Graded doped structure fabricated by vacuum spray method to improve the luminance of polymer light-emitting diodes

An increase in luminance of a polymer light-emitting diode(PLED) was obtained by fabricating a graded doping structure using a vacuum spray method. The small electron transport molecule, Tris(8-hydroxyquinolinato) aluminum(III)(Alq3), was graded dispersed along the film in the direction of growth in the hole transport polymer poly(3-hexylthiophene-2,5-diyl)(P3HT, regiorandom) layer of the PLED, despite being dissolved in the same organic solvent as the polymer. The PLED reported here, which is composed of a graded structure, emitted brighter light than PLEDs composed of pure polymer or of a blend of active layers prepared by spin coating and/or vacuum spray methods.

Solution Processable Material Derived from Aromatic Triazole, Azomethine and Tris: Preparation and Hole-Buffering Application in Polymer Light-Emitting Diodes

This work presents the synthesis of a new hole-buffering material TAZS and its successful application in polymer light-emitting diodes to enhance device performance. The TAZS is composed of aromatic 1,2,4-triazolylcore linked with three trihydroxy tert-butyl terminals via azomethine linkages. The TAZS forms ashomogeneous film deposited by spin-coating process. The HOMO and LUMO levels of TAZS are -5.23 eV and -2.40 eV, respectively, as estimated from cyclic voltammogram. The current density results of hole-only and electron-only devices confirm strong hole-buffering capability of TAZS layer. Multilayer PLEDs with different thickness of TAZS (ITO/PEDOT: PSS/TAZS (x nm)/SY/ETL/LiF/Al) have been successfully fabricated, using spin-coating process to deposit hole-injecting PEDOT: PSS, TAZS, and emissive SY layers. The PLED with 16 nm TAZS reveals the optimal device performance, with maximum luminance and maximum current efficiency of 19,046 cd/m2 and 4.08 cd/A, respectively, surpassing those without TAZS as HBL (8484 cd/m2, 2.13 cd/A). The hole-buffering characteristic of TAZS contributes greatly to improved charges’ recombination ratio and enhanced emission efficiency.

Hole-Buffer Material Derived from Pyrene, Schiff Base and Tris to Enhance Emission Efficiency of Polymer Light-Emitting Diodes

Inserting a hole-buffer layer is an effective way to enhance emission efficiency of electroluminescence devices. We have successfully synthesized a new hole-buffer material PSB composed of pyrene, Schiff base and trihydroxy tert-butyl groups by the Suzuki-coupling reaction. The HOMO and LUMO lev-els were -6.33 eV and -2.55 eV, respectively, as estimated from cyclic volt-ammograms. In addition, homogeneous films (rms roughness ~2 nm) were readily obtained by spin-coating process. Multilayer polymer light-emitting diodes, ITO/PEDOT:PSS/PSB/SY/LiF/Al, have been fabricated using PSB as hole-buffer layer (HBL). Inserting PSB as HBL significantly enhances the per-formance (maximum luminance: 26,439 cd/m2, maximum current efficiency: 7.03 cd/A), compared with the one without PSB (9802 cd/m2, 2.43 cd/A). It is also superior to the device with conventional BCP as hole-blocking layer (ITO/PEDOT:PSS/SY/BCP/LiF/Al: 15,496 cd/m2, 5.56 cd/A). Current results strongly indicate that the PSB is a potential hole-buffer material for electrolu-minescent devices.

A novel crosslinkable electron injection/transporting material for solution processed polymer light-emitting diodes

A novel crosslinkable water/alcohol soluble conjugated polymer PFN-C containing oxetane groups and aminoalkyl groups in the side chains has been developed and used as highly efficient electron injection and transporting material for polymer light-emitting diodes (PLEDs). The unique solubility in polar solvents and crosslinkable ability of PFN-C render it a good can- didate for solution processed multilayer PLEDs. It was found that PFN-C can greatly enhance the electron injection from high work-function metal cathode, due to its pendant amino groups. As a result, PLEDs with PFN-C/Al cathode exhibited compara- ble device performance to the devices with Ba/Al cathode. The resulting green light-emitting device showed promising perfor- mance with a maximum luminance efficiency of 13.53 cd A-1.

Fluorene pendant-functionalization of poly(N-vinylcarbazole)as deep-blue fluorescent and host materials for polymer light-emitting diodes

π-Electron coupling of pendant conjugated segment inπ-stacked semiconducting polymers always causes the formation of defect trapped sites and further quenched high-band excitons,which is harmful to the performance and stability of deep-blue polymer light-emitting diodes(PLEDs).Herein,considerate of“defect”carbazole(Cz)electromers in poly(N-vinylcarbazole)(PVK),a series of fluorene units are introduced into pendant segments(PVCz-DMeF,PVCz-FMeNPh and PVCz-DFMeNPh)to suppress the strongπ-electron coupling of pendant Cz units and enhance radiative transition toward fabricating sable PLEDs.Compared to PVCz-FMeNPh and PVCz-DFMeNPh,PVCz-DMeF spin-coated films show a relatively efficient deep-blue emission,completely similar to its single pendant chromophore,confirmed an extremely weak charge-transfer and electron coupling between adjacent pendant segments.Therefore,PLEDs based on PVCz-DMeF present stable and deep-blue emission with a high color purity(0.17,0.08),associated with extremely weak defect emission at 600∼700nm(induced by carbazole electromers).Finally,PLEDs based on PVCz-DMeF/F8BT blended films(1:1)also present the high maximum luminance(Lmax)of 6261 cd/m2 and current efficiency(CE_(max))of 2.03 cd/A,confirmed slightly trapped sites formation.Therefore,precisely control the arrangement and packing model of pendant units inπ-stacked polymer is an essential prerequisite for building efficient and stable emitter for optoelectronic devices.

Low-cost photoacoustic imaging systems based on laser diode and light-emitting diode excitation

Photoacoustic imaging,an emerging biomedical imaging modality,holds great promise for preclinical and clinical researches.It combines the high optical contrast and high ultrasound resolution by converting laser excitation into ultrasonic emission.In order to generate photoacoustic signal e±-ciently,bulky Q-switched solid-state laser systems are most commonly used as excitation sources and hence limit its commercialization.As an alternative,the miniaturized semiconductor laser system has the advantages of being inexpensive,compact,and robust,which makes a signi¯cant e®ect on production-forming design.It is also desirable to obtain a wavelength in a wide range from visible to nearinfrared spectrum for multispectral applications.Focussing on practical aspect,this paper reviews the state-of-the-art developments of low-cost photoacoustic system with laser diode and light-emitting diode excitation source and highlights a few representative installations in the past decade.

FLUORENE-BASED LIGHT-EMITTING POLYMERS

Several series of fluorene-based light-emitting polymers with the emphasis on achieving efficient and stable bluelight emission are reported. Spiro-functionalization may narrow the emission spectra (with smaller tail at Ionger wavelengths)of fluorene homopolymers to provide purer blue emission. The thermal spectral stability of the polymers could also beimproved because of the elevation of the glass transition temperature caused by the spiro-functionalization. However, theexcimer emission in fluorene homopolymers is not suppressed by the spiro-functionalization. Alternate copolymers of 9,9-dihexylfluorene and substituted phenylenes may emit efficient blue ligh both in solution and in film. The optical propertiesare dependent on the substituion on the phenylene ring. The alkoxy-substituted polymers displayed efficient PL and EL andgood thermal spectral stability. The HOMO and LUMO energy levels of the polymers based on the backbone structure couldbe tuned in a wide range by attaching different functional groups on the phenylene ring. By attaching europium(III) complexat the ends of the side chains in the alternate copolymers, we have demonstrated a new approach to achieving red emissionwith a very narrow spectrum. The copolymers of 9,9-dihexylfluorene and thiophene and bithiophene with differentsubstitutions were also synthesized to study the effect of substitution and regioregularity on the optical and other physicalproperties of the polymers.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Since polymer-based light-emitting diodes(PLEDs)arewellsuited building blocks for large-area and low-cost flexible display equipment,state-of-the-art thermally activated delayed fluorescence(TADF)PLEDs are in high demand.To respond to this demand,light-emitting TADF units have initially been modified with electron-transporting units to balance the carrier transport of regiorandom TADF polymers,and simultaneously,an intramolecular sensitizing strategy has also been employed by covalently incorporating TADF sensitizers with light-emitting TADF units and hosts in conjugated polymers to accelerate the spin-flip of triplet excitons.Superior photophysical properties have been achieved by a rational regulation of the proportions of each component,achieving a photoluminescence quantumyield of 90%,an extremely high rate of reverse intersystem crossing of 3×106 s−1,and a relatively low nonradiative decay rate of around 105 s−1.As a result,the solutionprocessed PLEDs can attain an external quantum efficiency(EQE)value of 25.4%with emission peaks of around 550 nm,representing record-high performance for PLEDs.The efficiency roll-off can also be significantly suppressed,maintaining an EQE value of 24.2%at 1000 cd/m2 with ideal efficiency roll-off of lower than 5%.Encouragingly,this work provides a valid strategy to tackle the imperative need for PLEDs with high EQE and low efficiency roll-off.

Germafluorene conjugated copolymer——synthesis and applications in blue-light-emitting diodes and host materials

A germafluorene-fluorene copolymer was successfully obtained via Suzuki polymerization.The ger-manium containing copolymer has an efficient blue light emission under the ultraviolet irradiation and its single layer EL device showed the highest brightness of 2630 cd/m2 at 7.8 V and the highest effi-ciency of 0.301 lm/W at 6.2 V.The copolymer can also serve as the host material for phosphorescent metal complexes with the maximum brightness of 15600 cd/m2 and the quantum efficiency of 8.5%.The results are quite promising and promise that as its analogs of fluorene and silafluorene,germafluorene is an excellent building block for blue light-emitting polymers and host materials.

Through-space charge transfer blue polymers containing acridan donor and oxygen-bridged triphenylboron acceptor for highly efficient solution-processed organic light-emitting diodes

Three kinds of through-space charge transfer(TSCT)blue polymers containing non-conjugated polystyrene backbone together with spatially-separated acridan donor and oxygen-bridged triphenylboron acceptors having different substituents of tert-butyl,hydrogen and fluorine are designed and synthesized.The designed TSCT blue polymers possess photoluminescence quantum yields up to 70%in solid-state film,single-triplet energy splitting below 0.1 eV,and typical thermally activated delayed fluorescence(TADF)effect.Meanwhile,the resulting polymers exhibit aggregation-induced emission(AIE)effect with emission intensity increased by up to^27 folds from solution to aggregation state.By changing the substituent of acceptors to tune the charge transfer strength,blue emission with peaks from 444 to 480 nm can be realized for the resulting polymers.Solution-processed organic light-emitting diodes based on the polymers exhibit excellent device performance with Commission Internationale de L’Eclairage(CIE)coordinates of(0.16,0.27),together with the maximum luminous efficiency of 30.7 cd A-1 and maximum external quantum efficiency of 15.0%,which is the best device efficiency for blue TADF polymers.

Through-space charge transfer polymers for solution-processed organic light-emitting diodes

Through-space charge transfer(TSCT)polymers are an attractive class of luminescent polymers with spatial donor/acceptor architecture and thermally activated delayed fluorescence effect,different from conventional luminescent polymers with conjugated donor-acceptor structure and through-bond charge transfer emission.Their emission comes from the intramolecular charge transfer by through-space pathway because the donor and acceptor segments are spatially proximate to each other in each repeating unit but are physically separated by nonconjugated polymer backbone.In this review,recent advances in TSCT polymers with linear,bottlebrush,and dendritic architectures are presented,with the focus on their molecular design,photophysical behavior,and device performance.We hope that this review shall provide a useful insight of new luminescent polymers with TSCT effect for use in solution-processed organic light-emitting diodes.

Synthesis and optical properties of an alternating copolymer composed of MEHPV and 1,3,4-oxadiazole

A poly （ p-phenylenevinylene ） （ PPV ） alternating copolymer, poly [ （ 2, 5-diphenylene-1, 3, 4-oxadiazole ）-4-4＇ - vinylene-alt-2-methoxy-5-（ 2-ethylhexyloxy ）-1, 4-phenylenevinylene] （oxa-MEHPV）, is synthesized by Heck coupling reaction and characterized with UV-vis, Fourier transform infrared （ FT-IR ）, ^1H-NMR and photoluminescence （ PL ） spectroscopy, oxa-MEHPV possesses an outstanding thermal stability and shows excellent solubility in common organic solvents such as dichloromethane, chloroform, toluene, and tetrahydrofuran（THF）. The introduction of the electron-deficient 1, 3, 4-oxadiazole units into the MEH-PPV backbone also increases the electron affinities of the conjugated segment, which leads to the blue-shift of the maximum absorption wavelength and makes the polymer have a high optical band-gap energy, good electron-transporting stability and high PL quantum yield.

Triphenyl Phosphine Oxide and Carbazole-based Polymer Host Materials for Green Phosphorescent Organic Light-emitting Diodes

Four novel polymers, poly（3,6-9-decyl-carbazole-alt-1,3-benzene） （PB13CZ）, poly（3,6-9-decyl-carbazole-alt- bis（4-phenyl） （phenyl） phosphine oxide） （PTPPO38CZ）, poly（3,6-9-decyl-carbazole-alt-2,4-phenyl（diphenyl） phosphine oxide） （PTPPO13CZ） and poly（3,6-9-decyl-carbazole-alt-bis（3-phenyl） （phenyl） phosphine oxide） （PTTPO27CZ） were synthesized, and their thermal, photophysical properties and device applications were further investigated to correlate the chemical structures with the photoelectric performance of bipolar host materials for phosphorescent organic light emitting diodes. All of them show high thermal stability as revealed by their high glass transition temperatures and thermal decomposition temperatures at 5% weight loss. These polymers have wide band gaps and relatively high triplet energy levels. As a result, the spin coating method was used to prepare the green phosphorescent organic light emitting diodes with polymers PTPPO38CZ, PTPPO13CZ and PTTPO27CZ as the typical host materials. The green device of polymer PTPPO38CZ as host material shows electroluminescent performance with maximum current efficiency of 2.16 cd.A-1, maximum external quantum efficiency of 0.7%, maximum brightness of 1475 cd.m-2 and reduced efficiency roll-off of 7.14% at 600 cd.m-2, which are much better than those of the same devices hosted by polymers PTTPO27CZ and PTPPO13CZ.

The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes

In recent years,organic light-emitting device technology has expanded from organic light-emitting diodes(OLEDs)to organic semiconductor laser diodes(OSLDs)with the progress of sophisticated molecular and device architectural designs.In OLEDs,the development of thermally activated delayed fluorescence molecules has been intensively investigated recently.As a result,the internal quantum efficiency of OLEDs containing relatively simple aromatic compounds without precious metals has reached almost 100%.Furthermore,incorporating a distributed feedback resonator structure into the OLED architecture has yielded OSLDs that exhibit the features of current-pumped lasing.In this short review,the authors describe the recent paradigm shift from OLEDs to OSLDs,mainly fromthe perspective of materials innovation.

Synthesis of Polyfluorene-Polytriarylamine Block Copolymer with Emitting Part at Junction Point for Light Emitting Applications

A block copolymer consisting of polyfluorene (PF) and polytriarylamine (PTAA) functionalized with green emitting phenoxazine moiety at the junction point of two blocks was designed and prepared for electroluminescent application. PF homopolymer was synthesized by Suzuki coupling polymerization, and was reacted with brominated phenoxazine. In the presence of the resulting PF functionalized with phenoxazine, C-N coupling polymerization of 4-(4’-bromophenyl)-4’’-butyldiphenylamine was carried out to afford a triblock copolymer, PTAA-phenoxazine-PF-phenoxazine-PTAA (PF-Ph-PTAA). Two types of random copolymers were also synthesized with fluorene and phenoxazine (PF2) by Suzuki coupling polymerization for comparison. All the polymers were soluble in common organic solvents and readily formed thin films by a solution processing. Prepared polymers exhibited similar UV absorption and PL emission in chloroform solutions. In a film state, the existence of phenoxazine unit drastically changed PL spectra. Although the content of phenoxazine unit in PF-Ph-PTAA was relatively high (13 mol%), it showed similar PL spectrum to that of PF2(phenoxazine content, 0.2 mol%) indicating that phenoxazine unit is isolated in single polymer chain nevertheless the high content. EL device based on PF-Ph-PTAA showed green-emission, suggesting that emission sites predominantly located in the vicinity of phenoxazine moiety because of its shallow HOMO level.

New applications of poly(arylene ether)s in organic light-emitting diodes

Compared with conventional π-conjugated polymers,poly（arylene ether）s（PAEs） may take advantages of excellent thermal properties,well-defined effective conjugated length and no catalyst contamination.Recently,their applications have been extended from engineering plastics to optoelectronic materials.In this review,various kinds of functional PAEs used as fluorescent polymers,host polymers and phosphorescent polymers in organic light-emitting diodes（OLEDs） are outlined,and their molecular design,synthesis and device performance are overviewed.

Polymer light-emitting devices based on europium(Ⅲ) complex with 11-bromo-dipyrido[3,2-a:2′,3′-c]phenazine

Polymer light-emitting diodes(PLEDs) containing Eu(DBM)3(Br DPPz)(DBM is dibenzoylmethane, and Br DPPz is 11- bromo-dipyrido[3,2-a:2′,3′-c]phenazine) doped in a blend of poly(9,9-dioctylfluorene)(PFO) and 2-tert-butylphenyl-5- biphenyl-1,3,4-oxadiazole(PBD) as the host matrix were reported. Eu(DBM)3(Br DPPz) exhibited high thermal stability and intense UV-Vis absorption. Narrow-bandwidth red emission at 612 nm with a full width at half-maximum(FWHM) of 14.0 nm was observed from Eu(DBM)3(Br DPPz) in these double-layered PLEDs at dopant concentrations from 1 wt% to 8 wt%. For the PLED containing 1 wt% Eu(DBM)3(Br DPPz), a maximum luminance of 829 cd/m2 at 153.5 m A/cm2, highest external quantum efficiency of 1.70% at 2.1 m A/cm2 and maximum luminance of 0.74 cd/A at 4.31 m A/cm2 were obtained.

Luminescent metal-halide perovskites:fundamentals,synthesis,and light-emitting devices

Metal-halide perovskites have garnered considerable research attention as highly efficient light emitters in recent years due to their outstanding optoelectronic properties with remarkable tunability and excellent solution processabilities.Substantial advancements have been achieved in the development of novel halide perovskites,and the exploitations of these materials in lightemitting devices.This review comprehensively outlines recent breakthroughs in metal-halide perovskites,encompassing the rational design of perovskite materials with tunable light emission properties,the controllable growth of single crystal for a deeper understanding of their structure-property relationships,as well as the fundamental insights into the photophysics and carrier dynamics in perovskite systems.Additionally,it provides an overview of recent applications of perovskite materials in high-performance light-emitting diodes(LEDs)and lasers.

Bipolar Blue Light-emitting Polyfluorenes Containing Dibenzothiophene-S,S-dioxide/Carbazole Units

Bipolar blue light-emitting polyfluorenes（PFSO-Cz） containing electron-deficient dibenzothiophene-S,S- dioxide（SO） and electron-rich carbazole（Cz） unit were synthesized. All the polymers show a high thermal stability with the decomposition temperatures over 400 ℃ and higher photoluminescence quantum yields. The highest occupied molecular orbital energy levels（EHMH＇s） slightly enhance and the lowest unoccupied molecular orbital energy levels（ELuMo＇S） gently depress with the increase of Cz content in the polymers. PL spectra of the polymers display remarkable red shift and broadening with the increase of solvent polarities, indicating significant intramolecular charge transfer（ICT） effect in the polymers. Electroluminescence（EL） spectra of the polymers exhibit a broadening tendency with increasing the content of Cz unit in the polymers. The superior device performances were obtained with the maximum lumhlous efficiency（LEmax） of 5.2 cd/A, the maximum external quantum efficiency（EQEmax） of 4.8% and the Internationale de I＇Eclairage（CIE）（x,y） coordinates of （0.16, 0.17） for PFSO15-Cz10 based on the single-layer device of ITO/PEDOT：PSS/EL/CsF/A1. The results indicate that the efficient bipolar blue light-emitting polyfluorenes are also constructed by Suzuki copolymerization using the monomers in common use.

Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide unit in alkyl side chain

Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide(SO) unit in alkyl side chain(PF-FSOs and PF-CzSOs) were synthesized. All the polymers show high thermal stability with the decomposition temperatures over400 °C. The highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital(LUMO) energy levels of the copolymer slightly decrease with the increase of SO content in side chain. PL spectra of the polymers show slightly red shift and broadening with the increase of solvent polarities, indicating unremarkable intramolecular charge transfer(ICT) effect in the polymers containing SO unit in alkyl side chain. EL spectra of the polymers are almost unchanged in the current densities from 100 to 400 mA cm.2, indicating the superb EL stability of the resulted polymers. The EL spectra of the copolymers exhibit obvious blue-shift and narrowing with the CIE of(0.18, 0.11) for PF-FSO10 and(0.17, 0.11) for PF-CzSO10, respectively,compared with PF-SO10 containing SO unit in main chain with the CIE of(0.16, 0.17) and PFO with the CIE of(0.18, 0.18).The superior device performances were obtained with the luminous efficiency(LEmax) of 1.17 and 0.68 cd A.1 for PF-FSO15 and PF-CzSO20, respectively, compared with the LEmax of 0.37 cd A.1 for PFO. The results indicate that linking SO unit to alkyl side chain of the polyfluorene is a promising strategy for efficient blue light-emitting polymers.