Improved electron injection in organic LED with lithium quinolate/aluminium cathode

Lithium quinolate (Liq), covered with aluminium, was used as an electron injection layer in a double layer organic light emitting diode consisting of NPD as the hole transport layer and Alq as the emitting layer resulting in lower turn on voltage and increased power efficiency. The driving voltage required to achieve a luminance of 100 cd/m² decreased from 5.8 V for the Ca/Al to 4.2 V when Liq/Al was used, improving device power efficiency from 2.3 to 4.1 lm/W. The performance tolerance to layer thickness of Liq devices is also better than that of the devices with lithium fluoride (LiF). Due to the highly insulating nature of LiF, it can only be used when deposited as an ultra-thin layer, while the Liq can be deposited into layers as thick as 5 nm without significantly affecting the EL properties. An Liq electron injecting layer has also been tried in combination with Ca, Mg and Ag cathodes. Our experiments support the assumption that free lithium is released from lithium quinolate, as in the case of lithium fluoride, when Liq is over coated with active metals such as Al.     

Organic tunable electroluminescent diodes from polyfluorene derivatives

We report on the electroluminescent properties of recently synthesized fluorine-based π-conjugated polymers. The spectral emission varies from blue to yellow depending on the composition of the alternated copolymers containing thiophene or phenylene moieties. The luminance of the devices can be enhanced by adequate balancing of the hole and electron injection/transport. Incorporation of a hole transporting molecule in the polymer and insertion of an insulating buffer layer in the device resulted in enhancement of the luminous efficiency. A 30-fold enhancement of the luminance was obtained by inserting an electron transporting layer. The highest luminance reached was 1640 cd/m² at 17 V and was obtained with a green emitter, poly(2,2′-(5,5′-bithienylene)-2,7-(9,9-dioctylfluorene)) (PBTF).     

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.     

Conjugated palladium complex with poly(3-heptylpyrrole) and its application

Poly(3-heptylpyrrole) was demonstrated to serve as an efficient π-conjugated ligand to afford a conjugated complex with Pd(MeCN)₂Cl₂, which was successfully applied to organic light emitting diode (OLED) devices. An OLED device with the conjugated complex film as a hole injection layer performed the maximum luminance of 11,000 cd/m² at 10 V, which was 2 V lower than a device with the conventional copper phthalocyanine (CuPc) hole injection layer.     

Efficient single layer organic light emitting diodes based on a Terbium pyrazolone complex

Single layer devices of the organolanthanide complex, terbium Tris-(1-phenyl-3-methyl-4-(tertiarybutyryl)pyrazol-5-one)triphenylphosphine oxide [(tb-PMP)₃Tb(Ph₃PO)] were made to investigate its light emission and current transporting properties. Ca and Mg layers were used for the cathode contact. A higher current density at much lower voltages can be attained with Ca cathode because of the enhanced electron injection. The maximum brightness of a single layer device with a Ca cathode was 226 cd/m² at 18 V and the best electroluminescence (EL) efficiency was 0.67 cd/A at 14 V and 70 cd/m².     

Effect of SPFGraphene dopant in MEH–PPV organic light-emitting devices

In this paper, graphene acts as the acceptor material in the luminous layer. The doping behavior of graphene in MEH–PPV has improved the device performance due to the efficient electron injection and transport through highly conductive graphene. When the graphene content is 0.02 wt%, the highest EL brightness reaches 1960 cd/m² and the threshold voltage declines from 8 V to 5 V. When the graphene content is 0.02 wt%, the device has the highest EL brightness compared with the devices of other graphene content at the same current density. The doping graphene into MEH–PPV results in the best luminous efficiency and balanced electron and hole mobilities in the active layer.     

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.     

Highly efficient bilayer blue phosphorescent organic light-emitting devices

We have developed highly efficient blue phosphorescent organic light-emitting devices comprising of two organic layers. Hole transporting 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) was used as an emitting host for iridium(III)bis[(4,6-di-fluorophenyl)-pyridinato-N,C²′]picolinate (FIrpic) guest. In our bilayer system, the host–guest energy transfer process leads to a low optimal doping concentration of 2 wt%, while the better charge balance is achieved by the better electron injection into the host layer from electron transport layer. Using these bilayer structures, we demonstrate a maximum current efficiency of 34 cd/A in the device structure of ITO/TAPC: FIrpic (30 nm, 2 wt%)/3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (50 nm)/LiF/Al.     

Near-IR electromer emission from new ambipolar carbazole containing phosphorescent dendrimer based organic light emitting diode

The solution process is more suitable for large-sized OLED in terms of small material usage and no need for fine mask patterning technique. Among soluble materials, dendrimers are acknowledged as a new class of emissive OLED materials that can carry both the emissive chromophore and the charge transporting moiety simultaneously. Here, we report on a new Ir(III)-based dendrimer and an efficient single emission layer OLED device by spin-coating the dendrimer itself on PEDOT:PSS polymer. This dendrimer carries a pair of carbazole functional groups on the pyridine side and a fluorinated phenyl on the other side of the Ir(ppy)₃ ligand. The latter functional group acts as an electron transporting material and the former as a host and hole-transporting material. The dendrimer device has a green emission at 540 nm with current efficiency as high as 25 Cd/A. Interestingly, we have found that the two neighboring carbazole form the electromer under the applied electrical current and, thus, that the EL device shows new near-IR emission over 720 nm.     

Effects of casting solvents on characteristics of organic EL devices containing poly(l-glutamate) as hole-transport material

The characteristics of EL devices containing α-helical poly(l-glutamate) and having a carbazole side chain (PCELG) were found to strongly depend on the casting solvents. Among EL devices fabricated using chlorine-containing casting solvents such as 1,2-dichloroethane (DCE), 1,1,2,2-tetrachloroethane (TCE), chloroform (CHCl₃), and monochlorobenzene (?-Cl), the EL device fabricated using the DCE solvent exhibited the maximum luminance (65.8 cd/m²). The difference between the threshold voltages of the devices ranged up to 6.0 V, despite their fabrication by using solutions with the same composition ratio. The maximal efficiency of the devices fabricated using the DCE solvent was found to be 20 times greater than that of the devices fabricated using TCE solvents. A comparison of the current densities and voltages among devices fabricated using different casting solvents at their maximal efficiencies showed that the maximal efficiencies tended to significantly increase in the order TCE < CHCl₃ < DCE solvents at similar voltages (～15 V), despite a lowering of the current density. The current density was considered to be directly related to the number of carriers injected into the device. The above-mentioned observations suggested that the maximal efficiency in these devices was not the hole and electron injection efficiencies, but the values of some parameters subsequent to carrier injection, such as the recombination rate, amount of excitons generated, and the diffusion length.     

Synthesis and optical properties of quinoxaline-containing poly(aryl ether)s

Three novel poly(aryl ether)s were synthesized from the reaction of three bisphenols with 2,3-bis(4-fluorophenyl)-quinoxaline via nucleophilic aromatic substitution. The polymers contained electron transporting and emissive moieties separated by ether linkages. Quinoxaline was used as electron transporting segment, while p-distyrylbenzene, 2,6-distyrylpyridine and p-quinquephenyl were used as emissive segments. The low reactivity 2,3-bis(4-fluorophenyl)-quinoxaline towards nucleophilic aromatic substitution results in polymers with limited molecular weights. The polymers were soluble in polar organic solvents and strong organic acids. In THF solutions the polymers showed absorption maxima at 349–354 nm and emission maxima at 417–454 nm, with quantum yields of 22–41%. In solid state the polymers showed absorption maxima at 350–353 nm and emission maxima at 427–461 nm. The optical properties of polymers were influenced by intrachain interactions in solution, while interchain interactions were important in solid state.     

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).     

Highly efficient red electroluminescence induced by efficient electron injection and exciton confinement

Highly efficient DCJTB-doped device was realized by enhanced electron injection and exciton confinement. A fluorine end-capped linear phenylene/oxadiazole oligomer 2,5-bis(4-fluorobiphenyl-4′-yl)-1,3,4-oxadiazole (1) and a trifluoromethyl end-capped oligomer 2,5-bis(4-trifluoromethylbiphenyl-4′-yl)-1,3,4-oxadiazole (2) were designed and incorporated as an electron transporting/hole blocking material in the device structure ITO/NPB (60 nm)/DCJTB:Alq₃ (0.5%, 10 nm)/1 or 2 (20 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm). The devices showed highly efficient red luminescence. In particular, the device based on 1 achieved pure red luminescence at 620 nm originating from DCJTB, with a narrow FWHI of 65 nm, maximal brightness of 13,300 cd/m² at voltage of 20.8 V and current density of ca. 355 mA/cm². High current and power efficiencies (>3.6 cd/A, 1.0 lm/W) were retained within a wide range of current densities. Our results show efficient and stable DCJTB-doped red electroluminescence could be anticipated for practical applications by taking advantage of the present approaches. The control experiments using BCP were also studied.     

Micrometer-scaled OFET channel patterns fabricated by using PEDOT/PSS microfibers

We have easily fabricated channel patterns of organic field-effect transistors (OFETs), in which channel lengths were 5 μm, by using wet-spun poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) microfibers with diameters of ca. 5 μm. Pentacene-based FETs with a top-contacted configuration, showed a hole mobility of 0.13 cm² V^?1 s^?1 and on/off current ratio of 9.4 × 10⁴. The device also showed large current of ca. 160 μA (V_D = ?50 V; V_G = ?50 V), reflecting shorter channel length of the devices. We have evaluated self-assembled monolayer (SAM) through measurements of water contact angle and by Zisman plot. As a result, critical surface tension of the octadecyltrichlorosilane (OTS)-treated SiO₂ surface is 17 mN m^?1 which is consistent with that of well-known SAM. We also well analyzed the cross-sections of the device by a scanning transmission electron microscopic (STEM) technique. The results indicated that the thicknesses of the pentacene layer in the channel part and Au layer in the source/drain part were ca. 30 and 30 nm, respectively. Furthermore, it is also indicated that the 100-nm grains of the pentacene were well adhered on the surface of the SAM-formed SiO₂ layer.     

Depletion- and ambipolar-mode field-effect transistors based on the organic heterojunction composed of pentacene and hexadecafluorophtholocyaninatocopper

We investigated heterojunction organic field-effect transistors (OFETs) using pentacene and hexadecafluorophtholocyaninatocopper (F₁₆CuPc) as double active layers. Two operation modes including depletion- and ambipolar-type were observed depending on the deposition order of two organic films. Depletion-mode was firstly observed from that devices with pentacene as the bottom layer and F₁₆CuPc as the top layer, which was attributed to dipole effects originated from the pentacene/F₁₆CuPc interface. Then improved device performances were obtained with mobility from 0.87 to 1.06 cm²/V s, and threshold voltage shifted from ?20 to +25 V as compared with conventional pentacene-based devices. Furthermore, the heterojunction OFETs exhibited typical ambipolar transport when alternating the deposition order of two films, which exhibited ambipolar mobilities with 0.06 cm²/V s for electron and 0.0025 cm²/V s for hole, respectively. All results implied the utilization of heterojunction can effectively improve the device performances of OFET, and operation mode strongly on the deposition order of two films.     

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.     

Highly efficient blue electroluminescence based on a new anthracene derivative

A novel blue-light-emitting material, 2,3,6,7-tetramethyl-9,10-dinaphthyl-anthracene (TMADN), was synthesized and characterized. Organic light-emitting diode (OLED), which has a double-layer structure, has been fabricated. In this OLED, the homemade TMADN was used as the light-emitting material and 4,7-diphenyl-1,10-phenanthroline (DPA) was used as the hole blocking/electron transporting material, N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphenyl]-4,4′-diamine (NPB) was used as the hole transporting material. The peak emission of electroluminescence (EL) is at about 456 nm and the CIE coordinates are (0.171, 0.228). The brightness of the device is up to 5600 cd/m² at 17 V with the maximum EL efficiency of 2.2 cd/A.     

N-channel operation of pentacene thin-film transistors with ultrathin polymer gate buffer layer

N-channel operation of pentacene thin-film transistors with ultrathin poly(methyl methacrylate) (PMMA) gate buffer layer and gold source–drain electrode was observed. We prepared pentacene thin-film transistors with an 8-nm thick PMMA buffer layer on SiO₂ gate insulators and obtained electron and hole field-effect mobilities of 5.3 × 10^?2 cm²/(V s) and 0.21 cm²/(V s), respectively, in a vacuum of 0.1 Pa. In spite of using gold electrodes with a high work function, the electron mobility was considerably improved in comparison with previous studies, because the ultrathin PMMA film could decrease electron traps on SiO₂ surfaces, and enhance the electron accumulation by applied gate voltages.     

Study on the characteristics of metal–organic interface for organic thin-film transistors

In this study, the interfacial characteristics between pentacene and Au layers were investigated with varying of the deposition rate of Au layer from 1.0 to 15.0 Å/s. For the devices with the structure of bottom-Au/pentacene/top-Au, it was observed that the electrical characteristics could be improved by increasing the deposition rate of top-Au, and the highest electrical conductivity value, 1.5 × 10⁻⁶ S/cm, was obtained for the device with the top-Au-deposited at 15.0 Å/s. AES results showed that the integrated atomic content of Au in top-Au layer is substantially increased with the deposition rate of top-Au, but there was no critical difference in the depth profile of Au atoms regardless of the deposition rate of top-Au. And also, we fabricated pentacene-based Schottky diodes and measured the hole injection barrier heights from Au electrode into pentacene layer using Fowler-Nordheim theory. Upon the investigations, it was observed that the hole injection barrier was reduced with increasing the deposition rate of Au electrode and the lowest value of 0.12 eV was obtained for the device with the Au electrode deposited at 15.0 Å/s. As a result, the performance of top-contact OTFT could be improved with increasing the deposition rate of Au electrodes (source and drain).     

Polymer light-emitting diodes with a phenyleneethynylene derivative as a novel hole blocking layer for efficiency enhancements

This paper reports on the use of an electron transport layer (ETL) in polymer light-emitting diodes based on poly(2,5-bis(3′,7′-dimethyl-octyloxy)1,4-phenylene-vinylene) (BDMO-PPV). This ETL is inserted between BDMO-PPV and a calcium cathode as a hole blocking layer (HBL). A novel phenyleneethynylene derivative (ImPE) is proposed and compared to well-known materials such as tris(8-hydroxyquinoline) aluminum (Alq₃) and bathocuproïne (BCP). Efficient hole blocking is achieved leading to yield improvements at low luminances. With a 8 nm-thick ImPE layer, at 1 cd/m², the power efficiency reaches 1.2 lm/W whereas a BDMO-PPV-only PLED exhibits a 0.13 lm/W power efficiency. ImPE enables to reach higher performances than Alq₃ for low luminances (<20 cd/m²). However, for luminances higher than 350 cd/m², it is demonstrated that the hole blocking in no more efficient because of a too strong electric field.