•A novel self-host near-infrared TADF dendrimer MPPA-MCBP with four-arm encapsulation was designed and synthesized.•Bipolar MCBP dendrons can effectively reduce the unwanted concentration quenching and the unbalanced charge transfer.•TADF dendrimers with bipolar dendrons will be a feasible approach to develop efficient nondoped solution-processed OLEDs.A self-host thermally activated delayed fluorescence (TADF) dendrimer, namely MPPA-MCBP, for solution-processed nondoped near-infrared organic light-emitting diodes (OLEDs) was designed and synthesized, in which the bipolar CBP derivatives is introduced to render balanced charge transportation. This structural modification that the dendrons are well-bounded by the flexible alkyl chains, which can efficiently reduce the intermolecular interactions between the emissive cores and ensure the stable operation of the solution-processed device. As a result of the spin-coated OLEDs employing MPPA-MCBP as the host-free near-infrared emitter exhibits a highest external quantum efficiency (EQE) of 0.62% and a peak wavelength at 698 nm, which indicates the design of self-host TADF dendrimers containing bipolar dendrons will be a promising strategy to improve the electroluminescent performance of solution-processed nondoped device.Here, a novel self-host TADF dendrimer for solution-processed nondoped near-infrared OLEDs was designed and synthesized, in which the bipolar CBP derivatives is introduced to render balanced charge transportation.Download high-res image (277KB)Download full-size image

The molecular aggregation and exciton–polaron interaction of the host–guest system were successfully restricted by efficient molecular encapsulation. The solution-processed blue and green TADF OLEDs have been realized with external quantum efficiencies above 23% by employing the encapsulated TADF host and guest as emission layers.

The molecular aggregation and exciton–polaron interaction of the host–guest system were successfully restricted by efficient molecular encapsulation. The solution-processed blue and green TADF OLEDs have been realized with external quantum efficiencies above 23% by employing the encapsulated TADF host and guest as emission layers.

A self-host thermally activated delayed fluorescence (TADF) dendrimer POCz-DPS for solution-processed nondoped blue organic light-emitting diodes (OLEDs) was designed and synthesized, in which the bipolar phosphine oxide carbazole moiety was introduced by alkyl chain to ensure balanced charge transfer. The investigation of physical properties showed that the bipolar dendrons not only improve the morphological stability but also restrain the concentration quenching effect of the TADF emissive core. The spin-coated OLEDs featuring POCz-DPS as the host-free blue emitter achieved the highest external quantum efficiency (7.3%) and color purity compared with those of doped or nondoped devices based on the parent molecule DMOC-DPS, which indicates that incorporating the merits of encapsulation and bipolar dendron is an effective way to improve the electroluminescent performance of the TADF emitter used for a solution-processed nondoped device.Keywords: bipolar; dendrimer; organic light-emitting diodes; self-host; solution process; TADF;

A benzimidazole/phosphine oxide hybrid 1,3,5-tris(1-(4-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazol-2-yl)benzene (TPOB) was newly designed and synthesized as the electron-transporting component to form an exciplex-type host with the conventional hole-transporting material tris(4-carbazoyl-9-ylphenyl)amine (TCTA). Because of the enhanced triplet energy and electron affinity of TPOB, the energy leakage from exciplex-state to the constituting molecule was eliminated. Using energy transfer from exciplex-state, solution-processed blue phosphorescent organic light-emitting diodes (PHOLEDs) achieved an extremely low turn-on voltage of 2.8 V and impressively high power efficiency of 22 lm W–1. In addition, the efficiency roll-off was very small even at luminance up to 10 000 cd m–2, which suggested the balanced charge transfer in the emission layer. This study demonstrated that molecular modulation was an effective way to develop efficient exciplex-type host for high performanced PHOLEDs.Keywords: blue emission; exciplex; organic light emitting diode; solution-process; turn-on voltage

•Due to the meta-linking configuration, the triplet energy of the newly designed electron-transporting material is 3.15 eV.•The small ΔEST of the exciplex leads to the successive triplet up-conversion in the transient decay curves.•Due to the high triplet energy, the energy leakage from exciplex-state to the constituting molecule was eliminated.A high triplet energy electron transporting material 1,3,5-tris(diphenylphosphoryl)benzene (TPO) was successfully designed and synthesized to form an efficient exciplex with the commonly used hole transporting molecule tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The singlet-triplet energy difference in this exciplex was only 0.03 eV, which leads to the successive triplet up-conversion and delayed fluorescence. In addition, due to the high triplet energies of TPO and TCTA, the energy leakage from exciplex-state to the constituting molecule was eliminated. By employing this exciplex as host, solution-processed white phosphorescent OLEDs have been realized with a low turn-on voltage of 3 V and a high power efficiency of 20.5 lm W−1. These results indicate that the well-designed exciplex can be used as efficient host material for low-cost solution-processed OLEDs.Due to the high triplet energies of constituting molecules and exciplex, the energy leakages from the excited-state to the components or the dopants were successfully eliminated.

A series of bipolar hosts based on carbazole and phenyl benzimidazole (PBI) moieties, collectively named xCz–nPBI, were designed and synthesized. On the basis of different numbers, ratios and link-configurations of the functional groups, the influence of substitution on the chemical, photophysical and electrochemical properties of the host materials were investigated in detail. Both DFT calculations and single carrier devices demonstrate that the strategy of introducing more electron-withdrawing PBI groups in the molecules can effectively enhance the electron injection and transport ability of the bipolar host, while an increased number of carbazole units endows the hosts with a much smaller ΔEST for efficient hole injection at the cost of sacrificing their charge balance property. As a result, the solution-processed green-emitting PHOLEDs based on Cz–6PBI show an extremely low turn on voltage of 2.9 V and the highest current and power efficiency of 47.8 cd A−1 and 29.6 lm W−1, respectively. Even at luminance as high as 1000 cd m−2, their efficient roll-off was only 4.2%, which was far better than the 6Cz–PBI host device. Because the T1 energy levels and triplet state locations of these hosts are similar, their ΔEST and charge balance property should be the main factors that influence their EL performances. We conclude that it is not necessary to achieve a very small ΔEST by introducing more carbazole moieties at the cost of weakening of electron transporting ability. As for solution-processed devices, which suffer from solvent impurities and oxygen diffusion induced strong electron trapping effect, a systemic increase in the number of electron-withdrawing PBI groups in their host materials can significantly enhance the charge balance of their emission layers (EMLs) for highly power efficient solution-processed PHOLEDs.

Three solution-processable exciplex-type host materials were successfully designed and characterized by equal molar blending hole transporting molecules with a newly synthesized electron transporting material, which possesses high thermal stability and good film-forming ability through a spin-coating technique. The excited-state dynamics and the structure–property relationships were systematically investigated. By gradually deepening the highest occupied molecular orbital (HOMO) level of electron-donating components, the triplet energy of exciplex hosts were increased from 2.64 to 3.10 eV. Low temperature phosphorescence spectra demonstrated that the excessively high triplet energy of exciplex would induce a serious energy leakage from the complex state to the constituting molecule. Furthermore, the low energy electromer state, which only exists under the electroexcitation, was found as another possible channel for energy loss in exciplex-based phosphorescent organic light-emitting diodes (OLEDs). In particular, as quenching of the exciplex-state and the triplet exciton were largely eliminated, solution-processed blue phosphorescence OLEDs using the exciplex-type host achieved an extremely low turn-on voltage of 2.7 eV and record-high power efficiency of 22.5 lm W–1, which were among the highest values in the devices with identical structure.Keywords: blue phosphorescence; exciplex; power efficiency; solution process; turn-on voltage

Two soluble bipolar host materials (mCP-BPBI and CP-QPBI), comprising different proportions of hole-transporting carbazole and electron-transporting benzimidazole, were synthesized. Their thermal, physical, and electrochemical properties were characterized. The designated bulky star-shaped structures efficiently suppress the direct intramolecular interaction between the donor and acceptor subunits to give high triplet energies. Through computational studies, varying the ratio of hole- and electron-transporting moieties could significantly change the carrier injection/transporting abilities and charge balance properties of the host materials. Indeed, CP-QPBI with more benzimidazole units shows extremely enhanced current density at the same voltage when compared to mCP-BPBI. The operating voltage of solution-processed phosphorescent light-emitting diodes with CP-QPBI as host were dramatically reduced by ∼3 V compared with the similar devices of mCP-BPBI. At the same time, the power efficiencies were improved for 2–2.5 times at the corresponding voltage. Importantly, both blue and green devices maintain their high efficiencies even at brightness up to 1000 cd m–2, which clearly demonstrates that the new strategy applied to improve electron-transporting ability and charge-balance property of the solution-processable host material by tuning the ratio of donor and acceptor unit is profitable.Keywords: bipolar host; high power efficiency; low operating voltage; OLEDs; reorganization energy; solution-process

A novel material TPA-BPhPO with a photoluminescence quantum yield of 68% and a triplet energy of 2.48 eV has been designed and synthesized. High external quantum efficiencies of 1.41% and 9.3% were achieved in solution-processed blue electrofluorescent and green electrophosphorescent devices, respectively.

A series of bipolar hosts based on carbazole and phenyl benzimidazole (PBI) moieties, collectively named xCz–nPBI, were designed and synthesized. On the basis of different numbers, ratios and link-configurations of the functional groups, the influence of substitution on the chemical, photophysical and electrochemical properties of the host materials were investigated in detail. Both DFT calculations and single carrier devices demonstrate that the strategy of introducing more electron-withdrawing PBI groups in the molecules can effectively enhance the electron injection and transport ability of the bipolar host, while an increased number of carbazole units endows the hosts with a much smaller ΔEST for efficient hole injection at the cost of sacrificing their charge balance property. As a result, the solution-processed green-emitting PHOLEDs based on Cz–6PBI show an extremely low turn on voltage of 2.9 V and the highest current and power efficiency of 47.8 cd A−1 and 29.6 lm W−1, respectively. Even at luminance as high as 1000 cd m−2, their efficient roll-off was only 4.2%, which was far better than the 6Cz–PBI host device. Because the T1 energy levels and triplet state locations of these hosts are similar, their ΔEST and charge balance property should be the main factors that influence their EL performances. We conclude that it is not necessary to achieve a very small ΔEST by introducing more carbazole moieties at the cost of weakening of electron transporting ability. As for solution-processed devices, which suffer from solvent impurities and oxygen diffusion induced strong electron trapping effect, a systemic increase in the number of electron-withdrawing PBI groups in their host materials can significantly enhance the charge balance of their emission layers (EMLs) for highly power efficient solution-processed PHOLEDs.