A spiro-linked hyperbranched architecture has been incorporated into electrophosphorescent conjugated polymers for the first time, aiming at simultaneously tailoring the intra- and intermolecular triplet energy back transfer from the phosphorescent guest to the conjugated polymer host. Based on a prototype with this unique structure, slower decay of triplet excitons, and 5-8 fold enhancement of device efficiencies are obtained compared with the conventional blending counterpart.