An adamantane-based host material, namely, 4-{3-[4-(9H-carbazol-9-yl)phenyl]adamantan-1-yl}benzonitrile (CzCN-Ad), was prepared by linking an electron-donating carbazole unit and an electron-accepting benzonitrile moiety through an adamantane bridge. In this approach, two functional groups were attached to tetrahedral points of adamantane to construct an “sp³” topological configuration. This design strategy endows the host material with a high triplet energy of 3.03 eV due to the disruption of intramolecular charge transfer. Although CzCN-Ad has a low molecular weight, the rigid nonconjugated adamantane bridge results in a glass transition temperature of 89 °C. These features make CzCN-Ad suitable for fabricating blue phosphorescent organic light-emitting diodes (PhOLEDs). The devices based on sky-blue phosphor bis[(4,6-difluorophenyl)pyridinato-N,C^2′](picolinato)iridium(III) (FIrpic) achieved a high maximum external quantum efficiency (EQE) of 24.1 %, which is among the best results for blue PhOLEDs ever reported. Furthermore, blue PhOLEDs with bis(2,4-difluorophenylpyridinato)-tetrakis(1-pyrazolyl)borate iridium(III) (FIr6) as dopant exhibited a maximum EQE of 14.2 % and a maximum luminance of 34 262 cd m⁻². To the best of our knowledge, this is the highest luminance ever reported for FIr6-based PhOLEDs.

Organic thin-film electroluminescent (EL) devices, such as organic light-emitting diodes (OLEDs), typically operate using constant voltage or direct current (DC) power sources. Such approaches require power converters (introducing power losses) and make devices sensitive to dimensional variations that lead to run away currents at imperfections. Devices driven by time-dependent voltages or alternating current (AC) may offer an alternative to standard OLED technologies. However, very little is known about how this might translate into overall performance of such devices. Here, a solution-processed route to creating highly efficient AC field-induced polymer EL (FIPEL) devices is demonstrated. Such solution-processed FIPEL devices show maximum luminance, current efficiency, and power efficiency of 3000 cd m⁻², 15.8 cd A⁻¹, and 3.1 lm W⁻¹ for blue emission, 13 800 cd m⁻², 76.4 cd A⁻¹, and 17.1 lm W⁻¹ for green emission, and 1600 cd m⁻², 8.8 cd A⁻¹, and 1.8 lm W⁻¹ for orange-red emission. The high luminance and efficiency, and solution process pave the way to industrial roll-to-roll manufacturing of solid state lighting and display.

The effect of solution-processed p-type doping of hole-generation layers (HGLs) and electron-transporting layer (ETLs) are systematically investigated on the performance of solution-processable alternating current (AC) field-induced polymer EL (FIPEL) devices in terms of hole-generation capability of HGLs and electron-transporting characteristics of ETLs. A variety of p-type doping conjugated polymers and a series of solution-processed electron-transporting small molecules are employed. It is found that the free hole density in p-type doping HGLs and electron mobility of solution-processed ETLs are directly related to the device performance, and that the hole-transporting characteristics of ETLs also play an important role since holes need to be injected from electrode through ETLs to refill the depleted HGLs in the positive half of the AC cycle. As a result, the best FIPEL device exhibits exceptional performance: a low turn-on voltage of 12 V, a maximum luminance of 20 500 cd m⁻², a maximum current and power efficiency of 110.7 cd A⁻¹ and 29.3 lm W⁻¹. To the best of the authors' knowledge, this is the highest report to date among FIPEL devices driven by AC voltage.

Two host materials of {4-[diphenyl(4-pyridin-3-ylphenyl)silyl]phenyl}diphenylamine (p-PySiTPA) and {4-[[4-(diphenylphosphoryl)phenyl](diphenyl)silyl]phenyl}diphenylamine (p-POSiTPA), and an electron-transporting material of [(diphenylsilanediyl)bis(4,1-phenylene)]bis(diphenylphosphine) dioxide (SiDPO) are developed by incorporating appropriate charge transporting units into the tetraarylsilane skeleton. The host materials feature both high triplet energies (ca. 2.93 eV) and ambipolar charge transporting nature; the electron-transporting material comprising diphenylphosphine oxide units and tetraphenylsilane skeleton exhibits a high triplet energy (3.21 eV) and a deep highest occupied molecular orbital (HOMO) level (-6.47 eV). Using these tetraarylsilane-based functional materials results in a high-efficiency blue phosphorescent device with a three-organic-layer structure of 1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane (TAPC)/p-POSiTPA: iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6)/SiDPO that exhibits a forward-viewing maximum external quantum efficiency (EQE) up to 22.2%. This is the first report of three-organic-layer FIr6-based blue PhOLEDs with the forward-viewing EQE over 20%, and the device performance is among the highest for FIr6-based blue PhOLEDs even compared with the four or more than four organic-layer devices. Furthermore, with the introduction of bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzoimidazol-N,C³)iridium acetylacetonate [(fbi)₂Ir(acac)] as an orange emitter, an all-phosphor warm-white PhOLED achieves a peak power efficiency of 47.2 lm W⁻¹, which is close to the highest values ever reported for two-color white PhOLEDs.

Great efforts have been devoted to seek novel approaches for constructing blue fluorescent materials, which is one of the most important prerequisites for the commercialization of OLEDs. In recent years, various outstanding luminogens with aggregation-induced emission characteristic exhibit promising applications as emitters, but blue AIE fluorophores with excellent EL performance are still very scarce. Here, five hole-dominated blue AIE molecules are demonstrated by adopting construction approaches of changing linkage modes and increasing intramolecular torsion together, with the aim to restrict conjugation lengths without sacrificing good EL data. Device results show that the novel synthesized materials could be applied as bifunctional materials, namely blue light-emitting and hole-transporting materials, with comparable EL efficiencies, and the η_C,max and η_ext,max are up to 8.03 cd A⁻¹ and 3.99% respectively, which is among the best EL performance for blue AIE luminogens.

New heteroleptic cyclometalated iridium(III) 2-phenylpyridine-type complexes with trifluoromethyl substituents and various main-group moieties were synthesized and their photophysical, electrochemical, and electroluminescent (EL) properties studied. The emission color can be tuned by a facile derivatization of the phenyl moiety of 2-phenylpyridine with various main-group moieties, and we have prepared new yellowishgreen to orange triplet emitters with enhanced charge injection/charge transporting features, which can furnish attractive EL performance in phosphorescent organic light-emitting devices (OLEDs). Attempts were also made to fabricate two-color white-light OLEDs based on a combination of fluorescent blue and phosphorescent orange emitters.

Three TPE trimers with meta or para linkage modes have been successfully synthesized. When fabricated as emissive layers in non-doped OLEDs, they all exhibit blue or deep-blue emissions with maximum current efficiency up to 4.03 cd A⁻¹, further verifying the facile but ingenious approach by utilizing meta-linkage mode in longer conjugated systems.

To achieve high efficiencies in blue phosphorescent organic light-emitting diodes (PhOLEDs), the triplet energies (T₁) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S₁) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6-difluorophenylpyridinato-N,C^2′)iridium(III)] tetrakis(1-pyrazolyl)borate (FIr6; T₁=2.73 eV). In both the photo- and electro-excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic-layer device, the maximum current efficiency of 37 cd A⁻¹ and power efficiency of 40 Lm W⁻¹ were achieved for the FIr6-based blue PhOLEDs.

To achieve high efficiencies in blue phosphorescent organic light-emitting diodes (PhOLEDs), the triplet energies (T₁) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S₁) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6-difluorophenylpyridinato-N,C^2′)iridium(III)] tetrakis(1-pyrazolyl)borate (FIr6; T₁=2.73 eV). In both the photo- and electro-excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic-layer device, the maximum current efficiency of 37 cd A⁻¹ and power efficiency of 40 Lm W⁻¹ were achieved for the FIr6-based blue PhOLEDs.

The correspondence between triplet location effect and host-localized triplet–triplet annihilation and triplet–polaron quenching effects was performed on the basis of a series of naphthyldiphenylamine (DPNA)-modified phosphine oxide hosts. The number and ratio of DPNA and diphenylphosphine oxide was adjusted to afford symmetrical and unsymmetrical molecular structures and different electronic environments. As designed, the first triplet (T₁) states were successfully localized on the specific DPNA chromophores. Owing to the meso- and multi-insulating linkages, identical optical properties and comparable electrical performance was observed, including the same first singlet (S₁) and T₁ energy levels to support the similar singlet and triplet energy transfer and the close frontier molecular orbital energy levels. This established the basis of rational investigation on T₁ location effect without interference from other optoelectronic factors.

Grafting six fluorene units to a benzene ring generates a new highly twisted core of hexakis(fluoren-2-yl)benzene. Based on the new core, six-arm star-shaped oligofluorenes from the first generation T1 to third generation T3 are constructed. Their thermal, photophysical, and electrochemical properties are studied, and the relationship between the structures and properties is discussed. Simple double-layer electroluminescence (EL) devices using T1–T3 as non-doped solution-processed emitters display deep-blue emissions with Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.08) for T1, (0.16, 0.08) for T2, and (0.16, 0.07) for T3. These devices exhibit excellent performance, with maximum current efficiency of up to 5.4 cd A⁻¹, and maximum external quantum efficiency of up to 6.8%, which is the highest efficiency for non-doped solution-processed deep-blue organic light-emitting diodes (OLEDs) based on starburst oligofluorenes, and is even comparable with other solution-processed deep-blue fluorescent OLEDs. Furthermore, T2- and T3-based devices show striking blue EL color stability independent of driving voltage. In addition, using T0–T3 as hole-transporting materials, the devices of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)/T0–T3/tris(8-hydroxyquinolinato)aluminium (Alq₃)/LiF/Al achieve maximum current efficiencies of 5.51–6.62 cd A⁻¹, which are among the highest for hole-transporting materials in identical device structure.

Four 4,4′-bis(1,2,2-triphenylvinyl)biphenyl (BTPE) derivatives, 4,4′-bis(1,2,2-triphenylvinyl)biphenyl, 2,3′-bis(1,2,2-triphenylvinyl)biphenyl, 2,4′-bis(1,2,2-triphenylvinyl)biphenyl, 3,3′-bis(1,2,2-triphenylvinyl)biphenyl and 3,4′-bis(1,2,2-triphenylvinyl)biphenyl (oTPE-mTPE, oTPE-pTPE, mTPE-mTPE, and mTPE-pTPE, respectively), are successfully synthesized and their thermal, optical, and electronic properties fully investigated. By merging two simple tetraphenylethene (TPE) units together through different linking positions, the π-conjugation length is effectively controlled to ensure the deep-blue emission. Because of the minor but intelligent structural modification, all the four fluorophores exhibit deep-blue emissions from 435 to 459 nm with Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of, respectively, (0.16, 0.14), (0.15, 0.11), (0.16, 0.14), and (0.16, 0.16), when fabricated as emitters in organic light-emitting diodes (OLEDs). This is completely different from BTPE with sky-blue emission (0.20, 0.36). Thus, these results may provide a novel and versatile approach for the design of deep-blue aggregation-induced emission (AIE) luminogens.

Two coordination complex emitters as well as host materials Be(PPI)₂ and Zn(PPI)₂ (PPI = 2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenol) are designed, synthesized, and characterized. The incorporation of the metal atom leads to a twisted conformation and rigid molecular structure, which improve the thermal stability of Be(PPI)₂ and Zn(PPI)₂ with high T_d and T_g at around 475 and 217 °C, respectively. The introduction of the electron-donating phenol group results in the emission color shifting to the deep-blue region and the emission maximum appears at around 429 nm. This molecular design strategy ensures that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) HOMO and LUMO of Be(PPI)₂ and Zn(PPI)₂ localize on the different moieties of the molecules. Therefore, the two complexes have an ambipolar transport property and a small singlet–triplet splitting of 0.35 eV for Be(PPI)₂ and 0.21 eV for Zn(PPI)₂. An undoped deep-blue fluorescent organic light-emitting device (OLED) that uses Be(PPI)₂ as emitter exhibits a maximum power efficiency of 2.5 lm W⁻¹ with the CIE coordinates of (0.15, 0.09), which are very close to the National Television Standards Committee (NTSC) blue standard (CIE: 0.14, 0.08). Green and red phosphorescent OLEDs (PhOLEDs) that use Be(PPI)₂ and Zn(PPI)₂ as host materials show high performance. Highest power efficiencies of 67.5 lm W⁻¹ for green PhOLEDs and 21.7 lm W⁻¹ for red PhOLEDs are achieved. In addition, the Be(PPI)₂-based devices show low-efficiency roll-off behavior, which is attributed to the more balanced carrier-transport property of Be(PPI)₂.

A series of solution-processable small molecules PO1–PO4 were designed and synthesized by linking N-phenylnaphthalen-1-amine groups to a phenyl phosphine oxide core through a π-conjugated bridge, and their thermal, photophysical, and electrochemical properties were investigated. The phosphine oxide linkage can disrupt the conjugation and allows the molecular system to be extended to enable solution processability and high glass transition temperatures (159–181 °C) while preserving the deep-blue emission. The noncoplanar molecular structures resulting from the trigonal-pyramidal configuration of the phosphine oxide can suppress intermolecular interactions, and thus these compounds exhibit strong deep-blue emission both in solution and the solid state with high photoluminescent quantum yield (PLQY) of 0.88–0.99 in dilute toluene solution. Solution-processed nondoped organic light-emitting diodes featuring PO4 as emitter achieve a maximum current efficiency of 2.36 cd A⁻¹ with CIE coordinates of (0.15, 0.11) that are very close to the NTSC blue standard. Noticeably, all devices based on these small-molecular fluorescent emitters show striking deep-blue electroluminescent color stability and extremely low efficiency roll-off.

A series of tetraarylsilane compounds, namely p-BISiTPA (1), m-BISiTPA (2), p-OXDSiTPA (3), m-OXDSiTPA (4), are designed and synthesized by incorporating electron-donating arylamine and electron-accepting benzimidazole or oxadiazole into one molecule via a silicon-bridge linkage mode. Their thermal, photophysical and electrochemical properties can be finely tuned through the different groups and linking topologies. The para-disposition compounds 1 and 3 display higher glass transition temperatures, slightly lower HOMO levels and triplet energies than their meta-disposition isomers 2 and 4, respectively. The silicon-interrupted conjugation of the electron-donating and electron-accepting segments gives these materials the following advantages: i) relative high triplet energies in the range of 2.69–2.73 eV; ii) HOMO/LUMO levels of the compounds mainly depend on the electron-donating and electron-accepting groups, respectively; iii) bipolar transporting feature as indicated by hole-only and electron-only devices. These advantages make these materials ideal universal hosts for multicolor phosphorescent OLEDs. 1 and 3 have been demonstrated as universal hosts for blue, green, orange and white electrophosphorescence, exhibiting high efficiencies and low efficiency roll-off. For example, the devices hosted by 1 achieve maximum external quantum efficiencies of 16.1% for blue, 22.7% for green, 20.5% for orange, and 19.1% for white electrophosphorescence. Furthermore, the external quantum efficiencies are still as high as 14.2% for blue, 22.4% for green, 18.9% for orange, and 17.4% for white electrophosphorescence at a high luminance of 1000 cd m⁻². The two-color, all-phosphor white device hosted by 3 acquires a maximum current efficiency of 51.4 cd A⁻¹, and a maximum power efficiency of 51.9 lm W⁻¹. These values are among the highest for single emitting layer white PhOLEDs reported till now.

High-performance, green, orange, and red top-emitting organic light-emitting diodes (TOLEDs) with p–i–n homojunction are demonstrated. An excellent ambipolar host, 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD), which has good thermal and morphological stabilities, a high triplet energy level, and equally high electron and hole mobilities, is chosen as the organic host material for the homojunction devices. By electrical doping, the carrier injection and transporting characteristics are greatly improved. The optical structure is optimized in view of light emission of different colors to enhance the color purity and improve the view characte­ristics. As a result, high efficiency p–i–n homojunction TOLEDs with saturated intrinsic emission of the emitting materials and angular independence of the emission are realized. The performances of these p–i–n homojunction TOLEDs are even higher than the multi-layer heterojunction bottom-emitting devices using the same emitting layers.

With the aim of endowing triplet emitters in the development of organic light-emitting devices (OLEDs) with electron-injection/-transporting (EI/ET) features, the phenylsulfonyl moiety was introduced into the phenyl ring of a 2-phenylpyridine (Hppy) ligand and the yellow phosphorescent heteroleptic iridium(III) complex 1 was developed. It was shown that the SO₂Ph unit could provide EI/ET character to 1, as indicated from both electrochemical and computational data. Complex 1 is a promising yellow-emitting material for both monochromatic OLEDs and white OLEDs (WOLEDs). The outstanding electronic traits associated with 1, coupled with careful device design, afforded very attractive electroluminescent performances for two-element WOLEDs, including a low turn-on voltage of less than 3.7 V, a maximum brightness of 48 000 cd m⁻², an external quantum efficiency of 13.0 %, a luminance efficiency of 34.7 cd A⁻¹, and a power efficiency of 24.3 Lm W⁻¹. In addition, a good color rendering index (CRI) of about 74, a stable white color with a Commission Internationale de L′Eclairage (CIE_x,y) variation of Δ(x, y)<±(0.02, 0.02), and a correlated color temperature higher than 5130 K were obtained. These encouraging results indicate the potential of these WOLEDs as good candidates for warm indoor lighting sources, as well as the critical contribution of such key EI/ET properties to triplet emitters to advance new OLED research.

A series of bipolar transport host materials: 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) (1), 2,5-bis(4-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (p-CzOXD) (2), 2,5-bis(3-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (m-CzOXD) (3) and 2-(2-(9H-carbazol-9-yl)phenyl)-5-(4-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (op-CzOXD) (4) are synthesized through simple aromatic nucleophilic substitution reactions. The incorporation of the oxadiazole moiety greatly improves their morphological stability, with T_d and T_g in the range of 428–464 °C and 97–133 °C, respectively. The ortho and meta positions of the 2,5-diphenyl-1,3,4-oxadiazole linked hybrids (1 and 3) show less intramolecular charge transfer and a higher triplet energy compared to the para-position linked analogue (2). The four compounds exhibit similar LUMO levels (2.55–2.59 eV) to other oxadiazole derivatives, whereas the HOMO levels vary in a range from 5.55 eV to 5.69 eV, depending on the linkage modes. DFT-calculation results indicate that 1, 3, and 4 have almost complete separation of their HOMO and LUMO levels at the hole- and electron-transporting moieties, while 2 exhibits only partial separation of the HOMO and LUMO levels possibly due to intramolecular charge transfer. Phosphorescent organic light-emitting devices fabricated using 1–4 as hosts and a green emitter, Ir(ppy)₃ or (ppy)₂Ir(acac), as the guest exhibit good to excellent performance. Devices hosted by o-CzOXD (1) achieve maximum current efficiencies (η_c) as high as 77.9 cd A⁻¹ for Ir(ppy)₃ and 64.2 cd A⁻¹ for (ppy)₂Ir(acac). The excellent device performance may be attributed to the well-matched energy levels between the host and hole-transport layers, the high triplet energy of the host and the complete spatial separation of HOMO and LUMO energy levels.

A new triphenylamine/oxadiazole hybrid, namely m-TPA-o-OXD, formed by connecting the meta-position of a phenyl ring in triphenylamine with the ortho-position of 2,5-biphenyl-1,3,4-oxadiazole, is designed and synthesized. The new bipolar compound is applicable in the phosphorescent organic light-emitting diodes (PHOLEDs) as both host and exciton-blocking material. By using the new material and the optimization of the device structures, very high efficiency green and yellow electrophosphorescence are achieved. For example, by introducing 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) to replace 2, 9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP)/tris(8-hydroxyquinoline)aluminium (Alq₃) as hole blocking/electron transporting layer, followed by tuning the thicknesses of hole-transport 1, 4-bis[(1-naphthylphenyl)amino]biphenyl (NPB) layer to manipulate the charge balance, a maximum external quantum efficiency (η_EQE,max) of 23.0% and a maximum power efficiency (η_p,max) of 94.3 lm W⁻¹ are attained for (ppy)₂Ir(acac) based green electrophosphorescence. Subsequently, by inserting a thin layer of m-TPA-o-OXD as self triplet exciton block layer between hole-transport and emissive layer to confine triplet excitons, a η_EQE,max of 23.7% and η_p,max of 105 lm W⁻¹ are achieved. This is the highest efficiency ever reported for (ppy)₂Ir(acac) based green PHOLEDs. Furthermore, the new host m-TPA-o-OXD is also applicable for other phosphorescent emitters, such as green-emissive Ir(ppy)₃ and yellow-emissive (fbi)₂Ir(acac). A yellow electrophosphorescent device with η_EQE,max of 20.6%, η_c,max of 62.1 cd A⁻¹, and η_p,max of 61.7 lm W⁻¹, is fabricated. To the author’s knowledge, this is also the highest efficiency ever reported for yellow PHOLEDs.

By incorporating two phosphorescent dyes, namely, iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C²′]picolinate (FIrpic) for blue emission and bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzoimidazol-N,C³)iridium(acetylacetonate) ((fbi)₂Ir(acac)) for orange emission, into a single-energy well-like emissive layer, an extremely high-efficiency white organic light-emitting diode (WOLED) with excellent color stability is demonstrated. This device can achieve a peak forward-viewing power efficiency of 42.5 lm W⁻¹, corresponding to an external quantum efficiency (EQE) of 19.3% and a current efficiency of 52.8 cd A⁻¹. Systematic studies of the dopants, host and dopant-doped host films in terms of photophysical properties (including absorption, photoluminescence, and excitation spectra), transient photoluminescence, current density–voltage characteristics, and temperature-dependent electroluminescence spectra are subsequently performed, from which it is concluded that the emission natures of FIrpic and (fbi)₂Ir(acac) are, respectively, host–guest energy transfer and a direct exciton formation process. These two parallel pathways serve to channel the overall excitons to both dopants, greatly reducing unfavorable energy losses. It is noteworthy that the introduction of the multifunctional orange dopant (fbi)₂Ir(acac) (serving as either hole-trapping site or electron-transporting channel) is essential to this concept as it can make an improved charge balance and broaden the recombination zone. Based on this unique working model, detailed studies of the slight color-shift in this WOLED are performed. It is quantitatively proven that the competition between hole trapping on orange-dopant sites and undisturbed hole transport across the emissive layer is the actual reason. Furthermore, a calculation of the fraction of trapped holes on (fbi)₂Ir(acac) sites with voltage shows that the hole-trapping effect of the orange dopant is decreased with increasing drive voltage, leading to a reduction of orange emission.