An exciplex forming cohost system is employed to achieve a highly efficient organic light-emitting diode (OLED) with good electroluminescent lifetime. The exciplex is formed at the interfacial contact of a conventional star-shaped carbazole hole-transporting material, 4,4',4″-tris(N-carbazolyl)-triphenylamine (TCTA), and a triazine electron-transporting material, 2,4,6-tris[3-(1H-pyrazol-1-yl)phenyl]-1,3,5-triazine (3P-T2T). The excellent combination of TCTA and 3P-T2T is applied as the cohost of a common green phosphorescent emitter with almost zero energy loss. When Ir(ppy)(acac) is dispersed in such exciplex cohost system, OLED device with maximum external quantum efficiency of 29.6%, the ultrahigh power efficiency of 147.3 lm/W, and current efficiency of 107 cd/A were successfully achieved. More importantly, the OLED device showed a low-efficiency roll-off and an operational lifetime (τ) of ∼1020 min with the initial brightness of 2000 cd/m, which is 56 times longer than the reference device. The significant difference of device stability was attributed to the degradation of exciplex system for energy transfer process, which was investigated by the photoluminescence aging measurement at room temperature and 100 K, respectively.

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Quantum dot light-emitting devices (QLEDs), originally developed for displays, were recently demonstrated to be promising light sources for various photomedical applications, including photodynamic therapy cancer cell treatment and photobimodulation cell metabolism enhancement. With exceptional emission wavelength tunability and potential flexibility, QLEDs could enable wearable, targeted photomedicine with maximized absorption of different medical photosensitizers. In this paper, we report, for the first time, the in vitro study to demonstrate that QLEDs-based photodynamic therapy can effectively kill Methicillin-resistant Staphylococcus aureus, an antibiotic-resistant bacterium. We then present successful synthesis of highly efficient quantum dots with narrow spectra and specific peak wavelengths to match the absorption peaks of different photosensitizers for targeted photomedicine. Flexible QLEDs with a peak external quantum efficiency of 8.2% and a luminance of over 20,000 cd/m2 at a low driving voltage of 6 V were achieved. The tunable, flexible QLEDs could be employed for oral cancer treatment or diabetic wound repairs in the near future. These results represent one fresh stride toward realizing QLEDs’ long-term goal to enable the wide clinical adoption of photomedicine.

Excited states in organic light-emitting diodes (OLEDs) are inevitably formed with both singlets and triplets under electrical excitation. Singlets and triplets are allowed and forbidden to recombine, respectively, due to spin selection rule. It has been shown that the triplets can be almost 100% converted into singlets in thermally activated delayed fluorescence (TADF) molecules based on the design of chemically combining donor and acceptor moieties to enable intramolecular chargetransfer states. [1][2][3][4] Recently, various TADFmolecule-based OLEDs with extremely high external quantum efficiency (EQE) exceeding 35% have been successfully demonstrated. [5,6] Similarly, high EQEs can also be conveniently realized by physically mixing donor and acceptor components to form intermolecular charge-transfer states in exciplex systems, where the nonradiative triplets are also largely converted into radiative singlets. The advantages and versatile applications of exciplex systems for giving high-efficiency OLEDs have been highlighted recently. [7] More significantly, OLEDs with exciplex-forming systems as emitting layer have been reported to achieve EQE higher than 19%, [8][9][10] manifesting their bright and promising prospects in OLED technology based on physically Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge-transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex lightemitting diodes are reported. Magneto-photoluminescence studies reveal that the triplet-to-singlet conversion in exciplexes involves an artificially generated spin-orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge-transfer occurs with forming electric dipoles (D +• →A −• ), providing the ionic polarization to generate SOC in exciplexes. By having different singlet-triplet energy differences (ΔE ST ) in 9,9′-diphenyl-9H,9′H-3,3′-bicarbazole (BCzPh):3′,3′″,3′″″-(1,3,5-triazine-2,4,6-triyl) tris(([1,1′-biphenyl]-3-carbonitrile)) (CN-T2T) (ΔE ST = 30 meV) andBCzPh:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methyl-pyrimidine (B3PYMPM) (ΔE ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge-transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE max (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE ST , and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D +• →A −• ) size, enhanced SOC, increased orbital polarization, and decreased ΔE ST can simultaneously occur to cooperatively operate the triplet-to-singlet conversion.