Chunmiao Han

Ternary ambipolar phosphine oxide hosts based on indirect linkage for highly efficient blue electrophosphorescence: towards high triplet energy, low driving voltage and stable efficiencies.

The effective strategy of indirect linakge for constructing ternary ambipolar phosphine oxide (PO) hosts with the high first triplet energy levels (T(1)) was successfully demonstrated. The interplay between the chromophore, hole and electron transporting moieties was effectively restrained. Both of T(1) as high as 3.0 eV and ambipolar characteristics were perfectly realized, which consequently resulted in the highly efficient blue-emitting phosphorescent organic light-emitting diodes with low driving voltage and stable efficiencies.

Controllably Tuning Excited‐State Energy in Ternary Hosts for Ultralow‐Voltage‐Driven Blue Electrophosphorescence

Phosphorescent organic light-emitting diodes (PHOLEDs), with 100% theoretical internal efficiency, are being rapidly developed as a most promising approach to meet the urgent and extensive demand of energy-efficient and portable digital terminals and lighting sources. Thanks to the recent breakthrough of highly efficient blue PHOLEDs and outcoupling technologies, PHOLEDs in full color can already realize extremely high efficiencies that approach those of fluorescent tubes (about 70 LmW ). Nevertheless, as the hosts in the emitting layers (EMLs) should have higher triplet excited energy levels (T1) to confine the excitons on phosphorescent guests, the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gaps in PHOLEDs are often much larger than their fluorescent counterparts, which consequently result in poor energy-level alignment and thus higher driving voltages. This drawback not only complicates the design of driving circuit, but also directly reduces power efficiency (PE). Thus, the low-voltage-driving high-efficiency PHOLED remains the biggest challenge. Su, Kido, et al. have reported green PHOLEDs with extremely low operating voltages of 2.18 V for onset and 2.41 V at 100 cdm 2 through good management of the interfacial contact between electron transporting layers and anodes. However, there are only a few blue PHOLEDs that achieve low driving voltages; for example, applicable luminance at a driving voltage of less than 3 V. The formidable challenge is the high barriers for carrier injection and transportation deriving from the prerequisite of extremely high T1 of the hosts, for example, 2.85 eV (0.2 eV higher than that of blue phosphor iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic; Scheme 1). This issue actually reflects the intrinsic

Short-Axis Substitution Approach Selectively Optimizes Electrical Properties of Dibenzothiophene-Based Phosphine Oxide Hosts

Two dibenzothiophene (DBT)-based phosphine oxide hosts, named 4-diphenylphosphoryl dibenzothiophene (DBTSPO) and 4,6-bis(diphenylphosphoryl) dibenzothiophene (DBTDPO), were prepared by short-axis substitution with the aim to selectively adjust electrical properties. The combined effects of short-axis substitution and the involvement of electron-donating S atom in conjugation effectively suppress the influence of electron-withdrawing diphenylphosphine oxide (DPPO) moieties on the frontier molecular orbitals and the optical properties. Therefore, DBTSPO and DBTDPO have the nearly same hole injection ability and the excited energy levels, while more electron-transporting DPPOs and the symmetrical configuration endow DBTDPO with enhanced electron-injecting/transporting ability. As the result, on the basis of this short-axis substitution effect, the selective adjustment of electrical properties was successfully realized. With the high first triplet energy level (T(1)) of 2.90 eV, the suitable energy levels of the highest occupied molecular orbital and the lowest unoccupied molecular orbital of -6.05 and -2.50 eV and the improved carrier-transporting ability, DBTDPO supported its blue- and white-emitting phosphorescent organic light-emitting diodes as the best low-voltage-driving devices reported so far with the lowest driving voltages of 2.4 V for onset and <3.2 V at 1000 cd m(-2) (for indoor lighting) accompanied with the high efficiencies of >30 lm W(-1) and excellent efficiency stability.

A Single Phosphine Oxide Host for High‐Efficiency White Organic Light‐Emitting Diodes with Extremely Low Operating Voltages and Reduced Efficiency Roll‐Off

White light can be realized by mixing red, green, and blue (RGB; the three primary colors) light [ 1 ] or by mixing two complementary colors, such as sky blue and yellow lights. [ 3 ] Because of the ideal characteristics of the phosphors, in particular their ability to harvest triplet excitons and potential 100% internal quantum effi ciency, [ 4 ] electrophosporescent emitters are widely used in WOLEDs to improve device performance. [ 5 ] After decades of research and development, WOLEDs with power effi ciency (PE) beyond 100 lm W − 1 with outcoupling technology and 30 lm W − 1

Highly efficient multifluorenyl host materials with unsymmetrical molecular configurations and localized triplet States for green and red phosphorescent devices.

Highly efficient green and red electro-phosphorescence is achieved in devices with the host material DPESPODEF3. The multiple fluorenyl moieties of the host material are arranged such that it has an unsymmetrical molecular configuration, and its triplet-state location is tuned such that it has independent energy (ET) and charge transfer (CT) channels. As a result, DPESPODEF3 can suppress triplet-triplet annihilation and triplet-polaron quenching. In the resulting green and red phosphorescent devices, impressive external quantum efficiencies of ca. 20% and 16% and power efficiencies of ca. 75 and 20 lm W(-1) , respectively, are observed.

Insulated donor–π–acceptor systems based on fluorene-phosphine oxide hybrids for non-doped deep-blue electroluminescent devices

An ambipolar ternary deep-blue emitter with CIE coordinates of (0.15, 0.07) and high electroluminescent performance was constructed on the basis of an insulated donor-π-acceptor system through an indirect linkage.

A New Phosphine Oxide Host based on ortho‐Disubstituted Dibenzofuran for Efficient Electrophosphorescence: Towards High Triplet State Excited Levels and Excellent Thermal, Morphological and Efficiency Stability

An efficient host for blue and green electrophosphorescence, 4,6-bis(diphenylphosphoryl)dibenzofuran (o-DBFDPO), with the structure of a short-axis-substituted dibenzofuran was designed and synthesised. It appears that the greater density of the diphenylphosphine oxide (DPPO) moieties in the short-axis substitution configuration effectively restrains the intermolecular interactions, because only very weak π-π stacking interactions could be observed, with a centroid-to-centroid distance of 3.960 Å. The improved thermal stability of o-DBFDPO was corroborated by its very high glass transition temperature (T(g)) of 191 °C, which is the result of the symmetric disubstitution structure. Photophysical investigation showed o-DBFDPO to be superior to the monosubstituted derivative, with a longer lifetime (1.95 ns) and a higher photoluminescent quantum efficiency (61 %). The lower first singlet state excited level (3.63 eV) of o-DBFDPO demonstrates the stronger polarisation effect attributable to the greater number of DPPO moieties. Simultaneously, an extremely high first triplet state excited level (T(1)) of 3.16 eV is observed, demonstrating the tiny influence of short-axis substitution on T(1). The improved carrier injection ability, which contributed to low driving voltages of blue- and green-emitting phosphorescent organic light-emitting diodes (PHOLEDs), was further confirmed by Gaussian calculation. Furthermore, the better thermal and morphological properties of o-DBFDPO and the matched frontier molecular orbital (FMO) levels in the devices significantly reduced efficiency roll-offs. Efficient blue and green electrophosphorescence based on the o-DBFDPO host was demonstrated.

Convergent modulation of singlet and triplet excited states of phosphine-oxide hosts through the management of molecular structure and functional-group linkages for low-voltage-driven electrophosphorescence.

The controllable tuning of the excited states in a series of phosphine-oxide hosts (DPExPOCzn) was realized through introducing carbazolyl and diphenylphosphine-oxide (DPPO) moieties to adjust the frontier molecular orbitals, molecular rigidity, and the location of the triplet excited states by suppressing the intramolecular interplay of the combined multi-insulating and meso linkage. On increasing the number of substituents, simultaneous lowering of the first singlet energy levels (S(1)) and raising of the first triplet energy levels (T(1), about 3.0 eV) were achieved. The former change was mainly due to the contribution of the carbazolyl group to the HOMOs and the extended conjugation. The latter change was due to an enhanced molecular rigidity and the shift of the T(1) states from the diphenylether group to the carbazolyl moieties. This kind of convergent modulation of excited states not only facilitates the exothermic energy transfer to the dopants in phosphorescent organic light-emitting diodes (PHOLEDs), but also realizes the fine-tuning of electrical properties to achieve the balanced carrier injection and transportation in the emitting layers. As the result, the favorable performance of blue-light-emitting PHOLEDs was demonstrated, including much-lower driving voltages of 2.6 V for onset and 3.0 V at 100 cd m(-2), as well as a remarkably improved E.Q.E. of 12.6%.

Suppressing triplet state extension for highly efficient ambipolar phosphine oxide host materials in blue PHOLEDs

A series of phosphine oxide hosts were constructed to investigate the influence of the triplet state extension in hosts on electrophosphorescence, in which DPESPOPhCz with the carbazolyl-localized triplet state endowed its blue-emitting PHOLEDs with favourable performance, including an external quantum efficiency more than 13%.

Controlling optoelectronic properties of carbazole–phosphine oxide hosts by short-axis substitution for low-voltage-driving PHOLEDs

Preserved high first triplet energy levels and improved electrical properties of two donor-acceptor type carbazole-phosphine oxide hosts were achieved through short-axis substitution to realize efficient PHOLEDs with extremely low driving voltages of 2.6 V for onset and <3.2 V at 100 cd m(-2).