Juewen Zhao

A novel bipolar phenanthroimidazole derivative host material for highly efficient green and orange-red phosphorescent OLEDs with low efficiency roll-off at high brightness

A new bipolar fluorophore, N,N-diphenyl-4′-(9-(4′-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1′-biphenyl]-4-yl)-9H-fluoren-9-yl)-[1,1′-biphenyl]-4-amine (PPI-F-TPA), consisting of an electron-withdrawing phenanthro[9,10-d]imidazole (PI) chromophore and an electron-donating triphenylamine group, based on an indirect linkage, has been designed and synthesized. The sp3-hybridized C9 atom of the fluorene linkage efficiently interrupts molecular conjugation and inhibits π–π intermolecular interactions, resulting in efficient violet-blue emission, excellent thermal stability and high triplet energy. Equipped with balanced carrier mobility, PPI-F-TPA shows impressive performance as the emitting layer in non-doped OLEDs, which achieved an external quantum efficiency (EQE) of 3.11% with a CIE coordinate of (0.16, 0.05). Furthermore, the high triplet energy allows PPI-F-TPA to be used as a host for PhOLEDs. High performance green and orange-red PhOLEDs with the maximum EQEs, current efficiencies (CE) and power efficiencies (PE) of 15.6% and 12.5%, 57 cd A−1 and 27 cd A−1, 60 lm W−1 and 28.3 lm W−1, respectively, have been successfully obtained. More importantly, all the devices exhibit low efficiency roll-off; in particular, that of the orange-red PhOLEDs is extremely small. The orange-red PhOLED has a decay rate of EQE less than 1% at 1000 cd m−2, 13.6% at 10 000 cd m−2 and 29.5% even at 50 000 cd m−2, which is very rare among orange or orange-red PhOLEDs at such high brightness.

High-performance fluorescent/phosphorescent (F/P) hybrid white OLEDs consisting of a yellowish-green phosphorescent emitter

The color rendering index (CRI) of a white organic-light emitting device (WOLED) employing standard red + green + blue emitters is typically limited by the deep valley between the red and the green emission peaks. To address this issue without increasing device complexity, we synthesized a yellowish-green iridium emitter, iridium(III) bis(2-phenylpyridine)(2-(benzo[d]oxazol-2-yl)phenol) (Ir(ppy)2bop), for replacing the standard green emitter. By combining emissions from Ir(ppy)2bop with those from a blue fluorescent emitter and a red phosphor, a high performance fluorescent/phosphorescent (F/P) WOLED has been fabricated. The device gives white emission with a maximum efficiency of 55.2 cd A−1 (49.6 lm W−1) and an EQE value of 20% without any light extraction technologies. It is noteworthy that the color rendering index (CRI) of the white OLED reaches up to 89. Considering both the efficiency and the CRI, these results are among the best-reported white OLEDs.

Non-doped deep blue emitters based on twisted phenanthroimidazole derivatives for organic light-emitting devices (CIE y ≈ 0.04)

The π-conjugation length of donor–acceptor molecules is not conducive to blue emission and the color purity of devices. Hence, by using a twisted donor–acceptor molecular design, we developed three deep-blue emitters, mtp, Tmtp and Cmtp. Compared to the TPA-BPI we reported previously, the subtle molecular modification and optimization shows extremely good color purity without impairing the excellent photophysical and electrical properties. The nondoped mtp-based device emitted deep-blue emission at 436 nm with CIE of (0.15, 0.05) and a maximum EQE of 3.89%. The Cmtp-based device emitted blue light of at 445 nm with CIE of (0.15, 0.07). Especially, the Tmtp-based device showed a violet-blue CIE coordinate of (0.15, 0.04).

Mechanochromic asymmetric sulfone derivatives for use in efficient blue organic light-emitting diodes

Typical π–π stacking is suppressed by the asymmetric molecular design for high fluorescence quantum yield (Φf) blue light emission, overcoming the aggregation caused quenching (ACQ) limitation. In this research, two novel blue fluorescent materials with asymmetric structure: 2-(4′-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)-[1,1′-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSC) and 2-(4′-((4′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)sulfonyl)-[1,1′-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSBC), consisting of a sulfone group as the electron acceptor and two different electron donors, carbazole and phenanthroimidazole, were designed and synthesized. The two compounds have high Φf (95.3% for PSC and 81.1% for PSBC) in film because of the restricting π–π stacking, and show apparent mechanochromic properties, i.e., an emission change from deep blue to blue-green resulting from external mechanical stimuli. The emissions display 50 nm/23 nm red shifts after grinding. Organic light emitting diodes (OLEDs) using the two compounds as emitters exhibited good efficiencies: the doped PSC-based device emitted blue light at 444 nm with CIE co-ordinates of (0.151, 0.072). The PSBC-based device also emitted blue light at 444 nm with CIE co-ordinates of (0.151, 0.068). A maximal external quantum efficiency (EQE) of 5.43% was also achieved.

Bromine-substituted triphenylamine derivatives with improved hole-mobility for highly efficient green phosphorescent OLEDs with a low operating voltage

Charge transport materials play a crucial role in organic light-emitting diodes (OLEDs) due to their effect on reducing the operating voltage and enhancing the power efficiency. To develop hole transport materials with improved hole-mobility, two novel bromine-substituted triphenylamine derivatives: ((4-bromo-phenyl)-bis-(4-quinolin-8-yl-phenyl)-amine) Br-DQTPA and (9,9-bis-(4-triphenyl-amine)-2,7-dibromo-9H-fluorene) Br-DTF were designed and synthesized. The hole mobilities of Br-DQTPA and Br-DTF show one order of magnitude enhancement compared with non-brominated (phenyl-bis-(4-quinolin-8-yl-phenyl)-amine) DQTPA and (9,9-bis-(4-triphenyl-amine)-9H-fluorene) DTF under the same electric field. Attributed to their improved hole mobilities, traditional green phosphorescent OLEDs that use Br-DQTPA or Br-DTF as a hole transport material both show an ultralow operating voltage at 1 cd m−2 (below 2.4 V) and very high luminous efficiencies (over 21% and 90 lm W−1) without light outcoupling improvement. Those results are much better than those of DQTPA-, DTF-, and even NPB-based green devices, indicating that bromine-substitution is a promising and convenient way to achieve novel hole transport materials with improved hole-mobility.

Novel phenanthroimidazole-based blue AIEgens: reversible mechanochromism, bipolar transporting properties, and electroluminescence

Multifunctional materials are crucial and have promising applications in a wide range of organic electronics. Herein, we designed and synthesized two bipolar blue molecules named 2-(4-(4,5-diphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazol-1-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (PPI-PIM-TPE) and 1-phenyl-2-(4-(2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-phenanthro[9,10-d]imidazol-1-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (2PPI-TPE) with aggregation-induced emission (AIE) and mechanochromism characteristics. They both have good thermal stability (Td is 505 °C for PPI-PIM-TPE and 510 °C for 2PPI-TPE), strong AIE properties and reversible mechanochromism. The quantum yields in the solid state were as high as 61.9% for PPI-PIM-TPE and 73.4% for 2PPI-TPE. In addition, the two pristine solid powders are white and emit blue light. After grinding, the solid becomes yellow and emits blue-green emission. The color changes are reversible by solvent fuming. The change in emission color can be observed by the naked eye, demonstrating that they are typical mechanochromic materials. Non-doped blue OLEDs based on 2PPI-TPE exhibit an external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) of 2.48%, 6.46 cd A−1 and 4.72 lm W−1, respectively. The doped device based on 2PPI-TPE as a dopant emitter exhibits a higher EQE, CE and PE of 3.55%, 6.67 cd A−1 and 5.52 lm W−1. The performances of the OLEDs with these emitters are among the best of recent reports based on blue materials with AIE and mechanochromism simultaneously.

A simple and broadly applicable synthesis of fluorene-coupled D–σ–A type molecules: towards high-triplet-energy bipolar hosts for efficient blue thermally-activated delayed fluorescence

An effective two-step synthesis of high-triplet-energy (ET) bipolar hosts that is simple and broadly applicable is introduced. Electron-donating (D = (9-phenyl-9H-carbazol-3-yl/4-(9H-carbazol-9-yl)phenyl)) and -accepting (A = (tetrafluoropyridin-4-yl/4,6-diphenyl-1,3,5-triazin-2-yl)) units have been coupled to the C9 atom of fluorene to afford the four hosts 3CzFTFP, 9PhFTFP, 3CzFDPhTz, and 9PhFDPhTz with sp3 C9-centered bulky ternary structures, excellent thermal/morphological stabilities, tunable abilities for bipolar charge carrier injection/transport, and identical high ET (2.87 eV). Blue thermally-activated delayed fluorescence (TADF) devices using these new hosts and 2CzPN as the host–guest system exhibited efficient electroluminescence efficiencies of 38 cd A−1/30 lm W−1/17%, which are among the best for 2CzPN-based TADF devices. The generality of this synthetic strategy to high-ET fluorene-coupled D–σ–A type hosts was confirmed by successfully appending other D/A groups ((diphenylamino)phenyl/phenylsulfonyl) to fluorene, affording TPAFBSO.

Ternary Acceptor-Donor-Acceptor Asymmetrical Phenanthroimidazole Molecule for Highly Efficient Near-Ultraviolet Electroluminescence with External Quantum Efficiency (EQE) >4 %

A new ternary acceptor (A)-donor (D)-acceptor (A) asymmetrically twisted deep-blue emitting molecule, PPI-2BI, was synthesized by attaching two electrophilic benzimidazole (BI) units to the C2 and N1 positions of a phenanthroimidazole (PI) donor unit. Profiting from the enhanced D-A electronic coupling, the electron injecting and transporting abilities of the new triangle-shaped A-D-A molecule are considerably improved and the molecule shows high photoluminescence (PL) and electroluminescence (EL) efficiencies. By using PPI-2BI as a non-doped emitting layer (EML), the resulting organic light-emitting device exhibits emission with color coordinates of (0.158, 0.124) and a maximum external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 4.63 %, 4.98 cd A-1 , and 4.82 lm W-1 , respectively. Additionally, a simple bilayer device using PPI-2BI as both the EML and the electron-transporting layer (ETL) also shows an EQE of 3.81 % with little changes to the color purity. Remarkably, a PPI-2BI-based doped device emits efficient near-ultraviolet EL with color coordinates of (0.154, 0.047) and an EQE of 4.12 %, which is comparable to that of the best reported near-UV emitting devices.