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Vapor-Deposited Organic Light-Emitting Devices
Published in Zhigang Rick Li, Organic Light-Emitting Materials and Devices, 2017
Previously, there had been a perception in the OLED research field that PHOLEDs, although highly desirable for use in active-matrix applications because of their high efficiency, were unsuitable for passive-matrix applications. The first generation of PHOLEDs contained PtOEP as the phosphorescent dopant. However, these devices had a spectral dependence on applied current. This was a result of the long radiative lifetime of PtOEP (>10 μs). As the applied current was increased, dopant sites within the device became saturated, resulting in the inability of excitons to transfer from the host material. Some of these excitons then decayed, emitting light characteristic of the host material. The devices also exhibit a steep roll-off in efficiency that has been mainly interpreted as the result of triplet–triplet annihilation. However, the latest generations of PHOLEDs have efficiency roll-offs at high drives that are comparable to or better than the conventional fluorescent OLED or PLED devices [138], and they show higher efficiency at the high luminance values needed in a passively addressed display. The main advantages of passive addressing are that it is relatively inexpensive and that, in principle, it is possible to drive small- to medium-sized displays. However, large-area, high-resolution displays (>5″) are problematic because of the high current densities and hence increased power consumption (I2R) described above. For a more detailed discussion of these issues, see, for example, Gu and Forrest [139].
Nanomaterial-Based Electrochemiluminescence Biosensors
Published in Li Jun, Wu Nianqiang, Biosensors Based on Nanomaterials and Nanodevices, 2017
From the view of thermodynamics, ion annihilation process involves two routes: “S-route” and “T-route.” If the enthalpy related to the ion annihilation reaction exceeds the energy required to produce the lowest excited states from the ground state, then the reaction is defined as “energy sufficient” singlet route “S-route.” In contrast, if the enthalpy is lower than the energy required to produce the lowest excited state but still exceeds the triple state energy, the emitting species is formed via a triplet–triplet annihilation “T-route.” The typical example of “T-route” annihilation is the ECL of rubrene and of Ru(bpy)32+ (bpy = bi-pyridine) or its derivatives. In addition, when excimers (excited dimers) and exciplexes (excited complexes) are formed in ion annihilation, the system is said to follow the “E-route”. The major advantage of the annihilation process is that it requires only the ECL species, solvent, and supporting electrolyte in order to generate light. However, the potential window of water is often not sufficiently wide to allow the species to be both oxidized and reduced, making it necessary to use organic solvents such as acetonitrile or N,N-dimethylformamide (DMF).
A novel anthracene derivative with an asymmetric structure as an electron transport material for stable Rec. 2020 blue organic light-emitting diodes
Published in Journal of Information Display, 2020
Cheng Liu, Dongdong Zhang, Lian Duan
Since the pioneering works of Tang and Van Slyke in 1987, organic light-emitting diodes (OLEDs) have been the subject of extensive research and have undergone rapid development [1]. With the great progress of phosphorescence (Ph) and thermally activated delayed fluorescence (TADF) emitters, OLEDs with external quantum efficiency (EQE) values as high as ∼40% have been widely reported [2]. The mutual high efficiency and long lifetimes of blue OLEDs, however, still greatly hinder the further development of such technology. Despite their high EQE, blue phosphorescent OLEDs (PhOLEDs) and thermally activated delayed fluorescence (TADF)-OLEDs suffer from short operation lifetimes. To date, the practically applied blue OLEDs rely on the conventional fluorescent organic dopants with triplet–triplet annihilation (TTA) to partly recycle the triplet excitons. Yet, the stability of blue TTA-OLEDs still lags behind that of the green/red ones and especially that of the deep-blue ones.
Pure-organic phosphine oxide luminescent materials
Published in Journal of Information Display, 2020
All the transitions irrelevant to radiation lead to non-radiative energy losses (e.g. VR, IC, and ISC), causing luminescent efficiency reduction. Particularly, as the remarkably longer lifetime of the triplet states increases the collisional possibility, the triplet-involved processes are extremely sensitive to the quenching effects, which are embodied as the singlet–triplet annihilation (STA), triplet–triplet annihilation (TTA), and triplet-polaron quenching (TPQ) in electrical devices. Therefore, both PH and TADF emitters suffer seriously from triplet concentration quenching. Actually, few pure-organic compounds can emit room-temperature phosphorescence, which largely depends on the assistance of the heavy-atom effect for SOC enhancement. It was recently found that based on delicately modulated intermolecular interactions, the triplet states of pure-organic aggregates could be stabilized to improve the PH lifetimes and efficiencies, even though most high-efficiency phosphors are cyclometalated heavy-metal complexes.
Recent advances with optical upconverters made from all-organic and hybrid materials
Published in Science and Technology of Advanced Materials, 2019
Roland Hany, Marco Cremona, Karen Strassel
The operating mode of upconversion devices is different from the several known photon upconversion processes. Photon upconversion is described as the process that converts two or more sequentially absorbed low-energy photons into a photon of higher energy. Sensitized triplet-triplet annihilation is one such topical example of an efficient optical upconversion process that allows light to be converted into radiation of higher energy at low power excitation. Such materials are developed, i.a., to capture the infrared region of sunlight, thereby increasing the efficiency of photovoltaic cells [11–13].