•Four donor-acceptor structure isomers based on triphenylamine and acridine are synthesized.•Theoretical combined experimental characterizations are carried out for the excited state properties of the four isomers.•Substitution sites, twisted angles and conjugations make difference on the non-radiative rates.•Structure-property relationship of acridine based donor-acceptor molecules are further clarified for OLED applications.Charge-transfer (CT) state is becoming a useful excited state to design next-generation luminescence materials for high-performance organic light-emitting diode (OLED). However, strong CT state usually causes serious decrease in photoluminescence (PL) efficiency due to the small overlap between HOMO and LUMO. In order to harvest both high exciton utilization and high PL efficiency in fluorescent OLEDs, figuring out the strategy of fine modification of CT component in excited state through structural modulation is important. In this work, four donor–acceptor structure triphenylamine (TPA) - acridine (AC) isomers (TPA-9AC, TPA-1AC, TPA-2AC and TPA-3AC) were designed and synthesized to investigate structure-property relationship between isomerization effect and excited state properties. Density functional theory (DFT) calculations and solvatochromic absorption, emission and photoluminescence decay spectra are carried out for the deep understanding of their emissive state character. The four isomers exhibit gradually increased PL efficiency from low-polarity hexane to medium-polarity tetrahydrofuran (THF), which could be assigned to the formation of hybridized local and charge-transfer (HLCT) states and effectively suppressed non-radiative transition arised from acridine in medium-polarity solvents. Among four isomers, TPA-3AC achieved the best EL performance, due to the proper LE and CT compositions in the emissive state, demonstrating that isomerization of donor and acceptor functional moieties is an effective approach for structural modification for high-efficiency fluorescent OLED emitters.Download high-res image (210KB)Download full-size image

Molybdenum oxide (MoO3) thin films were prepared by sol–gel methods at room temperature from the precursors of MoO3 powder mixing into NH3 or H2O2 solution and then directly treated by UV-ozone instead of widely used high-temperature annealing. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) characteristics demonstrated that the room-temperature sol–gel derived MoO3 thin films exhibited excellent uniformity, unchanged chemical structure, and high work function. For the first time, the novel solution-processed MoO3 thin films were successfully applied as the hole injection layers (HILs) for solution-processed organic light-emitting diodes (OLEDs). The efficiencies of the resulting OLEDs were comparable or even higher than that of the device using PEDOT:PSS as the HIL. More importantly, the lifetimes of the solution-processed OLEDs are improved by nearly 2 orders of magnitude. This study should provide a potential approach to develop low-cost, high-performance, and long-lifetime OLEDs for practical applications.Keywords: hole injection; interface stability; molybdenum oxide; organic light-emitting diodes; solution process;

•Two bipolar host materials mCPpPO and mCPmPO have been disigned and synthesized.•High efficiency and low roll off blue device were fabricated.•The high efficiency single emission layer white PhOLED basd on mCPmPO was obtained.Two bipolar host materials, mCPpPO and mCPmPO have been synthesized by Ni(II)/Zn-catalyzed cross-coupling of diphenylphosphine oxide and corresponding aryl bromide. The photophysical properties, HOMO/LUMO orbital distribution and triplet levels of these host materials are investigated and optimized by tuning the linking modes between electron acceptor triphenylphosphine oxide and electron donor N,N′-dicarbazolyl-3,5-benzene (mCP). When mCP is linked to the meta-position of benzene of triphenylphosphine oxide, the hybrid (mCPmPO) shows much higher steric hinderance than the para-position linked analogue (mCPpPO) so that it possesses a higher triplet energy. Equipped with the bipolar transport properties, mCPmPO-based blue PhOLED doped FIrpic shows a maximum current efficiency (ηc,max) of 40.0 cd/A, a maximum power efficiency (ηp,max) of 39.7 lm/W, corresponding the maximum external quantum efficiency (ηEQE,max) of 20.3%, and the current efficiency still maintain to 34.8 cd/A even at 1000 cd/m2. Based on the optimized triplet energy level, the single emission layer white PhOLED hosted by mCPmPO shows ηc,max, ηp,max and ηEQE,max of 46.9 cd/A, 39.7 lm/W and 17.6%, respectively.Graphical abstract

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode are of great interest for large-size active-matrix displays due to their easy integration with n-type thin film transistors (TFTs) based on low-cost and highly-uniform amorphous silicon and oxide. In this work, a solution-processable electron transporting material 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13) is employed to blend with a solution-processable hole transporting material 4,4′,4′′-tri(9-carbazoyl)triphenylamine (TCTA) to be used as a universal bipolar co-host for blue, green and red phosphors, and for the first time, phosphorescent IOLEDs are fabricated by solution-processing small molecules. High efficiency and reduced efficiency roll-off are achieved in the solution-processed IOLEDs, which mainly contribute to the high quality of the solution-processed small molecule films as well as the balanced charge injection in the co-host system. Importantly, the solution process is advantageous over vacuum evaporation to deposit multi-component small molecule films, and can be expected to reduce manufacturing costs. Our results demonstrate a promising approach to fabricate low-cost and high-performance IOLEDs for n-type TFT-based displays.

Two novel simple electron-transport (ET) type host materials, 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)benzene (MDBIP) and 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)pyridine (MDBIPy) have been designed and synthesized. The two compounds exhibit high decomposition temperatures (Td: 444 °C for MDBIP and 450 °C for MDBIPy) and a stable amorphous glassy state (Tg: 108 °C for MDBIP and 110 °C for MDBIPy). In the typical device ITO/MoO3 (10 nm)/4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB, 80 nm)/TCTA (5 nm)/Host: Ir(ppy)3 (9 wt%, 20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), when MDBIPy was used as host material, the device B showed a maximum efficiency of 21.1%, 75.8 cd A−1 and 83.6 lm W−1. When MDBIPy was utilized as both the electron transporting and host layer, the unilateral homogeneous device C exhibited a maximum efficiency of 20.6%, 74.2 cd A−1 and 71.8 lm W−1. More interestingly, device C showed low efficiency roll-off relative to device B. When the brightness of the device is over 1000 cd m−2, the current efficiency of device C is higher than device B. At a brightness of 5000 cd m−2, the current efficiency of device C is only roll off 5.6%. These results demonstrate that utilizing electron-transport type host materials to fabricate unilateral homogeneous PhOLEDs is a promising way to simplify the device configuration and optimize the performance of OLED devices.

A series of novel bipolar blue phosphorescent host materials mBICP, mBINCP and mBIPhCP have been designed and synthesized, which comprehensively outperform the widely used phosphorescent host, 1,3-di(9H-carbazol-9-yl)benzene (mCP). The thermal, photophysical and electrochemical properties of these host materials were finely tuned through linking different carbazole moieties to the benzimidazole. mBICP (Tg = 84 °C) and mBIPhCP (Tg = 103 °C) exhibit high morphological stabilities in comparison with mCP. Theoretical calculations show that the HOMO/LUMO orbitals of these materials are mainly dispersed on the electron donating and electron accepting groups, respectively. A blue PhOLED device fabricated using mBICP as the host exhibits a maximum external quantum efficiency (ηEQE,max) of 18.7% and a maximum power efficiency (ηP,max) of 33.6 lm W−1. Interestingly, the external quantum efficiencies (ηEQE) are still as high as 17.1% at a high luminance of 1000 cd m−2. Furthermore, the two-color, all-phosphor and single-emitting-layer white device hosted by mBICP achieved a maximum external quantum efficiency (ηEQE,max) of 20.5% corresponding to a maximum power efficiency (ηP,max) of 53.3 lm W−1.

A new bromination method, where butterfly-shaped tetrasubstituted carbazole derivatives TSPFCz and TTPhCz have been designed and synthesized, which possess the twist butterfly skeletons and exhibit excellent thermal and morphological stabilities, has been adopted. By utilizing these novel compounds as host materials, high efficiency solution-processed green phosphorescent organic light-emitting diodes (PhOLEDs) have been achieved.

The widely used hole-transporting host 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) blended with either a hole-transporting or an electron-transporting small-molecule material as a mixed-host was investigated in the phosphorescent organic light-emitting diodes (OLEDs) fabricated by the low-cost solution-process. The performance of the solution-processed OLEDs was found to be very sensitive to the composition of the mixed-host systems. The incorporation of the hole-transporting 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) into TCTA as the mixed-host was demonstrated to greatly reduce the driving voltage and thus enhance the efficiency due to the improvement of hole injection and transport. On the basis of the mixed-host of TCTA:TAPC, we successfully fabricated low driving voltage and high efficiency blue and white phosphorescent OLEDs. A maximum forward viewing current efficiency of 32.0 cd/A and power efficiency of 25.9 lm/W were obtained in the optimized mixed-host blue OLED, which remained at 29.6 cd/A and 19.1 lm/W at the luminance of 1000 cd/m2 with a driving voltage as low as 4.9 V. The maximum efficiencies of 37.1 cd/A and 32.1 lm/W were achieved in a single emissive layer white OLED based on the TCTA:TAPC mixed-host. Even at 1000 cd/m2, the efficiencies still reach 34.2 cd/A and 23.3 lm/W and the driving voltage is only 4.6 V, which is comparable to those reported from the state-of-the-art vacuum-evaporation deposited white OLEDs.Keywords: blue and white OLEDs; mixed-host; phosphorescence; small molecules; solution-processed;

In this paper, we successfully improved the spectral stability in blue/orange complementary white organic light-emitting diodes (OLEDs) by utilizing hole-type single host double emissive layer structure. The demonstrated double emissive layer structure effectively suppresses the direct recombination of electron–hole pairs on the hole-trapping orange phosphor and thus reduces the deteriorated effect of charge trapping on electroluminescence spectrum stability by controlling exciton recombination zone. It is shown that the white light emission is a cascade energy transfer process from host to blue phosphor and then to orange phosphor, which seems to be less affected by the driving conditions. Thus, the change in Commission Internationale de L’Eclairage coordinates (CIE) in the white OLEDs is less than (±0.010, ±0.007) as the voltage increases from 4 V to 9 V, which correspond to the luminance increasing from 200 cd m−2 to about 20,000 cd m−2. This is superior to that of co-doped single emissive layer devices, which show much larger CIEs variation of (±0.05, ±0.02) in the same driving voltage range. We gave detailed analysis on the exciton recombination processes and well elucidated the working mechanism of the fabricated double emissive layer structure white OLEDs.Graphical abstractHighlights► It is found that multi-emission layer white OLED shows better color stability than single layer one. ► It is important to suppress the direct electron/hole recombination on the charge trapping orange phosphorescent dye. ► The width of the recombination zone in our single hole-type host white OLED is found to be about 3 nm.

A series of new blue materials based on highly fluorescent di(aryl)anthracene and electron-transporting phenanthroimidazole functional cores: 2-(4-(anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (ACPI), 2-(4-(10-(naphthalen-1-yl)anthracen-9-yl)phenyl)-1-p-henyl-1H-phenanthro[9,10-d]imidazole (1-NaCPI), 2-(4-(10-(naphthalen-2-yl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (2-NaCPI) were designed and synthesized. These materials exhibit good film-forming and thermal properties as well as strong blue emission in the solid state. To explore the electroluminescence properties of these materials, three layer, two layer and single layer organic light-emitting devices were fabricated. With respect to the three layer device 4 using ACPI as the emitting layer, its maximum current efficiency reaches 4.36 cd A−1 with Commission Internationale del’Eclairage (CIE) coordinates of (0.156, 0.155). In the single layer device 10 based on ACPI, maximum current efficiency reaches 1.59 cd A−1 with Commission Internationale del’Eclairage (CIE) coordinates of (0.169, 0.177). Interestingly, both device 4 and 10 has low turn on voltage and negligible efficiency roll off at high current densities.Graphical abstractHighlights► A series of new robust pure blue fluorescence materials were developed. ► Anthracene and phenanthro[9,10-d]imidazole functional cores were structured. ► Non-doped devices based on these new compounds exhibit good pure blue emission. ► The blue device show negligible efficiency roll off at high current densities.

The hole-transporting material 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) is widely used as a host in phosphorescent organic light-emitting diodes (PhOLEDs). Based on the unipolar TCTA, we synthesized a novel bipolar host material, (9-(4-(bis(4-(9H-carbazol-9-yl)phenyl)amino)phenyl)-9H-carbazol-3-yl) diphenylphosphine oxide (TCTAPO) by directly imparting an electron-transporting diphenylphosphine oxide moiety to TCTA. It was found that TCTAPO can be used as an ideal versatile host for blue, orange, and white PhOLEDs due to its high triplet energy (ET = 2.83 eV), high glass transition temperature (Tg = 165 °C), and bipolar charge-transporting ability. The maximum power efficiencies of 40.7 and 43.7 lm/W were achieved in the blue and orange PhOLEDs with FIrpic and (fbi)2Ir(acac) as the blue and orange dopants, respectively. Furthermore, the single-emitting-layer white organic light-emitting diode (WOLED) based on the emitter of TCTAPO:FIrpic:(fbi)2Ir(acac) exhibited a maximum current efficiency of 43.7 cd/A, a maximum power efficiency of 47.0 lm/W, and a maximum external quantum efficiency (ηext) of 17.2%. Compared to the PhOLEDs with TCTA as the host, the maximum power efficiencies increased about 56%, 36%, and 81%, for blue, orange, and white devices, respectively.

This study describes the synthesis and characterization of a series of new blue fluorescent materials, with propeller-like topology, consisting of 1,3,5-tri(9-anthracene)benzene core and various aromatic dendrons, such as naphthalene, 3,5-diphenylbenzene, carbazole, and N,N-diphenylamine. These compounds show excellent thermal and morphological stability with high glass transition temperatures (Tg) (166–231 °C) and high thermal decomposition temperatures (Td) (427–504 °C). Solution-processable double-layered OLEDs fabricated with these materials as the light-emitting layer show stable blue emission and good performance. The nondoped electronic device fabricated using compound 5c exhibits a maximum brightness of 4754 cd/m2 and maximum current efficiency of 2.0 cd/A (power efficiency, 1.71 lm/W) with Commission Internationale d’Eclairage (CIEx,y) color coordinates of (x = 0.16, y = 0.19) and the devices’ threshold voltage are only 3.4 eV. Compound 5d shows an even higher efficiency of up to 4.90 cd/A with CIEx,y color coordinates of (x = 0.17, y = 0.31) when doped with a blue fluorescent dopant, 4,4′-bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi).Graphical abstractHighlights► We synthesized a series of new high Tg propeller-shaped blue fluorescent materials. ► The properties of these materials could be tuned by the different surface groups. ► Solution-processable double-layered OLEDs fabricated with these materials show stable blue emission and good performance.

A series of starburst materials (T1−T3) bearing a 1,3,5-tri(anthracen-10-yl)benze-ne core (T0) and three oligofluorenes arms have been synthesized and characterized. Single-crystal diffraction analysis has shown that the core of these starburst materials possess a propeller twist topology, which made the starburst materials exhibit good film-forming capabilities and display deep blue emission both in solution and in the thin solid film. The compounds (T1−T3) possess high glass transition temperatures (Tg’s) at 107, 109, and 110 °C, and high decomposition temperatures (Td’s) at 438, 440, and 434 °C, respectively. In addition, the double-layered devices fabricated with the three materials as the emitter show a stable deep-blue emission and the device performance increases with arm length at some extent. The double-layered device based on T2 has a maximum brightness of over 3400 cd/m2 and a maximum current efficiency of 1.80 cd/A with CIE coordinates of (0.149, 0.098), which is among the best of the deep-blue starburst material devices reported so far in the current available literature.

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode are of great interest for large-size active-matrix displays due to their easy integration with n-type thin film transistors (TFTs) based on low-cost and highly-uniform amorphous silicon and oxide. In this work, a solution-processable electron transporting material 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13) is employed to blend with a solution-processable hole transporting material 4,4′,4′′-tri(9-carbazoyl)triphenylamine (TCTA) to be used as a universal bipolar co-host for blue, green and red phosphors, and for the first time, phosphorescent IOLEDs are fabricated by solution-processing small molecules. High efficiency and reduced efficiency roll-off are achieved in the solution-processed IOLEDs, which mainly contribute to the high quality of the solution-processed small molecule films as well as the balanced charge injection in the co-host system. Importantly, the solution process is advantageous over vacuum evaporation to deposit multi-component small molecule films, and can be expected to reduce manufacturing costs. Our results demonstrate a promising approach to fabricate low-cost and high-performance IOLEDs for n-type TFT-based displays.

Two novel simple electron-transport (ET) type host materials, 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)benzene (MDBIP) and 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)pyridine (MDBIPy) have been designed and synthesized. The two compounds exhibit high decomposition temperatures (Td: 444 °C for MDBIP and 450 °C for MDBIPy) and a stable amorphous glassy state (Tg: 108 °C for MDBIP and 110 °C for MDBIPy). In the typical device ITO/MoO3 (10 nm)/4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB, 80 nm)/TCTA (5 nm)/Host: Ir(ppy)3 (9 wt%, 20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), when MDBIPy was used as host material, the device B showed a maximum efficiency of 21.1%, 75.8 cd A−1 and 83.6 lm W−1. When MDBIPy was utilized as both the electron transporting and host layer, the unilateral homogeneous device C exhibited a maximum efficiency of 20.6%, 74.2 cd A−1 and 71.8 lm W−1. More interestingly, device C showed low efficiency roll-off relative to device B. When the brightness of the device is over 1000 cd m−2, the current efficiency of device C is higher than device B. At a brightness of 5000 cd m−2, the current efficiency of device C is only roll off 5.6%. These results demonstrate that utilizing electron-transport type host materials to fabricate unilateral homogeneous PhOLEDs is a promising way to simplify the device configuration and optimize the performance of OLED devices.

A series of novel bipolar blue phosphorescent host materials mBICP, mBINCP and mBIPhCP have been designed and synthesized, which comprehensively outperform the widely used phosphorescent host, 1,3-di(9H-carbazol-9-yl)benzene (mCP). The thermal, photophysical and electrochemical properties of these host materials were finely tuned through linking different carbazole moieties to the benzimidazole. mBICP (Tg = 84 °C) and mBIPhCP (Tg = 103 °C) exhibit high morphological stabilities in comparison with mCP. Theoretical calculations show that the HOMO/LUMO orbitals of these materials are mainly dispersed on the electron donating and electron accepting groups, respectively. A blue PhOLED device fabricated using mBICP as the host exhibits a maximum external quantum efficiency (ηEQE,max) of 18.7% and a maximum power efficiency (ηP,max) of 33.6 lm W−1. Interestingly, the external quantum efficiencies (ηEQE) are still as high as 17.1% at a high luminance of 1000 cd m−2. Furthermore, the two-color, all-phosphor and single-emitting-layer white device hosted by mBICP achieved a maximum external quantum efficiency (ηEQE,max) of 20.5% corresponding to a maximum power efficiency (ηP,max) of 53.3 lm W−1.