Liduo Wang

H2O effect on the stability of organic thin-film field-effect transistors

Degradation of organic thin-film field-effect transistors (OTFTs) with pentacene as the active material has been studied. It was found that the field-effect mobility of the device decreased by 30% and the on/off current ratio decreased to one fifth after the OTFTs had been stored in atmosphere for 500 h. Through surface morphology analysis by atomic force microscopy and absorption analysis by infrared spectroscopy, it was found that the adsorption of H2O on the pentacene layer was the main reason for the degradation. Remarkable improvement in the device performance was achieved by device encapsulation with UV curable resin.

Tuning of Charge Balance in Bipolar Host Materials for Highly Efficient Solution-Processed Phosphorescent Devices

A novel bipolar host material, which meets the requirements of high triplet energy, good charge carrier transport properties, high solubility, and film-forming ability at the same time, has been designed and synthesized. Utilizing a new compound as host material, high-efficiency solution-processed blue and white phosphorescent organic light-emitting diodes (PHOLEDs) have been achieved.

Novel star-shaped host materials for highly efficient solution-processed phosphorescent organic light-emitting diodes

Two novel star-shaped host materials for solution processed blue phosphorescent organic light-emitting devices, 9-(5′,5‴-diphenyl[1,1′:3′,1″:3″,1‴:3‴,1⁗-quinquephenyl]-5″-diyl)-9H-carbazole (DQC) and 9,9′-(5′-phenyl[1,1′:3′,1″-terphenyl]-3,5-diyl)bis-9H-carbazole (PTC), were synthesized by the Suzuki coupling reaction. These compounds both exhibited high glass-transition temperatures (Tg ≥ 128 °C) and excellent film-forming ability. The nonplanar star-shaped configuration of DQC and PTC limited the effective extension of their π–conjugation, leading to the same triplet energy of 2.81 eV. The solution processed single layer devices using DQC and PTC as the host for the phosphorescence emitter iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) showed the maximum luminance efficiencies of 9.2 and 12.8 cd A−1, respectively. By introducing a thin 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI) electron-transporting and exciton-confining layer, the maximum efficiencies of the solution processed devices based on DQC and PTC were further improved to 21.7 and 25.7 cd A−1 with the maximum external quantum efficiencies up to 9.2% and 11.9%, respectively. Furthermore, the DQC- and PTC-based devices showed significantly high performance compared with the corresponding devices based on 1,3-bis(9-carbazolyl)benzene (mCP).

Low-voltage pentacene thin-film transistors with Ta2O5 gate insulators and their reversible light-induced threshold voltage shift

We have fabricated pentacene thin-film transistors using Ta2O5 films prepared by magnetron reactive sputtering as gate insulators. These transistors exhibit good electrical characteristics at an operating voltage as low as 5 V, with a field-effect mobility of 0.32cm2∕Vs, an on∕off ratio of 104, and a subthreshold slope of 0.5V∕decade. We have also investigated the optical properties of these transistors and observed a reversible light-induced threshold voltage shift. Under illumination, the threshold voltage shifts towards the positive direction while the field-effect mobility and on∕off ratio remain almost unchanged. In the dark, however, the threshold voltage can slowly be restored to its original state. At a gate voltage of −5V, the transistors show a broadband responsivity of 3.7A∕W after illumination at 60μW∕cm2 for 10 min.

Dependency of organic phototransistor properties on the dielectric layers

Organic phototransistors with pentacene semiconductor and Ta2O5 or polymethyl methacrylate (PMMA) dielectric layer have been investigated. It was found that the phototransistor properties strongly depend on the dielectric layer. Under a broadband light with 10mW∕cm2, the sensitivity of the Ta2O5 based transistor is much higher than that of the PMMA based transistor. For Ta2O5 based transistor, the photosensitivity (the ratio of photocurrent to dark current) and the threshold voltage shift are 4000 and 13.5V, respectively. While for PMMA based transistor, the corresponding values are only 0.5 and 2.9V, respectively. That large difference is attributed to the electron trapping ability of Ta2O5.

Synthesis, Characterization, and Photophysical and Electroluminescent Properties of Blue-Emitting Cationic Iridium(III) Complexes Bearing Nonconjugated Ligands

The development of pure-blue-to-deep-blue-emitting ionic phosphors is an ultimate challenge for full-color displays and white-light sources. Herein we report two series of short-wavelength light-emitting cationic iridium(III) complexes with nonconjugated ancillary and cyclometalating ligands, respectively. In the first series, nonconjugated 1-[(diphenylphosphino)methyl]-3-methylimidazolin-2-ylidene-C,C2 (dppmmi) is used as the ancillary ligand and 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dfppy), and 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz) are used as cyclometalating ligands. In the second one, nonconjugated 2,4-difluorobenzyl-N-pyrazole (dfbpz) is used as the cyclometalating ligand and 3-methyl-1-(2-pyridyl)benzimidazolin-2-ylidene-C,C(2) (pymbi) as the ancillary ligand. The synthesis and photophysical and electrochemical properties, together with the X-ray crystal structures of these complexes, have been investigated. At room temperature, blue-emitting complexes [Ir(ppy)2(dppmmi)]PF6 (1) and [Ir(dfppy)2(dppmmi)]PF6 (2; PF6(-) is hexafluorophosphate) show much larger photoluminescence quantum yields of 24% and 46%, respectively. On the contrary, for complexes [Ir(dfppz)2(dppmmi)]PF6 (3) and [Ir(dfbpz)2(pymbi)]PF6 (4), deep-blue luminescence is only observed at low temperature (77 K). Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon changes of the ligands. It is shown that the electronic transition dipoles of cationic iridium complexes 1 and 2 are mainly confined to cyclometalated ligands ((3)MLCT and LC (3)π-π*) and those of complex 3 are confined to all of the ligands ((3)MLCT, LC (3)π-π*, and (3)LLCT) because of the high LUMO energy level of dfppz. The emission of 4 mainly originates from the central iridium(III) ion and cyclometalated ligand to ancillary ligand charge transfer ((3)MLCT and (3)LLCT), in contrast to commonly designed cationic complexes using carbene-type ancillary ligands, where emission originates from the cyclometalated main ligands. Solution-processed organic light-emitting diodes based on complexes 1 and 2 gave blue-green (498 nm) and blue (478 nm) electroluminescence with maximum current efficiencies of 3.8 and 3.4 cd A(-1), respectively. The results indicate that introducing nonconjugated ligands into cationic iridium complexes is an effective means of achieving short-wavelength light-emitting phosphors.

High-efficiency orange to near-infrared emissions from bis-cyclometalated iridium complexes with phenyl-benzoquinoline isomers as ligands

We report the synthesis and characterization of three novel bis-cyclometalated iridium complexes with phenyl-benzoquinoline (pbq) analogs as ligands, namely, Ir(pbq-f)2acac, Ir(dpbq-f)2acac, and Ir(pbq-g)2acac, where pbq-f, dpbq-f and pbq-g representing 3-phenyl-benzo[f]quinoline, 1,3-diphenyl-benzo[f]quinoline, and 2-phenyl-benzo[g]quinoline, respectively. Interesting distinctions were observed in the electronic structures, photophysical and electroluminescent properties of these complexes. Ir(pbq-f)2acac and Ir(dpbq-f)2acac are orange-red emissive phosphors with strong metal–ligand charge transfer (3MLCT) emission bands centered at 577 and 604nm, respectively, while Ir(pbq-g)2acac shows the largest red-shift to near-infrared (NIR) region with a peak emission at 708nm and a shoulder around 780nm in solution. All the phosphors exhibit strong electrophosphorescence with negligible triplet-triplet annihilation due to quite short phosphorescent lifetimes (∼0.5µs) and high emission quantum yields. Orange-red emissive Ir(dpbq-f)2acac gives a maximum current efficiency of 17.4 cd/A and external quantum efficiency (ηext) of 10.5%. NIR emissive Ir(pbq-g)2acac shows a promising emission centered at 720nm with a shoulder above 780nm. Forward light output is 4.6 mW/cm2 at 13V and the maximum ηext is nearly 1.1%. Our study demonstrates that the constitutional isomers of cyclometallated ligand distinctly control the electronic structures and emissive properties of the corresponding Ir complexes and the obtained NIR emissive Ir(pbq-g)2acac implies the potential to realize highly efficient NIR OLEDs based on Ir(III) complexes.

Investigation of the spectra of phosphorescent organic light-emitting devices in relation to emission zone

The dependence of the electroluminescent spectra on the emission zone of blue electrophosphorscent light emitting diodes (PHOLEDs) was investigated. The light emission of a PHOLED was tuned from blue to greenish blue by adjusting the position of the emission zone in the PHOLED. Experimental results agreed well with the numerical simulation based on the effect of the wide-angle optical interference by the metal cathode. The comparison of the numerical results and the electroluminescent spectra of the PHOLED was then extended to serve as the basis of another method to determine the location of the emission zones of PHOLEDs.

Control of Intramolecular π–π Stacking Interaction in Cationic Iridium Complexes via Fluorination of Pendant Phenyl Rings

Intramolecular π-π stacking interaction in one kind of phosphorescent cationic iridium complexes has been controlled through fluorination of the pendant phenyl rings on the ancillary ligands. Two blue-green-emitting cationic iridium complexes, [Ir(ppy)(2)(F2phpzpy)]PF(6) (2) and [Ir(ppy)(2)(F5phpzpy)]PF(6) (3), with the pendant phenyl rings on the ancillary ligands substituted with two and five fluorine atoms, respectively, have been synthesized and compared to the parent complex, [Ir(ppy)(2)(phpzpy)]PF(6) (1). Here Hppy is 2-phenylpyridine, F2phpzpy is 2-(1-(3,5-difluorophenyl)-1H-pyrazol-3-yl)pyridine, F5phpzpy is 2-(1-pentafluorophenyl-1H-pyrazol-3-yl)-pyridine, and phpzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine. Single crystal structures reveal that the pendant phenyl rings on the ancillary ligands stack to the phenyl rings of the ppy ligands, with dihedral angles of 21°, 18°, and 5.0° between least-squares planes for complexes 1, 2, and 3, respectively, and centroid-centroid distances of 3.75, 3.65, and 3.52 Å for complexes 1, 2, and 3, respectively, indicating progressively reinforced intramolecular π-π stacking interactions from complexes 1 to 2 and 3. Compared to complex 1, complex 3 with a significantly reinforced intramolecular face-to-face π-π stacking interaction exhibits a significantly enhanced (by 1 order of magnitude) photoluminescent efficiency in solution. Theoretical calculations reveal that in complex 3 it is unfavorable in energy for the pentafluorophenyl ring to swing by a large degree and the intramolecular π-π stacking interaction remains on the lowest triplet state.

Organic photocouplers consisting of organic light-emitting diodes and organic photoresistors

We have fabricated one kind of organic photocoupler with organic light-emitting diode as the input unit and pentacene photoresistor as the output unit. The wavelength of the emitting light was 522nm. The output current of the photocoupler linearly increased with its input current and the ratio of the transfer current density reached 3. When the output voltages were 60V, the ratio of the maximum output current to the minimum output current was 150. The breakdown voltage between the input unit and the output unit was more than 10kV and the response time of the device was about 6.5s. We believe this kind of device is promising in the full organic optoelectronic integrated circuits.