Jinghong Peng

Synthesis and characterization of arylamino end-capped silafluorenes for blue to deep-blue organic light-emitting diodes (OLEDs)

Diphenylamino- or cabazolyl-endcapped silafluorene derivatives which show a wide energy band gap, a high fluorescence quantum yield and high stability have been designed, synthesized, and characterized. Double layer electroluminescent devices of these silafluorene derivatives exhibited efficient blue emission. The non-doped double layer OLEDs containing TDMS, TDPS, CDMS, or CDPS exhibited better electroluminescence efficiencies than those of the devices using the reference emitter DPFL-NPB, among which the best device was with TDPS, which showed a maximum current efficiency of 1.62 cd A−1 and an external quantum efficiency (EQE) of 1.36%. The solution processed device using TDPS as dopant exhibited a high performance with an EQE of 2.48% and an obviously low turn-on voltage of 4 V, when compared to the results of the reference device. The replacement of the carbon atom of the fluorene unit with a silicon atom could lower the energy gap effectively and improve the thermal stability as well as optical performances. The results indicate that the end-capped arylamino groups affect the organic light-emitting diode (OLED) performances greatly and aryl or alkyl substitution on the 9-position of a silafluorene unit is also crucial to the OLED performances of this kind of silafluorene.

Improved efficiency in polymer light-emitting diodes using metal-enhanced fluorescence

Metal-enhanced fluorescence was realized in the emissive layer of organic electroluminescent devices. Core-shell Au nanoparticles (Au@SiO2) doped into the emissive layer of polymer light-emitting diodes (PLEDs) were used to enhance the luminous efficiency by a factor of 1.6 relative to the undoped reference devices (from 6.3 cd/A to 10.0 cd/A). The silica shell outside the Au nanoparticles was used to ensure that there was sufficient distance between the Au nanoparticles and the fluorescent polymer material to avoid quenching of the excitons. In addition, sufficient overlap of the energy of the localized surface plasmon resonance of the Au nanoparticles and the energy of the excitons formed in the emissive layer was guaranteed. These led to an enhanced PLED efficiency. This research provides a way to obtain high performance organic electroluminescent devices.

AIE-Active Fluorene Derivatives for Solution-Processable Nondoped Blue Organic Light-Emitting Devices (OLEDs)

A series of fluorene derivatives end-capped with diphenylamino and oxadiazolyl were synthesized, and their photophysical and electrochemical properties are reported. Aggregation-induced emission (AIE) effects were observed for the materials, and bipolar characteristics of the molecules are favored with measurement of carrier mobility and calculation of molecular orbitals using density functional theory (DFT). Using the fluorene derivatives as emitting-layer, nondoped organic light-emitting devices (OLEDs) have been fabricated by spin-coating in the configuration ITO/PEDOT:PSS(35 nm)/PVK(15 nm)/PhN-OF(n)-Oxa(80 nm)/SPPO13(30 nm)/Ca(8 nm)/Al(100 nm) (n = 2-4). The best device with PhN-OF(2)-Oxa exhibits a maximum luminance of 14 747 cd/m(2), a maximum current efficiency of 4.61 cd/A, and an external quantum efficiency (EQE) of 3.09% in the blue region. Investigation of the correlation between structures and properties indicates that there is no intramolecular charge transfer (ICT) increase in these molecules with the increase of conjugation length. The device using material of the shortest conjugation length as emitting-layer gives the best electroluminescent (EL) performances in this series of oligofluorenes.

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.

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.

Solution-processed organic light-emitting diodes with enhanced efficiency by using a non-conjugated polymer doped small-molecule hole-blocking layer

The small-molecule hole-blocking material SPPO13 has been doped with a non-conjugated polymer to act as the hole-blocking layer in solution-processed OLEDs. Such a doping strategy can significantly improve the electron injection in devices, resulting in an enhanced luminous efficiency and a reduced turn-on voltage.

Doping core–shell nanoparticles into a solution-processed electron transporting layer for polymer light-emitting diodes

Core–shell gold nanoparticles have been doped into the solution-processed electron-transporting layer (ETL) of polymer light-emitting diodes (PLEDs). By using this doping strategy, metal-enhanced fluorescence was realized in the device. The doped device has obtained enhanced luminance, enhanced luminous efficiency and a reduced turn-on voltage compared with that using the non-doped ETL.

Preparation of exciplex-based fluorescent organic nanoparticles and their application in cell imaging

In this paper, we report a novel organic fluorescent nanoparticle based on exciplexes for cell imaging. Through a reprecipitation method, we used a combination of 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC) and 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13) to form nanoparticles. In the aggregated structures, TAPC and SPPO13 were forced into proximity that led to the corresponding exciplex formation. A red-shifted fluorescence emission with considerably longer fluorescence lifetimes ascribed to exciplex emission can be achieved. Along with the good stability and low cytotoxicity of organic nanoparticles, the prepared TAPC/SPPO13 exciplex nanoparticles were successfully applied in live cell imaging. These properties make TAPC/SPPO13 exciplex nanoparticles good candidates for cellular labeling and imaging materials.