Hosts For Phosphorescent Organic Light-emitting Diodes Research Articles

High and Balanced Bipolar‐Transporting Deep‐Blue HLCT Material for Efficient Monochrome and White OLEDs based on a Simple Phenanthroimidazole‐Dibenzothiophene Derivative

AbstractThe irreconcilable contradiction between wide bandgap and charge carriers injection/transport in deep‐blue organic electroluminescence materials is one of the largest bottlenecks restricting the development of organic light‐emitting diodes (OLEDs). Here, a multifunctional deep‐blue hybridized local and charge‐transfer material containing a phenanthroimidazole‐dibenzothiophene derivative that can act as an emitter as well as a host for phosphorescent OLEDs is reported. Owing to its high and balanced bipolar‐transporting ability, remarkable fluorescence efficiency, and suitable triplet energy level, the deep‐blue device with it as a neat emitter provides an external quantum efficiency (EQE) of 6.68%, while yellow and red phosphorescent OLEDs offer the EQEs of 25.35% and 22.19%, respectively. All of them exhibit low‐efficiency roll‐off characteristics. What's more amazing is that the fluorescent/phosphorescent hybrid white OLEDs achieve high power efficiencies and EQEs over 95.9 lm W−1 and 26.8%, and the values are still higher than 77.7 lm W−1 and 25.6% at the high luminance of 1000 cd m−2.

Multifaceted Sulfone-Carbazole-Based D-A-D Materials: A Blue Fluorescent Emitter as a Host for Phosphorescent OLEDs and Triplet-Triplet Annihilation Up-Conversion Electroluminescence.

Electroluminescence (EL) from the singlet-excited (S1) state is the ideal choice for stable, high-performing deep-blue organic light-emitting diodes (OLEDs) owing to the advantages of an adequately short radiative lifetime, improved device durability, and low cost, which are the most important criteria for their commercialization. Herein, we present the design and synthesis of three donor-acceptor-donor (D-A-D)-configured deep-blue fluorescent materials (denoted as TC-1, TC-2, and TC-3) composed of a thioxanthone or diphenyl sulfonyl acceptor and phenyl carbazolyl donor. These systems exhibit strong deep-blue photoluminescence (422-432 nm) in solutions and redshifted emission (472-486 nm) in thin films. The solid-state photoluminescence quantum yield (PLQY) was estimated to be 78 and 94% for TC-2 and TC-3, respectively. TC-2 and TC-3 possess good molecular packing and large molecular cross-sectional areas, which not only improves the PLQY but enhances the triplet-triplet annihilation up-conversion (TTAUC) efficiency of fluorescent emitters. Furthermore, both compounds were applied as an acceptor for confirming their TTAUC property using bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) (Ir(MDQ)2acac) as the sensitizer. Non-doped OLEDs based on TC-2 and TC-3 exhibit blue EL in the 461-476 nm range. In particular, TC-3 exhibits a maximum external quantum efficiency (EQEmax) of 5.1%, and its EL maximum is 476 nm. In addition, the three emitters were employed as hosts in red OLEDs using bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2acac) as the phosphorescent dopant. The red phosphorescent OLEDs based on TC-1, TC-2, and TC-3 achieve excellent EQEmax values of 21.6, 22.9, and 21.9%, respectively, and peak luminance efficiencies of 12.0, 14.0, and 12.3 cd A-1. These results highlight these fluorophores' versatility and promising prospects in practical OLED applications.

Universal host materials based on carbazole-formate derivatives for blue, green and red phosphorescent organic light-emitting diodes

The design of universal hosts suitable for different color dopants is still desired for efficient organic light-emitting diodes (OLEDs). In this work, two novel carbazole-formate based isomers (2CzMC and 4CzMC) with planar donor‒π‒acceptor (D‒π‒A) structures were designed and synthesized as the hosts for phosphorescent OLEDs (PhOLEDs). Due to the weak intramolecular charge transfer character and limited conjugation extensions, both 2CzMC and 4CzMC showed wide-bandgap emissions around 380 nm and high triplet energy levels (2.83 eV for 2CzMC, and 2.90 eV for 4CzMC). The photoluminescent spectra of 2CzMC and 4CzMC doped films with blue, green and red phosphors exhibited completely energy transfer from the hosts to the dopants. PhOLEDs devices with similar structures employing these doped film as emitter layers achieved excellent performance. Especially for 4CzMC-FIrpic based blue device, the external quantum efficiency (EQE) reached as high as 18.9%. This study showed a molecular design strategy to develop novel carbazole-based hosts for different color PhOLEDs.

Efficient red phosphorescent OLEDs based on the energy transfer from interface exciplex: the critical role of constituting molecules

A novel acceptor material, 9-(4′-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-3-yl)-9H-carbazole (o-DTPPC) was developed to form interface exciplex with commonly used donors, to maximize the performances of red phosphorescent organic light emitting diodes (PHOLEDs). It is found that the exciplex involving 4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC) exhibits the most significant thermally activated delayed fluorescence (TADF) property, derived from the high triplet energy level as well as strong hole-transporting ability of TAPC. Intriguingly, it is the same donor-acceptor combination which achieved the highest device efficiency when adopted as the host for red PHOLEDs. Maximum efficiencies as high as 31.36 cd A−1,17.95 lm W−1, and 21.01% for the current efficiency, power efficiency and external quantum efficiency, respectively with low efficiency roll-off were realized. The improved performance can be attributed to the efficient TADF properties of the interface exciplex-forming host constituting TAPC, benefiting the Forster energy transfer. The article first underlines the importance of the constituting molecules in the interface exciplex-forming hosts, shedding new insight about the choice of interface exciplex as the host for PHOLEDs, which may lead to even higher performances, paving their ways towards practical applications.

Synthesis of novel twisted carbazole–quinoxaline derivatives with 1,3,5-benzene core: bipolar molecules as hosts for phosphorescent OLEDs

A series of carbazole/quinoxaline hybrids have been synthesized by classic Ullmann and Pd/Cu-catalyzed Sonogashira coupling reaction. Their photophysical, thermal, and electrochemical properties were investigated. The introduction of electron rich carbazole and electron deficient quinoxaline on to the 1,3,5-benzene center leads to a twisted structure with good glass forming property and imparts a bipolar character. The triplet energies in the range of 2.34–2.53eV indicate them as potential host materials in phosphorescent OLEDs.

Molecular design of host materials based on triphenylamine/oxadiazole hybrids for excellent deep-red phosphorescent organic light-emitting diodes

A series of triphenylamine/oxadiazole hybrids, p-TPA-p-OXD (1a), o-TPA-p-OXD (1b), p-TPA-m-OXD (2a) and o-TPA-m-OXD (2b), were designed, synthesized and characterized as bipolar transport host materials for deep-red phosphorescent organic light-emitting diodes (OLEDs). The ortho-TPA linked hybrids (1b and 2b) show less intramolecular charge transfer, blue-shifted emission, wider energy gap, and higher triplet energy as compared to their para-TPA linked analogues (1a and 2a). Phosphorescent organic light-emitting devices (PHOLEDs) fabricated by using the four hybrids as the hosts and the red emitter bis(1-phenylisoquinolinato)(acetylacetonate)iridium [(piq)2Ir(acac)] as the guest exhibit much higher EL performances with maximum external quantum efficiencies of 9.8–21.6% and lower turn-on voltages (2.7–3.1 V) compared with the reference device with common 4,4′-bis(N-carbazolyl)biphenyl (CBP) as a host material (4.3%, 5.3 V). The external quantum efficiency of 21.6% achieved by using o-TPA-m-OXD as host is the highest for deep-red electrophosphorescence with the Commission Internationale de l'Eclairage (CIE) coordinates of (0.68, 0.32) reported in the literature to date. Green electrophosphorescence devices by using Ir(ppy)3 as guest and 1b, 2a and 2b as hosts also show excellent EL performances with maximum external quantum efficiencies of 17.1–19.6%. This work demonstrates that tradeoffs among bipolar property, triplet energy, energy gap and energy level can be realized through judicious molecular design for a host in phosphorescent OLEDs.