Phosphorescent Host Materials Research Articles

Ultraefficient Non‐Doped Deep Blue Fluorescent OLED: Achieving a High EQE of 10.17% at 1000 cd m−2 with CIEy<0.08

AbstractThe pursuit for efficient deep blue material is an ever‐increasing issue in organic optoelectronics field. It is a long‐standing challenge to achieve high external quantum efficiency (EQE) exceed 10% at brightness of 1000 cd m−2 with a Commission International de L'Eclairage (CIEy) <0.08 in non‐doped organic light‐emitting diodes (OLEDs). Herein, this study reports a deep blue luminogen, PPITPh, by bonding phenanthro[9,10‐d]imidazole moiety with m‐terphenyl group via benzene bridge. The non‐doped OLED based on PPITPh exhibits an exceptionally high EQE of 11.83% with a CIE coordinate of (0.15, 0.07). The EQE still maintains 10.17% at the brightness of 1000 cd m−2, and even at a brightness as high as 10000 cd m−2, an EQE of 7.5% is still remained, representing the record‐high result among non‐doped deep‐blue OLEDs at 1000 cd m−2. The unprecedented device performance is attributed to the reversed intersystem crossing process through hot exciton mechanism. Besides, the maximum EQE of orange phosphorescent OLED with PPITPh as host is 32.02%, and remains 31.17% at the brightness of 1000 cd m−2. Such minimal efficiency roll‐off demonstrates that PPITPh is also an excellent phosphorescent host material. The result offers a new design strategy for the enrichment of high‐efficiency deep blue luminogen.

Synthesis and Performance of Bipolar Green Phosphorescent Host Material Based on Imidazopyridine

Synthesis and Performance of Bipolar Green Phosphorescent Host Material Based on Imidazopyridine

Critical Role of High‐Lying Triplet States for Efficient Excitons Utilization in High‐Performance Non‐Doped Deep‐Blue Fluorescent and Hybrid White Organic Light‐Emitting Diodes

AbstractFor organic light‐emitting diodes (OLEDs), the characteristics of high‐lying excited states of pure organic materials significantly affect the utilization of triplet excitons, which are critical in the process of electroluminescence. Herein, two novel molecules, PT‐1 and PT‐2, with deep‐blue emission are obtained, which exhibit nearly identical photophysical behavior in the photoluminescence process. However, the remarkable distinction in the characteristics of the high‐lying triplet excited states between PT‐1 and PT‐2 leads to a significant difference in the electroluminescence performance. Moreover, the non‐doped OLED based on PT‐1 exhibits maximum external quantum efficiency (ηext) of 6.63% with a low efficiency roll‐off. In addition, the authors employ PT‐1 as the phosphorescent host materials to fabricate two‐color hybrid white OLEDs (WOLEDs), from which they can realize the transformation from warm‐white to quasi‐white light by tuning the thickness of emission layer, with maximum ηext and power efficiency (ηp) of 23.93%/84.37 lm W−1 and 10.49%/33.96 lm W−1, respectively. These results deeply demonstrate the effects of high‐lying excited states on electroluminescence and facilitate the preparation of functional OLEDs.

Analysis of device performance and thin-film properties of thermally damaged organic light-emitting diodes

This paper reports the variation in the optical and geometrical properties of individual organic layers to be used for thermally damaged top-emission organic light-emitting diodes (TEOLEDs). The copper deposited on the back of TEOLEDs is employed as a thermal facilitator, and a certain thermal damage occurs to the organic layers and devices. The phosphorescent host material 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) is rapidly damaged to a significant extent owing to the low glass transition temperature (T g ), which also changes its optical and geometrical surface properties. Although the optical properties of the hole transport layer, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) were changed slightly, the surface morphology was changed significantly. Despite having a higher T g , the exciton blocking layer, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), shows notable variations in optical properties and surface morphology due to heat exposure. Surprisingly, the electroluminescence spectra and micro-cavity are affected by increasing temperature without any considerable changes in device performance. Hence, this study reveals that besides T g , the surface morphologies and thicknesses of the organic layers are also important factors in the annealing process and play a vital role in causing thermal damage to TEOLEDs. • Surface morphology variations with respect to temperature in the TEOLEDs. • The variation in optical and geometrical properties of organic layers depending on the thickness. • CBP, TCTA and NPB showing notable damage to the properties by thermal exposure. • The color purity of TEOLEDs was affected by micro-cavity after thermal exposure.

Cyanocarbazole-substituted di(9H-carbazol-9-yl)-1,1'-biphenyl derivatives for efficient blue phosphorescent host materials for organic light emitting diodes

To develop the efficient blue phosphorescent host materials for Organic light-emitting Diodes (OLEDs), cyanocarbazole-substituted 3',5'-di(9H-carbazol-9-yl)-1,1'-biphenyl derivatives are designed and synthesized. Multilayer devices were fabricated in the following sequence; ITO (180 nm)/Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (75 nm)/Blue phosphorescent host materials (15 nm)/Bis[2-(4,6-difluorophenyl)pyridinato-C 2,N](picolinato)iridium(III) (Firpic) (15 nm)/1,3,5-Tri(m-pyridine-3-ylphenyl)benzene (TmPyPB) (40 nm)/Liq (2 nm)/Al (100 nm). In particularly, blue phosphorescent device using 9-(3',5'-di(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)-9H-carbazole-3-carbonitrile as a host material exhibited efficient emission with a luminous efficiency, a power efficiency, an external quantum efficiency, and CIE coordinates of 11.6 cd/A, 4.10 lm/W, 5.62% at 20 mA/cm2, and (0.15, 0.33) at 8 V, respectively.

Synthesis and characterization of novel phosphorescent host materials based on triphenylpyridine derivatives

Three novel phosphorescent host materials, 4′,4‴-(4-phenylpyridine-2,6-diyl)bis(N,N-diphenyl-[1,1′-biphenyl]-4-amine) (P1), 2,6-bis(4′-(naphthalen-1-yl)-[1,1′-biphenyl]-4-yl)-4-phenylpyridine (P2), and 2,6-bis(3′,5′-dimethyl-[1,1′-biphenyl]-4-yl)-4-phenylpyridine (P3), based on triphenylpyridine with a symmetrical molecular conformation were designed and synthesized from 2,6-bis(4-bromophenyl)-4-phenylpyridine (3) reacting with [4-(diphenylamino)phenyl]boronic acid (A1), [4-(naphthalen-1-yl)phenyl]boronic acid (A2), and (3,5-dimethylphenyl)boronic acid (A3) through Suzuki coupling reaction, respectively. The compounds obtained were characterized by elemental analysis, 1H NMR, and 13C NMR spectroscopy. The thermal properties of these materials were studied by thermogravimetric analysis and differential scanning calorimetry measurements, which showed that they exhibited excellent thermal stability with glass transition temperatures over 94 °C. Their photophysical properties were also investigated, and the results indicated that all compounds possessed considerably high triplet energy level greater than 2.6 eV; they also emitted blue fluorescence in different solvents. Finally, the results of the density function theory calculations revealed that the compound P1 showed the well-separated frontier molecular orbital, which indicated that it possessed bipolar carrier transport ability for holes and electrons, whereas the compound P2 and P3 exhibited greater overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.

Synthesis and Photoelectronic Properties of Novel High-efficiency Bipolar Phosphorescent Host Material

Synthesis and Photoelectronic Properties of Novel High-efficiency Bipolar Phosphorescent Host Material

Design and Development of Highly Efficient Light-Emitting Layers in OLEDs with Dimesitylboranes: An Updated Review.

With excellent luminescent properties and transport properties, triarylborane compounds containing two mesitylenes (Mes) have gained much attention for their application in OLEDs as light-emitting layers. This study serves as an updated review summarizing recent developments in the design of fluorescent chromophores and phosphorescent host materials for OLEDs comprising small molecular compounds of dimesitylborane (BMes2 ) as luminescent layers, with attention to the performance of different light-emitting devices. Problems that need to be solved in the research and application of BMes2 in OLEDs are presented and the application prospects of such materials are suggested.

Indole-linked triazine-dibenzothiophene/dibenzofuran based host materials for high-efficiency green and red phosphorescent organic light-emitting diodes

Two host heterocyclic compounds 5-(dibenzo[b,d]furan-4-yl)-1-(4,6-diphenyl-1,3,5-triazin-2-yl)-1H-indole (FITRZ) and 5-(dibenzo[b,d]thiophen-4-yl)-1-(4,6-diphenyl-1,3,5-triazin-2-yl)-1H-indole (TITRZ) were synthesized by CN coupling reaction of triazine and dibenzothiophene/dibenzofuran moiety with a 1,5-bridged indole unit. Their photo-physical, thermal and electrochemical properties were fully explored, and the results indicate that both of them are promising phosphorescent host materials for green and red PhOLED devices with low on-set potentials. For example, the green PHOLED based on TITRZ showed the maximum current and external quantum efficiency up to 85.9 cd/A and 23.7%. Meanwhile, the red PhOLED based on FITRZ yielded the maximum current and external quantum efficiency of 41.3 cd/A and 18.8% with a turn-on voltage of 3.0 V.

Two novel blue phosphorescent host materials containing phenothiazine-5,5-dioxide structure derivatives.

Two novel D–A bipolar blue phosphorescent host materials based on phenothiazine-5,5-dioxide: 3-(9H-carbazol-9-yl)-10-ethyl-10H-phenothiazine-5,5-dioxide (CEPDO) and 10-butyl-3-(9H-carbazol-9-yl)-10H-phenothiazine-5,5-dioxide (CBPDO) were synthesized and characterized. The photophysical, electrochemical and thermal properties were systematically investigated. CEPDO and CBPDO not only have a high triplet energy but also show a bipolar behavior. Moreover, their fluorescence emission peaks are in the blue fluorescence region at 408 nm and the fluorescence quantum efficiency (Φ) of CEPDO and CBPDO were 62.5% and 59.7%, respectively. Both CEPDO and CBPDO showed very high thermal stability with decomposition temperatures (Td) of 409 and 396 °C as well as suitable HOMO and LUMO energy levels. This preferable performance suggests that CEPDO and CBPDO are alternative bipolar host materials for the PhOLEDs.

Synthesis and device properties of mCP analogues based on fused-ring carbazole moiety

Novel mCP analogues consisting of blue phosphorescent host materials with fused-ring, 1,3-bis(5H-benzofuro[3,2-c]carbazol-5-yl)benzene (BFCz) and 1,3-bis(5H-benzo[4,5]thieno[3,2-c]carbazol-5-yl)benzene (BTCz) were designed and synthesized using benzofurocarbazole and benzothienocarbazole donor moieties. BFCz and BTCz exhibit high glass transition temperatures of 147 and 157 °C, respectively, and high triplet bandgaps of 2.94 and 2.93 eV, respectively. To explore the electroluminescence properties of these materials, multilayer blue phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated in the following device structure: indium–tin-oxide (ITO)/PEDOT:PSS/4,4’-cyclohexylidene bis[N,N-bis(4-methylphenyl)aniline] (TAPC)/1,3-bis(N-carbazolyl) benzene (mCP)/host:FIrpic/diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1)/LiF)/Al. The PHOLEDs with BTCz exhibited efficient blue emission with luminous and quantum efficiencies of 30.9 cd/A and 15.5% at 1000 cd/m2, respectively.

Abnormal temperature dependent behaviors of intersystem crossing and triplet-triplet annihilation in organic planar heterojunction devices

Anomalous temperature dependent magneto-electroluminescence was observed at low and high magnetic field strength from organic planar heterojunction devices incorporated common phosphorescent host materials of N,N′-dicarbazolyl-3,5-benzene (mCP) or 4,4′-N,N′-dicarbazole-biphenyl (CBP) as an emissive layer. We found that intersystem crossing became stronger with decreasing temperature and that triplet-triplet annihilation (TTA) occurred at room temperature but ceased at low temperature. Analyses of the electroluminescence spectra of these devices and their temperature dependences indicated that the population of exciplex states increased at low temperature, which caused the abnormal behavior of intersystem crossing. Additionally, long lifetime of the excitons within mCP or CBP layer may allow TTA to occur at room temperature, while the reduced population of excitons at low temperature may account for the disappearance of TTA even though the excitons had increased lifetime.

Design, synthesis, characterization and application of a novel electron-deficient moiety 1,5-diazacarbazole in high triplet energy host materials

A novel electron deficient, high triplet energy moiety 1,5-diazacarbazole (NCz) was designed, synthesized and firstly introduced for the construction of phosphorescent host materials.

First-principles study of intrinsic vacancy defects in Sr2MgSi2O7 phosphorescent host material

Electronic structures of intrinsic vacancy defects in Sr2MgSi2O7 phosphorescent host material are investigated using first-principles calculations. Si vacancies are too high in energy to play any role in the persistent luminescence of Sr2MgSi2O7 phosphor. Mg vacancies form easier than Sr vacancies as a result of strain relief. Among all the vacancies, O1 vacancies stand out as a likely candidate because they are the most favorable in energy and introduce an empty triply degenerate state just below the CBM and a fully-occupied singlet state at ~1 eV above the VBM, constituting in this case effective hole trap level and electron trap levels, respectively. Mg vacancies are unlikely to explain the persistent luminescence because of its too shallow electron trap level but they may compensate the hole trap associated with O1 vacancies. We yield consistent evidence for the defect physics of these vacancy defects on the basis of the equilibrium properties of Sr2MgSi2O7, total-energy calculations, and electronic structures. The persistent luminescence mechanism of Sr2MgSi2O7:Eu2+, Dy3+ phosphor is also discussed based on our results for O1 vacancies trap center. Our results provide a guide to more refined experiments to control intrinsic traps, whereby probing synthetic strategies toward new improved phosphors.

28.3: Invited Paper: Progress on Phosphorescent OLED Materials

In this paper, we present the development of phosphorescent host materials, having triplet energy more than 2.5 eV, to sensitize red phosphorescent dopants. Some of the new phosphorescent hosts exhibited lifetimes (T70) of about 40,000h at an initial luminance of 1000 cd/m2, with efficiencies as high as 28 cd/A at a driving voltage of about 4.2 V.

Charge conduction study of phosphorescent iridium compounds for organic light-emitting diodes application

The charge conduction properties of a series of iridium-based compounds for phosphorescent organic light-emitting diodes (OLEDs) have been investigated by thin-film transistor (TFT) technique. These compounds include four homoleptic compounds: Ir(ppy)3, Ir(piq)3, Ir(Tpa-py)3, Ir(Cz-py)3, and two heteroleptic compounds Ir(Cz-py)2(acac) and FIrpic. Ir(ppy)3, Ir(piq)3 and FIrpic are commercially available compounds, while Ir(Tpa-py)3, Ir(Cz-py)3 and Ir(Cz-py)2(acac) are specially designed to test their conductivities with respect to the commercial compounds. In neat films, with the exception of FIrpic, all Ir-compounds possess significant hole transporting capabilities, with hole mobilities in the range of about 5×10−6–2×10−5cm2V−1s−1. FIrpic, however, is non-conducting as revealed by TFT measurements. We further investigate how Ir-compounds modify carrier transport as dopants when they are doped into a phosphorescent host material CBP. The commercial compounds are chosen for the investigation. Small amounts of Ir(ppy)3 and Ir(piq)3 (<10%) behave as hole traps when they are doped into CBP. The hole conduction of the doped CBP films can be reduced by as much as 4 orders of magnitude. Percolating conduction of Ir-compounds occurs when the doping concentrations of the Ir-compounds exceed 10%, and the hole mobilities gradually increase as their values reach those of the neat Ir films. In contrast to Ir(ppy)3 and Ir(piq)3, FIrpic does not participate in hole conduction when it is doped into CBP. The hole mobility decreases monotonically as the concentration of FIrpic increases due to the increase of the average charge hopping distance in CBP.

螺芴氧杂蒽(SFX)类有机半导体及其OLEDs

One-pot protocol to spiro-based organic semiconductors has several advantages such as low-cost stuffs available, pot-atom-step economic (PASE) procedures, and others over the conventional o -halobiaryl method, representing the paradigm of green organic semiconductors (GOS) with the application of information technology. Herein, this review traces back to the history of spiro-compounds, including synthesis, structures and the application of electronics, followed by the discovery on one-pot synthesis of spirocyclic arenes with the merit of tandem mechanism that involves three bonding at once. We demonstrated the SFX-based molecular design platform under the four-element theory to organic semiconductors, affording a chance to get insight into structure-parameter relationship. Finally, we highlighted the SFX-based electroluminescent materials, including various fluorescent materials, RGB phosphorescent host and guest materials as well as printable OLEDs materials. Among them, the best deep-blue fluorescent OLED exhibits the maximum current efficiency (CE), the maximum power efficiency (PE) and external quantum efficiency (EQE) of 7.4 cd/A, 4.1 lm/W, 4.6%, respectively; the obtained green phosphorescent OLED with the CE of 70.0 cd/A, PE of 77.0 lm/W, EQE of 19.2%, as well as universal host materials for phosphorescent OLEDs. Finally, we outlook the spiro-based GOSs, including the obstacle of synthetic methodology to spiro-arenes, application bottleneck for OLEDs display and lighting source, potential explorations in other electronics, molecular circuits as well as smart devices.

Controlling singlet–triplet splitting in carbazole–oxadiazole based bipolar phosphorescent host materials

A rational molecular design strategy for carbazole–oxadiazole based bipolar host materials was developed to improve the device efficiency of blue phosphorescent organic light-emitting diodes (PHOLED). Steric effects of strategically placed methyl groups led to an increase of triplet energies (o-2MPCzPOXD: 2.66eV and o-3MPCzPOXD: 2.73eV versus the initial host material o-PczPOXD: 2.62eV) while less pronouncedly affecting singlet energies and, therefore, retaining low driving voltages, high power efficiencies and remarkably low efficiency roll-offs in PHOLEDs. The maximum quantum efficiencies (EQE) for blue devices (FIrpic) were significantly raised for o-2MPCzPOXD (13.6%) and o-3MPCzPOXD (11.5%) versus o-PCzPOXD (9.0%) although yielding comparable values for green devices (Ir(ppy)3; 12.9% and 15.4% versus 13.2%). Supported by theoretical calculations a structure–property relationship was established from photo-physical properties, PHOLED performance measurements and structural characterization from single crystal data.

Novel diarylborane–phenanthroimidazole hybrid bipolar host materials for high-performance red, yellow and green electrophosphorescent devices

In this work, two novel bipolar host materials p-BPPI and m-BPPI containing phenanthroimidazole/dimesitylborane (Mes2B) with para- and meta-linkage have been designed, synthesized and characterized. The appending Mes2B moiety improves the thermal stability, electrochemical stability and carrier injection/transport ability of both target compounds. The test results of time-of-flight (TOF) and single-carrier devices show that both the new hosts possess bipolar charge-transporting characteristics. As a result, series of highly efficient green (66.3cdA−1, 63.1lmW−1, 18.2%), yellow (55.2cdA−1, 66.6lmW−1, 14.5%) and red (20.1cdA−1, 20.4lmW−1, 13.5%) PhOLEDs are achieved by using them as the universal host materials. The results indicate that bipolar host p-BPPI and m-BPPI have high potential in fabricating various color OLEDs for displays and lighting applications. Our study further enriches the selection of D and A group for phosphorescent host materials. The relationship between molecular structures and optoelectronic properties is discussed experimentally and theoretically.

Control of conjugation degree via position engineering to highly efficient phosphorescent host materials.

The C3 meta-position of fluorene is utilized to construct high-triplet energy compounds. Incorporating a spiroacridine structure, two new host materials SAFDPA and SAFCz were facilely obtained. Their thermal and photophysical properties are fully investigated. The best efficiencies of 19.4%/21.5% of blue/white devices are achieved by SAFCz.