Numerical Study of Triplet Dynamics in Organic Semiconductors Aimed for the Active Utilization of Triplets by TADF under Continuous-Wave Lasing.

The limitation of lasing duration less than nanosecond order has been a major problem for realizing organic solid-state continues-wave (CW) lasers and organic semiconductor laser diodes. Triplets accumulation under CW excitation has been well recognized as a critical inhibiting factor. To overcome this issue, the utilization of thermally activated delayed fluorescence (TADF) emitters is a promising mechanism because of efficient reverse intersystem crossing. Herein, we model the triplet accumulation processes under lasing and propose the active utilization of TADF for lasing based on our simulation analysis. We used the rate constants experimentally determined from the optical properties of a boron difluoride curcuminoid fluorophore showing both TADF and lasing. We demonstrate that the intersystem crossing efficiency is gradually increased after the convergence of relaxation oscillation, i.e., terminating laser oscillation. In addition, we found that when the reverse intersystem crossing rate is close to the intersystem crossing rate, CW lasing becomes dominant.

Bis(Triphenylamine)Benzodifuran Chromophores: Synthesis, Electronic Properties and Application in Organic Light-Emitting Diodes.

A series of bis(triphenylamine)benzodifuran chromophores have been synthesized and fully characterised. Starting from suitably functionalized benzodifuran (BDF) precursors, two triphenylamine (TPA) moieties are symmetrically coupled to a central BDF unit either at 4,8-positions through double bonds (1) and single bonds (2) respectively, or at 2,6-positions through double bonds (3). Their electronic absorption and photoluminescence properties as well as redox behaviour have been investigated in detail, indicating that the π-extended conjugation via vinyl linkers in 1 and 3 leads to comparatively strong electronic interactions between the relevant redox moieties TPA and BDF. Due to intriguing electronic properties and structural planarity, 3a has been applied as a dopant emitter in organic light-emitting diodes. A yellowish-green OLED exhibits a high external quantum efficiency (EQE) of 6.2%, thus exceeding the theoretical upper limit most likely due to energy transfer from an interface exciplex to an emissive layer and/or favorable horizontal orientation.

Molecular Orientations of Delayed Fluorescent Emitters in a Series of Carbazole-Based Host Materials.

Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (ηext) of 26% using an emission layer with horizontal orientation ratio (Θ) of 79%, which is higher than that of an almost randomly oriented emission layer with Θ of 62% (ηext = 22%).

F8BT Oligomers for Organic Solid-State Lasers.

The yellow-green emissive poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) polymer is widely used because of its suitability for a variety of applications. However, we have found that F8BT shows huge performance variations that depend on the chemical supplier, with photoluminescence quantum yields (PLQYs) ranging from 7 to 60% in neat films. Polymers generally face problems including purity, polydispersity, and reproducibility, which also affect F8BT polymers. Therefore, to overcome these problems, we investigated oligomers of F8BT, which can easily be purified and can thus be obtained in a high-purity form. In the three oligomers (M1-M3) that we synthesized, the PLQYs were much higher than those of conventional F8BT (>80% in their neat films) although their PL spectra were nearly the same as that of F8BT, and their amplified spontaneous emission (ASE) thresholds were lower than that of the polymer (e.g., 1.9 µJ cm-2 for M3 and 2.7 µJ cm-2 for F8BT) because of a higher net gain and better film morphology. Furthermore, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of the oligomers were found to be similar to those of F8BT, making them candidate materials for use as hosts in light-emitting devices. The ASE using a near-infrared laser emitter doped in F8BT and oligomer hosts showed a clear difference despite nearly the same properties for steady-state emission.

Manipulating the Electronic Excited State Energies of Pyrimidine-Based Thermally Activated Delayed Fluorescence Emitters To Realize Efficient Deep-Blue Emission.

The development of efficient and robust deep-blue emitters is one of the key issues in organic light-emitting devices (OLEDs) for environmentally friendly, large-area displays or general lighting. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention. However, only a handful of examples of deep-blue TADF emitters have been reported to date, and the emitters generally show large efficiency roll-off at practical luminance over several hundreds to thousands of cd m-2, most likely because of the long delayed fluorescent lifetime (τd). To overcome this problem, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τd. We then systematically investigated the relationships among the chemical structure, properties, and device performances. The resultant novel pyrimidine emitters, called Ac-XMHPMs (X = 1, 2, and 3), contain different numbers of bulky methyl substituents at acceptor moieties, increasing the excited singlet (ES) and triplet state (ET) energies. Among them, Ac-3MHPM, with a high ET of 2.95 eV, exhibited a high external quantum efficiency (ηext,max) of 18% and an ηext of 10% at 100 cd m-2 with Commission Internationale de l'Eclairage chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs. Our molecular design strategy provides fundamental guidance to design novel deep-blue TADF emitters.

Highly Luminescent π-Conjugated Terpyridine Derivatives Exhibiting Thermally Activated Delayed Fluorescence.

Typically, luminescent π-conjugated 2,2':6',2''-terpyridine (tpy) derivatives are versatile components for tridentate metal ligands, supramolecular materials, two-photon absorption bioimaging probes, fluorescent ion sensors, and organic light-emitting devices. However, a limited number of luminescent tpy materials, other than metal complexes, have been reported. This study introduces a series of π-conjugated tpy derivatives that exhibit strong thermally activated delayed fluorescence (TADF). We have observed that a blue tpy emitter outperforms conventional fluorescent emitters. Additionally, a green tpy emitter has exhibited a performance that is almost comparable to that of its green phosphorescent counterparts, realizing an external quantum efficiency close to 25 % and a power efficiency exceeding 80 lm W-1 with an exceptionally low efficiency roll-off. This study demonstrates the first example of highly luminescent tpy-based TADF emitters.