Highly Efficient and Stable Phosphorescent Organic Light‐Emitting Diodes Utilizing Reverse Intersystem Crossing of the Host Material

Abstract

high reliability, their internal electroluminescence quantum effi ciency ( η int ), defi ned as the number of photons generated per injected carrier, is limited to 25% because of the exciton-branching ratio of singlet excited states under electrical excitation. [ 2 ] To increase the value of η int , several methods were proposed, such as employing phosphorescent materials with heavy atoms, utilizing triplet-triplet exciton annihilation for extra singlet generation, and employing thermally activated delayed fl uorescence (TADF) materials. [3][4][5][6] In particular, an η int value of 100% is expected by employing phosphorescent and TADF materials. In 1999, effi cient electrophosphorescence was fi rst demonstrated using iridium complexes, and an η int value of almost 100% was achieved afterwards. [ 7,8 ] Many highly effi cient phosphorescent OLEDs (PHOLEDs) with values of η int of about 100% were demonstrated by using a device architecture that can confi ne charges and excitons inside the phosphorescence-emitting layer. [ 9,10 ] Although PHOLEDs have a longer history than OLEDs fabricated with TADF materials, the basic concept for improving both the effi ciency and operational stability is not clear; this is the greatest challenge for the practical application of PHOLEDs.In recent years, several PHOLEDs with high effi ciency and high operational stability were reported. [ 11,12 ] The confi guration of the emitting layer, i.e., the combination of the phosphorescent dopant and the surrounding host material, was proposed as a key parameter determining the effi ciency and stability of PHOLEDs. Particular metal complexes such as bis(benzo[h]quinolin-10-olato-kN,kO)beryllium( II ) (Bebq 2 ) and bis[2-(2-hydroxyphenyl) benzothiazolato]zinc( II ) [Zn(BTZ) 2 ] are effective hosts for red PHOLEDs; however, an effective host for green/blue PHOLEDs with high operational stability was not yet reported. If the role of effective hosts such as metal complexes in a highly effi cient and stable PHOLED is revealed, the development of useful host materials for green/blue PHOLEDs is expected to be accelerated.Herein, to investigate a possible strategy for realizing highly effi cient and stable PHOLEDs, we examine the energy-transfer mechanism in such PHOLEDs. A highly effi cient green PHOLED that exhibits a half lifetime of over 10000 h with an initial luminance of 1000 cd m -2 is realized by using a Phosphorescent organic-light emitting diodes (PHOLEDs) exhibit an internal quantum effi ciency of 100% and their early practical realization is expected. One of the main challenges in PHOLEDs is to improve the operational stability of green and blue devices. Triplet exciton dynamics in stable PHOLEDs, which are different from those in unstable PHOLEDs, are shown. An effi cient and stable green PHOLED is demonstrated by employing a suitable host that surrounds the phosphorescent dopant. The transient photoluminescence characteristics show that the triplet excitons of the host, which are generally unstable, are rapidly transferred to the dop...

Blue Thermally Activated Delayed Fluorescence‐Emitting Phosphine Oxide Hosts for Ultrasimple and Highly Efficient White Organic Light‐Emitting Diodes

“…However, in this kind of WOLEDs, optimal exciton allocation is formidable due to the unidirectional singlet–triplet conversion and the exclusion of blue emitter in triplet harvesting, in turn forcing the employment of multi‐EML for spatial exciton allocation, which can be solved through adopting blue thermal‐activated delayed fluorescence (TADF) emitters with triplet utilization capability instead of blue fluorophors. Actually, using blue TADF dye as host in IV ‐type EML can support the triplet–singlet conversion through reverse intersystem crossing (RISC), thereby adaptably regulating the exciton population of blue emission for optimal exciton allocation ( Scheme b). Nevertheless, in comparison to “FL+PH” hybrid WOLEDs, the involvement of triplet exciton in both blue and yellow emitting processes doubles nonradiative channels, and meanwhile dramatically worsens the host–dopant interaction induced quenching, especially triplet–triplet annihilation .…”

Molecular Design and Device Design to Improve Stabilities of Organic Light-Emitting Diodes

“…Although there have been many reports on PHOLEDs with IQE of about 100%, the basic concept for improving both the efficiency and operational stability was first discussed as late as 2014. Operationally stable PHOLEDs have been demonstrated using two types of host: exciplexes with a small energy gap between the singlet (S 1 ) and triplet (T 1 ) excited statesE ST  [15,18], and single TADF materials [14,16,17,20]. An emitting layer (EML) consisting of a single TADF material and a phosphorescent dopant is ideal for low-cost, highperformance PHOLEDs because strict control of the co-evaporation rate is essential when the coevaporation of the EML involves the exciplex and the emitter [15,18].…”

“…However, in recent years, the stability of OLEDs has begun to be intensively discussed from two viewpoints: operational stability and air-stability. Since high operational stability is essential for almost all OLEDs used for practical applications, there have been many reports on materials suitable for achieving operationally stable OLEDs [12][13][14][15][16][17][18][19][20][21][22][23]. Some OLEDs employing flexible substrates or simple encapsulation structures have been applied for optoelectronic sensors and biophotonics in recent years [24][25][26].…”

High efficiency and low roll-off green OLEDs with simple structure by utilizing thermally activated delayed fluorescence material as the universal host

“…TADF materials, which could achieve 100% IQE through the efficient reverse intersystem crossing (RISC) of triplet excitons, had a hugely successful harvest as the emitter due to the small energy difference (ΔE S-T ) between the singlet and triplet excited state [7][8][9][10]. Research on TADF material as the host has also received attention in the past few years [11][12][13]. Qiu et al fabricated highly efficient orange Fl-OLEDs with an EQE as high as 12.2% by utilizing TADF materials of 2,4-diphe-nyl-6-bis (12-phenylindolo) [2,3-a]carbazole-11-yl)-1,3,5-triazine (DIC-TRZ) as the host [11].…”