Highly efficient electroluminescence from a solution-processable thermally activated delayed fluorescence emitter

Wada, Yoshimasa; Shizu, Katsuyuki; Kubo, Syozo; Suzuki, Katsuaki; Tanaka, Hiroyuki; Adachi, Chihaya; Kaji, Hironori

doi:10.1063/1.4935237

Cited by 81 publications

(55 citation statements)

References 25 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Nevertheless, the device (ITO/α‐NPD/6 wt% emitter :mCBP/TPBi/LiF/Al) using tri‐PXZ‐TRZ shows a green emission with an EQE of 13.3% and EL max at 553 nm. A solution‐processable green emitter, 3ACR‐TRZ (λ max : 504 nm; PLQY: 98%; τ d : 6.7 µs in 16 wt% CBP; Δ E ST : 0.02 eV), was reported by Adachi et al (Figure ), which shows a very efficient triplet utilization of 96% together with a PLQY close to unity in doped CBP . It is worth noting that although the dihedral angle between the acridan donor and the phenyl bridge is nearly 90°, the PLQY of the emitter remains exceptionally high.…”

Section: Tadf Emittersmentioning

confidence: 78%

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

2017

View full text Add to dashboard Cite

The earliest account of electroluminescence, the process of converting electrical energy into light, using organic materials can be traced back to 1963 when Pope et al. applied a direct current to an anthracene single crystal under a bias of 400 V using silver-paste electrodes. [3] Although anthracene fluorescence was observed, a driving voltage of 400 V is evidently not viable in practical applications. The seminal breakthrough in the development of OLEDs appeared in 1987 when Tang and Van Slyke reported a double-layered device using tris(8-hydroxyquinoline)aluminum (Alq 3 ) as the emitting and electron-transporting layer. [1] The green-emitting device showed an external quantum efficiency (EQE) of about 1% when driven at less than 10 V. This marked the dawn of OLED development and tremendous interest and effort from both academia and industry have followed subsequent to this pioneering work, resulting in the ultimate wide-scale commercialization of OLEDs, particularly for display applications.In order to make OLEDs commercially viable for lighting applications, where the cost per unit must be competitive with presently used technology, there are a number of challenges that must be overcome aside from reducing the production cost. The organic emitters should have high photoluminescence quantum yields (PLQYs), which directly impact the device efficiency. The energy levels of the frontier molecular orbitals (i.e., highest occupied and lowest unoccupied molecular oribitals (HOMOs and LUMOs)) of each of the layers in the device should be optimally relatively aligned in order to: i) minimize the barrier to charge injection, and ii) control the recombination region within the device, which greatly affects both the device efficiency and lifetime. [4] The organic materials must demonstrate sufficient thermal stability to be compatible with their vacuum deposition during device fabrication or produce thin films of suitable morphology when spin-coated during solution processing. Regardless of the fabrication method, the organic material must be morphologically stable during device operation when Joule heat is produced in the device. [5] Aside from the aforementioned challenges, another key issue to address is the management of hole and electron recombination within the device, each possessing their own spin. Unlike photoexcitation, in which mainly singlet excited states are produced in the organic emitters, exciton formation through charge (hole and electron) recombination in OLED devices results in 25% singlets and 75% triplets, according to The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low-cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a...

show abstract

Section: Tadf Emittersmentioning

confidence: 78%

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

2017

View full text Add to dashboard Cite

show abstract

“…Thus, the planar D and A groups are generally used to construct TADF emitters. Till now, 9,9‐dimethyl‐9,10‐dihydroacridine (DMAC) is one of the most popular planar and rigid D groups used to construct high performance TADF emitters . Plenty of DMAC‐based D–A structure compounds successfully realize extremely small Δ E ST below 0.1 eV and deliver high efficiencies in the devices .…”

Section: Physical Properties Of Molecules In This Workmentioning

confidence: 99%

Avoiding Energy Loss on TADF Emitters: Controlling the Dual Conformations of D–A Structure Molecules Based on the Pseudoplanar Segments

et al. 2017

View full text Add to dashboard Cite

The recent introduction of thermally activated delayed fluorescence (TADF) emitters is regarded as an important breakthrough for the development of high efficiency organic light-emitting devices (OLEDs). The planar D and A groups are generally used to construct TADF emitters for their rigid structure and large steric hindrance. In this work, it is shown that many frequently used nonaromatic (noncontinuous conjugation or without satisfying Hückel's rule) planar segments, such as 9,9-dimethyl-9,10-dihydroacridine, are actually pseudoplanar segments and have two possible conformations-a planar form and a crooked form. Molecules constructed from pseudoplanar segments can thus have two corresponding conformations. Their existence can have significant impact on the performance of many TADF emitters. Two design strategies are presented for addressing the problem by either (1) increasing the rigidity of these groups to suppress its crooked form or (2) increasing the steric hindrance of the linked group to minimize energy of the emitters with the highly twisted form. Following these strategies, two new emitters are synthesized accordingly and successfully applied in OLEDs demonstrating high external quantum efficiencies (20.2% and 18.3%).

show abstract

“…According to previous studies, TADF involves thermal activation process not only from T 1 to S 1 , but also from T n ( n ≥ 1) to S m ( m ≥ 1) if the molecule contains quasi‐degenerate orbits . Nevertheless, a small energy difference between T n and S m is required to obtain high rate constant of UISC ( k uisc ) from T n to S m .…”

Section: Resultsmentioning

confidence: 99%

Pure Organic Emitter with Simultaneous Thermally Activated Delayed Fluorescence and Room‐Temperature Phosphorescence: Thermal‐Controlled Triplet Recycling Channels

Zhong

et al. 2017

Advanced Optical Materials

View full text Add to dashboard Cite

can up-conversion through intersystem crossing (UISC) to the lowest singlet excited state (S 1 ) via thermal activation. [1,2] A separated HOMO (the highest occupied molecular orbital)/LUMO (the lowest unoccupied molecular orbital) distribution is derived from the twisted linkage of electron donor (D)/electron acceptor (A) and the significant energy offset between their corresponding frontier molecular orbitals in one molecule, [3] and a consequent small singlet-triplet splitting gap, which is believed to be an essential prerequisite for TADF molecule.The effective utilization of triplet excitons in TADF-based OLEDs inspired the community to explore more routes to take advantage of the triplet excitons of pure organic molecules, for example, triplettriplet annihilation (TTA), exciplex, and even organic phosphorescence. [4,5] Compared to the metal-involving phosphorescence, [6] pure organic room-temperature phosphorescence (RTP) is still in infancy stage. Several explanations for the unique phenomenon of organic RTP have been proposed, including crystallization-induced phosphorescence (CIP), [5] strong coupling in H-aggregated molecules to stablize triplet excited states, [7] and intermolecular electronic coupling of subunits (n and π*). [8] In terms of the chemical structures, pure organic RTP molecules have some common characteristics: (i) containing electron donor-acceptor (D-A) structure to promote the effective electronic communication; (ii) containing O, N, and/or P heteroatoms in molecular structures to facilitate the spin-forbidden singlet-triplet intersystem crossing via n-π* transition; (iii) a rigid crystalline state by introducing the intensive intermolecular interactions to suppress the nonradiative deactivation process by oxygen and moisture, which may lead to the RTP emission. [9] The design principles of organic RTP molecules are analogous to those of TADF molecules in view of D-A structure, and thus we reasoned that rational molecular design may lead to metalfree pure organic emitter with simultaneous TADF and RTP. Such unique organic emitters will enable them not only fundamentally important but also promising for various applications.To validate our hypothesis, we designed and synthesized a new D-A type organic molecule with quinoxaline as electron acceptor and phenoxazine (PXZ) as electron donor. Quinoxaline A new route to utilize the triplet excitons by simultaneous thermally activated delayed fluorescence (TADF) and phosphorescence is demonstrated for a new quinoxaline/phenoxazine hybrid emitter. Moreover, the two triplet recycling channels are thermally controlled, and a clear threshold temperature of 170 K is observed. Below the threshold temperature, direct triplet radiation (phosphorescence) is the dominant process. In contrast, the channel of upconversion through intersystem crossing is activated above the threshold and the resulting TADF gradually becomes the predominant process. By using the new compound as emitter in organic light-emitting diodes, a high external quantum efficienc...

show abstract

Highly efficient electroluminescence from a solution-processable thermally activated delayed fluorescence emitter

Cited by 81 publications

References 25 publications

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

Avoiding Energy Loss on TADF Emitters: Controlling the Dual Conformations of D–A Structure Molecules Based on the Pseudoplanar Segments

Pure Organic Emitter with Simultaneous Thermally Activated Delayed Fluorescence and Room‐Temperature Phosphorescence: Thermal‐Controlled Triplet Recycling Channels

Contact Info

Product

Resources

About