Organic Nanoparticles: Preparation,Self‐Assembly,and Properties

The strong tendency of organic nanoparticles to rapidly self‐assemble into highly aligned superlattices at room temperature when solution‐cast from dispersions or spray‐coated directly onto various substrates is described. The nanoparticle dispersions are stable for years. The novel precipitation process used is believed to result in molecular distances and alignments in the nanoparticles that are not normally possible. Functional organic light‐emitting diodes (OLEDs)—which have the same host–dopant emissive‐material composition—with process‐tunable electroluminescence have been built with these nanoparticles, indicating the presence of novel nanostructures. For example, only changing the conditions of the precipitation process changes the OLED emission from green light to yellow.     

Impact of forward bias on electroluminescence efficiency in blue and green InGaN quantum well diodes: A comparative study

Electroluminescence (EL) spectral intensity in the high-brightness blue and green InGaN single-quantum-well (SQW) diodes has been comparatively studied over a wide temperature range and as a function of injection current. When the necessary forward bias conditions to get a certain current level are different, it is found that the anomalous EL quenching previously observed below 100 K for the SQW diodes strongly changes and shows a striking difference between the blue and green SQW diodes. This unusual EL evolution pattern is attributed to both internal and external fields, suggesting the importance of the internal piezoelectric field effects on the efficient carrier capture processes by localized tail states within the SQW under the presence of high-density misfit dislocations.     

Near‐Infrared Light‐Emitting Diodes (LEDs) Based on Poly(phenylene)/Yb‐tris(β‐Diketonate) Complexes

Near‐infrared‐emitting electroluminescent (EL) devices using blue‐light‐emitting polymers blended with the Yb complexes Yb(DBM)₃phen (DBM = dibenzoylmethane), Yb(DNM)₃phen (DNM = dinaphthoylmethane), and Yb(TPP)L(OEt) (L(OEt) = [(C₅H₅)Co{P(O)Et₂}₃]^–) have been studied. EL devices composed of Yb(DNM)₃phen blended with PPP‐OR11 showed enhanced near‐IR output at 977 nm when compared to those fabricated with Yb(DBM)₃phen/PPP‐OR11 blends. The maximum near‐IR external efficiencies of the devices with Yb(DBM)₃phen and Yb(DNM)₃phen are, respectively, 7 × 10^–5 (at 6 V and at 0.81 mA mm^–2) and 4 × 10^–4 (at 7 V, and 0.74 mA mm^–2). The optimal blend composition for EL device performance consisted of PPP‐OR11 blended with 10–20 mol‐% Yb(DNM)₃phen. A device fabricated using Yb‐(TPP)L(OEt)/PPP‐OR11 showed significantly enhanced near‐IR output efficiency, and future efforts will focus on devices fabricated using porphyrin‐based materials.     

High‐Tg Carbazole Derivatives as Blue‐Emitting Hole‐Transporting Materials for Electroluminescent Devices

A series of dicarbazolyl derivatives bridged by various aromatic spacers and decorated with peripheral diarylamines were synthesized using Ullmann and Pd‐catalyzed C–N coupling procedures. These derivatives emit blue light in solution. In general, they possess high glass‐transition temperatures (T_g > 125 °C) which vary with the bridging segment and methyl substitution on the peripheral amine. Double‐layer organic light‐emitting devices were successfully fabricated using these molecules as hole‐transporting and emitting materials. Devices of the configuration ITO/HTL/TPBI/Mg:Ag (ITO: indium tin oxide; HTL: hole‐transporting layer; TPBI: 1,3,5‐tris(N‐phenylbenzimidazol‐2‐yl)benzene) display blue emission from the HTL layer. The EL spectra of these devices appear slightly distorted due to the exciplex formation at the interfaces. However, for the devices of the configuration ITO/HTL/Alq₃/Mg:Ag (Alq₃ = tris(8‐hydroxyquinoline)aluminum) a bright green light from the Alq₃ layer was observed. This clearly demonstrates the facile hole‐transporting property of the materials described here.     

Cover Picture: Light‐Emitting Organic Solar Cells Based on a 3D Conjugated System with Internal Charge Transfer (Adv. Mater. 22/2006)

The cover image illustrates the dual photovoltaic and electroluminescence function of a single‐layer device based on a thienylenevinylene–triphenylamine with internal charge transfer (ICT), as reported by Cravino, Roncali, and co‐workers on p. 3033. The material forms an organic glass with isotropic electronic properties while ICT leads simultaneously to an extension of the photoresponse to the red and to an increase of the open circuit voltage. The use of an additional layer of C₆₀ further improves the photovoltaic. Images of the sun and moon courtesy NASA/JPL–Caltech.     

Electroluminescence from phenylenevinylene-based polymer blends

The electroluminescent behaviour of films of poly(phenylphenylenevinylene) (PPPV), of PPPV blended with polystyrene (PS) and of PS doped with oligo(phenylenevinylene) sandwiched between indium–tin oxide (ITO) and Al contacts has been investigated. Polymer blending increases the relative quantum efficiency by up to two orders of magnitude. Studying the cell performance under application of rectangular voltage pulses as a function of temperature indicates that (i) hole injection at the ITO contact occurs by tunnelling, (ii) tunnel injection of electrons at the cathode is promoted by a space charge field across an interfacial Al₂O₃ layers and (iii) leakage of holes through the cathodic barrier is the main loss mechanism for holes.     

Electroluminescence of a device based on europium β-diketonate with phosphine oxide complex

Rare earth (RE) ions have spectroscopic characteristics to emit light in narrow lines, which makes RE complexes with organic ligands candidates for full color OLED (Organic Light Emitting Diode) applications. In particular, β-diketone rare earth (RE³⁺) complexes show high fluorescence emission efficiency due to the high absorption coefficient of the β-diketone and energy transfer to the central ion. In this work, the fabrication and the electroluminescent properties of devices containing a double and triple-layer OLED using a new β-diketone complex, [Eu(bmdm)₃(tppo)₂], as transporting and emitting layers are compared and discussed. The double and triple-layer devices based on this complex present the following configurations respectively: device 1: ITO/TPD (40 nm)/[Eu(bmdm)₃(tppo)₂] (40 nm)/Al (150 nm); device 2: ITO/TPD (40 nm)/[Eu(bmdm)₃(tppo)₂] (40 nm)/Alq₃ (20 nm)/Al (150 nm) and device 3: ITO/TPD (40 nm)/bmdm-ligand (40 nm)/Al (150 nm), were TPD is (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1-biphenil-4,4-diamine) and bmdm is butyl methoxy-dibenzoyl-methane. All the films were deposited by thermal evaporation carried out in a high vacuum system. These devices exhibit high intensity photo- (PL) and electro-luminescent (EL) emission. Electroluminescence spectra show emission from Eu³⁺ ions attributed to the ⁵D₀ to ⁷F_J (J = 0, 1, 2, 3 and 4) transitions with the hypersensitive ⁵D₀ → ⁷F₂ transition (around 612 nm) as the most prominent one. Moreover, a transition from ⁵D₁ to ⁷F₁ is also observed around 538 nm. The OLED light emission was almost linear with the current density. The EL CIE chromaticity coordinates (X = 0.66 and Y = 0.33) show the dominant wavelength, λ_d = 609 nm, and the color gamut achieved by this device is 0.99 in the CIE color space.