Search

The electron injection material PF-EP was found to be thermo-cleavable. Its solubility in chlorobenzene can be adjusted by thermal treatment at different temperatures. By analyzing the results of TGA, FT-IR, and 13C solid-state NMR, we interpret that the solubility transition is caused by partial acidification, crosslinking through a hydrogen-bonded network and coordination of O, P and Li. Based on this thermo-cleavable approach, efficient fully solution-processed IBPLEDs were successfully fabricated. The maximum current efficiency of the device with 2.5% Li2CO3 doped PF-EP as the EIL reaches nearly 1.7 times of that of a conventional device. We attribute the high performance to the good electron injection and hole blocking abilities of PF-EP and Li2CO3.

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.

Two new hole-transporting materials, namely HFB-Cz and HFB-Dpa, were designed and synthesized by attaching carbazole and diphenylamine units to the hexakis(9,9-dihexyl-9H-fluoren-2-yl)benzene (HFB) core via Buchwald–Hartwig coupling reaction. The long alkyl chain and core rigidity endow these compounds with good solution processability and high thermal stability. HFB-Cz and HFB-Dpa exhibit significantly high glass transition temperatures (225 and 154 °C) relative to widely used hole-transporting materials, such as N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4, 4′-diamine (TPD, 65 °C) and 1,4-bis((1-naphthylphenyl)amino)biphenyl (NPB, 96 °C). Solution-processed green OLED devices using HFB-Cz and HFB-Dpa as hole-transporting materials exhibit very high efficiencies with a maximum current efficiency up to 6.2 cd A−1. These efficiencies are substantially higher than the NPB-based control device, and are among the highest for the hole-transporting materials in similar device configuration.

A new triphenylamine-based homoleptic iridium(III) complex is designed and synthesized by simply altering the ligation positions of the triphenylamine units. The theoretical calculations reveal that the difference in the ligation position has a significant influence on the optical and electronic properties of the complexes. Through dispersing the green phosphor G-Ir into PVK in the presence of an electron-transport material, 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD), the green phosphorescent device achieves maximum current/power/external quantum efficiencies of 43.8 cd A−1/20.5 lm W−1/15.1%. The new green phosphor and its counterparts OO-Ir and R-Ir have been demonstrated as active components for white polymer light-emitting diodes (WPLEDs). A single-layer white light-emitting device is fabricated by doping the sky-blue emitter iridium(III) bis(2-(4,6-difluorophenyl)-pyridinato-N,C2)picolinate (FIrpic), G-Ir, OO-Ir and R-Ir into a general polymer matrix, with the maximum current/power/external quantum efficiencies of 23.5 cd A−1/12.0 lm W−1/8.6%, which are comparable with the best results obtained from the conventional RGB primary color system under the same device structure.

A solution-processable host molecule TPO comprised of hole-transporting triphenylamine and electron-transporting oxadiazole has been synthesized. Through meta-linkage between the donor and the acceptor, complete charge localization of the HOMO and LUMO and proper triplet energy are imparted. The new compound shows good thermal stability with a high glass-transition temperature of 131 °C. Smooth and homogeneous film can be obtained by spin-coating from a TPO/iridium complex blend as probed by atomic force microscopy. The solution-processed red phosphorescent organic light-emitting device (PhOLED) achieves a maximum current efficiency of 13.3 cd A−1 with Commission Internationale de l'Eclairage coordinates of (0.64, 0.36); while the green device reaches a maximum current efficiency of 40.8 cd A−1, and the value is still as high as 39.6 cd A−1 at a practical luminance of 1000 cd m−2. The low roll-off can be attributed to the bipolar nature of the host material TPO. An optimized device further elevates the efficiency to 56.8 cd A−1, which is among the highest ever reported for small-molecule based green PhOLEDs fabricated by a wet process.

A series of carbazole-containing tetraarylsilane compounds, namely p-BISiPCz (1), m-BISiPCz (2), p-OXDSiPCz (3) and m-OXDSiPCz (4) were designed and synthesized by incorporating electron-donating carbazole 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 di-para-position compounds 1 and 3 display higher glass transition temperatures and slightly lower triplet energies than their di-meta-position isomers 2 and 4, respectively. The four compounds exhibit similar HOMO levels (5.60–5.63 eV), while the LUMO level of 3 (2.36 eV) is slightly lower than that of 4 (2.28 eV). The silicon-interrupted conjugation of the electron-donating and electron-accepting segments endows these materials with relative high triplet energies, good thermal and morphological stability, and bipolar transporting ability. For FIrpic-based blue PhOLEDs, the di-meta-position compounds 2 and 4 display better device performances than their di-para-position analogues 1 and 3, respectively. Device B using 2 as the host exhibits the best performance with a maximum current efficiency of 29.3 cd A−1, a maximum power efficiency of 19.8 lm W−1, and a maximum external quantum efficiency of 11.4%. Green phosphorescent devices using (ppy)2Ir(acac) as guest and 1–4 as hosts show excellent EL performances with maximum external quantum efficiencies of 18.3–22.2%. Remarkably, device H hosted by 4 still exhibits an external quantum efficiency of 19.4% at the extremely high luminance of 10 000 cd m−2. These efficiencies are significantly higher than those of blue and green control devices using mCP as host, respectively.

Four blue fluorescent host emitters with bifunctional charge transport groups appended to the 9- and 10-positions of the anthracene core have been designed and synthesized. By introducing peripheral bulky aryl-substitution groups to the emissive core, the four compounds show a decreased tendency to crystallise and have high glass transition temperatures ranging from 154 to 226 °C. The theoretical calculations reveal that the four self-hosted blue emitters possess noncoplanar structures to suppress intermolecular interaction within the films. The amorphous compounds exhibit strong deep-blue emission both in solution and the solid state. With different end-capping groups, the photophysical and electrochemical properties are tuned to produce efficient deep-blue performance with a simple device architecture. Organic light-emitting diodes (OLEDs) featuring 4 as the emitter achieve a maximum power efficiency of 2.0 lm W−1 with Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.10) that are very close to the National Television Standards Committee's blue standard. The well-matched energy level between the anode and 4 as well as the intrinsic good charge transport abilities results in a very low driving voltage (2.7 V), making the nondoped deep-blue electroluminescent device power efficient.

Two bridged triphenylamine/fluorene hybrids, BTPAF1 and BTPAF2, were designed and synthesized through Friedel–Crafts reaction. Their thermal, electrochemical, electronic absorption and photoluminescent properties were fully investigated. Very high glass transition temperatures (Tg) were observed at 204 °C for BTPAF1 and 211 °C for BTPAF2, owing to the introduction of rigid fluorene and bridged triphenylamine unit. The encapsulation of a fluorene unit at the para positions of bridged triphenylamine greatly enhances their electrochemical stability. The linkage by the quaternary carbon atom of the fluorene moiety (C-9) effectively prevents the extension of π-conjugation of the bridged triphenylamine core, and consequently means that the compounds have a high triplet energy of 2.86 eV. Phosphorescent organic light-emitting devices (PHOLEDs) fabricated by using the two compounds as the hosts and the blue emitter bis[2-(4′,6′-difluorophenyl)pyridinato-N,C2′]iridium(III) picolate (FIrpic) as the guest exhibit good EL performances with a maximum current efficiency of 20 cd A−1, a maximum power efficiency of 14 lm W−1, and a maximum external quantum efficiency of 9.4%. Green electrophosphorescent devices by using green-emitter iridium(III) fac-tris(2-phenylpyridine) [Ir(ppy)3] as guest and the two new compounds as the hosts display excellent EL performances with a maximum current efficiency of 75 cd A−1, a maximum power efficiency of 60 lm W−1, and a maximum external quantum efficiency of 19.5%. The device figures of merit, together with the excellent morphological and electrochemical stabilities, make the new compounds ideal host materials for PHOLEDs, especially for high-temperature applications of devices.

A novel compound (BCBP) based on the modification of a well-known host material 4,4′-(bis(9-carbazolyl))biphenyl (CBP) through arylmethylene bridge linkage was synthesized, and fully characterized. Its thermal, electrochemical, electronic absorption and photoluminescent properties were studied. A high glass transition temperature (Tg) of 173 °C is observed for BCBP due to the introduction of the bridged structure, remarkably contrasting with a low Tg of 62 °C for CBP. Furthermore, the bridged structure enhances the conjugation and raises the HOMO energy, thus facilitating hole-injection and leading to a low turn-on voltage in an electroluminescent device. With the device structure of ITO/MoO3/NPB/Ir complex: BCBP/BCP/Alq3/LiF/Al, maximum power efficiencies of 41.3 lm/W and 6.3 lm/W for green- and blue-emitting OLED were achieved, respectively.

A bipolar transport compound, 2,5-bis(4-(9-(2-ethylhexyl)-9H-carbazol-3-yl)phenyl)-1,3,4-oxadiazole (CzOXD), incorporating both electron- and hole-transport functionalities, was synthesized and fully characterized by 1H NMR, 13C NMR, elemental analysis and mass spectrometry. Its thermal, electrochemical, electronic absorption and photoluminescent properties were studied. The atomic force microscopy images indicate that smooth and homogeneous films can be obtained by spin-coating from a chloroform solution of CzOXD/iridium complex blends. Highly efficient small-molecule-based organic light emitting devices were fabricated by a wet process. With the device structure of ITO/PEDOT:PSS/Ir complex : CzOXD/BCP/Alq3/LiF/Al, a maximum luminance of 15232 cd m−2 and current efficiency of 20 cd A−1 for a yellow-emitting OLED, and 4896 cd m−2, 4.6 cd A−1 for a red-emitting OLED were achieved under ambient conditions.

A series of novel fluorene-alt-carbazole or fluorene-based copolymers have been synthesized by Suzuki polycondensation. The iridium complexes with the cyclometalated ligand 2-p-tolyl-benzothiazole were incorporated into the copolymers by either embedding or end-capping into the backbone via an ancillary β-diketonate ligand. The copolymers were characterized by 1H NMR, 13C NMR, and GPC. The TGA/DSC measurements indicate that the copolymers with carbazole units in the main chain show better thermal stability than those with only fluorene units. Electrochemical investigations reveal that the HOMO and LUMO energy levels of the monomeric iridium complexes fall within those of the parent polymers. The absorption spectra of the polymers are mostly characteristic of the polymer backbone. The PL spectra of the polymers show phosphorescent emission at ca. 560 nm with a shoulder at ca. 594 nm, and their PL decay measurements indicate that the end-capped polymerP4 has a longer triplet lifetime with a mono-exponential mode than those of the embedded polymersP1–3 with biexponential modes. Light-emitting diodes using the copolymers as the emitting layer under different device configurations were fabricated. For P1–3, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) doping in copolymers shows remarkably enhanced efficiencies, whereas for P4, the introduction of a PVK layer significantly improves the device performance. Noticeably, the devices using P4 as the emitting layer display significantly higher efficiencies than those based on P1–3, which is mainly attributed to the fact that P4 suffers much less from triplet exciton back transfer from the iridium complex to the polymer backbone than P1–3 do. An orange–red-emitting PLED with an emission peak at 599 nm, a maximum external quantum efficiency of 2.19% at a current density (J) of 0.3 mA cm−2, and a maximum luminance of 2347 cd m−2 at 17 V was achieved from the device ITO/PEDOT/PVK/P4/Ba/Al.

A series of morphology-stable carbazole-based iridium(III) complexes with green to red emission have been prepared and characterized by elemental analysis, nuclear magnetic resonance, and mass spectroscopy. Their thermal, electrochemical, electronic absorption, and photoluminescent properties have been studied. Highly efficient polymer light-emitting devices by using these complexes as dopant emitters, both non-conjugated polymer (PVK) and conjugated polymer, polyhedral oligomeric silsesquioxane-terminated poly(9,9-dioctylfluorene) [PFO(poss)], as the host materials, have been achieved. With the device structure of ITO/PEDOT/(PFO(poss) + 30% PBD)–2 wt.% 1/Ba/Al, a maximum external quantum efficiency of 6.4% and a maximum luminous efficiency of 6.00 cd A−1 with red emission at 608 nm were obtained. With the device configuration of ITO/PEDOT/(PFO(poss) + 30% PBD)–4 wt.% 4/Ba/Al, a maximum external quantum efficiency of 9.9% and a maximum luminous efficiency of 22.4 cd A−1 with yellow–green emission at 544 nm were realized. The increased morphology stability of 1 and 2 imparted by the N-decyl long chains at the N atom of carbazole results in significantly better device performance than their short chain analogues 1a and 2a under identical device configurations.

A series of novel iridium(III) complexes with two 2-arylquinoline derivatives as cyclometalated ligands and one monoanionic ligand, such as acetylacetonate (acac), N,N′-diethyldithiocarbamate (Et2dtc) and O,O′-diethyldithiophosphate (Et2dtp), as ancillary ligands have been synthesized and structurally characterized by 1H NMR, MS and elemental analysis (EA). The cyclic voltammetry, absorption, emission and electroluminescence properties of these complexes were systematically investigated. Through extending π-conjugation, introducing electron-donating groups in the ligand frame, or changing the ancillary ligands, the HOMO energy levels of the iridium(III) complexes can be tuned, while their LUMO levels remain little affected; in consequence, the emission wavelengths of the iridium(III) complexes can be tuned in the range 606–653 nm. The highly efficient organic light-emitting diodes (OLEDs) with saturated red emission have been demonstrated. A maximum current efficiency of 10.79 cd A−1, at a current density of 0.74 mA cm−2, with an emission wavelength of 616 nm and Commisioon Internationale de L'Eclairage (CIE) coordinates of (0.65, 0.35), which are very close to the National Television System Comittee (NSTC) standard red emission, have been achieved when using complex (DPQ)2Ir(acac) as a phosphor dopant.

Five new inorganic–organic hybrid nanocomposite compounds were obtained by inserting stilbazolium chromophores into the interlamellar space of VOPO4. The intercalation compounds were characterized by elemental analysis, X-ray powder diffraction (XRD), infrared spectroscopy and X-ray photoelectron spectroscopy (XPS). The second-order nonlinear optical property of these intercalation compounds are briefly discussed.