Joong Hwan Yang

Highly Efficient Red Phosphorescent Dopants in Organic Light‐Emitting Devices

Phosphorescent organic light-emitting diodes (PHOLEDs) have been developed for more than 10 years. As a result, the highly effi cient red PHOLED materials are now utilizing in commercial active-matrix organic light-emitting diodes (AMOLEDs) and lighting. Nevertheless, the development of much more effi cient PHOLEDs is still required due to a dramatic reduction of power consumption by LED backlight in thin fi lm transistor-liquid crystal display (TFT-LCDs) and the generation of new lighting applications with comparable effi ciency of fl uorescent lamps. There have been many studies about improving the effi ciency of red PHOLEDs by developing host and dopant materials. However, the radiative and nonradiative rate constants ( k r , k nr ) have a strong dependence on the energy gap ( Δ E ) between the emissive excited state and the ground state in accordance with “the energy gap law”, which indicates that k nr increases with a decrease in Δ E . Hence it is obvious that highly effi cient red emission with smaller Δ E tends to give larger k nr and smaller k r , leading to the lower emission quantum yield. [ 1 , 2 ]

Small single–triplet energy gap bipolar host materials for phosphorescent blue and white organic light emitting diodes

We report extremely high efficiency tandem white organic light-emitting diodes (OLEDs) with newly synthesized host materials. Our new host materials have well balanced bipolar characteristics and very small singlet to triplet splitting energies (∼0.4 eV) due to almost no orbital overlapping between ground and excited states. The fabricated blue phosphorescent device with one of these host materials shows very high external quantum efficiency of 25.7%, low onset and driving voltages of 2.47 V and 3.49 V at 1 and 1000 cd m−2, respectively. High performance asymmetric tandem OLEDs could be fabricated by adopting these efficient and low driving voltage blue unit and very efficient yellow unit devices. Power efficiency of 63.1 lm W−1 and current efficiency of 128.8 cd A−1 at 1000 cd m−2 were achieved without any light out-coupling technology. We expect that blue phosphorescent OLEDs with our new narrow band-gap bipolar host materials could be a promising solution to make high efficiency white OLEDs for display and lighting applications.

High-performance bipolar host materials for blue TADF devices with excellent external quantum efficiencies

New bipolar host molecules composed of carbazole, pyridoindole, and dibenzothiophene (DBT) were synthesized for blue thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs). 2,8-Di(9H-carbazol-9-yl)dibenzo[b,d]thiophene, 9-(8-(9H-carbazol-9-yl)dibenzo[b,d]thiophen-2-yl)-9H-pyrido[2,3-b]indole, and 2,8-bis(9H-pyrido[2,3-b]indol-9-yl)dibenzo[b,d]thiophene were prepared based on the selective reactivity at the 2,8-positions of DBT. The new symmetric and asymmetric host materials exhibited high triplet energies (2.89–2.95 eV). 4,5-Di(9H-carbazol-9-yl)phthalonitrile (2CzPN) was selected as an emitting dopant for achieving sky-blue emissions in TADF-OLEDs. 2CzPN-doped TADF-OLEDs, whose configuration is ITO (50 nm)/HATCN (7 nm)/TAPC (75 nm)/host:6% 2CzPN (20 nm)/TmPyPB (50 nm)/LiF (15 nm)/Al (100 nm), showed low driving voltages and high external quantum efficiencies (EQEs). These results are attributed to the well-controlled bipolar character of the host giving a better charge balance in the emitting layer. In particular, the device containing ZDN:6% 2CzPN showed an unprecedentedly high EQE of 25.7% (at 0.074 mA cm−2).

Optimized structure of silane-core containing host materials for highly efficient blue TADF OLEDs

Three new derivatives containing silane cores, viz. 9,9′,9′′-(((4-(pyridin-3-yl)phenyl)silanetriyl)tris(benzene-4,1-diyl))tris(9H-carbazole) (SiCz3Py1), bis(4-(9H-carbazol-9-yl)phenyl)bis(4-(pyridin-3-yl)phenyl)silane (SiCz2Py2), and 9-(4-(tris(4-(pyridin-3-yl)phenyl)silyl)phenyl)-9H-carbazole (SiCz1Py3), were designed and synthesized. Carbazole as a donor and pyridine as an acceptor were tethered to tetraphenylsilane at different mole ratios. All three host materials showed high glass transition temperatures between 118 and 164 °C, which are different from those of the previous silane-based host materials (e.g., diphenyldi-o-tolylsilane (UGH-1), 1,4-bis(triphenylsilyl)benzene (UGH-2), and 1,3-bis(triphenylsilyl)benzene (UGH-3)). The triplet energies of these three hosts are observed at 2.85–2.90 eV, which is high enough for them to act as blue host materials in thermally activated delayed fluorescence organic light emitting diodes (TADF OLEDs). In particular, SiCz2Py2 and SiCz1Py3 hosted-TADF OLEDs demonstrated excellent performances, with a maximum external quantum efficiency (EQEmax) of 18.7 and 18.8%, respectively. Such good performances of SiCz2Py2 and SiCz1Py3 are originated by suppressing the non-radiative triplet decay and high reverse intersystem crossing (RISC) rate constant for efficient triplet to singlet up-conversion. This work demonstrates that tetraphenylsilane is a promising non-conjugate (i.e., sp3-hybridized) linkage core for developing a variety of high Tg host materials, particularly for blue TADF OLEDs.