Jiuyan Li

Highly Efficient Orange and White Organic Light-Emitting Diodes Based on New Orange Iridium Complexes

Highly Efficient Orange and White Organic Light-Emitting Diodes Based on New Orange Iridium Complexes

Deep-blue and white organic light-emitting diodes based on novel fluorene-cored derivatives with naphthylanthracene endcaps

Novel fluorene based deep-blue-emitting molecules with naphthylanthracene endcaps, namely 2,7-di(10-naphthylanthracene-9-yl)-9,9-dioctylfluorene (NAF1) and 7,7′-di(10-naphthylanthracene-9-yl)-9,9,9′,9′-tetraoctyl-2,2′-bifluorene (NAF2), are synthesized by a Suzuki cross-coupling reaction. These materials exhibit excellent thermal and amorphous stabilities, and high fluorescence quantum yield of over 70%. Organic light-emitting devices (OLEDs) using NAF1 or NAF2 as non-doped emitter exhibit bright deep blue electroluminescence with CIE coordinates of (0.15, 0.13) for NAF1, (0.16, 0.13) for NAF2. A maximum power efficiency of 2.2 lm W−1 (4.04 cd A−1, 4.04%) is achieved for NAF1, which is among the highest values ever reported for deep-blue fluorescent OLEDs. A further improved coordinates of (0.15, 0.09) with efficiencies of 3.56 cd A−1 and 2.10 lm W−1 are achieved for NAF1 upon tuning device thickness, which are also among the best data for non-doped deep blue fluorescent OLEDs with a CIE coordinate of y < 0.1. NAF1 serves as excellent host emitter when doped with an orange fluorophore (4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran, DCJTB). Upon careful tuning the doping level, the two-emitting-component (NAF1 : DCJTB) OLED realizes efficient white light emission with a power efficiency of 3.01 lm W−1 (7.66 cd A−1), a brightness of 12090 cd m−2, and a standard white light coordinates of (0.33, 0.33). This performance is among the best results ever reported for two-emitting-component white OLEDs based on fluorescent materials.

Homoleptic tris-cyclometalated iridium complexes with 2-phenylbenzothiazole ligands for highly efficient orange OLEDs

Homoleptic tris-cyclometalated iridium(III) complexes containing 2-phenylbenzothiazole derivatives as ligands have been successfully synthesized and characterized for the first time. Electron-donating (CH3, OCH3) and -withdrawing groups (F) were introduced into the 6-position of the benzothiazole moiety in the ligands to verify their influence on the optical and electronic properties of the complexes. Organic light-emitting diodes using these iridium complexes as doped emitters exhibited orange electrophosphorescence with excellent performances. An extremely high brightness of 95 800 cd m−2 and a maximum luminance efficiency of 87.9 cd A−1 (46.0 lm W−1) were achieved for the pristine complex without any substituent in the ligand. These performances represent a significant improvement for vacuum-deposited orange OLEDs and the new record of the efficiencies for orange OLEDs reported so far. The substituents in the ligand were observed to be rather unimportant to influence the performance of this series of iridium complexes.

Universal Host Materials for High-Efficiency Phosphorescent and Delayed-Fluorescence OLEDs

A series of bipolar hosts, namely, 5-(2-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (o-CzDPz), 5-(3-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (m-CzDPz), 5-(9-phenyl-9H-carbazol-3-yl)-1,3-dipyrazolbenzene (3-CzDPz), and 5-(3,5-di(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (mCPDPz), are developed for phosphorescent and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). They are designed by selecting pyrazole as n-type unit and carbazole as p-type one. The triplet energy (E(T)), the frontier molecular orbital level, and charge transporting abilities, are adjusted by varying the molar ratio of pyrazole to carbazole and the linking mode between them. They have high E(T) values of 2.76-3.02 eV. Their electroluminescence performance is evaluated by fabricating both phosphorescent and TADF devices with blue or green emitters. The m-CzDPz hosted blue phosphorescent OLEDs achieves high efficiency of 48.3 cd A(-1) (26.8%), the 3-CzDPz hosted green phosphorescent device exhibits 91.2 cd A(-1) (29.0%). The blue and green TADF devices with 3-CzDPz host also reach high efficiencies of 26.2 cd A(-1) (15.8%) and 41.1 cd A(-1) (13.3%), respectively. The excellent performance of all these OLEDs verifies that these pyrazole-based bipolar compounds are capable of being universal host materials for OLED application. The influence of molar ratio of n-type unit to p-type one and the molecular conformation of these hosts on their device performance is discussed and interpreted.

Iridium complexes containing 2-aryl-benzothiazole ligands: color tuning and application in high-performance organic light-emitting diodes

Three 2-aryl-benzothiazole chromophores were designed and synthesized for use as major cyclometalating ligands of iridium complexes, in which the aryl groups were N-phenyl-3-carbazolyl, 2-(9,9-dioctyl)fluorenyl and N-phenyl-2-carbazolyl. The homoleptic tris-cyclometalated and heteroleptic bis-cyclometalated iridium complexes, 1–5, were synthesized using these ligands. By adjusting the chemical structures and then the electronic state of these complexes, we were able to continuously tune the phosphorescence from yellow to saturated red with peak wavelengths in the order of 1 < 2 < 3 < 4 < 5. The quantum chemical calculations and the electrochemical data clearly demonstrate the origin of the phosphorescence color tuning. The organic light-emitting diodes (OLEDs) containing these iridium complexes as doped emitters exhibited yellow to red electrophosphorescence with excellent performance. Particularly, the complex 1 based device produced high efficiencies of 75.9 cd A−1, 48.2 lm W−1, and 23.0% with CIE (0.46, 0.53), which represent the highest efficiencies for yellow OLEDs up to now. Furthermore, 1 was used to fabricate two-element white OLEDs in combination with a blue phosphor and high efficiencies of 57.9 cd A−1 and 21.9% were achieved, which are among the best efficiencies for two-emitting-component white OLEDs reported so far.

Diphenylphosphorylpyridine-functionalized iridium complexes for high-efficiency monochromic and white organic light-emitting diodes

Four new cyclometalated iridium(III) complexes containing 2-aryl-5-diphenylphosphorylpyridine ligands, in which the aryl was difluorophenyl (Ir-1), phenyl (Ir-2), 4-(diphenylamino)phenyl (Ir-3) and 2-naphthyl (Ir-4), have been synthesized for application in organic light-emitting diodes (OLEDs). The incorporation of diphenylphosphoryl on the pyridine causes a decrease of the lowest unoccupied molecular orbital (LUMO) for the iridium complexes and the universal bathochromic shift by as much as 50 nm in their phosphorescence. In particular, the presence of the diphenylphosphoryl group on the 2-difluorophenylpyridine ligand of iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) redistributed the LUMO from the ancillary ligand entirely to the cyclometalating ligand. These complexes were used as doped emitters to fabricate OLEDs. The bluish-green device based on Ir-1 exhibited a maximum luminance efficiency of 43.6 cd A−1 (22.8 lm W−1, 9.5%). Highly efficient yellow devices were obtained with 51.6 cd A−1 (27 lm W−1, 14.5%) and 29.6 cd A−1 (15.5 lm W−1, 8.8%) for Ir-2 and Ir-3, respectively. The maximum luminance efficiency of 27.2 cd A−1 (15.5 lm W−1, 9.7%) was achieved for the orange-red device containing Ir-4. Furthermore, a two-emitting-component white OLED was fabricated with Ir-4 in combination with the traditional sky-blue FIrpic and a high efficiency of 23.9 cd A−1 (13.9 lm W−1, 5.4%) with CIE (0.29, 0.43) was realized.

A CBP derivative as bipolar host for performance enhancement in phosphorescent organic light-emitting diodes

N,N′-Dicarbazolyl-4,4′-biphenyl (CBP) is one of the most successful uni-polar host materials for phosphorescent organic light-emitting diodes (PhOLEDs). We report the synthesis and properties of one novel CBP derivative, CBP-CN, with two cyano groups (CN) at the 3-site of carbazole rings. The strong electron-withdrawing CN group was introduced with the expectation to promote electron-injecting/-transporting abilities and to achieve bipolar features for CBP-CN. In comparison with the parent CBP, CBP-CN possesses lowered HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels and dramatically increased Tg (glass transition temperature, 162 °C), but unaltered HOMO–LUMO band gap and triplet energy (2.69 eV). Green and red PhOLEDs were fabricated with CBP-CN as hosts for traditional iridium phosphors. The maximum luminance efficiency (ηL) of 80.61 cd A−1 (23.13%) was achieved for the green PhOLED, and 10.67 cd A−1 (15.54%) for the red one, which represent efficiency increases of 25–33% compared with those of the best devices with CBP host and are even among the best data for phosphorescent OLEDs reported so far. The theoretical calculation and the carrier-only devices investigation confirmed that the electron-injecting/-transporting character and the bipolar nature of CBP-CN should be responsible for the performance enhancements.

Novel ZnII complexes of 2-(2-hydroxyphenyl)benzothiazoles ligands: electroluminescence and application as host materials for phosphorescent organic light-emitting diodes

A group of novel zinc complexes containing 2-hydroxyphenylbenzothiazole (BTZ) ligands were designed and synthesized, in which different substituents (OCH3, CH3, F, CF3, COOCH2CH3) were attached at the 6-position of the benzothiazole ring in the BTZ ligands. Both photoluminescence (PL) and electroluminescence (EL) behaviors of these zinc complexes were investigated. The emission colors of these zinc complexes were readily tuned from bluish-green to yellow by simply varying the substituent, with strong electron-withdrawing substituents being favorable for longer-wavelength fluorescence. Efficient EL was obtained when these zinc complexes were used as non-doped emitting layers in organic light-emitting diodes (OLEDs). Furthermore, these zinc complexes were proved to be capable of acting as triplet hosts for iridium phosphor in red phosphorescent OLEDs. A high external quantum efficiency of 17.5% was realized for the red phosphorescent OLED with the present zinc complexes as hosts and tris(2-phenylisoquinoline)iridium as doped emitter, which is greatly enhanced compared to that (12.6%) of the device with the traditional 4,4′-bis(N-carbazoly)biphenyl (CBP) as host. The present study successfully exploited novel zinc complexes as electron-transporting host materials for phosphorescent OLEDs.

Simple Bipolar Host Materials for High-Efficiency Blue, Green, and White Phosphorescence OLEDs

3-(1H-Pyrazol-1-yl)pyridine is used as electron-transporting unit to construct bipolar host materials o-CzPyPz, m-CzPyPz, and p-CzPyPz for application in phosphorescent organic light-emitting diodes (PhOLEDs). By varying the ortho-, meta-, or para-linking mode between the n-type 3-(1H-pyrazol-1-yl)pyridine and the p-type carbazole on phenylene bridge, the optoelectronic parameters are tuned to large extent. The highly twisted o-CzPyPz has high triplet energy of 2.95 eV, while the isomer p-CzPyPz with more coplanar conformation has smaller triplet energy of 2.67 eV. The m-CzPyPz-hosted blue PhOLED exhibits a peak current efficiency of 49.1 cd A(-1) (corresponding to an external quantum efficiency of 24.5%) and low-efficiency roll-off, while the p-CzPyPz-hosted green PhOLEDs turns on at 2.8 V and exhibits high efficiencies of 91.8 cd A(-1) (96.1 lm W(-1) and 27.3%). Furthermore, two-emitting-layer white OLEDs are fabricated with m-CzPyPz or p-CzPyPz as common hosts for both blue and orange phosphors, which realize high efficiencies of 57.8 cd A(-1) (45.4 lm W(-1) and 23.6%) and 60.7 cd A(-1) (38.1 lm W(-1) and 23.1%). The optimization of host structure for good matching of host and dopant and finally for the ideal performance is discussed.

Bipolar host materials for high-efficiency blue phosphorescent and delayed-fluorescence OLEDs

A series of small molecular isomers, namely o-CzCN, m-CzCN, and p-CzCN, are developed for use as bipolar hosts in blue phosphorescent and fluorescent organic light-emitting diodes (OLEDs). Cyano (CN) substituted phenyl is selected as the n-type unit and N-phenyl-substituted carbazole as the p-type unit. By adjusting the ortho-, meta-, and para-linking styles of the functional units, the physical parameters are regularly tuned to a large extent. The study of complete spatial separation of frontier molecular orbitals and single-carrier devices confirm the bipolar feature. Blue phosphorescent and thermally activated delayed fluorescence (TADF) OLEDs were fabricated using iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN) as doped emitters. A maximum current efficiency of 46.81 cd A−1 and an external quantum efficiency of 23.14% were achieved for the phosphorescent OLED with the m-CzCN host. Furthermore, high efficiencies of 29.23 cd A−1 and 14.98% were obtained for the 2CzPN based blue TADF device with the o-CzCN host, which are higher than the best literature value of 13.6% for 2CzPN devices. Both m-CzCN and o-CzCN always perform better than p-CzCN. The influence of the chemical structures on their properties and performance is interpreted for these CN-decorated host materials.