Guifang Dong

Solution processable small molecules for organic light-emitting diodes

Organic light-emitting diodes (OLEDs) based on vacuum deposited small molecules have undergone significant progress since the first efficient double-layered OLEDs were reported in 1987 by Tang and Van Slyke. Recently, solution processed small molecular OLEDs are also drawing more and more research attention, as such a technology combines advantages of the facile synthesis of small molecules and the low-cost solution process like polymers. The performance of OLEDs made by solution process is gradually catching up with their vacuum deposited counterparts. This feature article will review the device structures adopted to achieve high performance solution processed OLEDs, the development of solution processable small molecules, and the comparisons of the different nature of the films and devices fabricated by solution-process or by vacuum deposition. Finally, the prospects and remaining problems will be discussed.

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.

Use of a 1H-benzoimidazole derivative as an n-type dopant and to enable air-stable solution-processed n-channel organic thin-film transistors.

We present here the development of a new solution-processable n-type dopant, N-DMBI. Our experimental results demonstrated that a well-known n-channel semiconductor, [6,6]-phenyl C(61) butyric acid methyl ester (PCBM), can be effectively doped with N-DMBI by solution processing; the film conductivity is significantly increased by n-type doping. We utilized this n-type doping for the first time to improve the air-stability of n-channel organic thin-film transistors, in which the doping can compensate for the electron traps. Our successful demonstration of n-type doping using N-DMBI opens up new opportunities for the development of air-stable n-channel semiconductors. It is also potentially useful for application on solution-processed organic light-emitting diodes and organic photovoltaics.

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.

High-triplet-energy tri-carbazole derivatives as host materials for efficient solution-processed blue phosphorescent devices

A novel series of solution-processible carbazole-based host materials, 3,6-bis(N-carbazolyl)-N-phenylcarbazole (BCC-36), 3,6-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-36), 2,7-bis(N-carbazolyl)-N-phenylcarbazole (BCC-27), and 2,7-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-27), is designed and synthesized. Owing to the highly twisted configuration, these hosts exhibit high triplet energy levels (2.90–3.02 eV) and high glass transition temperatures (147–210 °C). They also exhibit appropriate HOMO energy levels (−5.21–−5.36 eV), resulting in an improved hole-injection property. These novel compounds are employed to fabricate phosphorescent organic lighting-emitting diodes (OLEDs) as the host materials doped with the guests of iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) and iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) by spin coating. The best device performance of FIrpic based blue-emitting devices has a rather low turn-on voltage of 3.9 V, a maximum efficiency of 27.2 cd A−1 (11.8 lm W−1), and a maximum external quantum efficiency of 14.0%. Moreover, the best device performance of FIr6 based deep-blue-emitting devices exhibits a turn-on voltage of 4.9 V, a maximum efficiency of 11.5 cd A−1 (4.9 lm W−1), and a maximum external quantum efficiency 6.8%. The performance data are outstanding for solution-processed blue phosphorescent OLEDs.

High-efficiency organic light-emitting diodes with tunable light emission by using aromatic diamine/5,6,11,12-tetraphenylnaphthacene multiple quantum wells

Organic light-emitting diodes (OLEDs) with multiple-quantum-well (MQW) structures, which consist of N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1, 1′ biphenyl 4, 4′-diamine and 5,6,11,12-tetraphenylnaphthacene (rubrene), and tris(8-hydroxyquinoline) aluminum (Alq3) as the electron transporting material, have been fabricated. The results demonstrate that the MQW structure can sharply increase the performance of OLEDs. The diode with a MQW number of 4 exhibits efficiency up to 8.1 cd/A, which is four times that of the conventional diodes without the MQW structure. It is also interesting to find that the light emission from Alq3 and rubrene can be obtained together and the electroluminescent spectrum, which is independent of the driving voltage, varies with the well number. For the device with a well number of 6, pure rubrene light emission is obtained. Our work demonstrates that the organic MQW structure not only can efficiently control the carrier transporting, thus conducive to achieve an electron–hole balance,...

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).

High performance organic-inorganic perovskite-optocoupler based on low-voltage and fast response perovskite compound photodetector

Organic-inorganic hybrid photodetectors attract considerable attention because they can combine the advantages of both organic and inorganic systems. Here, a perovskite compound with a broad absorption spectrum and high power conversion efficiency is used as a photosensitive layer in an organic/inorganic hybrid heterojunction photodetector with a high and fast response. The high sensitivity exceeding 104 is obtained at bias of 0–4 V. Using a tandem organic light-emitting diode (OLED) as the light source, we fabricated an optocoupler device. The optocoupler achieved a maximum photoresponsivity of 1.0 A W−1 at 341.3 μWcm−2 at an input voltage of 6 V. The device also exhibits rapid response times of τrise ~ 20 μs and τfall ~ 17 μs; as well as a high current transfer ratio (CTR) of 28.2%. After applying an amplification circuit, the CTR of the optocoupler increases to 263.3%, which is comparable with that of commercial inorganic optocouplers. The developed hybrid optocoupler thus shows great promise for use in photonics.

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.

Bright single-active layer small-molecular organic light-emitting diodes with a polytetrafluoroethylene barrier

Single-layer organic light-emitting diodes (OLEDs) with a small molecule, tris(8-hydroxyquinoline) aluminum (Alq3) as the only active material, have been prepared. In order to achieve an efficient hole injection, a thin layer of polytetrafluoroethylene (Teflon) was inserted between the anode and the Alq3 layer. The effect of the Teflon layer thickness upon the device performance has also been investigated. A brightness of 16 000 cd/m2 with 6 nm thick Teflon layer was achieved, whereas the conventional double-layer OLEDs with N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-diamine and Alq3 showed only 9500 cd/m2 in our experiments. The single-layer structure technology is of great importance to the OLED’s commercialization due to its possible lower cost and higher production efficiency. And it is reasonable to infer that, based on this work, higher device performance could be realized by screening both the active material and the barrier layer material.