Organic light-emitting device using new distyrylarylene host materials

Novel distyrylarylene-based blue host materials, 4,4′-bis(1,2-diphenylvinyl)-p-terphenyl (2), 4,4′-bis[1-phenyl-2-(p-tolyl)vinyl]-p-terphenyl (3) and 4,4′-bis[1-phenyl-2-(4,4′-biphenyl)vinyl]-p-terphenyl (4) were successfully synthesized using 4,4′-bisbenzoyl-p-terphenyl (1) through Wittig–Hornor reaction and a blue organic light-emitting diode (OLED) was made from them. The structure of the blue device is ITO/DNTPD/NPB/Host:DSA-Ph/Alq₃/Al-LiF. Here, NPB is used as a hole transport layer, DNTPD as the hole injection layer, DSA-Ph as the blue dopant, Alq₃ as the electron transport layer and Al as the cathode. The blue device doped with 5%-DSA-Ph shows a blue EL spectrum at 463 nm and a high efficiency of 4.14–5.13 cd/A.     

Injection and charge transport effects on electroluminescence characteristics of molecularly-doped polymer light-emitting diodes

Electrical and optical characteristics of single-layer light-emitting diodes (LEDs) comprised of a solid solution of electron transport and emitter molecules of Alq₃, and hole transport molecules of TPD dispersed in polycarbonate (PC) have been studied using varying proportions of solutes and thickness of samples. The results show that the LEDs operate in the injection-controlled electroluminescence (EL) (ICEL) mode, the current being dominated by holes injected from the ITO anode. Electrons injected from the Mg/Ag cathode are trapped on Alq₃ molecules serving as recombination centers for mobile holes. In particular, the current–voltage characteristics and their evolution with sample thickness allowed us to conclude that the hole injection follows the one-dimensional Onsager model. The EL efficiency, expected to be a function of the driving current due to the ICEL operation mode of the LEDs, appears to be quasi-constant for each given cell showing up in the quasi-linear brightness–current relationship. This behaviour can be explained by the field-increasing mobility of holes as shown in analytical considerations relating the EL efficiency to the driving current and hole mobility. The EL yield and its current dependence appeared to be only slightly affected by the concentration ratio TPD:Alq₃. Its increase lowers the EL yield but strongly increases the absolute EL intensity. The conditions are examined to fabricate the polymer LEDs for different applications: for these where power consumption is of priority importance (high efficiency LEDs) and those where the high brightness of the EL display is required (high brightness LEDs). The results allow us to show a connection of polymer LED characteristics with their structure and technological parameters. This is a necessary step in fabrication of improved performance organic LEDs.     

A novel white organic electroluminescent device based on a thin LiF interlayer

A novel white organic electroluminescent device was fabricated by inserting a thin lithium fluoride (LiF) layer in the emitting layer (tris-(8-hydroxyquinoline) aluminum (Alq₃)). The electroluminescence device with the configuration indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(l-napthyl)-1,1′-biphenyl-4,4′-diamine (NPB; 45 nm)/tris-(8-hydroxyquinoline) aluminum (Alq₃; x nm)/LiF (0.3 nm)/Alq₃ ((45 − x) nm)/Al (150 nm) showed expanded electroluminescence (EL) spectra. The spectra contain tricolor, so this should be a simple method to realize white light emitting. We also elucidated the mechanism of expanded EL spectra formation and investigated the properties of these devices.     

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.     

Low voltage organic light emitting diode using p–i–n structure

Efficient n-type doping has been achieved by doping Liq in electron transport material Alq₃. Detailed investigation of current density–voltage characteristics of electron only devices with different doping concentrations of Liq in Alq₃ has been performed. An increase in current density by two orders of magnitude has been achieved with 33 wt% of Liq doped in Alq₃. Organic light emitting diode with p–i–n structure was fabricated using F₄-TCNQ doped α-NPD as hole transport layer, Ir(ppy)₃ doped CBP as emitting layer and 33 wt% Liq doped Alq₃ as electron transport layer. Comparison of OLEDs fabricated using undoped Alq₃ and 33 wt% Liq doped Alq₃ as electron transport layer shows reduction in turn on voltage from 5 to 2.5 V and enhancement of power efficiency from 5.8 to 10.6 lm/W at 5 V.     

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.     

Interfacial charges in organic hetero-layer light emitting diodes probed by capacitance–voltage measurements

The bias dependent capacitance of organic hetero-layer light emitting diodes (LEDs) based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1-biphenyl-4,4 diamine (NPB) and tris(8-hydroxyquinoline)aluminium (Alq₃) shows below the built-in voltage, a step-like change from a value corresponding to the total organic layer thickness to a higher value given by the Alq₃ layer thickness. The bias and frequency dependent behaviour of the capacitance can be explained by the presence of negative charges with a density of −6×10¹¹ e/cm² at the NPB–Alq₃ interface. This leads to an inhomogeneous potential distribution inside the device with a discontinuity of the electric field at the organic–organic interface.     

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.     

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.     

Top emission organic light emitting diode with a Cr/Al/Cr anode

A top emission organic light emitting diode (TEOLED) comprised of a Cr anode on PES film/NPB/Alq₃/cathodes has been fabricated. The triple layer structure of Cr/Al/Cr allowed for fabrication of a crack-free anode and provided better thermal stability and higher work function than a conventional ITO anode. For the Cr/Al/Cr anodes, a series of Cr layers with various surface morphology has been tested. A Cr layer with a smooth surface morphology was found to be optimal. The TEOLED fabricated on PES film having good anode surface morphology showed similar device characteristics to that on a Si wafer. TEOLEDs on PES film and Si wafer exhibited a maximum luminous efficiency of 2.87 and 3.0 cd/A, respectively, at 1000 cd/m² with a NPB/Alq₃/LiF/Al/Ag structure on a Cr/Al/Cr anode.     

New rhenium complexes containing 4,5-diazafluorene ligand for high-efficiency green electrophosphorescence

Two novel tricarbonyl rhenium complexes featuring 4,5-diazafluorene (DF)-based ligand, i.e., Re-DF and Re-EPDF (EPDF, 9,9-di-(4-ethoxyphenyl)-9-H-4,5-diazafluorene), were designed, synthesized and characterized by ¹H NMR and mass spectroscopy. The green organic light-emitting diodes (OLEDs) based on these complexes with the configuration of ITO/m-MTDATA (10 nm)/NPB (20 nm)/CBP: Re-complex (30 nm)/Bphen (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) were fabricated. The devices based on Re-DF showed a maximum current efficiency of 20.7 cd/A and a peak luminance of 2506 cd/cm², respectively. And the Re-EPDF doped devices exhibited a maximum current efficiency of 13.5 cd/A and a luminance of 3208 cd/cm². Moreover, the 20 wt.% Re-EPDF doped device still provided a maximum current efficiency of 13.2 cd/A.     

Improved electron injection into Alq3 based OLEDs using a thin lithium carbonate buffer layer

A cathode buffer layer of lithium carbonate (Li₂CO₃) we used to improve the electro-optical properties of organic light-emitting diodes (OLEDs). Li₂CO₃ layers with various thicknesses were prepared by thermally evaporating Li₂CO₃ powders. When a 1-nm-thick Li₂CO₃ layer was inserted between the aluminum (Al) cathodes and tris(8-hydroxyquinolinato)aluminum (Alq₃) electron-transporting layers, device properties such as the turn-on voltage, the maximum luminance, and the device efficiency were improved, becoming better than and comparable to those of devices with LiF and Cs₂CO₃ buffer layers. The surface of the Alq₃ film became smoother after the Li₂CO₃ layer was deposited. The reaction mechanisms between Li₂CO₃ and Alq₃ were also investigated. X-ray and ultraviolet photoelectron spectroscopy results show that some electrons transfer from Li₂CO₃ into Alq₃, which increases the electron concentration in Alq₃ films and moves the Fermi level close to the lowest unoccupied molecular orbital (LUMO) of Alq₃. Thus, the electron injection efficiency was enhanced due to a lower electron injection barrier, which improves the charge carrier balance in OLEDs and leads to better device efficiency.     

Spiro[fluorene-7,9′-benzofluorene] host and dopant materials for blue light-emitting electroluminescence device

New spiro-type 5-biphenyl-spiro[fluorene-7,9′-benzofluorene] (BH-1BP) and 5-diphenyl amine-spiro[fluorene-7,9′-benzofluorene] (BH-1DPA) were synthesized for use as blue organic light-emitting host and dopant materials, respectively. Their optical properties, including their UV absorption, photoluminescence and energy levels, were measured and blue OLEDs were made from them. The structure of the blue device is ITO/DNTPD/α-NPD/BH-1BP:5% dopant/Alq₃ or ET4/Al–LiF. Here, α-NPD is used as the hole transport layer, DNTPD as the hole injection layer, BH-1DPA or diphenyl-[4-(2-[1,1;4,1]terphenyl-4-yl-vinyl)-phenyl]-amine (BD-1) as the blue dopant materials, Alq₃ or ET4 as the transporting layer and Al as the cathode. The blue devices doped with 5% BH-1DPA and BD-1 show blue EL emissions at 444–448 nm at 7 V. A high efficiency of 3.28 cd/A and the CIE coordinates (0.14, 0.11) at 7 V can be achieved from the devices composed of BH-1BP:5% BD-1/ET4.     

Spectral tuning of light emitting diodes with phenyl-thiophenes

We report electroluminescence data on single and double layer devices based on well-defined phenyl-thiophenes (PTs) (PPPPT, PPPTP, PPTPP), wherein 2,5-thiophene (T) is systematically translated through a p-quinquephenyl (PPPPP) framework. Single layer light emitting diodes (LEDs) were prepared by vacuum deposition of the PTs and an aluminum top electrode onto an ITO substrate; double layer devices were prepared using Alq₃ as the electron-transport and emitter layer. The single layer LEDs exhibited light emission that could be tuned across the spectral range from blue to green depending upon the location of the thiophene moiety. All of the double layer devices emit in the 510-nm range which suggests that this emission originates from Alq₃ and that the PTs function as hole-transport materials. For both types of devices we present spectroscopic data, the wavelength dependence of the electroluminescence, and the current–voltage characteristics. Cyclic voltammetry was used to determine the redox behavior and the pertinent electronic energy levels of the PTs. The experimental UV λ_max values and HOMO values of the PTs were contrasted with ab initio calculations and found to be in good agreement.     

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.     

Lanthanide imidodiphosphinate complexes: Synthesis,structure and new aspects of electroluminescent properties

The lanthanide imidodiphosphinate complexes Ln(pip)₃ (Ln = Ce, Nd, Tb, Ho) were prepared from the corresponding Ln[N(SiMe₃)₂]₃ with [Ph₂P(O)]₂NH in a quantitative yield. X-ray single structure analysis of Ce(pip)₃ has revealed that the lanthanide ion in these compounds coordinated by three bidentate [Ph₂P(O)NP(O)Ph₂]⁻ ligands and THF molecule. With respect to the organic light emitting diodes (OLED) application, the simple electroluminescent (EL) devices with a multilayer configuration ITO/TPD/Ln(pip)₃/Yb, ITO/CBP/Ln(pip)₃/Yb, ITO/TPD/Ln(pip)₃/Alq₃/Yb, ITO/TPD/Alq₃/Ln(pip)₃/Yb were fabricated. It was found that the lanthanide imidodiphosphinate complexes possess the hole-blocking and electron-transporting ability and it was also observed the exciplex emission between these complexes and TPD.     

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.     

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).     

Air stable and low temperature evaporable Li3N as a n type dopant in organic light-emitting diodes

N doped organic light-emitting diodes were developed by using Li₃N as a n type dopant in electron transport layer. Driving voltage was greatly lowered by using Li₃N doped electron transport layer and combination of MoO₃ doped hole transport layer with Li₃N doped electron transport layer gave high quantum efficiency of 15% and low driving voltage of 4 V at 1000 cd/m² in green phosphorescent organic light-emitting diodes. Decomposition of Li₃N during evaporation into Li and N₂ was found to be responsible for n doping effect of Li₃N.     

Inhomogeneous transport property of Alq3 thin films: Local order or phase separation?

Steady-state current–voltage (I–V) and impedance–voltage (Z–V) measurements were performed on in situ (UHV) prepared metal (Ag, Al)/Alq₃/indium-tin oxide (ITO) devices after exposure to air. When increasing the positive bias on the top metal electrode to a relatively well-defined critical value, a transition from semiconducting to semi- or even insulating behavior of the contacted Alq₃ thin film is observed by means of I–V measurements. The final insulating state remains stable when applying negative bias to the Ag electrode. In the case of the Al electrode, there is a voltammetric current wave under a well-defined negative bias indicating a redox reaction of mobile ions at the Al electrode.The Z–V measurements reveal a peculiar feature of ac transport through the Alq₃ thin films, namely the equivalent series capacitance is equal to its parallel counterpart in the frequency range from 100 to 1 MHz and amounts to only a fraction (0.3–0.5) of the expected geometrical capacitance of the device. An equivalent electrical circuit has been developed, based on the existence of two parallel transport paths: an insulating (amorphous) Alq₃-phase shunted by a semiconducting (semi-insulating) one, both running into the impedance of the back contact. The equivalent circuit model composed exclusively of frequency independent elements is useful for predicting the maximum frequency for retaining the full geometrical capacitance. Even though the model is capable of describing the bias dependence of the impedance correctly, it does not shine light on the nature of the (ordered) phase or domain responsible for the dielectric loss. The possibility of local order connected with dipole–dipole interaction in the metal/Alq₃ interface zone is discussed. In any case, the ordered portion of the organic material seems to form the huge interface dipole of about 1 eV with Ag or Al [M.A. Baldo, S.R. Forrest, Phys. Rev. B 64 (2001) 085201], the direction of the dipole promoting electron injection to Alq₃. Then the semiconductor-to-insulator transition could be initiated by a damage of the interface dipole under a critical positive dc bias of the metal, preventing the flow of both dc and the real component of low-frequency ac current. The transition is not accompanied by any significant change in the impedance of the back contact common to both phases.