Exciplex emission and energy transfer in white light-emitting organic electroluminescent device

We report a white light organic electroluminescent (EL) device achieved through exciplex formation between organic materials as a green color light source. Exciplex formation at 500 nm originated from poly(N-vinylcarbazole) (PVK) and 2,5-bis(5-tert-bytyl-2-benzoxazolyl)thiophene (BBOT), exciplex formation at 550 nm between from N,N′-diphenyl-N,N′-bis(3-metylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and BBOT was assumed to correspond to the complex formation covering the green color region. In the mixed emitting materials, the PL decay lifetimes of BBOT and poly(3-hexylthiophene) (P3HT) were increased as much as 35–68 and 1.5–19 ns, respectively. This indicates that the energy transfer occurred from PVK to BBOT and P3HT, enhancing the quantum efficiency of these materials. We achieved white light emission with brightness as great as 12.3 μW/cm² at 20 V, corresponding to CIE coordinates of x=0.29 and y=0.353. The EL spectrum of the device was changed with a function of the applied voltage.     

Photoluminescence and electroluminescence properties of poly(9-vinylcarbarzole) doped with anthracence derivatives containing bis(ethynylphenyl oxadiazole) or bis(vinylphenyl oxadiazole) substituents

The photoluminescence (PL) and electroluminescence (EL) properties of two novel light-emitting anthracence derivatives containing bis(ethynylphenyl oxadiazole) or bis(vinylphenyl oxadiazole) substituents (ANOs) blended with poly(9-vinylcarbazole) (PVK) are characterized. The energy transfer process in the blends is discussed. By employing 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (BBOT) and tris(8-hydroxyquinoline) aluminum (Alq3) as electron transporting layers (ETLs) in the EL devices, remarkable improvements in EL efficiency were observed. The effect of the transporting layer on the EL intensity of the devices is also discussed.     

A new coumarin derivative used as emitting layer in organic light-emitting diodes

The photoluminescence and electroluminescence properties of a new coumarin derivative N-(7-diethylamino-coumarin-3-carboxyl)-N,N′-dicyclohexylurea (DCDU) were investigated. The compound showed a maximum quantum efficiency of 0.943 for 400 nm excitation. The cleavage of intramolecular hydrogen bond led to conformational transition of coumarin derivative during vacuum deposition, and the deposited film was formed with less crystalline phase. A stable yellow light emitted from a triple-layer EL device with a maximum brightness of 1100 cd/m² and an external efficiency of 0.4%.     

Vinyl polymer containing 1,4-distyrylbenzene chromophores: Synthesis,optical, electrochemical properties and its blend with PVK and Ir(ppy)3

Vinyl polymer PDSB containing pendant 1,4-distyrylbenzene derivatives was prepared from its precursor poly(4-vinylbenzyl chloride) (PVBC: M_w = 10,400, PDI = 1.12), which had been synthesized by the controlled radical polymerization (RAFT). PDSB is readily soluble in common organic solvents and basically amorphous material with 5% weight-loss temperature higher than 390 °C. The HOMO level of PDSB, estimated from cyclic voltammetric data, is −5.15 eV, which is higher than −5.8 eV of PVK and thus effective in reducing the hole injection barrier between the PEDOT:PSS and emitting layer. The blend films of fluorescent isolated blue emitter (PDSB) and phosphorescent green system [PVK/Ir(ppy)₃] were prepared to tune color emission. The blend films [PVK:PDSB:Ir(ppy)₃] show composition-dependent behavior in PL spectral properties and EL characteristics. The PL spectra of the blend films [PVK:PDSB:Ir(ppy)₃] show a major emission at 515 nm and a minor peak at 454 nm, which are attributed to Ir(ppy)₃ and PDSB, respectively. The C.I.E. 1931 coordinates of the blend LED devices shift from (0.29, 0.61) for PVK:PDSB = 10:0 to (0.25, 0.35) for PVK:PDSB = 0:10 with the increase of PDSB content.     

On the energy transfer from a polymer host to the rhenium(I) complex in OLEDs

Photoluminescence and electroluminescence of PVK films doped with fac-[ClRe(CO)₃(bpy)], bpy = 2,2′-bipyridine, are investigated. Photoluminescence spectra of spin-coated PVK films (λ_exc = 290 nm) exhibit a broad band centered at 405 nm. As the concentration of dopant increases, the polymer emission is quenched and a band at 555 nm appears (isosbestic point at 475 nm). In OLEDs with ITO/PEDOT:PSS/PVK/butylPBD/Al architecture doped with fac-[ClRe(CO)₃(bpy)], the polymer host emission is completely quenched even at the lowest concentration of dopant. The electroluminescence spectra of the devices show that there is an efficient energy transfer from the host to the dopant, which exhibits a very intense emission at 580 nm.     

Solution processable organic electro-phosphorescent iridium complex based on a benzothiazole derivative

We have synthesized a new solution processable iridium complex, di[2-(4′-octyloxyphenyl) benzothiazole]iridium(III)acetoacetone, [(OPBT)₂Ir(acac)], based on benzothiazole derivative for organic electro-phosphorescent devices. The synthesized molecule was identified by ¹H NMR and ¹³C NMR, and readily soluble in common organic solvents such as chlorobenzene. The UV–visible absorption and photoluminescence properties of pristine [(OPBT)₂Ir(acac) thin film as well as poly(N-vinylcarbzole) (PVK) thin film doped with the iridium complex were studied. The maximum UV–visible absorption and photoluminescence (PL) spectra are found to be at 337 nm and 547 nm, respectively. We have fabricated phosphorescent organic light-emitting devices using the ITO/PEDOT:PSS (40 nm)/PVK:(OPBT)₂Ir(acac) (40 nm)/Balq (40 nm)/LiF (1 nm)/Al (80 nm) configuration with the iridium complex as a triplet emissive dopant in poly(N-vinylcarbazole) (PVK) host. The electroluminescence (EL) devices showed greenish yellow light emission with maximum peak at 551 nm. Especially, the maximum external quantum and current efficiency of 1 mol% doped device were 1.74% and 4.89 cd/A, respectively.     

Polymer-based blue electrophosphorescent light-emitting diodes based on a new iridium(III) diazine complex

Polymer-based blue electrophosphorescent light-emitting diodes are reported by doping iridium(III) diazine complex (DFPPM)₂Ir(pic) (DFPPM = 2-(2,4-difluorophenyl)-pyrimidine, pic = picolinate) in poly(vinylcarbazole) (PVK) as the source of emission. The iridium(III) diazine complex shows sky blue emission with its maximum emission peak at 476 nm originating from an admixture of triplet metal-to-ligand charge-transfer and ligand-centered states. The (DFPPM)₂Ir(pic)-doped device furnishes a maximum external quantum efficiency of 2.2% and power efficiency of 1.9 lm/W at a 10 wt% doping concentration.     

Broad band and white phosphorescent polymer light-emitting diodes in multilayer structure

Efficient phosphorescent polymer light-emitting diode with poly(vinylcarbazole) (PVK) doped by two or three iridium complexes in single and bilayer structures are studied. With (tris-(2-4(4-toltyl) phenylpyridine) (Ir(mppy)₃) as the green emitter and (1-phenylisoquinoline) (acetylacetonate) iridium(III) (Ir(piq)₂) as the red emitter the efficiency is as high as 23 cd/A with broad band emission from 500 nm to 720 nm. For white emission a second layer is added with blue emitter ((III) bis[(4,-6-di-fluorophenyl-pyridinato)N,C²] picolinate) (FIrpic) doped in PVK. White light containing three spectral peaks results with efficiency 8.1 cd/A. As the second blue layer is replaced by the fluorescent (poly(9,9-dioctylfluorene)) (PFO) white emission with high color rendering index 86 is achieved. The efficiency is 5.7 cd/A with peak luminance 8900 cd/m². For a given iridium complexes ratio the relative intensity of the green and red emission depends sensitively on the second blue layer.     

Luminescence and photovoltaic cells of benzoselenadiazole-containing polyfluorenes

In this work, light-emitting and photovoltaic properties of new low band-gap copolymers (PFO-DBSe) based on 9,9-dioctylfluorene and 4,7-di-2-thienyl-2,1,3-benzoselenadiazole (DBSe) were investigated. The peak positions of photoluminescence and electroluminescence (EL) of the copolymers reach 758 and 727 nm, respectively, and locate in near-infrared light region. At a current of ∼5 mA, EL emission with peak position of 701 nm and a maximum external quantum efficiency of 1.62% was achieved from EL device with poly(9,9-dioctylfluorene) and the copolymer with DBSe content of 5% as the blend-type emitter, by utilizing of a configuration of ITO/PEDOT/PVK/emitter/CsF/Al. This is a high efficiency for deep red EL emission. The copolymer with DBSe content of 35% possesses good solar light spectral coverage according to its absorption spectrum, and a bulk-heterojunction photovoltaic cell with the copolymer as the electron donor and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor displayed an energy conversion efficiency of 0.91% under an AM1.5 solar simulator at 100 mW/cm².     

Photo-, and electroluminescence studies of 2,5-bis[2′-(4″-(6-hexoxy benzyl))-1′-ethenyl]-3, 4-dibutyl thiophenes

We have synthesized a thiophene-based compound 2,5-bis[2′-(4″-(6-hexoxy benzyl))-1′-ethenyl]-3, 4-dibutyl thiophene (HBDT) and a copolymer poly(2,5-bis(2′-(4″-(6-hexoxy benzyl))-1′-ethenyl)-3,4-dibutyl thiophene-1,6-diisocyanatohexane) (HBDT-PU) consisting of alternating HBDT and urethane spacer units. Absorption and photoluminescence (PL) spectra of the compounds coincide indicating that emission in HBDT-PU occurs from the thiophene containing unit. PL is emitted in the blue–green region with a maximum at 460 nm. Concentration quenching occurs in pure materials: PL efficiency is strongly enhanced when the compounds are dispersed in a polymer matrix like poly(N-vinyl carbozole) (PVK) or poly(methyl metacrylate) (PMMA). Light emitting devices were fabricated using a PVK:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) blend doped with HBDT as the emitter material. Efficient energy transfer from PVK:PBD to HBDT molecules takes place in blended films. The electroluminescence (EL) spectra coincided with the PL spectra of HBDT indicating that EL emission comes solely from HBDT molecules. The influence of the doping concentration on the EL efficiency was found to coincide with the concentration dependence of the PL quantum yield.     

Poly(N-vinylcarbazole) (PVK) deposited by evaporation for light emitting diodes thin films structures

The sandwich structures have been obtained by deposition of organic thin films on commercial tin oxide. Then the organic films were covered by a thin aluminium film. All the films were deposited by evaporation. The organic component was either a thin (150 nm) poly(N-vinylcarbazole) (PVK) film or a bilayer constituted of a thin PVK layer covered with a 8-hydroxyquinoline, aluminium (Alq₃). When PVK is deposited by classical thermal evaporation there is a shortening effect of the chain length, however, the main properties of the vinyl-carbazole molecules are preserved. These films are photoluminescent with blue light emission. The I–V characteristics of the sandwich structures are typical of such organic sample. They exhibit a rectifying behaviour, with a forward polarisation when the SnO₂ electrode is positively biased. They also exhibit electroluminescence. When a single PVK film is used, the signal is quite faint. It is improved when an organic bilayer is used. However, simultaneously there is a red shift of the signal.     

Poly(4-vinyltriphenylamine): Optical,electrochemical properties and its new application as a host material of green phosphorescent Ir(ppy)3 dopant

Vinyl polymer PTPA containing pendant triphenylamine chromophore, which possesses hole-transport characteristics, was used as a host material in green phosphorescent light-emitting diodes. The PL spectrum of PTPA showed an intrinsic peak (375 nm) attributed to triphenylamine groups and an excimer emission (440 nm). The PL and EL spectra of the blends [PTPA:Ir(ppy)₃] showed dominant green emission attributed to Ir(ppy)₃ due to efficient energy transfer from PTPA to Ir(ppy)₃. The HOMO level of PTPA, estimated from cyclic voltammetric data, was −5.36 eV, which is higher than −5.8 eV of PVK owing to hole-affinity characteristics of the pendant triphenylamine groups. The best performance was obtained with the EL device (ITO/PEDOT:PSS/PTPA:Ir(ppy)₃ (1 wt%):PBD (40 wt%)/Ca/Al), the maximal luminance and the maximal luminance efficiency were 8358 cd/m² and 4.5 cd/A, respectively. The present study suggests that the polymer PTPA is versatile host materials for green phosphorescent polymer light-emitting diodes applications.     

Photogeneration and photovoltaic effect in poly(N-vinylcarbazole):TiO2 bulk-heterojunction elaborated by hydrolysis–condensation reactions of TiO2 precursors

TiO₂:poly(N-vinylcarbazole) (PVK) bulk-heterojunctions (BHJ) have been elaborated from a blend of a TiO₂ precursor with the polymer in solution. The TiO₂ precursor is hydrolyzed with the surrounding air humidity in thin film. Titanium isopropoxide [Ti(ⁱOPr)₄] and Tetrakis (9H-carbazole-9-yl-ethyl-oxy) titanium [Ti(OeCarb)₄] were used as TiO₂ precursor. Photogeneration quantum yield in BHJ of TiO₂:PVK were determined by the surface potential decay (SPD) technique. Photovoltaic effect was studied in ITO/PEDOT:PSS/TiO₂:PVK/Al structures. It was found that the photovoltaic performance is strongly dependent on the homogeneity of the blends. The improved dispersion of the TiO₂ phase obtained with the precursor bearing a carbazole group allows efficient electron transport and fill factor enhancement of the photovoltaic devices.     

Spectrally-narrow blue light-emitting organic electroluminescent devices utilizing thulium complexes

Organic electroluminescent (OEL) devices with trivalent thulium (Tm³⁺) complex (Tm(acetylacetonato)₃(monophenanthroline) [Tm(AcA)₃phen]) as an emitting layer were fabricated. Double-layer type cells with a structure of glass substrate/indium–tin oxide (ITO)/poly-(vinylcarbazole) (PVK)/Tm complex/Al exhibit blue luminescence when forward bias dc voltage was applied. Spectrally-narrow OEL emission at 482 nm from Tm³⁺ ion in Tm complex was observed.     

Preparation of poly(N-vinylcarbazole) (PVK) nanoparticles by emulsion polymerization and PVK hollow particles

PVK nanoparticles were obtained by oil-in-water emulsion polymerization of N-vinylcarbazole (VCz). VCz is a white crystalline material with a melting point of 65 °C, which is an unsuitable monomer for conventional emulsion polymerization because of its crystallinity. However, we could successfully synthesize PVK nanoparticles by emulsion polymerization from VCz solution with organic solvent. And then, we found that the concentration of VCz influenced the particle size. PVK hollow particles were also obtained by etching of PMMA core from PMMA/PVK core–shell particles, and the size of a hollow particle was enlarged by two times compared with a core–shell particle due to the swelling process. PVK nanoparticles were probed by UV–vis absorption and fluorescence spectrometers, and hollow particles were characterized by FT-IR spectra. Both of them were confirmed by TEM, and the light-scattering instrument.     

White emission from a ternary polymer blend by incomplete cascade energy transfer

White light emission was obtained from a light-emitting diode (LED) prepared from a ternary polymer blend (19:1:1 by weight) consisting of poly(9-vinylcarbazole) (PVK), poly(9,9′-dihexlyfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-phenylenevinylene) (CPDHFPV), and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV), where the order of bandgap energy is PVK>CPDHFPV>MEH-PPV. The major component PVK acted not only as the matrix or diluent but also as the excitation energy donor to help the blend generate white light with high efficiency. Good miscibility between PVK and CPDHFPV facilitated the Förster-type excitation energy transfer from PVK to CPDHFPV enhancing the quantum efficiency. However, poor miscibility between CPDHFPV and MEH-PPV resulted in partial energy transfer between the polymers causing the blend to emit two colors simultaneously. Consequently, the incomplete cascade energy transfer in the blend generated a pure white color near CIE coordinate (0.33, 0.33) and the emissive color of this system showed a low sensitivity to the drive voltage.     

Synthesis and characterization of novel graft copolymers of Poly(N-vinylcarbazole) and Poly(3-methylthiophene) for optoelectronic applications

In this paper, we report the synthesis and characterization of a new material based on Poly(N-vinylcarbazole) (PVK) and Poly(3-methylthiophene) (PMeT), which has been chemically prepared by cross-linking of PVK in the presence of 3-methylthiophene monomers in chloroform with anhydrous FeCl₃. Thus, two samples were prepared, PVK–3MeT1 and PVK–3MeT2, obtained firstly in the fully doped state and successfully dedoped by chemical treatment. They were characterized by various spectroscopic methods as well as thermal analysis and conductivity measurements. Then, the formation of PMeT by oxidative coupling and its grafting into the skeleton of PVK is shown. Furthermore, the optical analysis shows that the new materials exhibit a blue-shift compared to that of the PMeT homo-polymer. The obtained graft copolymer presents interesting optical and thermal properties compared to that of the homo-polymers.     

Effects of Ag layers on the SiO2/FePt thin films deposited by magnetron sputtering

The effects of Ag layers with different locations and thicknesses on the structural and magnetic property of SiO2/FePt multilayer films were investigated.The non-magnetic Ag layer plays an important role in inducing（001） orientation and ordering of FePt grains,as well as the SiO2-doping reducing the grain size and the magnetic exchange coupling between grains.When the 10 nm Ag layer is moved from the bottom to the top of the SiO2/FePt multilayer film,the coercivity gradually decreases;the largest difference between the out-of-plane coercivity and the in-plane one is obtained in the sample of [SiO2（2 nm）/FePt（3 nm）]3/Ag（10 nm）/[SiO2（2 nm）/FePt（3 nm）]2.Furthermore,the location of Ag layers was fixed and the thickness was changed.The XRD curves suggest that the intensity of the（001） peak becomes the strongest with the addition of 10 nm Ag layers.     

Thermal stability and electric properties of Ba0.7Sr0.3TiO3 parallel plate capacitor with nano-Cr interlayer

A novel sandwich structure of Ba_0.7Sr_0.3TiO₃/Cr/Ba_0.7Sr_0.3TiO₃ (BST/Cr/BST) was sputtered onto Pt/Ti/SiO₂/Si substrate. With the insertion of a Cr layer, the leakage currents are decreased and the thermal stability of the specimens is enhanced. Temperature coefficient of capacitance (TCC) of specimens with BST(200 nm)/Cr(2 nm)/BST(200 nm) multifilms can achieve about 83% lower than those with BST (400 nm) monolayer. However, the dielectric constant of the BST(200 nm)/Cr(2 nm)/BST(200 nm) multifilms decreases to about 37% of that BST monolayer. The leakage current densities under an electric field of 125 kV/cm at 90 °C are 4 × 10^− 4 A/cm² and 9 × 10^− 1 A/cm² for BST (200 nm)/Cr (2 nm)/BST (200 nm) and monolayer BST (400 nm), respectively. X-ray diffraction results indicate the formation of a CrO₃ secondary phase after annealing at 700 °C or above in O₂ atmosphere. The root causes for the improvement of leakage currents and thermal stability with the insertion of nano-Cr interlayer are explored. The results show the insertion of Cr-nanolayer improves the electric properties for application in capacitors.     

Hole-injection properties of annealed polythiophene films to replace PEDOT–PSS in multilayered OLED systems

Hole-injection properties of annealed poly(alkoxy- and alkylthiophene) films in OLEDs were studied. Among them, annealed poly(3,3′-dihexyloxy-2,2′-bithiophene) (aPHOBT) film exhibited good hole-injection properties and a triple-layered OLED with the structure ITO/aPHOBT/PVK/Alq3/Mg–Ag (device I) showed much higher performance than a double-layered device without the aPHOBT layer (device II, ITO/PVK/Alq3/Mg–Ag). Device I was slightly inferior to a device having a PEDOT–PSS layer as the hole injector (device III, ITO/PEDOT– PSS/PVK/Alq3/Mg–Ag) in the low-intermediate region of the applied voltage (6–11 V), but gave comparable luminance to III when the applied voltage exceeded 11 V.