Chuang Yao

A sky-blue fluorescent small molecule for non-doped OLED using solution-processing

Fluorescent molecule 9,9-bis(4-bromobutyl)-2,7-bis(4-(1,2,2-triphenylvinyl)phenyl)-9H-fluorene, TPEF, comprising a fluorene unit and two tetraphenylethene moieties, has been utilized to serve as a sky-blue emitter in a solution-processed non-doped organic light-emitting diode (OLED), and its optical and electrical properties are investigated. The TPEF is an aggregation-induced emission (AIE)-active molecule. It is nearly non-emissive when dissolved in solution while emits strong fluorescence in solid state, indicating that it could be a promising candidate for electrofluorescence use. The solution-processed TPEF-based OLED with a simple non-doped structure exhibits sky-blue fluorescence emission, showing a maximum luminance of 2618 cd m−2, a maximum current efficiency of 4.55 cd A−1, and a maximum external quantum efficiency (EQE) of 2.17%.

Solution-processed oxadiazole-based electron-transporting layer for white organic light-emitting diodes

A novel alcohol-soluble electron-transporting small-molecule material comprising oxadiazole and arylphosphine oxide moieties, ((1,3,4-oxadiazole-2,5-diyl)bis(4,1-phenylene))bis(diphenylphosphine oxide) (OXDPPO), has been synthesized and characterized. Its single crystal structure, together with the photophysical, electrochemical and thermal properties, has been investigated. This material not only possesses a wide bandgap with a low HOMO level but also exhibits a strong π–π stacking with a distance of 3.35 A. Moreover, this compound shows excellent thermal stability with a high glass transition temperature of 104 °C and a decomposition temperature of 384 °C. The unique solubility in 2-propanol makes it a good candidate for fabricating fully solution-processed multilayer organic light-emitting diodes (OLEDs). Efficient solution-processed white OLEDs have been fabricated with this compound as an electron-transporting layer (ETL). It was found that this ETL can greatly balance the electrons and holes in devices with the high work-function metal cathode (Al) and an increase in luminous efficiency of ∼70-fold can be achieved. The maximum luminous efficiency of devices with an ETL/Al configuration is even higher than that of devices using a Ca cathode.

Solution processed blue phosphorescent organic light emitting diodes using a Ge-based small molecular host

Two kinds of host materials, 4,4′-(diphenylgermanediyl)bis(N,N-diphenylaniline) and bis(4-(9H-carbazol-9-yl)phenyl)diphenylgermane (DCzGe), for blue phosphorescent organic light emitting diodes (PhOLEDs) were designed by incorporating electron donating groups (carbazole and triphenylamine) into tetraphenylgermane, which is a new type of core moiety that has never been studied for use in this field. This molecular structure endows the compounds with a wide energy bandgap, high thermal/morphological stability and good solution processability. Based on the theoretic calculations, DCzGe was selected and synthesized as a host material which demonstrates a wide bandgap (Eg: 3.56 eV) and a high triplet energy (ET: 3.02 eV). It also exhibits a high glass transition temperature (110 °C), which is beneficial for resisting the Joule heat in devices. All solution processed, blue emitting PhOLEDs were fabricated by using a mixed host combining DCzGe and an electron-transporting material, with a maximum luminance of 10 000 cd m−2 and a maximum current efficiency of 15.2 cd A−1. Furthermore, the devices showed a very low current efficiency roll-off, which remained as high as 15.2 cd A−1 at the luminance of 1000 cd m−2, and the roll-off is only 2.6% even at the higher luminance of 2000 cd m−2.

An air-stable microwire radial heterojunction with high photoconductivity based on a new building block

Organic semiconductor materials with one-dimensional (1D) radial (core–shell) heterojunction structures are highly desired for their expected excellent optoelectronic properties. However, currently, such structures are still in a fledgling period for optoelectronic applications due to the absence of both good materials and suitable preparation methods. Here we have synthesized a p-type organic semiconductor based on a new electron-donating unit (dithienopyrazine) and utilized it as a shell material to construct organic 1D radial p–n heterojunctions. This p-type compound shows a higher oxidation potential and is more resistant to photooxidation in air than its analogs with the commonly-used benzodithiophene unit. Moreover, we prepared organic microwires with radial heterojunctions via a solution-processed method by self-assembly of our p-type material on the surface of n-type cores. Thus, photoconductive devices based on an individual microwire with the radial heterojunction can be fabricated and demonstrate a high photoconductivity. Our work provides a path for preparing 1D radial heterojunctions suitable for optoelectronic applications.

Obtaining highly efficient single-emissive-layer orange and two-element white organic light-emitting diodes by the solution process

By attaching two electron-withdrawing trifluoromethyl (CF3) groups to the 2-phenylbenzothiazole cyclo-metalated ligand, a bis-trifluoromethyl-functionalized orange-emitting phosphorescent iridium(III) complex bis-(6-(trifluoromethyl)-2-(4-(trifluoromethyl)phenylbenzothiozolato))iridium(acetylacetonate) [(F3BT-CF3P)2Ir(acac)] was successfully synthesized. The optical, electrochemical and electroluminescence (EL) properties of this new complex were studied. The experimental results support the theoretical expectation that incorporating electron-withdrawing trifluoromethyl groups at the 4-site of the phenyl ring directly bonded to the metal center, and at the 6-site of 2-phenylbenzothiazole, cause a bathochromic shift in the emission peak and bring the emission color much closer to long-wavelength orange light. Moreover, such trifluoromethyl substituents can hinder the π–π stacking or self-polarization effect occurring from the aggregation of the molecules. The new iridium complex gives an unchanged luminescence spectrum, regardless of whether it is in solution, in untreated film or in film doped at different concentrations. Using this iridium complex as a dopant emitter, solution-processed single emissive layer orange and two-element white OLEDs with good performance can be obtained. Highly efficient orange electroluminescence was obtained with a maximum efficiency of 10.5 cd A−1 and CIE coordinates (0.48, 0.51). When combined with a commercial sky-blue phosphorescent emitter, (CF3BT–CF3P)2Ir(acac) can be utilized to achieve two-element white OLEDs that exhibited a high efficiency of 28.3 cd A−1. Such OLEDs retain high efficiency at a luminance suitable for lighting (e.g. 5000 cd m−2).

Bi-layer hole-injecting layer composed of molybdenum oxide and polyelectrolyte for solution-processed OLEDs with prolonged stability

Currently, poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) is predominantly used as the hole-injecting layer in solution processed organic light-emitting diodes (OLEDs). However, its strong acidity is detrimental to device stability. Here, a bi-layer hole-injecting layer (BHIL) composed of molybdenum trioxide (MoO3) and an anionic polyelectrolyte has been used in solution processed OLEDs to replace PEDOT:PSS. The MoO3 layer was firstly solution deposited using low temperature combustion processing, which ensured that the device had a good hole-transporting ability. Then, an anionic polyelectrolyte was deposited on to the MoO3 layer also using a solution process, resulting in work function increase which was verified using peak force Kelvin probe force microscopy. Thus, the hole-injecting ability of the BHIL was found to be enhanced. It was found that BHIL-based OLEDs possessed a comparable electroluminescence performance but a better shelf-stability relative to the PEDOT:PSS based devices. This strategy gives a promising approach to obtaining high performance solution processed OLEDs with long-term stability.

Design, synthesis and characterization of a new blue phosphorescent Ir complex

Being incompatible with host materials in a physically blended emitting layer, phosphorescent dyes are prone to form aggregates induced by Joule heat in devices under work. In this work, a new and efficient blue phosphorescent dye Cz-C8-FIrpic was designed and synthesised by incorporating 9-phenyl-9H-carbazole into a commonly used blue emissive iridium complex bis(4,6-(difluorophenyl)pyridine-N,C2′)picolinate (FIrpic) via an alkyl chain linkage. This phosphorescent dye exhibits similar photophysical properties to the units of FIrpic and 9-phenyl-9H-carbazole in solutions. In solid films of Cz-C8-FIrpic, the energy transfer from 9-phenyl-9H-carbazole to FIrpic units is effective. The Cz-C8-FIrpic doped emissive layer was investigated by AFM, STEM-EDS, transient photoluminescence decay curves and molecular dynamics simulations. The results show that in the Cz-C8-FIrpic doped film the phase aggregation of FIrpic units is less severe than that in the typically used FIrpic film. In addition, the optimized Cz-C8-FIrpic based device achieved a maximum luminance of 25 142 cd m−2, a maximum EQE of 8.5% and a maximum current efficiency of 22.5 cd A−1 which is about 15% higher than that of the control device based on FIrpic. We conclude that grafting a typically used dye to functional groups with alkyl chains is useful to restrict phase separation in physically blended emitting layers, and thus can achieve high electroluminescence performances.

Facile fabrication of an organic semiconductor/graphene microribbon heterojunction by self-assembly

Herein, we report a facile self-assembly strategy to prepare a novel 1D organic semiconductor/graphene microribbon heterojunction by coating a layer of graphene sheets on the organic semiconductor microribbon. The organic semiconductor microribbon composed of a p-type small molecule 3,7-bis(5-(2-ethylhexyl)thiophen-2-yl)dithieno[2,3-b:2′,3′-e]pyrazine (BEHT-DTP) was prepared by evaporation-induced self-assembly. Subsequently the graphene nanosheets, as an electron acceptor, were self-assembled onto the surface of a BEHT-DTP microribbon in aqueous solution to form a 1D p–n junction. The device based on the single microribbon heterojunction demonstrated enhanced photoconductivity properties. This preliminary work points out a new path to fabricate 1D organic nano/micro-heterojunctions, avoiding complex molecular design and equipment.

Organic photodiodes constructed from a single radial heterojunction microwire

Organic nano/micro one-dimensional (1D) materials are generally considered as promising materials for flexible, portable optoelectronic devices due to their well-known distinctive feature. Over the past few years, numerous organic nano/micro 1D photosensitive resistors have been developed; however, as one of the important photoelectronic components for fabricating organic nano/microelectronic circuits, organic nano/micro 1D photodiodes have not been reported yet. Herein, on the basis of our previous work about an organic photosensitive radial heterostructure microwire, we tried to prepare another kind of radial heterostructure microwire and explore its photodiode properties. Excitingly, the organic radial heterostructure microwire using aluminum tris(8-hydroxyquinoline) as the core and poly(3-hexyl thiophene) as the shell, which was prepared by a solution-based method, showed excellent performance with a large rectification ratio, a high on/off ratio and a good photoresponsivity under ambient conditions. Our work has developed a convenient method to prepare microwire photodiodes based on an all-organic heterojunction.

A novel ternary organic microwire radial heterojunction with high photoconductivity

We fabricated a unique ternary organic hybrid microwire radial heterojunction by a facile method. First, 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA) microwires were prepared by solvent-evaporation-assisted self-assembly. Then, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoparticles were adsorbed onto the surface of the TCTA microwires, forming interesting corncob-like binary hybrid microwires. Finally, (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) was adsorbed on the surface of the binary hybrid microwires to form ternary hybrid microwire radial heterojunctions. The thermally activated delayed fluorescence (TADF) material 4CzIPN was introduced into the donor–acceptor (D–A) system to form the ternary hybrid microwire radial heterojunction for the first time. The morphology has been confirmed by fluorescence microscopy, SEM and TEM. Interestingly, we found that this ternary hybrid microwire exhibited efficient photoconductivity by fabricating a bottom contact device; the photocurrent increased by more than 3 times compared with the reference device without 4CzIPN. By examining some reference devices, it can be inferred that the enhancement of the photoconductivity originates from the reversed intersystem crossing (RISC) process in 4CzIPN. This process can promote the formation of triplet excitons, thereby increasing the charge carrier concentration in the conductive channel of the microwire radial heterojunction. These high photoconductivity ternary microwires provide an efficient approach to improve the performance of photovoltaic devices and show promise for applications in organic integrated optoelectronics.