Interchain polaron complexes in conducting polymers: Optical transitions and reactions

The binding energy E_β(ξ) of two polarons (P) (or solitons (S)) on adjacent chains arising due to interchain electron hopping t_⊥ is studied at various 3-d orders as a function of PP distance ξ. For out-of-phase order such PP bond is shown to be formed of Ps shifted along the chains. The stability of PP bonds and various decay processes for PP complexes are discussed. The type of 3-d order is found to determine the interchain optical transitions between P levels (through ξ-dependence) which may influence the recombination rate for Ps and could cause quasichemical reactions.     

Spin relaxation and magnetoresistance in disordered organic semiconductors

Because of the light elements involved, the spin–orbit coupling in organic materials is small. Therefore, the spin of charged polarons in these materials is expected to be a well conserved quantity. The conviction in the community grows that the main source of spin relaxation is in fact the coupling of the polaron spin to the random hyperfine fields of the hydrogen nuclei. By considering reactions between polarons forming bipolarons or excitons in the presence of these hyperfine fields we explain line shapes of the intrinsic magnetoresistance observed in disordered organic semiconductors. We also show how these hyperfine fields determine the spin-diffusion length in these semiconductors and how this affects the magnetoresistance line shapes of organic spin valves.     

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.     

Spin polarized injection and transport in organic polymers

Organic spintronics has been an attractive topic since the experiment in which injection of spin polarized electrons from ferromagnetic metals (FM) into organic polymers was reported. By considering both the spin polarons and spinless bipolarons as carriers in organic polymers, we studied the spin injection into organic polymers from both an electrocircuit model and the diffusion theory separately. It was found that a high current polarization could be obtained by adjusting the proportion of the polarons over the carriers. Effect of conductivity matching of the FM layer and the polymer layer on the spin injection was discussed.     

Spin dynamics control of recombination current in organic semiconductors

The recombination rate of coulombically bound electron–hole pairs depends on their spin configuration. Because of low spin–orbit coupling the spin dynamics of these well separated coulombic pairs is determined by weak hyperfine and exchange interactions. In this case a weak magnetic field produces strong effect on the spin dynamics and hence on the recombination rate of e–h pairs. We have shown that the recombination current in organic semiconductors may have a maximum as a function of recombination constant. For high recombination constants the current is space charge limited and decreases with increasing the e–h recombination constant. This decay of current is due to decrease of the region where the recombination takes place. At a low recombination constant the recombination takes place in the whole volume and the current increases with increasing the recombination constant. The characteristic recombination constant separating those two regimes depends on the thickness of sample, applied voltage, and charge carrier mobilities. The model predictions are consistent with experimental data for magnetoresistance of organic semiconductors.     

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 effect on the characteristics of organic diodes

In this paper, we study the effect of self-assembled monolayers (SAMs) on the electric behavior of organic diodes based on sexithiophene (6T) sandwiched between indium tin oxide (ITO) and aluminum. We have used molecules of SAMs based on a thiol with functional groups of oligothiophene (3T(CH₂)₆SH). Wettability measurements have been performed to characterize ITO surface energy and its modification upon deposition of SAMs. The results of contact angle measurements and surface energies demonstrate the homogeneity and rigidity of grafting surface. The current vs. applied voltage characteristics of devices show that conduction in weak biasing follow Richardson–Schottky behavior. Beyond 1.5 V, J–V characteristics can be successfully modeled by space-charge limited current (SCLC) theory followed by a trap charge limited current (TCLC). The electrical as well as optical characteristics of 6T layer are clearly affected by the presence of the SAM. The differences between ITO/SAM and bare ITO samples are interpreted in terms of structural effect induced by the self-assembled monolayer of 3T(CH₂)₆SH.     

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.     

Uniform dispersion of triplet emitters in multi-layer solution-processed organic light-emitting diodes

The influence of the iridium complex solubility on the efficiency of multi-layer solution-processed organic light-emitting diode is demonstrated by synthesized orange triplet iridium complexes with the same core. The solubility of the iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C²′) acetylacetonate is increased and uniform dispersion of iridium complex in polymer host poly(vinylcarbazole) is achieved by tert-butyl and n-hexyl group modification. Blade coating technique is utilized to achieve tri-layer structures with a polymer hole-transporting layer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-s-butylphenyl))diphenylamine)], a host–guest emissive layer, and small-molecule hole-blocking layer 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene. The efficiency as high as 20 cd/A is achieved for orange-emitting device.     

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.     

Transient electroluminescence in multilayer organic light-emitting diodes: experiment and theory

We have investigated a multilayer organic light-emitting diode (OLED) with 1,3,5-tris(N,N-bis-(4,5-methoxy-phenyl)-aminophenyl)-benzene (TAPB) acting as hole transporting layer (HTL) and tris(8-hydroxy-quinolinolato) aluminium (Alq₃) as electron transporting layer (ETL). Positive charge carriers in the HTL were detected optically as a function of the applied bias. Furthermore, we investigated the DC-characteristics of current and brightness as well as the onset behaviour of the electroluminescence (EL) as a function of the applied bias. An analytical model is developed to describe the observed carrier concentrations as well as the current–voltage characteristics and the transient EL measurements quantitatively.     

Grain size dependent mobility in polycrystalline organic field-effect transistors

Thin film transistors were made with vacuum evaporated sexithiophene and octithiophene. Scanning electron microscopy shows that the films are polycrystalline, with a grain size that increases when the temperature of the substrate is increased. The carrier mobility of the film increases with grain size. It is thermally activated for small grains, but becomes temperature independent for larger grains. These data are interpreted in terms of a model where charge transport is limited by trapping and thermal release at localized states located at grain boundaries.     

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%.     

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.     

Synthesis of novel Ir complexes and their application in organic light emitting diodes

Three new substituted arylpyridine coordinated iridium complexes Ir(DPP)₂(acac), Ir(BPP)₂(acac) and Ir(FPP)₂(acac) have been synthesized and used as emitter in OLED. The EL devices based on these complexes emitted in the green to yellow spectral range. The results showed that these complexes have strong phosphorescent characters. For the device based on Ir(FPP)₂(acac), the maximum brightness reached 14180 cd m⁻² and the maximum external efficiency reached 17.0 cd A⁻¹.     

Nanoengineering of organic semiconductors for light-emitting diodes: control of charge transport

The effect of intermolecular interactions on the properties of organic semiconductors is investigated using a family of conjugated dendrimers as model systems. Increasing the amount of branching, or generation number, of these molecules reduces the degree of interaction between the chromophores. The effect of this on both photophysical and charge transporting properties is reported. It is found that an increase in generation gives rise to a reduction in the red tail emission of the dendrimer. Time of flight measurements show a slowing of charge transport with increasing generation, which is found to be related to the films becoming more insulating. The results show that dendrimer generation provides an elegant way of controlling intermolecular interactions.     

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.