Efficient blue-to-violet organic light-emitting diodes

Organic light-emitting diodes emitting in the range of 400 nm (violet) to 460 nm (blue) are reported. The basic device structure consists of indium–tin oxide/N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/lithium fluoride (LiF)/aluminum. Offset of the energy levels at the TPD/BCP interface favors blocking of holes on the TPD side of the interface. Voltage-induced color change is observed and explained in terms of a switching from emission dominated by interfacial exciplex-induced recombination at low applied bias to one dominated by bulk exciton-induced recombination at high applied bias. With the addition of copper(II) phthalocyanine (CuPc) as an anode buffer layer and tris-8-(hydroxyquinoline) aluminum (Alq₃) as a cathode buffer layer, external quantum efficiencies as high as 0.5% at blue emission and 0.4% at violet emission have been obtained.     

Deep blue organic light-emitting diodes based on triphenylenes

We report about new easy-to-synthesize deep blue light-emitting organic materials. Various substituted low-molecular-weight triphenylene-derivatives have been prepared in a one-step procedure and are easily available on large scale and high purity. Furthermore, the synthesis of an oxetane functionalized, photo-crosslinkable triphenylene-based emitter material with enhanced film-forming properties is described. The low-molecular-weight emitters were vacuum-deposited, whereas the photo-crosslinkable emitter material derivative was processed from solution. The optical and electrical properties of the compounds were investigated. The corresponding photoluminescence emission spectra exhibit λ_max,ems values around 400 nm. Organic light-emitting multi layer devices were fabricated and characterized. OLED devices from these molecules emit deep blue light of 436–456 nm.     

Self-assembled monolayer of 2-(octadecylthio)benzothiazole for corrosion protection of copper

Self-assembled monolayer (SAM) of 2-(octadecylthio)benzothiazole (2-OTBT) was formed on a fresh copper surface obtained by nitric acid etching. Optimum conditions for formation of SAM have been established. XPS, AFM and FTIR studies have been used to characterize the SAM. Corrosion protection ability of the SAM has been evaluated in aqueous NaCl solution using electrochemical impedance, EQCN, potentiodynamic polarization and weight-loss studies. 2-OTBT SAM is found to have excellent corrosion protection efficiency in the aq. NaCl solution. The mechanism of corrosion inhibition of copper by 2-OTBT SAM is discussed.     

Field effect transistor behavior of organic light-emitting diodes with a modified configuration of ITO anode

We have developed the organic light-emitting diodes (OLEDs) with a modified configuration of ITO anode in which a thin channel was etched to form a bottom-contact field effect transistor (FET) using ITO and MgAg as a source/drain electrode and a gate electrode, respectively. The hole injection layer in OLEDs functioned as an active layer of FET and the other organic layers as insulator-like layer. The devices were found to exhibit a behavior of FET due to horizontal charge migration between source and drain, and an electro-optical transfer characteristic due to vertical charge transport and recombination. We have investigated the dependence of drain current on the channel length from 5 to 30 μm and found that the modified channel length could change drain current directionally and quantitatively.     

Spirobifluorene derivatives for ultraviolet organic light-emitting diodes

Four new spirobifluorene (SBF) derivatives, 1,3,5-tris(9,9′-spirobifluoren-2-yl)benzene (TSBFB), 1,3-bis(9,9′-spirobifluoren-2-yl)benzene (BSBFB), 2-(1-naphthyl)-9,9′-spirobifluorene (1SBFN), and 2-(2-naphthyl)-9,9′-spirobifluorene (2SBFN), were prepared as ultraviolet (UV)-emitting materials for organic light-emitting diodes (OLEDs). These SBF derivatives all have high glass transition temperatures (T_g) above 95 °C, affording high-quality amorphous films with good morphological stability, and exhibit strong UV photoluminescence (PL) in solid state, showing no formation of excimers. In multilayer OLEDs using these SBF compounds as the emissive layers, the desired UV electroluminescence was achieved with high external quantum efficiencies of up to 2.9%. These results demonstrate that these compounds are promising UV-emitting materials for OLEDs.     

Structural effects on the characteristics of organic field effect transistors based on new oligothiophene derivatives

We report on a comparative investigation of a new series of oligothiophene derivatives as active semiconductor in organic field effect transistors (OFET). Quater- and sexithiophenes end-capped with linear hexyl chains or fused phenyl rings have been synthesized. The oligomers have been deposited by vacuum evaporation onto Si-p⁺⁺ substrates. The gate dielectric was a bilayer PMMA/Ta₂O₅ which ensures both good field effect mobility and rather low operating voltage. The thickness of the oligothiophene films was 80 nm. OFET performances were analyzed as a function of the nature of molecules. The analysis of the characteristics of the various devices shows that the nature and the number of the end group exert a considerable influence on the field effect mobility of the resulting devices. Adding hexyl end group improves the mobility up to 0.1 cm² V⁻¹ s⁻¹. Reference di-hexyl-sexithiophene OFETs exhibit a mobility of 0.13 cm² V⁻¹ s⁻¹. Fused phenyl ring end terminations have also been studied. Poorer performances are obtained with this termination. Our results are discussed in terms of steric interactions and also on the basis of the influence of the end group in the oligothiophene on the molecular arrangement on the surface of the substrate.     

The influence of bulk traps on the subthreshold characteristics of an organic field effect transistor

The subthreshold characteristics of fabricated organic field effect transistors based on regioregular poly(3-dodecylthiophene) (P3DDT) as the active layer and poly-4-vinylphenol (P4VP) as the gate insulator have been investigated. The transistor turn-on occurs at a threshold voltage of around V_th=0 V. The (hole) mobility of 0.002–0.005 cm²/(V s) has been estimated from the linear region of the transfer characteristics. As usually observed for organic transistors, the inverse subthreshold slope is very high, in our case S≈7 V/dec. Furthermore, the subthreshold current depends on the drain voltage although the transistor is a long channel device. One possibility to explain these peculiarities are interface traps, as demonstrated recently by Scheinert et al. [J. Appl. Phys. 92 (2002) 330]. In this paper, the influence of bulk traps is shown. It turns out that both the high inverse subthreshold slope and the drain voltage dependence can be explained also by recharging of bulk traps. Therefore, other frequency and temperature dependent dynamic measurements have to be applied to distinguish between the different possible influences.     

Transient electroluminescence in organic alloy light emitting diodes

In this paper, we report on transient electroluminescence (EL) studies in (ITO/TPD/alloy/Alq₃/Al) organic light emitting diodes. The alloy in the active layer is a co-evaporated mixture of TPD + Alq₃ in the ratio TPD:Alq₃ = 1:4. These results were compared with the transient EL response of a standard device (ITO/TPD/Alq₃/Al). The EL response of the alloy device consists of two components – a fast component (10–20 μs) and a slow component (200–300 μs). It is shown that the slow component arises due to the leakage of electrons from the Alq₃ layer into the alloy layer and subsequent exciton formation in the alloy layer. The magnitude of the fast component depends on the pulse repetition rate and temperature. This is shown to be related to the presence of deep traps in the alloy layer. The presence of deep traps is also confirmed by current transients in the alloy device.     

Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes

The performances of blue phosphorescent organic light-emitting diodes (PHOLEDs) at high current densities have been investigated with double emissive layer structures (D-EMLs). The D-EMLs are comprised of two emissive layers with a hole-transport-type host of N,N′-dicarbazolyl-3,5-benzene (mCP) and an electro transport-type ultrawide band-gap host of m-bis-(triphenylsilyl)benzene (UGH3) both doped with a blue electro-phosphorescent dopant of iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C^2′) picolinate (FIrpic). The expansion of hole/electron recombination zone in D-EMLs has been successfully achieved by controlling of each EML properties, therefore external quantum efficiency, especially at high current density region was significantly enhanced. Moreover, the blue PHOLED with D-EMLs showed substantially reduced roll-off with the external quantum efficiency of 10.0% at 5000 cd/m².     

Observations on the memory effect in diodes fabricated from carbon-bridged dithiophenes

A number of laboratories including our own have reported hysteresis, i.e. a memory effect, in the current–voltage characteristics of semiconducting polymers. Here we report further results on the hysteresis observed in electropolymerised films of a carbon-bridged dithiophene. In particular, we present spectroscopic evidence that shows the effect requires a high degree of conjugation and moderate doping of the polymer. The presence of mobile ions per se in the film is insufficient to cause the effect. Successive film samples produced from the same monomer solution show progressive changes in both the UV and FT-IR spectra consistent with reduced conjugation in the polymer coupled with reduced doping by the BF₄⁻ counterions. These changes correlate with a reduction in hysteresis and a general lowering of film conductivity. From the spectroscopic evidence we develop a possible band energy scheme which suggests that both aluminium and ITO should make rectifying contacts to the polymer thus confirming our earlier findings.     

Microcavity effects on emissive color and electroluminescent performance in organic light-emitting diodes

Microcavity organic light-emitting diodes having a top metal mirror and a bottom dielectric mirror, which was distributed Bragg reflectors (DBR) fabricated by using TiO₂–SiO₂ alternative dielectric multilayer with a central stop-band and two sub-stop-bands, were fabricated. In the devices, the active layers consisted of a hole-transporting layer N,N′-di(naphthalene-l-yl)-N,N′-diphenylbenzidine (NPB) and an electron-transporting/emitting layer tris(8-hydroxy-quinoline) aluminum (Alq₃). The relationship of the electroluminescent (EL) spectrum and efficiency with the thickness of the active layer and metal layer was studied. It was found that the EL emissive color did not strongly depend on the thickness of the organic layer and metal layer, which was attributed to the excellent photon confinement role of the narrow stop-band of the used dielectric mirror. Thus, high efficiency microcavity organic light-emitting diodes were achieved, and the peak wavelength and color purity were not obviously changed, via optimizing the thickness of organic layer and metal electrode.     

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 organic light-emitting diodes using novel hole-transporting materials

A new class of tetraminobiphenyl derivatives, which contains a 3,3′,5,5′-tetraminobiphenyl core, has been synthesized and examined as a hole-transporting material for organic light-emitting diodes. We fabricated the organic light-emitting diode (OLED) cells with tetraminobiphenyl derivatives as hole-injecting layer, hole-transporting layer and hole-injecting and transporting layer for green device with tris(8-quinolinolato)aluminum (Alq₃) doped with 1% of Coumarin 545T (C545T) as green emitting layer. Tetraminobiphenyls were found to be useful as a novel hole-transporting material. The electroluminescent device with the 3,3′,5,5′-tetrakis(p-tolyldiamino)biphenyl (TTAB) as a hole-transporting layer was more efficient than that with the analogous triarylamine, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (NPB). The external luminous efficiency of the device IV having TTAB as one hole-injecting and transporting layer can reach 14.55 cd/A, which is higher than the standard device I (11.66 cd/A) using two layers, a hole-injecting layer and a hole-transporting layer.     

Pure red electrophosphorescent organic light-emitting diodes based on a new iridium complex

A new iridium complex was synthesized and demonstrated a saturated red light emission in organic light-emitting diodes (OLEDs). The maximum brightness of 2800 cd/m² and the external quantum efficiency of 5.5% were achieved in multilayer OLEDs. The peak wavelength of the emission was found to be at 677 nm with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.71, 0.27).     

Improved performance of organic light-emitting diodes using advanced hole-transporting materials

A new class of aryl amine derivatives, which contains phenylnaphthyldiamine core, has been synthesized and examined as a hole-transporting material (HTM) for organic light-emitting diodes (OLEDs). These phenylnaphthyldiamine derivatives possess high radical cation stabilities and high morphologic stabilities relative to their biphenyldiamine analogs. Theoretical experiments and OLED device fabrication were carried out to study their better hole-transporting properties. The electroluminescent devices with the phenylnaphthyldiamine derivatives HTM 2–4 as the hole-transporting layer were more efficient than that with the biphenyldiamine HTM 1.     

Spin dependent reactions of polaron pairs in PPV-based organic diodes

We characterize spin dependent kinetics of polaron pairs (PP) reactions in polymer organic light emitting diodes (OLED) based on a derivative of poly(phenylene-vinylene) [PPV], namely MEH-PPV as the active layer, using the dynamic response of the magnetic resonance effect on electroluminescence, injected current, and photocurrent. We found that at 10 K and under forward bias, the in-phase component of both electroluminescence detected magnetic resonance (ELDMR) and current detected magnetic resonance (CDMR) responses are positive at low microwave modulation frequency, f; but both reverse sign at a frequency f₀ that depends on the microwave power, current density, and device architecture. The similarity between ELDMR and CDMR response dynamics show that the two phenomena share a common origin. We identify the underlying ELDMR mechanism as due to current density increase under resonance conditions. From a model fit to the data that involves both spin singlet and triplet PP dynamics, we obtained their effective recombination and spin-lattice relaxation rates. We found that the spin-lattice relaxation rate in the active layer increases with the current density in the device, showing the importance of spin–spin interaction in spin dependent reactions in OLED. Unlike ELDMR and CDMR the reverse bias photocurrent detected magnetic resonance (PCDMR) is negative at low f. The negative sign is the result of the dynamic equilibrium between the resonantly reduced PP density and the photogenerated mobile charge carrier density.     

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).     

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.     

Organic interlayer for high power efficiency in organic light-emitting diodes

The power efficiency of organic light-emitting diodes (OLEDs) was improved using hexaazatriphenylene-hexacarbonitrile (HAT) as the interlayer between hole injection layer and hole transport layer for efficient hole injection. The hole injection was enhanced and the driving voltage was lowered by the HAT interlayer. A high power efficiency of 73.3 lm/W was obtained from the green phosphorescent organic light-emitting diode with the HAT interlayer.