Organic light-emitting diodes and organic light-emitting electrochemical cells based on silole–fluorene derivatives

Silole groups are known to present a high electron affinity. Initially, copolymerization of siloles with fluorene was aimed at improving electron injection into the polymer layer and so improving the electroluminescent properties of organic light-emitting diodes (OLED's) made from fluorene. But it also provides the ability to turn the light emission colour to the green part of the spectrum and to stop the well-known spectral shift degradation occurring in fluorene-based materials. In this paper we report the synthesis and the characterisation of 1,1-dimethyl-2,5-bis(fluoren-2-yl)-3,4-diphenylsilole 4, and of two soluble conjugated random copolymers derived from 9,9-ditetradecylfluorene and 1,1-dialkyl-2,5-diphenylsilole, where the alkyl group is either methyl 11a or n-hexyl 11b. Silole 4 crystallizes in the triclinic P-1 space group with a = 9.8771(8), b = 10.6240(10), c = 16.585(2) Å, α = 95.775(8), β = 97.025(7), and γ = 111.738(8)°. The results obtained with this molecule, operating in a single-layer OLED (luminance ≈450 Cd/m² at 12 V; η_max = 0.2 Cd/A), give evidences for the complementarity of the silole and the fluorenyl moieties in the improvement of the charge injection processes when compared with 1,1-dimethyl-2,3,4,5-tetraphenylsilole. The results obtained from organic light-emitting electrochemical cells (LEC's) made from silole–fluorene copolymers 11a, 11b and molten salts show an improvement of both the device lifetime and the spectral stability when compared with polyfluorene. To explain devices performances electrical characterisation data and atomic force microscope (AFM) imaging were combined.     

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.     

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.     

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.     

Color stable and interlayer free hybrid white organic light-emitting diodes using an area divided pixel structure

Color stable and interlayer free hybrid white organic light-emitting diodes were fabricated by using an area divided pixel structure. The area divided pixel structure was realized by stacking red and blue emitting layers using a fine metal mask. A phosphorescent red emitting layer was patterned by a metal mask and a blue fluorescent emitting layer was commonly deposited on the patterned red emitting layer. The blue fluorescent emitting layer could play a role of a hole-blocking layer and a white emission could be obtained due to separate emission of red and blue emitting layers. The interlayer free hybrid WOLEDs showed color stability from 100 cd/m² to 10,000 cd/m².     

High efficiency and long lifetime in organic light-emitting diodes using bilayer electron injection structure

A lithium quinolate-based electron injection structure was developed to improve luminance efficiency and lifetime of organic light-emitting diodes (OLEDs). A electron injection material based on Li complex, 8-hydroxyquinolinato lithium (Liq), was introduced as an electron injection material for OLEDs and the efficiency and lifetime of OLEDs were investigated according to the structure of the electron injection layer. A bilayer electron injection structure, a mixed layer of tris(8-hydroxyquinoline) aluminium (Alq₃) and Liq and a Liq layer, showed high efficiency of 11.6 cd/A compared with 9.8 cd/A for lithium fluoride (LiF). In addition, the extrapolated lifetime of OLED with the bilayer electron injection structure was improved by 40% at 1000 cd/m².     

Organic light-emitting diodes based on multilayer structures

We study various organic structures and explain the obtained improvements by the use of protective layer or hole transport layer in multilayer OLEDs based on Alq₃ as emitter.     

High efficiency deep blue phosphorescent organic light-emitting diodes using a double emissive layer structure

High efficiency deep blue phosphorescent organic light-emitting diodes (PHOLEDs) have been developed with a double emissive layer structure (D-EML). Using bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) as an electro-phosphorescent dopant, we achieved a maximum external quantum efficiency of 14.8% in the deep blue PHOLEDs with D-EMLs, which is 50% higher value than that of 9.76% with single emissive layer structure (S-EML). Moreover, the external quantum efficiency at high current density region of 10 mA/cm² was maintained up to 11.3% in this D-EML device. We attributed this enhancement to the expansion of carrier recombination region and the effective confinement of exciton within the emissive layer.     

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.     

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.     

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).     

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.     

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.     

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.     

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.     

White organic light-emitting diodes via mixing exciplex and electroplex emissions

The authors report the fabrication of white organic light-emitting devices and discuss their electroluminescence (EL) properties. The device structure is ITO/TPD (50 nm)/BCP (8 nm)/Rubrene (0.5 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al. In the EL spectra of this device, two new emissions peaking at 590 and 630 nm have been observed. These two emissions should be attributed to triplet exciplex and electroplex occurring at TPD/BCP interface. White emission was obtained based on this device under 12 V driving voltage, the Commission Internationale de l’Eclairage (CIE) coordinates arrives to (0.31, 0.33).     

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.     

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%.