Manufacture of brightness enhancement films (BEFs) by ultraviolet (UV) irradiation and their applications for organic light emitting diodes (OLEDs)

By ultraviolet (UV) irradiation, brightness enhancement films (BEFs) have been successfully manufactured with UV-curable polymers and applied for organic light emitting diodes (OLEDs).With BEFs, either green OLEDs (BEF/ITO glass/NPB (30 nm)/Alq₃ (65 nm)/LiF (0.5 nm)/Al (100 nm)) or white OLEDs (BEF/ITO glass/TAPC (40 nm)/mCP:Os:Firpic mixture (weight ratio = 82:17:1; 25 nm)/BCP (15 nm)/Alq₃ (30 nm)/LiF (0.5 nm)/Al(150 nm)) exhibit better electroluminescent performances than those without BEFs. In case of green OLEDs, the luminance and electroluminescent yield with 45° compound BEFs are, respectively, 1.51-fold and 1.42-fold (at 9 V, 60 mA/cm²) larger than those without BEFs. In case of white OLEDs, moreover, the luminance and electroluminescent yield with 45° compound BEFs are, respectively, 1.28-fold and 1.21-fold (at 9 V, 16 mA/cm²) larger than those without BEFs.     

Highly efficient energy transfer to a novel organic dye in OLED devices

The neutral 4,4-difluoro-8-(2,2′:6′,2″-terpyridine-4′-yl)-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (Boditerpy) molecule was synthesized and incorporated as dopant (<1%) in double-layer organic light emitting diodes (OLEDs) with the configuration ITO/α-NPD(60 nm)/Alq₃(60 nm):Boditerpy (0.4 nm)/LiF(0.02 nm)/Al(80 nm). This device exhibits green emission with a brightness of 545 cd/m² at 8 V and a maximum power efficiency of 0.9 lm/W. A full quantitative energy transfer process is indicated by a complete quenching of light emission from Alq₃ in photoluminescence. However, I–V characteristics indicate some losses during the charge transfer processes in OLED configuration     

Synthesis of 2-phenylquinoline-based ambipolar molecules containing multiple 1,3,4-oxadiazole spacer groups

A series of donor–acceptor type ambipolar electroluminescence dyes with the general structure PQ(OXD)_nT (where n = 1, 2 and 3) were prepared, in which PQ is 2-phenylquinoline, T is diphenylamine which constituted the hole transporting triphenylamine moiety with an adjacent phenyl ring, and OXD is an electron transporting 2-phenyl-1,3,4-oxadiazole repeating unit. The compounds fluoresced bluish green to green hue in solid-state, exhibited a positive solvatochromism in solution and their quantum efficiency decreased rapidly with increase in n. The materials are thermally stable with glass transition temperature (T_g) ranging from 83 °C (n = 1) to 130 °C (n = 3). Cyclic voltammetry studies indicated the HOMO remained relatively unchanged with n while the LUMO decreased (away from the vacuum level) with an increase in the number of OXD. For single layer homojunction OLEDs, highest efficiency was obtained when n = 1 (max luminous 3300 cd/m² and current efficiency 0.9 cd/A), whereas for multilayer heterojunction OLEDs, best results was achieved for compounds with n = 1 or 2 assuming the role of the HT layer (over 8200 cd/m² max and 2.0 cd/A). Formation of exciplexes led to significant red-shift and lower emission efficiency for the compound with n = 3.     

Cyclometalated iridium(III) complexes with 5-acetyl-2-phenypyridine derived ligands for red phosphorescent OLEDs

A series of new iridium complexes with 5-acetyl-2-phenylpyridine derivatives as ligands was developed. The complexes exhibited high EL performance when applied to OLEDs. These materials showed red emission with a peak at 575–636 nm. In particular, one of the devices in this study showed a maximum luminous efficiency, maximum power efficiency, external quantum efficiency and CIE coordinates of 29.0 cd/A, 6.13 lm/W, 8.86% at 20 mA/cm² and (0.57, 0.43) at 10 V, respectively. In addition, a deep red OLED with CIE coordinates of (0.67, 0.32) at 10 V exhibited a maximum luminous efficiency, maximum power efficiency and external quantum efficiency of 5.61 cd/A, 1.02 lm/W and 5.35% at 20 mA/cm², respectively.     

Undoped yellow-emitting organic light-emitting diodes from a phenothiazine-based derivative

The single layer and multilayer undoped light-emitting devices were fabricated using a new soluble phenothiazine-based derivative, poly(3,7-N-octyl phenothiozinyl terephthalylidene) (POPTP). Through the optimization of device structures, the multilayer device has a maximum luminance of 1203 cd/m² at the bias voltage of 9.3 V, using 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a hole-blocking layer and tris-(8-hydroxyquinoline)aluminium (Alq₃) as a electron-injection/transporting layer. The Commision International de L’Eclairage (CIE) coordinates stabilized at (x, y) = (0.46, 0.53) at various bias voltages. Additionally, the dominant wavelength (λ_D) of around 575 nm and the color purity of approximately 100% indicated a pure yellow emission property. Therefore, POPTP is a stable candidate material with a pure yellow emission for the undoped organic light-emitting diodes (OLEDs).     

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.     

OLEDs based on some mixed-ligand terbium carboxylates and zinc complexes with tetradentate Schiff bases: Mechanisms of electroluminescence degradation

Mixed-ligand terbium carboxylates—Tb(Carb)₃(TPPO)₂ (HCarb = HSal (salicylic acid), Hpobz (2-phenoxybenzoic acid); TPPO = triphenylphosphine oxide) and zinc complexes with tetradentate Schiff bases—ZnSB (H₂SB = H₂SAL1, H₂SAL2 (derivatives of salicylic aldehyde); H₂MO1, H₂MO2 (derivatives of o-vanillin)) were used as electroluminescent (EL) layers in organic light emitting diodes (OLEDs). These devices have undergone reversible and irreversible mechanisms of EL degradation. The reversible one is mainly associated with charge carrier trap filling. At the same time several mechanisms of irreversible degradation were observed under UV light irradiation, heating and ageing in ambient conditions. The degradation mechanisms of OLEDs under heating from 293 K to 320 K were related to the changes in the interface regions. The initial degradation was eliminated by (1) an application of alternating bias voltage; (2) a decrease of the substrate temperature and the velocity of the thermal evaporation of materials. The degradation of OLEDs was not observed under low UV irradiation that is promising for OLEDs operation under day light.     

Highly efficient deep-blue organic light-emitting diodes with doped transport layers

We demonstrate highly efficient, vapor-deposited blue organic light-emitting diodes (OLEDs) operating at low voltage. For reaching deep-blue color, we used two new fluorophores, 9,10-bis(9,9′-spirobi[9H-fluorene]-2-yl)anthracene (Spiro-Anthracene) from Covion, and 4,4′-bis-(N,N-diphenylamino)-tetraphenyl (4P-TPD) from Syntec-Sensient, sandwiched in between p- and n-type doped wide band-gap transport layers and appropriate blocking layers. These p-i-n OLED devices show high luminance and efficiency at low operating voltages. Both dyes emit deep-blue light at color coordinates of x = 0.15 and y = 0.09 (4P-TPD) and x = 0.15 and y = 0.18 (Spiro-Anthracene). Optimized devices containing Spiro-Anthracene reach a luminance of 100 and 1000 cd/m² already at a voltage of 2.9 and 3.4 V, respectively. At the same time, a deep-blue color with CIE color coordinates of x = 0.14 and y = 0.14 as well as good current efficiencies (3.9 cd/A at 100 cd/m²) and quantum efficiencies (3.7% at 100 cd/m²) are reached, which shows that the concept of doped transport layers and appropriate fluorescent emitters can be applied successfully to the preparation of blue OLEDs.     

Multilayer blue polymer light-emitting devices with spin-coated interlayers

Employment of multilayer heterostructures is a common approach to achieve efficiency and stable organic light emitting diodes (OLEDs). In this work, we report multilayer blue polymer light-emitting devices (PLEDs) by using spin-coated fluorene-triarylamine copolymers as interlayers between the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT) and the emitting layer. A blue PLED with stepped hole injection profile yields an external quantum efficiency of 6.0% at a luminance of 9500 cd/m² at 5.5 V and an extrapolated lifetime of more than 18,000 h from 100 cd/m².     

Luminescent properties of a novel naphthalimide-fluorene molecule

A novel naphthalimide-fluorene molecule, 4-(N,N-dimethylamino)-N-(2′-fluorenyl)-1,8-naphthalimide (DFN), has been synthesized and characterized, and its luminescent properties have been studied. DFN has an absorption maximum at 420 nm and possesses solvent polarity dependent changes. The environmental sensitivity exhibited the characteristics of an excited state charge transfer complex. DFN also showed strong luminescence, good electron-affinity, and temperature independence of fluorescence. The application of DFN in organic light-emitting diodes (OLEDs) as an electron-transporting electroluminescent material was investigated. The OLED with a structure of ITO/N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/DFN/Al shows a yellow-green emission (chromaticity coordinates: x = 0.424, y = 0.543) with a brightness of 3563 cd/m². The external quantum efficiency and the highest luminous efficiency of the device reach 0.2% and 0.55 lm/W, respectively.     

Synthesis,photophysical and electrophosphorescent properties of a novel fluorinated rhenium(I) complex

A novel fluorinated rhenium complex, i.e., Re-BFPP (BFPP, 2, 3-bis(4-fluorophenyl)pyrazino[2,3-f][1,10]phenanthroline) was designed, synthesized and characterized by ¹H NMR and mass spectroscopy. The light-emitting and electrochemical properties of this complex were studied. The organic light-emitting diodes (OLEDs) employing Re-BFPP as a dopant emitter with the structures of ITO/m-MTDATA (10 nm)/NPB (20 nm)/CBP: X wt.% Re-BFPP (30 nm)/Bphen (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) were successfully fabricated and a broad electroluminescent peak at 553 nm was observed. The 10 wt.% Re-BFPP doped device exhibited the maximum luminance of 6342 cd/m² and a peak current efficiency of 17.9 cd/A, corresponding to the power efficiency of 8.1 lm/W.     

3,12-Dimethoxy-7,8-dicyano-[5]helicene as a novel emissive material for organic light-emitting diode

3,12-Dimethoxy-7,8-dicyano-[5]helicene (DDH) was introduced as a novel emissive material for organic light-emitting diode. It shown good thermal stability and no glass transition temperature was observed. The LUMO, HOMO and energy band gap (?3.3, ?5.9 and 2.6 eV) of this compound were determined using cyclic voltammetry technique. Fluorescence quantum yield of DDH in chloroform is 0.27. The turn-on voltage of OLEDs with a configuration of ITO/PEDOT:PSS/DDH/Ca/Al was not a function of DDH thickness in a range of 60–100 nm. The best OLED, in which DDH thickness was 100 nm, exhibited a turn-on voltage of 3.7 V with maximum brightness of 1587 cd/m² at 8.0 V and 281 mA/cm². The maximum current efficiency and power efficiency were 0.64 cd/A and 0.29 lm/W, respectively. The CIE coordinates of the OLED electroluminescence, however, appeared to depend on the applied voltage as they were (0.38,0.47) at 5.0 V and (0.51,0.44) at 8.0 V.     

Electro-active monomers and polymers containing 3-arylcarbazol-9-yl fragments

Monomers and polymers containing 3-arylcarbazol-9-yl fragments were synthesized. The materials were examined by various techniques including differential scanning calorimetry, thermogravimetry, electron photoemission spectrometry and xerographic time of flight technique. The electron photoemission spectra of the layers of the derivatives showed ionization potentials of 5.35–5.45 eV. Hole drift mobilities in the layers of the monomers dispersed in polymeric host bisphenol Z polycarbonate (50%) range from 10^?6 to 1.6 × 10^?5 cm² V^?1 s^?1 at high electric fields at room temperature. The synthesized polymers have been tested as hole-transporting materials in bilayer OLEDs with Alq₃ as the emitter. The device with polymer containing (1-naphtyl)phenylamino substituted carbazolyl fragments exhibited the best overall performance with turn-on voltage of ～3 V, a maximum photometric efficiency of 4.1 cd/A and maximum brightness of ca. 22,900 cd/m².     

Preparation of Eu-activated strontium orthosilicate (Sr1.95SiO4:Eu0.05) phosphor by a sol–gel method and its luminescent properties

Eu²⁺-activated Sr₂SiO₄ phosphor was successfully synthesized by a sol–gel method using sodium silicate and SrO as the starting materials. The wavelength of the emission peak and the emission intensity of the phosphor powders were influenced by the pre-treating temperature. The maximum emission intensity of the phosphor was found as pre-treated at 1200 °C in air and then heated at 1300 °C in the reducing atmosphere (10% H₂ + 90% He). As the pre-treating temperature was <1200 °C, the composition of the phosphor powder was not uniform, which leads to decrease of the emission intensity, whereas >1200 °C, the decrease of the emission intensity may be caused from the reversible phase transformation of Sr₃SiO₅ → Sr₂SiO₄ at 1300 °C, which also shows the red-shift behavior.     

Low driving voltage in organic light-emitting diodes using MoO3 as an interlayer in hole transport layer

Driving voltage of organic light-emitting diodes (OLEDs) was lowered by applying MoO₃ as an interlayer between hole injection layer (HIL) and hole transport layer (HTL). MoO₃ was effective as an interlayer between HIL and HTL due to its valence band of around 5.3 eV which is suitable for hole injection. Hole injection from HIL to HTL was enhanced by MoO₃ interlayer and driving voltage of green fluorescent device could be lowered by 1.3 V at 1000 cd/m² by using thin MoO₃ interlayer.     

The z value dependence of photoluminescence in Eu2+-doped β-SiAlON (Si6−zAlzOzN8−z) with 1 ≤ z ≤ 4

Eu²⁺-doped β-SiAlON (Si_6?zAl_zO_zN_8?z) phosphors with 1 ≤ z ≤ 4 were synthesized by gas pressure sintering. The emission spectra exhibit two broad bands with maxima at about 415 nm (violet) and 540 nm (green) under ultraviolet excitation. The green emission is dominant at z ≤ 2, while the violet emission becomes dominant at z > 2. The combination of two emission bands in a compound could lead to a white emission in Eu²⁺-doped β-SiAlON with 2 < z ≤ 3.     

Direct emissive pattern formation in PPV-type polymer with built-in photoresist properties and the application to light emitting devices

The principle of photobleaching in the conjugated polymer material didodecylstilbenevinylene (didodecyl-PSV) was used as a built-in photoresist property to control the extent of the emissive layer in polymer light emitting devices (PLEDs). Illumination of thin films of the conjugated polymer material on optically transparent conducting substrates through a positive mask with short exposure times (20–60 s) using a 150 W Xenon lamp and subsequent application of the and the second electrode enabled the formation of an electroluminescent image of the mask. We demonstrate a direct method where no photoresist, intermediate steps or mechanical manipulation is required to generate a complex emissive pattern employing a homogenous film and homogenous electrodes. The fluorescence of the film showed complex lifetime decays. We found that the lifetime increased with emission wavelength indicating the presence of energetically disordered excitons. The fine structure observed in the solution emission is absent in the film emission. However, after photobleaching the film emission becomes very similar to the solution emission. The appearance of fine structure could indicate that only one state became responsible for the photoluminescence spectrum. Infrared spectroscopy was used to confirm the presence of a degradation route similar to that reported for dialkoxypolyphenylenevinylene materials where the vinylene bonds are react with molecular oxygen and form carboxylic acid groups. Finally the effect of using different electrode metals was established and a comparison made between the photoluminescence and electroluminescence properties. The electroluminescence emission maximum (580 nm) of didodecyl-PSV light emitting devices was found to be redshifted by 20 nm compared to the photoluminescence, exhibited turn-on voltages below 3 V and reached a luminance of 100 cd m⁻² at current densities below 10 mA cm⁻².     

A highly efficient deep blue fluorescent OLED based on diphenylaminofluorenylstyrene-containing emitting materials

We have designed and synthesized five blue emitters based on diphenylaminofluorenylstyrene emitting core groups. Multilayered OLEDs were fabricated using these materials as dopants in a 2-methyl-9,10-di(naphthen-2-yl)anthracene (MADN) host. One of them in particular a deep blue OLED using dopant 9-[4-(2-diphenylamino-9,9-diethylfluoren-7-yl)phenyl]-9-phenylfluorene (3) at 15% doping concentration exhibited a maximum luminance of 4720 cd m^?2 at 9.0 V, a luminous efficiency of 5.3 cd A^?1 at 20 mA cm^?2, a power efficiency of 2.9 lm W^?1 at 20 mA cm^?2, an external quantum efficiency of 4.8% at 20 mA cm^?2, and CIE coordinates (x = 0.15, y = 0.13) at 8.0 V. Furthermore, this deep blue device had very stable CIE coordinates of (x = 0.15, y = 0.13) that did not vary with doping concentration from 5% to 15%.     

Broadly tunable deep blue laser based on a star-shaped oligofluorene truxene

We report amplified spontaneous emission (ASE) and optically pumped deep-blue-emitting distributed feedback (DFB) lasers based on a star-shaped oligofluorene truxene molecule. A low ASE threshold of 2.1 kW/cm² at 439 nm was achieved. The material exhibits a high net gain of 38 cm^?1 and also low optical loss coefficient of 3.5 cm^?1. Second-order DFB lasers show tuning of the emission wavelength from 422 to 473 nm, and a minimum threshold density of 515 W/cm². This is the broadest tuning range (51 nm) reported for organic deep-blue/blue lasing materials.     

Polysilane based ultraviolet light-emitting diodes with improved turn-on voltage,stability and color purity

We demonstrate ultraviolet organic light-emitting diodes (OLEDs) with improved stability, low turn-on voltage and color purity by changing the cathode and annealing temperature of the polymer film. The electron injection process, which limits the electroluminescent performance of organic devices, has been enhanced tremendously by inserting a layer of LiF with appropriate thickness between the cathode and a poly(n-butylphenylsilane) (PS-4) layer, whose device structure is ITO/PEDOT:PSS/polysilane (PS-4)/LiF/Al. Devices with a LiF (6 Å) have the turn-on voltage of 4 V, which is lower than 7 V of devices made with Ca/Al layer. By inserting LiF as the anode interfacial layer, there is increase in the injection of electrons from Al (cathode) side due to tunneling effect and also act as hole blocking layer which enhance the recombination of electron and hole in the emissive layer. PS-4 is spin coated and annealed in vacuum for 1 h at different temperatures (90–120 °C). EL Spectra from these devices consists of white emission along with the UV peak. White emission is significantly suppressed when PS-4 is annealed at higher temperature and threshold voltage is lowest at 110 °C annealing temperature.