Small Molecular Host Materials Research Articles

Synthesis, Properties, Calculations and Applications of Small Molecular Host Materials Containing Oxadiazole Units with Different Nitrogen and Oxygen Atom Orientations for Solution-Processable Blue Phosphorescent OLEDs

A series of new small molecules based on symmetric electron-acceptor of 1,3,4-oxadiazole moiety or its asymmetric isomer of 1,2,4-oxadiazole unit were successfully synthesized and applied to solution-processable blue phosphorescent organic light-emitting diodes for the first time, and their thermal, photophysical, electrochemical properties and density functional theory calculations were studied thoroughly. Due to the high triplet energy levels (ET, 2.82–2.85 eV), the energy from phosphorescent emitter of iridium(III) bis[(4,6-difluorophenyl)-pyridinate-N,C2′]picolinate (FIrpic) transfer to the host molecules could be effectively suppressed and thus assuring the emission of devices was all from FIrpic. In comparison with the para-mode conjugation in substitution of five-membered 1,3,4-oxadiazole in 134OXD, the meta-linkages of 1,2,4-isomer appending with two phenyl rings cause the worse conjugation degree and the electron delocalization as well as the lower electron-withdrawing ability for the other 1,2,4-oxadiazole-based materials. Noting that the solution-processed device based on 134OXD containing 1,3,4-oxadiazole units without extra vacuum thermal-deposited hole/exciton-blocking layer and electron-transporting layer showed the highest maximum current efficiency (CEmax) of 8.75 cd/A due to the excellent charge transporting ability of 134OXD, which far surpassed the similar devices based on other host materials containing 1,2,4-oxadiazole units. Moreover, the device based on 134OXD presented small efficiency roll-off with current efficiency (CE) of 6.26 cd/A at high brightness up to 100 cd/m2. This work demonstrates different nitrogen and oxygen atom orientations of the oxadiazole-based host materials produce major impact on the optoelectronic characteristics of the solution-processable devices.

Structure effect of carbazole-oxadiazole based organic small molecule hosts on the solution-processed phosphorescent OLEDs performance

A group of solution-processed bipolar small molecular host materials, namely 1,4-bis(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (p-BCOB), 1,3-bis(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (m-BCOB) and 1,3,5-tris(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (TCOB) were synthesized by means of connecting carbazole-oxadiazole substituents on the para-, meta- and 1,3,5-meta- positions of benzene ring respectively. The influence of different linking positions of carbazole-oxadiazole substituents on the thermal, photophysical, electrochemical and charge transport properties of the compounds is comprehensively studied. For verifying the host configuration effect on the performance of phosphorescent organic light-emitting diodes (PHOLEDs), solution-processed green and red PHOLEDs with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) and bis(1-phenylisoquinolinato)(acetylacetonate) iridium (Ir(piq)2acac) as phosphorescent dopants were fabricated. The m-BCOB:Ir(ppy)3 and TOCB:Ir(ppy)3 PHOLEDs show excellent device performance with the maximum luminance efficiencies of 15.0cdA−1 and 13.7cdA−1, respectively, which are increased by 9.8 and 9.0 times compared with the p-BCOB:Ir(ppy)3 PHOLED because of effective Förster and Dexter energy transfer. However, the p-BCOB:Ir(piq)2acac PHOLED is the best among three hosts with a maximum external quantum efficiency of 7.9% due to the effective charge trapping. This work demonstrates that different host molecular structures have a big effect on the PHOLEDs performance.

Solution-processed bipolar small molecular host materials for single-layer blue phosphorescent organic light-emitting diodes

Three new solution processable small molecular host materials based on the bis-[3,5-di(9H-carbazol-9-yl)phenyl] structural moiety have been developed for blue phosphorescent (FIrpic dopant) organic light-emitting diodes. All three host materials have been characterized as having high glass transition temperatures (Tgs), 155–175 °C, indicative of good morphological stability of their amorphous thin films prepared from the solution process. Whereas N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]methylamine (CzPAMe) has the highest solid state triplet energy gap (ET) of 2.73 eV, tetrakis-[3,3′,5,5′-(9H-carbazol-9-yl)]triphenylphosphine oxide (CzPPO) and N,N-bis-[3,5-di(9H-carbazol-9-yl)phenyl]pyrimidin-2-amine (CzPAPm) are two host materials which are potentially bipolar for charge transport due to the electron deficient units of phenylphosphine oxide and pyrimidine, respectively. Due to the insufficient ET (2.56 eV) of CzPAPm, CzPPO or CzPAMe devices are significantly better than CzPAPm devices with or without a 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) co-host. Particularly, having no OXD-7 co-host and no vacuum thermal-deposited extra electron transporting layer, single-layer devices of CzPPO surpass CzPAMe devices and reach current efficiencies as high as 9.32 cd A−1 (or power efficiency of 4.97 lm W−1), one of the highest efficiencies among small molecular devices with the same fabrication process and same device configuration.

Solution-processible small-molecular host materials for high-performance phosphorescent organic light-emitting diodes

Two novel carbazole-based molecules were synthesized by attaching 3,6-bis(3,6-di-tert-butyl-carbazol-9-yl)-carbazole and pyrazole to the dimethylbiphenyl core in a symmetric and asymmetric way. Owing to highly twisted configuration, they exhibit high triplet energy of 2.90 eV. They are suitable for spin coating to make thin films. The complete spatial separation of frontier molecular orbitals and the single-carrier devices study confirm the bipolar feature of pyrazole-containing material. They were used as hosts to fabricate phosphorescent organic light-emitting diodes by wet method. The green devices exhibited maximum efficiencies of 33 cd A−1 and 35 cd A−1, which far exceed that (23 cd A−1) of the control device with the polyvinylcarbazole host. The versatility of these hosts also spread to orange devices and peak efficiencies of 35 cd A−1 and 39 cd A−1 were achieved. The pyrazole-containing material always performed better due to its bipolar charge transporting nature.

Synthesis of ring-structured polysiloxane as host materials for blue phosphorescent device

A ringed polysiloxane with a side chain of dibenzothiophene groups (BS-PSQ) is successfully synthesized as confirmed by MALDI-TOF MS and 1H-NMR. DSC, TGA, AFM and spectra reveal its good film-forming ability, high thermal and morphological stability, and good miscibility with the dopant FIrpic. In addition, it also has a high triplet energy and a wide band gap. The BS-PSQ may be used as a host for the blue light emitter iridium complex FIrpic. The electrophosphorescent device based on BS-PSQ as the active layer exhibits typical blue emission, and the performance of the device is superior to that of any other POSS-based host material reported and also to some of the small molecular host materials.