A rational molecular design strategy for tuning the emission color of phosphorescent complexes by functionalization of the bis(2-phenylpyridine)(2-(2′-oxyphenyl)-2-oxazoline/oxazole)iridium(III) framework is reported. Five new complexes (2–6) have been synthesized in good yields and characterized by cyclic voltammetry, absorption, and photoluminescence studies, by time-dependent density functional theory (TD-DFT) calculations, and by single-crystal X-ray diffraction studies for complexes 2, 4, and 6. An interesting feature of the complexes is that the HOMO is localized on the Ir d-orbitals and the phenoxylate part of the “ancillary” ligand, while the LUMO is located on the pyridyl ring of the ppy ligands. A few other complexes containing 2′-oxyphenyl-2-oxazoline/oxazole ancillary ligands have been reported previously; however, until now there has not been a systematic investigation into manipulating this unusual frontier orbital distribution to tune the emissive properties. It is shown that exchanging the phenylpyridine (ppy) ligand for 2,4-difluoro-ppy gives a blue shift of 21–22 nm (from 1 to 2 and from 4 to 5), and the introduction of electron-withdrawing substituents (SO2Me, CF3) onto the phenoxylate ring of the (2′-oxyphenyl)-2-oxazole ligand results in a further blue shift of 13–20 nm. Combining these functionalizations gives sky-blue emission with λmaxPL 476 and 479 nm for complexes 5 and 6 in dichloromethane solution. The solution quantum yields of all the complexes are within the range ΦPL 0.42–0.73. The observed lifetimes (τobs = 1.52–3.01 μs) and spectral profiles are indicative of phosphorescence from a mixture of ligand-centered and MLCT excited states. (TD-)DFT calculations are in close agreement with the observed photophysical and electrochemical properties of the complexes. Phosphorescent organic light-emitting diodes have been fabricated using complexes 2, 3, 5, and 6 as the emitter, doped in a 4,4′-bis(N-carbazolyl)biphenyl host, giving efficient emission in the blue-green region. Notably, complex 5 gives λmaxEL 480 nm with a maximum brightness of 26150 cd m–2.

Highly efficient long-wavelength thermally activated delayed fluorescence (TADF) materials are developed using 2,3-dicyanopyrazino phenanthrene (DCPP) as the electron acceptor (A), and carbazole (Cz), diphenylamine (DPA), or 9,9-dimethyl-9,10-dihydroacridine (DMAC) as the electron donor (D). Because of the large, rigid π-conjugated structure and strong electron-withdrawing capability of DCPP, TADF molecules with emitting colors ranging from yellow to deep-red are realized with different electron-donating groups and π-conjugation length. The connecting modes between donor and acceptor, that is, with or without the phenyl ring as π-bridge, are also investigated to study the π-bridge effect on the thermal, photophysical, electrochemical, and electroluminescent properties. Yellow, orange, red, and deep-red organic light-emitting diodes (OLEDs) based on DCPP derivatives exhibit high efficiencies of 47.6 cd A–1 (14.8%), 34.5 cd A–1 (16.9%), 12.8 cd A–1 (10.1%), and 13.2 cd A–1 (15.1%), with Commission Internationale de L’Eclairage (CIE) coordinates of (0.44, 0.54), (0.53, 0.46), (0.60, 0.40), and (0.64, 0.36), respectively, which are among the best values for long-wavelength TADF OLEDs.Keywords: dicyanopyrazino phenanthrene derivatives; intramolecular charge transfer excited states; long-wavelength emitters; organic light-emitting diodes; thermally activated delayed fluorescence;

Two new deep-red iridium(III) complexes, (fpiq)2Ir(dipba) (fIr1) and (f2piq)2Ir(dipba) (dfIr2), comprising two cyclometaling ligands of fluorophenyl-isoquinoline derivatives (fpiq and f2piq) and a N-heterocyclic carbene (NHC)-based ancillary ligand of N,N′-diisopropylbenzamidinate (dipba) are designed, synthesized, and characterized. Given the unique four-membered Ir–N–C–N backbone built by the metal center and the ancillary ligand, both phosphors achieve significant improvement for their comprehensive optoelectronic characteristics. Density function theory (DFT) calculations and electrochemical measurements support the genuine pure red phosphorescent emission of fIr1 and dfIr2 based on their clearly distinct electron density distributions of the HOMO/LUMO orbitals compared with other red-emitting Ir(III) derivatives. Both new phosphors show deep-red emission with λmax values in the region of 650–660 nm with high PLQYs and short excited-state lifetimes. The phosphorescent organic light emitting diodes (PhOLEDs) based on fIr1 and dfIr2 realize deep-red EL with the stable CIEx,y coordinates of (0.70, 0.30) and (0.69, 0.31), the peak EQE/PE values of 15.4%/9.3 lm W–1 and 16.7%/10.4 lm W–1, respectively, which maintain such high levels as 10.6%/3.5 lm W–1 and 10.8%/3.6 lm W–1 at the practical luminance of 1000 cd m–2. They are the highest EL values reported for the OLEDs with such deep-red CIE coordinates.Keywords: deep-red; electroluminescence (EL); four-membered heterocycles; iridium complex; phosphorescence;

AbstractThe design and synthesis of highly efficient deep red (DR) and near-infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a great challenge. A strategy was developed to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline-6,7-dicarbonitrile (QCN) were employed as electron donor (D) and acceptor (A), respectively, to synthesize a TADF compound, TPA-QCN. The TPA-QCN molecule with orange-red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light-emitting devices (OLEDs) were fabricated by regulating TPA-QCN dopant concentration in the emitting layers.

•Blue-emitting triarylcyclopentadiene derivatives were synthesized.•Compounds possess highly twisted conformations.•Compounds exhibited aggregation-induced emission enhancement characteristics.•A non-doped OLED displays good electroluminescent performance.Five highly efficient blue-emitting triarylcyclopentadiene derivatives 1–5 were designed and synthesized by introducing phenyl, methoxyphenyl, methoxybiphenyl and methoxynaphthyl groups to cyclopentadiene backbone, respectively. These compounds possess good thermal stability with decomposition temperatures of 264–340 °C and exhibit intense blue fluorescence with maximum emission peaked at 457–470 nm. All compounds emit weak fluorescence in solution, but they become strong emitters in the aggregated state indicating typical aggregation-induced emission enhancement (AIEE) properties. AIEE behaviour was further confirmed by fluorescent changes depended on temperature and solution viscosity as well as time-resolved fluorescence lifetime measurements. Unexpected substituent effect on aggregation structures and fluorescent properties of 1–5 was also observed. Single-crystal structure analysis revealed that highly twisted conformations and bulky substituents on cyclopentadiene core could dramatically suppress the intermolecular π-π interactions leading to intense blue emission of these compounds in aggregates. Furthermore, the non-doped and doped organic light-emitting diodes (OLEDs) using compounds 2 and 5 as the emitting layers have been fabricated, respectively. A non-doped OLED with compound 5 displays good electroluminescent performance with a light-blue emission and a maximum luminance of 3886 cd/m2.Five highly efficient AIEE-active blue-emitting triarylcyclopentadiene derivatives were designed and synthesized. AIEE mechanism were investigated by fluorescent changes depended on temperature and solution viscosity as well as time-resolved fluorescence and single-crystal structural analysis.Download high-res image (426KB)Download full-size image

A four-color warm-white organic light-emitting diode employing a simple adjacent two-emitting-layer structure as a blue host–orange dopant/green host–red dopant has been fabricated, which exhibited a stable high electroluminescent performance: an external quantum efficiency of 23.3% and a power efficiency of 63.2 lm W–1 at an illumination-relevant luminance of 1000 cd m–2 with a high color-rendering index (CRI) of 92 and maintained high levels of 21.6% and 48.8 lm W–1 with a CRI value of 93 at the extremely high luminance of 5000 cd m–2. To our knowledge, this should be the best result so far for a white-light organic light-emitting diode with CRI > 90, simultaneously exhibiting very high efficiencies based on a high luminance level for the solid-state lighting.Keywords: four-color; high CRI; phosphorescent material; warm-white; white-light organic light-emitting diode (WOLED)

•Three efficient blue-emitting cyclopentadiene derivatives have been synthesized.•Compounds are non-coplanar structures with bulky substituents.•Compounds exhibited aggregation-induced emission enhancement characteristics.•The non-doped organic light-emitting diode based on compound displayed blue emission.Three highly fluorescent blue-emitting molecules, namely 1,2-diphenyl-4-thiophenyl-1,3-cyclopentadiene (DPCP 1), 1,2-diphenyl-4-(4-thiophenyl)phenyl)-1,3-cyclopentadiene (DPCP 2) and 1,2-diphenyl-4-(4-dibenzothiphenyl)phenyl)-1,3-cyclopentadiene (DPCP 3), have been synthesized by using aryl-substituted cyclopentadiene and thiophene or dibenzothiphene as ingredients. The single crystal structure analysis reveals that DPCP 1–3 are non-coplanar structures and bulky substituents on cyclopentadiene core imposed a significant reduction on intermolecular interactions, hence leading to their intense blue emission in both solution and solid state. DPCP 1 and DPCP 3 also showed a typical aggregation-induced emission enhancement in mixed water/acetone solution. These compounds exhibited good thermal stability with decomposition temperatures between 239 and 383 °C. The non-doped organic light-emitting diodes using DPCP 3 as the emitting layer displayed a very low turn-on voltage at 3.2 V and pure blue emission with the Commission Internationale de I'Éclairage (CIEx,y) coordinates of (0.16, 0.16) and a maximum luminance of 2277 cd m−2.

A novel organosilane compound, bis(4-(1-phenylphenanthro[9,10-d]imidazol-2-yl)phenyl)diphenylsilane (Si(PPI)2), has been designed and synthesized. It has a high thermal decomposition temperature of 528 °C and is able to form an amorphous glass with a high glass-transition temperature of 178 °C. In addition, it possesses high singlet and triplet energies and displays efficient energy transfer to the selected blue fluorescent and green and red phosphorescent dopants when used as a host material. Electrochemical measurements and single-carrier devices indicate that Si(PPI)2 is a bipolar transport material, allowing the injection and transport of both electrons and holes. By using (Si(PPI)2) as a host, high-performance fluorescent blue (FB) and phosphorescent green (PG) and red (PR) OLEDs with a uniform and simple device configuration have been achieved. These OLEDs exhibit very high peak external quantum efficiency (EQE) and peak power efficiency (PE), i.e. 6.1% and 8.0 lm W−1 for FB, 19.2% and 51.1 lm W−1 for PG and 12.0% and 15.6 lm W−1 for PR. Moreover, the high-level EQE of 4.0, 19.1 and 10.6% and PE of 3.0, 41.6 and 7.5 lm W−1 can be maintained by FB, PG and PR, respectively, at the practical luminance of 100 cd m−2. Furthermore, the emission colors of these OLEDs remain almost unchanged within the whole range of driving voltages. Importantly, the blue OLED displays a pure blue emission (CIE: 0.18, 0.17).

A novel phosphorescent emitter BZQPG possessing bipolar charge transporting ability realizes the most efficient orange-red electroluminescence reported to date with the power efficiency of 77.1 lm W−1 and external quantum efficiency of 27.3% together with low efficiency roll-off (>50 lm W−1 and 26% at 5000 cd m−2).

A novel phosphorescent host FPYPCA possessing the bipolar charge transporting ability realizes the most efficient deep-red PhOLED, which maintains very high-level EQEs of >23% at rather a high and wide luminance range of 1000–10000 cd m−2.

Two deep blue emitting materials PPI-PPITPA and PPI-PPIPCz with dual carrier transport properties and small singlet–triplet splitting features are designed and synthesized. PPI-PPITPA and PPI-PPIPCz were used not only as non-doped emitting layers to fabricate highly efficient deep blue OLEDs, but also as hosts to construct high performance green, yellow and red phosphorescent OLEDs.

Achieving high power efficiencies at high-brightness levels is still an important issue for organic light-emitting diodes (OLEDs) based on the thermally activated delayed fluorescence (TADF) mechanism. Herein, enhanced electroluminescence efficiencies were achieved in fluorescent OLEDs using a TADF molecule, (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), as a host and quinacridone derivatives (QA) as fluorescent dopants.

Highly efficient phosphorescent organic light-emitting diodes (PhOLEDs) based on two iridium complexes constructed by the N^C^N-coordinated terdentate ligands (also called pincer ligands) have been achieved. They exhibit high peak power efficiency (PE) and external quantum efficiency (EQE) values of 35.5 lm W−1 & 15.8% for blue-green emission, 47.4 lm W−1 & 16.7% for green emission, which maintain the high levels of 19.2 lm W−1 & 14.5% and 30.6 lm W−1 & 16.1% at rather high and practical luminance of 500 cd m−2 with low driving voltages of less than 6 V. These values show almost a twofold enhancement over the most efficient PhOLEDs based on pincer iridium complexes ever reported. Here, the appropriate selection of a prominent electron-transport molecule TPBi as a host to match the dopant molecules (1 or 2) that possess sufficient hole-transport ability is critical in the remarkable EL-performance improvement compared to previous reports. We will present a comprehensive investigation that not only encompasses the conventional thermal, photophysical and electrochemical properties of both complexes, but also emphatically studies the charge carrier injecting/transporting and electroluminescent (EL) characteristics of two phosphorescent emitters doped in different hosts.

In this work, two novel hybrids of an electron-accepting phosphine oxide moiety attached to a phenanthroimidazole have been designed and synthesized. The PO group is used as a point of saturation between the PPI moiety and the outer phenyl groups, so the high triplet energy of PPI is preserved to act as a host for red and green phosphorescent dopants. The strong intermolecular interactions and steric effect of the diphenylphosphine oxide (DPO) moiety endows the films with high quantum yields in the deep-blue emission region. Compared to PPI, the carrier (hole- and electron-)injection/transport properties were greatly promoted by the appended DPO group according to single-carrier device measurement. Besides, the morphological and thermal stabilities were also improved. The multiple functions enable adaptation of several simplified device configurations. The undoped deep-blue fluorescent device exhibits an external quantum efficiency of 2.24% with CIE (0.16, 0.08), very close to the NTSC blue standard CIE (0.14, 0.08). High performance for green (65.4 cd A−1, 73.3 lm W−1 and 18.0%) and red (19.0 cd A−1, 21.3 lm W−1 and 13.5%) phosphorescent devices used as hosts have been achieved. The experimental and theoretical relationships between the molecular structures and the optoelectronic properties are discussed.

•Two novel bipolar host materials containing phenanthroimidazole and dimesitylborane are obtained.•High-performance green, yellow and red PhOLEDs based on two bipolar hosts have been demonstrated.•Very high and stable EL efficiencies are achieved.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 (66.3 cd A−1, 63.1 lm W−1, 18.2%), yellow (55.2 cd A−1, 66.6 lm W−1, 14.5%) and red (20.1 cd A−1, 20.4 lm W−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.Two novel bipolar hosts containing phenanthroimidazole and dimesitylborane 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. Within these merit, we have fabricated highly efficient green (66.3 cd A−1, 63.1 lm W−1, 18.2%), yellow (55.2 cd A−1, 66.6 lm W−1, 14.5%) and red (20.1 cd A−1, 20.4 lm W−1, 13.5%) PhOLEDs by using these compounds as 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.

Two new indolo[3,2-b]carbazole derivatives (DPDT-ICZ and DNDT-ICZ) with multifunctionality were designed and synthesized. They were employed as deep-blue emitters, hole-transporting materials and hosts to fabricate organic light-emitting devices (OLEDs). The devices which used them as emitters displayed deep-blue emissions with CIE coordinates of (0.15, 0.08). They have been employed as hole-transporting and host material simultaneously to construct high performance yellow and red phosphorescent OLEDs. High power efficiencies (78.3 lm W−1 for yellow devices and 20.4 lm W−1 for red devices) for phosphorescent OLEDs were achieved. Importantly, these devices displayed the feature of low roll-off of efficiencies. At the luminance of 1000 cd m−2, roll-off of current efficiencies was 1.3% for the yellow device and 14.6% for the red device.

A bipolar iridium complex, (ppy)2Ir(dipig), based on the ancillary ligand N,N′-diisopropyl-diisopropyl-guanidinate (dipig) with well-known cyclometalated (C^N) ligand ortho-(2-pyridyl)phenyl (ppy), is applicable in phosphorescent organic light-emitting diodes (PHOLEDs) as an efficient emitter, using easily available host materials and a simple device fabrication process. The corresponding PHOLEDs are dominated by an efficient direct-exciton-formation mechanism and show very high EL efficiency together with gratifying host- and doping-concentration-independent features. EL efficiency values of more than 93 lm W−1 for power efficiency (ηp) and 24% for external quantum efficiency (ηext) accompanied by little efficiency roll-off at high luminance are achieved in the (ppy)2Ir(dipig)-based devices by adopting the common materials 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as the host, with rather random concentration ranges of 8–15 wt% and 15–30 wt%, respectively. To the best of our knowledge, these values are the highest efficiencies ever reported for yellow PHOLEDs, and are even comparable with the highest levels for PHOLEDs in the scientific literature. Moreover, the ηp and ηext values of the non-doped device can reach 70 lm W−1 and 18% respectively. They are almost two times higher than those of the most efficient reported PHOLEDs based on a neat emitting layer (EML).

Two new homologous phosphorescent iridium complexes, bis-(2-phenylpyridine)(2-(2′-hydroxyphenyl)-2-oxazoline)iridium(III) [(ppy)2Ir(oz)] (1) and bis-(2-phenylpyridine)(2-(2′-hydroxyphenyl)-2-thiazoline)iridium(III) [(ppy)2Ir(thoz)] (2), have been obtained in good yields and characterized by single-crystal X-ray diffraction, cyclic voltammetry, photoluminescence and electroluminescence studies, and by time-dependent density functional theory (TD-DFT) calculations. Using the two complexes, which differ only by the heteroatom (O or S) substitution at the same site in the ancillary ligand, as the emitter, doped in a 4,4′-bis(N-carbazolyl)biphenyl (CBP) host, gave phosphorescent organic light-emitting diodes (PhOLEDs) with very efficient green and yellow emission, respectively. The turn-on voltages for both devices are low (3.5–3.7 V). The green-emitting (ppy)2Ir(oz) – based device has a maximum brightness of 61560 cd m−2 (at 16 V); maximum luminance efficiency of 66.2 cd A−1, 17.1% external quantum efficiency, 54 lm W−1 power efficiency and CIE coordinates of (0.35, 0.61) at a brightness of 10000 cd m−2. For the yellow-emitting (ppy)2Ir(thoz)-based device with a wide full spectral width at half maximum (FWHM) of 110 nm, the corresponding values are 21350 cd m−2 (at 14.5 V); 27.0 cd A−1, 8.5%, 18.0 lm W−1 and CIE coordinates of (0.46, 0.50). Colour tuning is primarily a consequence of the significantly wider emission bandwidth of complex 2 compared to complex 1.

•Highly efficient PHOLEDs based on a bipolar iridium emitter have been demonstrated.•PHOLEDs show gratifying concentration-insensitive and low-driving-voltage feature.•EQE values maintain at more than 20% in very wide luminance range of 1–15,000 cd m−2.A series of simplified trilayer phosphorescent organic light-emitting diodes (PHOLEDs) with high efficiency and little efficiency roll-off based on a bipolar iridium emitter Iridium(III) bis(2-phenylpyridinato)-N,N′-diisopropyl-diisopropyl-guanidinate (ppy)2Ir(dipig) has been demonstrated. They are dominated by the efficient direct-exciton-formation mechanism and show gratifying concentration-insensitive and low-driving-voltage features. In particular, very high and stable electroluminescence (EL) efficiencies (maximum power efficiency and external quantum efficiency >98 lm W−1 and 25% respectively, and external quantum efficiency >20% over a wide luminance range of 1–15,000 cd m−2) are achieved in the PHOLEDs based on emitting layers (EMLs) consisting of (ppy)2Ir(dipig) codeposited with common host CBP in an easily controlled doping concentration range (15–30 wt%). The EL performance of the PHOLEDs is comparable to the highest PHOLEDs reported in scientific literature.Graphical abstract

Organic magneto-electroluminescence (MEL) based on the charge-transfer (CT) states was investigated to clarify the electron–hole (e–h) pair mechanism for the organic magnetic field effects. The CT state is an ideal object because its emission is a direct intermolecular recombination process without forming intramolecular exciton. We found that the MEL of the CT states is not only greater than that of the exciton, but also exhibits almost no high-field decrease at low temperatures. Our results directly prove the e–h pair mechanism. Meanwhile, the transient electroluminescence measurements with and without magnetic fields confirm that magnetic field has no effect on the charge mobility but on the charge recombination process, implying the charge mobility-related mechanisms may be less dominant above the turn-on voltage.Graphical abstractHighlights► We report magneto-electroluminescence (MEL) of NPB:d(ppy)BF charge-transfer state. ► The MEL of NPB:d(ppy)BF CT state is larger than that of NPB exciton. ► The MEL of NPB:d(ppy)BF CT state does not change with injection current. ► The MEL of NPB:d(ppy)BF CT state exhibits no high-field decay at low temperature. ► Transient EL measurements revealed charge mobility was not changed by magnetic field.

A high-efficiency and pure white OLED has been realized by only doping one novel phosphorescent orange-light-emitting complex (bzq)2Ir(dipba) into a suitable deep-blue-emitting fluorescent complex Bepp2 as an emissive layer. The highest efficiency for white OLEDs with a simple HTL-EML-ETL architecture, with a peak power efficiency (PE) of 48.8 lm W−1 and a peak external quantum efficiency (EQE) of 27.8%, has been realized by employing both singlet and triplet excitons for emission. The PE and EQE at the applicable brightness of 1000 cd m−2 are 37.5 lm W−1 and 36.8%, respectively.

A phosphorescent material (Fppy)2Ir(dipba) possessing high PL efficiency in the solid state and superior hole/electron transporting property has acted successfully as an efficient neat phosphorescent emitter as well as an excellent host for high-performance PHOLEDs.

This paper reports series of highly efficient organic light-emitting diodes (OLEDs) with high-quality blue emission that employs an aromatic enyne derivative (E)-CPEY doped in the CBP (4, 4′-N, N′-dicarbazolylbiphenyl) host with the easily controlled doping concentrations as the emitters. The peak EL efficiencies (>8.5 cd A−1, >5.5 lm W−1 and ⩾6.0%) of these blue OLEDs with the stable CIE (0.15, 0.10 ± 0.01) are the highest values ever reported for the saturated deep-blue OLEDs up to now. Moreover, although the doping concentration of them changes such large span as 10–20 wt%, the deviation for both the EL efficiency and the brightness always keep within a very small range during almost the whole driving process, which should lead to the easy and reproducible fabrication process of high performance blue OLEDs.Graphical abstractHighly efficient blue OLEDs employing an aromatic enyne derivative (E)-CPEY doped in the CBP host with the easily controlled doping concentrations as the emitters, are developed. The resulting EL devices with the pure and stable blue light (0.15, 0.10 ± 0.01) not only show relatively insensitive EL performances to the doping concentration changing in a high and wide range of 10 (for device II)-20 (for device III) wt%, but also exhibit the highest EL efficiencies (>8.5 cd A−1, >5.5 lm W−1 and ⩾6.0%) ever reported for the saturated deep-blue OLEDs up to now.Highlights► OLEDs based on aromatic enyne compound exhibit pure blue emission. ► The blue OLEDs show high EL efficiency in wide doping-concentration range. ► The highest EL efficiency ever reported for the saturated deep-blue OLEDs are presented.

Efficient blue organic light-emitting diodes have been developed based on one novel fluorescent beryllium complex bis(2-(2-hydroxyphenyl)-4-methyl-pyridine)beryllium (Be(4-mpp)2). The simple double-layer device based on Be(4-mpp)2 as the EML as well as the ETL not only shows pure and stable blue emission with the CIE coordinates of (0.14, 0.09), but also presents very high EL efficiency in terms of both the peak values (5.4% for EQE and 4.2 lm W−1 for PE) and the EQE value remaining ⩾4.0% in very wide brightness range (10–10,000 cd m−2) that indicates very good operational stability. They are the highest EL efficiencies ever reported for such saturated and stable OLED (CIE: x < 0.15, y < 0.10) to the best of our knowledge.Graphical abstractEfficient blue organic light-emitting diodes have been developed based on one novel fluorescent beryllium complex bis(2-(2-hydroxyphenyl)-4-methyl-pyridine)beryllium (Be(4-mpp)2). The simple double-layer device based on Be(4-mpp)2 as the EML as well as the ETL not only shows pure and stable blue emission with the CIE coordinates of (0.14, 0.09), but also presents very high EL efficiency in terms of both the peak values (5.4% for EQE and 4.2 lm W−1 for PE) and the EQE value remaining ⩾4.0% in very wide the brightness range (10–10,000 cd m−2) that indicates very good operational stability. These values mentioned above are the highest EL efficiencies ever reported for such saturated deep-blue OLED to the best of our knowledge.Highlights► A novel fluorescent blue-emission complex Be(4-mpp)2 was developed. ► Be(4-mpp)2-based device exhibited deep-blue EL with the stable CIE (0.14, 0.09). ► The non-doped blue OLED showed very high EL efficiency of 5.4% and 4.2 lm W−1.

This paper reports a highly efficient organic light-emitting diode (OLED) with high-quality white emission that employs a deep blue-emitting beryllium complex bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp2) doped with a wide-bandwidth orange-emitting fluorescent dye 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4-H-pyran (DCM) through incomplete energy transfer from the blue host to the orange dopant. The two-component WOLED exhibits ideal and stable CIE coordinates (0.334 ± 0.002, 0.337 ± 0.007) with a high color rendering index equal to 79–81 upon variation in brightness from 10 to 10,000 cd m−2. As an all-fluorophore-doped white OLED, it yields very high EL efficiencies with the peak luminescence efficiency. The peak value of luminescence efficiency (LE), power efficiency (PE), and external quantum efficiency (EQE) is 14.0 ± 0.35 cd A−1, 9.2 ± 0.25 lm W−1, and 5.6 ± 0.15%, respectively.Graphical abstractResearch highlights► WOLED uses two commercial fluorescent materials as the emitting materials. ► Two-component WOLED yields stable and ideal white CIE coordinates. ► All-fluorophore-doped WOLED presents very high EL performance (LE, PE and EQE).

Four phenanthroimidazole derivatives (a: 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole; b: 2-phenyl-1-p-tolyl-1H-phenanthro[9,10-d]imidazole; c: 1-phenyl-2-p-tolyl-1H-phenanthro[9,10-d]imidazole; d: 1,2-di-p-tolyl-1H-phenanthro[9,10-d]imidazole) were synthesized and their single crystal structures, photophysical, electrochemical and mobility properties were carefully studied. Taking advantage of the thermal stability and the hole transporting (HT) ability, the highly efficient Alq3-based organic light-emitting diodes (OLEDs) have been achieved by employing the compounds a–d as a functional layer between NPB (4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl) and Alq3 (tris(8-hydroxyquinoline)aluminium) layers. For the device of [ITO/NPB/d/Alq3/LiF/Al], a maximum luminous efficiency (LE) of 8.1 cd A−1 was obtained with a maximum brightness of 65130 cd m−2, which exhibited much higher efficiency compared to the device with structure of [ITO/NPB/Alq3/LiF/Al]. The results demonstrated not only an alternative idea to design novel HT materials, but also a convenient way to improve the performance of the NPB/Alq3-based devices by introduction of a suitable organic buffer layer.

Highly efficient organic light-emitting diode have been developed by using a fluorinated quinacridone derivative: N,N′-di(n-butyl)-2,9-difluoroquinacridone doped in Alq3 as the emitting layer. The device shows much lower driving-voltage (the turn-on voltage is 2.5 V, the practical brightness of 100 and 1000 cd m−2 are realized at 3.2 and 4.2 V, respectively) and higher power efficiency (the peak value of 15.2 lm W−1 is obtained at 4 V and 598 cd m−2) than that of devices based on all the other QA derivatives. The pronounced EL performance enhancement might be attributed to the improved electron injection induced by introducing the strong electron-withdrawing fluorine atoms into the QA derivative.

Highly efficient, low driving-voltage, red phosphorescent OLEDs with a wide range of doping concentrations or even without doping are successfully fabricated by use of an easily available amidinate-ligated iridium complex.

Two new deep-blue iridium(III) complexes, (dfpypy)2IrFptz (Ir1) and (Medfpypy)2IrFptz (Ir2), comprising difluoro-bipyridyl (dfpypy) derivatives as cyclometaling ligands and a chelated pyridyl-triazole (Fptz) ancillary ligand are reported. The bipyridyl ligands lead to a significantly increased HOMO–LUMO gap and a hypsochromic shift of the phosphorescence compared to phenylpyridyl analogs. Density function theory (DFT) calculations and electrochemical measurements for Ir1 and Ir2 support their genuine blue phosphorescent emission. The combination of ancillary and cyclometalating ligands in Ir1 and Ir2 significantly influences the molecular orbitals of both complexes, leading to clearly distinct electron density distributions of the HOMO and LUMO compared with other blue-emitting Ir(III) derivatives. Both complexes Ir1 and Ir2 show deep-blue emission with λmax values in the region of 435–465 nm with high PLQYs and short excited-state lifetimes. The phosphorescent organic light emitting diodes (PhOLEDs) based on Ir1 and Ir2 achieve remarkably high EL performance with low efficiency roll-off at high luminance. The bluest color (CIEx,y 0.14, 0.11) and the highest EL efficiency were achieved in the device based on Ir2 (Device 2), where the peak EQE/PE of 13.0%/11.2 lm W−1 together with the corresponding values of 12.6%/8.8 lm W−1 and 10.1%/5.0 lm W−1 at the practical luminances of 100 and 1000 cd m−2 respectively, strongly compete with those of any deep-blue fluorescent and/or phosphorescent OLEDs with similar CIE coordinates previously reported.

Highly efficient, low driving-voltage, red phosphorescent OLEDs with a wide range of doping concentrations or even without doping are successfully fabricated by use of an easily available amidinate-ligated iridium complex.

A novel organosilane compound, bis(4-(1-phenylphenanthro[9,10-d]imidazol-2-yl)phenyl)diphenylsilane (Si(PPI)2), has been designed and synthesized. It has a high thermal decomposition temperature of 528 °C and is able to form an amorphous glass with a high glass-transition temperature of 178 °C. In addition, it possesses high singlet and triplet energies and displays efficient energy transfer to the selected blue fluorescent and green and red phosphorescent dopants when used as a host material. Electrochemical measurements and single-carrier devices indicate that Si(PPI)2 is a bipolar transport material, allowing the injection and transport of both electrons and holes. By using (Si(PPI)2) as a host, high-performance fluorescent blue (FB) and phosphorescent green (PG) and red (PR) OLEDs with a uniform and simple device configuration have been achieved. These OLEDs exhibit very high peak external quantum efficiency (EQE) and peak power efficiency (PE), i.e. 6.1% and 8.0 lm W−1 for FB, 19.2% and 51.1 lm W−1 for PG and 12.0% and 15.6 lm W−1 for PR. Moreover, the high-level EQE of 4.0, 19.1 and 10.6% and PE of 3.0, 41.6 and 7.5 lm W−1 can be maintained by FB, PG and PR, respectively, at the practical luminance of 100 cd m−2. Furthermore, the emission colors of these OLEDs remain almost unchanged within the whole range of driving voltages. Importantly, the blue OLED displays a pure blue emission (CIE: 0.18, 0.17).

A phosphorescent material (Fppy)2Ir(dipba) possessing high PL efficiency in the solid state and superior hole/electron transporting property has acted successfully as an efficient neat phosphorescent emitter as well as an excellent host for high-performance PHOLEDs.

Two new indolo[3,2-b]carbazole derivatives (DPDT-ICZ and DNDT-ICZ) with multifunctionality were designed and synthesized. They were employed as deep-blue emitters, hole-transporting materials and hosts to fabricate organic light-emitting devices (OLEDs). The devices which used them as emitters displayed deep-blue emissions with CIE coordinates of (0.15, 0.08). They have been employed as hole-transporting and host material simultaneously to construct high performance yellow and red phosphorescent OLEDs. High power efficiencies (78.3 lm W−1 for yellow devices and 20.4 lm W−1 for red devices) for phosphorescent OLEDs were achieved. Importantly, these devices displayed the feature of low roll-off of efficiencies. At the luminance of 1000 cd m−2, roll-off of current efficiencies was 1.3% for the yellow device and 14.6% for the red device.

Achieving high power efficiencies at high-brightness levels is still an important issue for organic light-emitting diodes (OLEDs) based on the thermally activated delayed fluorescence (TADF) mechanism. Herein, enhanced electroluminescence efficiencies were achieved in fluorescent OLEDs using a TADF molecule, (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), as a host and quinacridone derivatives (QA) as fluorescent dopants.

A high-efficiency and pure white OLED has been realized by only doping one novel phosphorescent orange-light-emitting complex (bzq)2Ir(dipba) into a suitable deep-blue-emitting fluorescent complex Bepp2 as an emissive layer. The highest efficiency for white OLEDs with a simple HTL-EML-ETL architecture, with a peak power efficiency (PE) of 48.8 lm W−1 and a peak external quantum efficiency (EQE) of 27.8%, has been realized by employing both singlet and triplet excitons for emission. The PE and EQE at the applicable brightness of 1000 cd m−2 are 37.5 lm W−1 and 36.8%, respectively.

A novel phosphorescent host FPYPCA possessing the bipolar charge transporting ability realizes the most efficient deep-red PhOLED, which maintains very high-level EQEs of >23% at rather a high and wide luminance range of 1000–10000 cd m−2.

In this work, two novel hybrids of an electron-accepting phosphine oxide moiety attached to a phenanthroimidazole have been designed and synthesized. The PO group is used as a point of saturation between the PPI moiety and the outer phenyl groups, so the high triplet energy of PPI is preserved to act as a host for red and green phosphorescent dopants. The strong intermolecular interactions and steric effect of the diphenylphosphine oxide (DPO) moiety endows the films with high quantum yields in the deep-blue emission region. Compared to PPI, the carrier (hole- and electron-)injection/transport properties were greatly promoted by the appended DPO group according to single-carrier device measurement. Besides, the morphological and thermal stabilities were also improved. The multiple functions enable adaptation of several simplified device configurations. The undoped deep-blue fluorescent device exhibits an external quantum efficiency of 2.24% with CIE (0.16, 0.08), very close to the NTSC blue standard CIE (0.14, 0.08). High performance for green (65.4 cd A−1, 73.3 lm W−1 and 18.0%) and red (19.0 cd A−1, 21.3 lm W−1 and 13.5%) phosphorescent devices used as hosts have been achieved. The experimental and theoretical relationships between the molecular structures and the optoelectronic properties are discussed.

Two deep blue emitting materials PPI-PPITPA and PPI-PPIPCz with dual carrier transport properties and small singlet–triplet splitting features are designed and synthesized. PPI-PPITPA and PPI-PPIPCz were used not only as non-doped emitting layers to fabricate highly efficient deep blue OLEDs, but also as hosts to construct high performance green, yellow and red phosphorescent OLEDs.

A bipolar iridium complex, (ppy)2Ir(dipig), based on the ancillary ligand N,N′-diisopropyl-diisopropyl-guanidinate (dipig) with well-known cyclometalated (C^N) ligand ortho-(2-pyridyl)phenyl (ppy), is applicable in phosphorescent organic light-emitting diodes (PHOLEDs) as an efficient emitter, using easily available host materials and a simple device fabrication process. The corresponding PHOLEDs are dominated by an efficient direct-exciton-formation mechanism and show very high EL efficiency together with gratifying host- and doping-concentration-independent features. EL efficiency values of more than 93 lm W−1 for power efficiency (ηp) and 24% for external quantum efficiency (ηext) accompanied by little efficiency roll-off at high luminance are achieved in the (ppy)2Ir(dipig)-based devices by adopting the common materials 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as the host, with rather random concentration ranges of 8–15 wt% and 15–30 wt%, respectively. To the best of our knowledge, these values are the highest efficiencies ever reported for yellow PHOLEDs, and are even comparable with the highest levels for PHOLEDs in the scientific literature. Moreover, the ηp and ηext values of the non-doped device can reach 70 lm W−1 and 18% respectively. They are almost two times higher than those of the most efficient reported PHOLEDs based on a neat emitting layer (EML).

A novel phosphorescent emitter BZQPG possessing bipolar charge transporting ability realizes the most efficient orange-red electroluminescence reported to date with the power efficiency of 77.1 lm W−1 and external quantum efficiency of 27.3% together with low efficiency roll-off (>50 lm W−1 and 26% at 5000 cd m−2).

Two new homologous phosphorescent iridium complexes, bis-(2-phenylpyridine)(2-(2′-hydroxyphenyl)-2-oxazoline)iridium(III) [(ppy)2Ir(oz)] (1) and bis-(2-phenylpyridine)(2-(2′-hydroxyphenyl)-2-thiazoline)iridium(III) [(ppy)2Ir(thoz)] (2), have been obtained in good yields and characterized by single-crystal X-ray diffraction, cyclic voltammetry, photoluminescence and electroluminescence studies, and by time-dependent density functional theory (TD-DFT) calculations. Using the two complexes, which differ only by the heteroatom (O or S) substitution at the same site in the ancillary ligand, as the emitter, doped in a 4,4′-bis(N-carbazolyl)biphenyl (CBP) host, gave phosphorescent organic light-emitting diodes (PhOLEDs) with very efficient green and yellow emission, respectively. The turn-on voltages for both devices are low (3.5–3.7 V). The green-emitting (ppy)2Ir(oz) – based device has a maximum brightness of 61560 cd m−2 (at 16 V); maximum luminance efficiency of 66.2 cd A−1, 17.1% external quantum efficiency, 54 lm W−1 power efficiency and CIE coordinates of (0.35, 0.61) at a brightness of 10000 cd m−2. For the yellow-emitting (ppy)2Ir(thoz)-based device with a wide full spectral width at half maximum (FWHM) of 110 nm, the corresponding values are 21350 cd m−2 (at 14.5 V); 27.0 cd A−1, 8.5%, 18.0 lm W−1 and CIE coordinates of (0.46, 0.50). Colour tuning is primarily a consequence of the significantly wider emission bandwidth of complex 2 compared to complex 1.