Zhensong Zhang

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

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%.

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).

Elevating the Triplet Energy Levels of Dibenzofuran-Based Ambipolar Phosphine Oxide Hosts for Ultralow-Voltage-Driven Efficient Blue Electrophosphorescence: From DA to DπA Systems

A series of donor (D)-π-acceptor (A)-type phosphine-oxide hosts (DBF(x) POPhCz(n)), which were composed of phenylcarbazole, dibenzofuran (DBF), and diphenylphosphine-oxide (DPPO) moieties, were designed and synthesized. Phenyl π-spacer groups were inserted between the carbazolyl and DBF groups, which effectively weakened the charge transfer and triplet-excited-state extension. As the result, the first triplet energy levels (T(1)) of DBF(x)POPhCz(n) are elevated to about 3.0 eV, 0.1 eV higher than their D-A-type analogues. Nevertheless, the electrochemical analysis and DFT calculations demonstrated the ambipolar characteristics of DBF(x)POPhCz(n). The phenyl π spacers hardly influenced the frontier molecular orbital (FMO) energy levels and the carrier-transporting ability of the materials. Therefore, these D-π-A systems are endowed with higher T(1) states, as well as comparable electrical properties to D-A systems. Phosphorescent blue-light-emitting diodes (PHOLEDs) that were based on DBF(x)POPhCz(n) not only inherited the ultralow driving voltages (2.4 V for onset, about 2.8 V at 200 cd m(-2), and <3.4 V at 1000 cd m(-2)) but also had much-improved efficiencies, including about 26 cd A(-1) for current efficiency, 30 Lm W(-1) for power efficiency, and 13% for external quantum efficiency, which were more than twice the values of devices that are based on conventional unipolar host materials. This performance makes DBFDPOPhCz(n) among the best hosts for ultralow-voltage-driven blue PHOLEDs reported so far.

A bulky pyridinylfluorene-fuctionalizing approach to synthesize diarylfluorene-based bipolar host materials for efficient red, green, blue and white electrophosphorescent devices

By incorporating electron-transporting pyridine and hole-transporting fluorene moieties into a nonplanar 3-dimensional molecule, via the sp3-hybridized carbon linkage by Friedel–Crafts reaction, three novel donor–acceptor (p–n) bipolar host materials 9-(pyridin-2-yl)-3-(9-(pyridin-2-yl)-fluoren-9-yl)-carbazole (CzPy-PyFM), 3,6-bis(9-(pyridine-2-yl)-fluoren-9-yl)-9-(pyridin-2-yl)-carbazole (CzPy-DPyFM) and 5,5′′-bis(9-(pyridin-2-yl)-fluoren-9-yl-)-2,2′:5′,2′′-terthiophene (T3DPyFM) have been synthesized successfully. These pyridinylfluorene end-capped materials with large steric hindrance have been applied as efficient universal hosts for blue, green and red PhOLEDs. The blue, green, and red PhOLEDs hosted by highly thermally stable CzPy-PyFM exhibit the maximum external quantum efficiencies (EQEs) of 10.49%, 11.04%, and 6.50%, respectively. On this basis, the single-emissive-layer three-color and all-phosphor CzPy-PyFM-based white PhOLEDs have been fabricated, which acquire a maximum current efficiency of 10.4 cd A−1, a power efficiency of 10.2 lm W−1 and an EQE of 5.58%. This is the first example of the diarylfluorenes as universal hosts for full-color RGB PhOLEDs as well as white PhOLEDs.

Nondoped deep-blue spirofluorenexanthene-based green organic semiconductors (GOS) via a pot, atom and step economic (PASE) route combining direct arylation with tandem reaction

Effective synthesis of organic semiconductors with pot, atom, and step economic (PASE) methods will be an indispensable part of green electronics. In this paper, we combined direct arylation with one-pot/tandem reaction to synthesize a green organic semiconductor (GOS), di(spiro[fluorene-9,9′-xanthene]-2-yl)-1,2,4,5-tetrafluorobenzene (DSFX-TFB), serving as the emitting layer for organic light-emitting devices (OLEDs). The maximum current efficiency (CE) of 3.2 cd A−1, power efficiency (PE) of 2.0 lm W−1 and external quantum efficiency (EQE) of 4.1% were obtained with the CIE coordinates (0.15, 0.08) in the nondoped deep-blue OLEDs. Meanwhile, good energy transfer and device performances with maximum CE of 8.03 cd A−1 and PE of 4.62 lm W−1 were achieved in typical sky-blue fluorescent OLEDs using DSFX-TFB as host and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph) as dopant material. This work provides a PASE synthetic route for GOSs and eco-friendly materials.

Spiro[fluorene-9,9′-xanthene]-based universal hosts for understanding structure–property relationships in RGB and white PhOLEDs

Ingenious construction of state-of-the-art models of electrophosphorescent hosts for uncovering the deep role of different molecular modifications in PhOLEDs performances is crucial for rational cumulative improvements of device efficiency and for accelerating their commercialization. A series of conjugation-interrupted spiro[fluorene-9,9′-xanthene](SFX)/9-arylfluorenes (AFs) hybrid host materials have been designed and synthesized by concise two-step reactions, and have been demonstrated to serve as universal hosts for low voltage blue, green, red and single-emitting layer white PhOLEDs. By varying the electronegativity and position of the bulky AF substitutes, two independent, selective methods for fine tuning frontier molecular orbital energy levels without affecting the high triplet energy level (T1) have been realized. This offers either facilitated hole- or electron-injection/blocking without influencing the optical properties, which can explain the performance of the corresponding RGB PhOLEDs. This investigation provides good guidance for the future rational design of high-performance PhOLED hosts by accumulative structural modifications.