Novel Strategy to Develop Exciplex Emitters for High‐Performance OLEDs by Employing Thermally Activated Delayed Fluorescence Materials

Though urgently required, high performance near‐infrared (NIR) emitters are still rare given the challenge of obtaining high photoluminance quantum efficiency (PLQY) at the same time ensuring NIR emission. The major issue lies in the design strategy for which strong electron donating/withdrawing moieties with high PLQY should be integrated with a scrumptious way. Herein, a novel donor–acceptor–acceptor (D–A–A) type NIR emitter comprising highly polar cyano group (CN) together with rigid benzo[c][1,2,5]thiadiazole as tandem acceptor and 4,4′‐dimethyltriphenylamine as donor is successfully designed. This constructing strategy not only allows the D/A maintain their intrinsic electron‐donating/withdrawing characteristics, but also retains high PLQY. In merits of these features, excellent external quantum efficiency (EQE) of 3.8% with peak emission at 692 nm for 15% doped device is achieved. Encouragingly, impressive EQE of 3.1% with the peak emission at 708 nm is also successfully achieved without doping technique. It is believed that these efficiencies are the best or among the best comparing to those of the reported NIR organic light‐emitting diodes with similar electroluminescence peak. Notably, efficiency roll‐offs of both doped and nondoped device are also quite flat.

Section: Introductionmentioning

confidence: 99%

D–A–A‐Type Emitter Featuring Benzo[c][1,2,5]thiadiazole and Polar CN Bond as Tandem Acceptor for High‐Performance Near‐Infrared Organic Light‐Emitting Diodes

Wang

et al. 2017

“…Noticeably, the hole and electron mobilities at molar ratio of 5:5 exhibit smaller difference, indicating more balanced carriers transporting. Therefore, all the OLEDs based on m‐CBP:B4PyPPM host were developed with the optimized molar ratio of 5:5 …”

Section: Resultsmentioning

confidence: 99%

White OLEDs with an EQE of 21% at 5000 cd m⁻² and Ultra High Color Stability Based on Exciplex Host

Zhao

Yuan

et al. 2018

Exciplex has drawn attention because of its great potential for use as a host in organic light‐emitting diodes (OLEDs). Here a novel exciplex host formed with 3,3′‐di(9H‐carbazol‐9‐yl) biphenyl (m‐CBP) and 4,6‐bis (3,5‐di(pyridin‐4‐yl) phenyl)‐2‐phenylpyrimidine (B4PyPPM) is achieved. By utilizing m‐CBP:B4PyPPM exciplex host, blue, cyan, green, and red phosphorescent OLEDs exhibit maximum external quantum efficiencies (EQEs) of 26.2%, 26.1%, 24.7%, and 26.5%, respectively. Moreover, single‐emitting layer (EML) white organic light‐emitting devices (WOLEDs) are achieved by doping blue and red phosphorescent emitters, which exhibit high maximum EQEs of 23.9% and 26.9%. Even at a practical brightness of 5000 cd m−2, the two WOLEDs still keep high EQEs of 19.1% and 21.0%, respectively. Furthermore, multiple‐EML WOLEDs are also constructed with efficiencies of 18.9% and 21.0% at the practical brightness of 5000 cd m−2. The devices show extremely stable white emission with small Commission Internationale de L'Eclairage (CIE) coordinate varying range of (±0.001, ± 0.000) from 100 to 100 00 cd m−2. The outstanding features presented here can provide a good reference for future design of high‐performance WOLEDs.

“…They are popular as the hosts of light‐emitting materials, but they can also efficiently emit by themselves through thermally activated delayed fluorescence (TADF) mechanism . Since the first demonstration of efficient light emission of the exciplexes by TADF process, the external quantum efficiency (EQE) of the exciplexes has been dramatically improved …”

Section: External Quantum Efficiency Current Efficiency and Color Cmentioning

confidence: 99%

“…The TADF material is a kind of bipolar material with a donor–acceptor structure and it could make the intermolecular type exciplex with the n‐type or p‐type material . For instance, 6‐(9,9‐dimethylacridin‐10(9H)‐yl)‐3‐methyl‐1H‐isochromen‐1‐one (MAC):PO‐T2T exciplexes were derived from a n‐type acceptor and an TADF emitter . The exciplexes could also be developed by integrating two TADF emitters which have the large highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) offset.…”

Section: External Quantum Efficiency Current Efficiency and Color Cmentioning

confidence: 99%

Ternary Exciplexes for High Efficiency Organic Light‐Emitting Diodes by Self‐Energy Transfer

Jeon

Jang

Lee

2019

High EQE exciplexes were also reported by mixing an n-type acceptor with an intramolecular TADF material. The TADF material is a kind of bipolar material with a donor-acceptor structure and it could make the intermolecular type exciplex with the n-type or p-type material. [24][25][26] For instance, 6-(9,9-dimethylacridin-10(9H)-yl)-3-methyl-1H-isochromen-1-one (MAC):PO-T2T exciplexes were derived from a n-type acceptor and an TADF emitter. [20] The exciplexes could also be developed by integrating two TADF emitters which have the large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) offset. Bis [4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS): 9,9′,9″-(5-(4,6-diphenyl-1,3, 5-triazin-2-yl)benzene-1,2,3-triyl)tris (9H-carbazole) (TCzTrz) is an example of the exciplex derived from two TADF emitters. [27] A high EQE of 15.3% was attained in the exciplex. However, the EQE of the exciplexes was rather limited because of low photoluminescence quantum yield (PLQY) of the exciplex. Unlike the common emitting layer system comprised of host and dopant, the exciplex emitting layer is like a nondoped emitting layer. Therefore, the exciton quenching of the exciplexes may occur in the emitting layer, decreasing the EQE of the exciplex organic light-emitting diodes (LEDs) (OLEDs). Another reason is the nature of the exciplex emission which is known as the intermolecular charge transfer emission. Compared with the intramolecular TADF emission, the intermolecular exciplex emission is relatively inefficient by loss mechanisms between two materials. Therefore, a new way of improving the EQE of the exciplexes is strongly required.Herein, we describe a host-dopant type ternary exciplex system with a high EQE of 17.5%. A p-type donor and a n-type acceptor were introduced to form a high energy exciplex and an additional donor-acceptor type TADF emitter was mixed as another p-type donor to generate a low energy exciplex with the n-type acceptor. The high energy exciplex functioned as a host and the low energy exciplex behaved as a dopant in the ternary exciplex. Final exciplex emission was observed from the low energy exciplex. It was demonstrated that the dispersion of the low energy exciplex in the high energy exciplex and the energy transfer from the high energy exciplex to the low energy exciplex increased the EQE of the exciplex devices. The EQE of the exciplex device achieved in this work is one of the best EQEs reported in the exciplex OLEDs.Ternary exciplexes with an external quantum efficiency of 17.5% are developed by mixing a p-type donor material, an n-type acceptor material, and a donor-acceptor type material together. In the ternary mixture, the n-type donor generates exciplexes with the p-type donor and the donor-acceptor type material at the same time. Two different exciplexes with dissimilar emission wavelengths are formed and the emission of the high-energy exciplex is transferred to the low-energy exciplex by energy transfer. The high-energy exciplex and low-energy ex...