Degradation mechanism of small molecule-based organic light-emitting devices

p-Type polymer-hybridized high-performance piezoelectric nanogenerators

Enhancing the output power of a nanogenerator is essential in applications as a sustainable power source for wireless sensors and microelectronics. We report here a novel approach that greatly enhances piezoelectric power generation by introducing a p-type polymer layer on a piezoelectric semiconducting thin film. Holes at the film surface greatly reduce the piezoelectric potential screening effect caused by free electrons in a piezoelectric semiconducting material. Furthermore, additional carriers from a conducting polymer and a shift in the Fermi level help in increasing the power output. Poly(3-hexylthiophene) (P3HT) was used as a p-type polymer on piezoelectric semiconducting zinc oxide (ZnO) thin film, and phenyl-C(61)-butyric acid methyl ester (PCBM) was added to P3HT to improve carrier transport. The ZnO/P3HT:PCBM-assembled piezoelectric power generator demonstrated 18-fold enhancement in the output voltage and tripled the current, relative to a power generator with ZnO only at a strain of 0.068%. The overall output power density exceeded 0.88 W/cm(3), and the average power conversion efficiency was up to 18%. This high power generation enabled red, green, and blue light-emitting diodes to turn on after only tens of times bending the generator. This approach offers a breakthrough in realizing a high-performance flexible piezoelectric energy harvester for self-powered electronics.

The mechanism of burn-in loss in a high efficiency polymer solar cell

Degradation in a high efficiency polymer solar cell is caused by the formation of states in the bandgap. These states increase the energetic disorder in the system. The power conversion efficiency loss does not occur when current is run through the device in the dark but occurs when the active layer is photo-excited.

Pure white organic light-emitting diode with lifetime approaching the longevity of yellow emitter

A high-efficiency pure white organic light-emitting diode was fabricated with lifetime approaching that of the low-excitation-energy (yellow) emitter containing counterpart, or six times that of the deep-blue counterpart. The white device was composed of two emission layers with mixed hosts of different compositions. They were respectively doped with yellow rubrene and deep-blue 4,4'-bis-[4-{N,N,N',N'- tetrakis-(4-fluoro-diphenylamino)-phenyl}-vinyl]-biphenyl. The resulting efficiency was 6.0 lm/W (12.4 cd/A) at 20 mA/cm(2). The long device lifetime may be attributed to the double mixed-host architecture employed that effectively dispersed the injected carriers into three different recombination zones and consequently diluted the damaging effect arising from the accumulated charge from un-recombined carriers, hence leading to a markedly improved lifespan.

Direct comparison of solution- and vacuum-processed small molecular organic light-emitting devices with a mixed single layer

It will be interesting and valuable information can be achieved if a direct comparison between organic light emitting devices (OLEDs) fabricated by vacuum evaporated method (vac) and solution-based manufacturing processes (sol) was realized. Small molecular OLEDs with a mixed organic layer structure (MOLOLEDs) make it possible for direct comparison between devices with the same materials but fabricated by the two processing methods. This article shows a direct comparison of the luminescence characteristics, charge conduction, and device physics between MOLOLEDs fabricated by vac- and sol-processing techniques. It gives an elementary explain how the organic/metal interfaces influence the charge conduction and device performance.

Four-coordinate organoboron compounds with a π-conjugated chelate ligand for optoelectronic applications

Four-coordinate organoboron compounds with a π-conjugated chelate backbone have emerged recently as highly attractive materials for a number of applications including use as emitters and electron-transport materials for organic light-emitting diodes (OLEDs) or organic field transistors, photoresponsive materials, and sensory and imaging materials. Many applications of this class of boron compounds stem from the electronic properties of the π-conjugated chelate backbone. Charge-transfer transitions from an aromatic substituent attached to the boron center of the π-conjugated chelate backbone and steric congestion have also been found to play important roles in the luminescent and photochromic properties of the four-coordinate boron compounds. This article provides an update-to-date account on the application aspects of this important class of compounds in materials science with the emphasis on OLED applications and photochromic switching.

Direct determination of energy level alignment and charge transport at metal-Alq3 interfaces via ballistic-electron-emission spectroscopy

Using ballistic-electron-emission spectroscopy (BEES), we directly determined the energy barrier for electron injection at clean interfaces of Alq(3) with Al and Fe to be 2.1 and 2.2 eV, respectively. We quantitatively modeled the sub-barrier BEES spectra with an accumulated space charge layer, and found that the transport of nonballistic electrons is consistent with random hopping over the injection barrier.

Strategies to design bipolar small molecules for OLEDs: donor-acceptor structure and non-donor-acceptor structure

Organic light-emitting diodes (OLEDs) have attracted great attention because of their potential applications in full-color displays and solid-state lights. In the continual effort to search for ideal materials for OLEDs, small molecules with bipolar transporting character are extremely attractive as they offer the possibility to achieve efficient and stable OLEDs even in a simple single-layer device. In this Research News, we review the two design strategies of bipolar materials for OLEDs: molecules with or without donor-acceptor structures. The correlation between the experimental results and theoretical calculations of some of the materials is also discussed.

Sonochemical fabrication of 8-hydroxyquinoline aluminum (Alq3) nanoflowers with high electrogenerated chemiluminescence

Well-defined Alq(3) nanoflowers were fabricated via a facile and fast sonochemical route. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the structure and shape of the as-prepared product. The results showed that the resulting Alq(3) was composed of nanobelts with thickness about 50 nm, average widths of 200 nm, and length up to 10 μm. The Alq(3) nanoflowers exhibited good electrogenerated chemiluminescence behavior.

Improved performance of organic light-emitting diodes with MoO3 interlayer by oblique angle deposition

We fabricated and demonstrated improved organic light emitting diodes (OLEDs) in a thin film architecture of indium tin oxide (ITO)/ molybdenum trioxide (MoO3) (20 nm)/N,N'-Di(naphth-2-yl)-N,N'-diphenyl-benzidine (NPB) (50 nm)/ tris-(8-hydroxyquinoline) (Alq3) (70 nm)/Mg:Ag (200 nm) using an oblique angle deposition technique by which MoO3 was deposited at oblique angles (θ) with respect to the surface normal. It was found that, without sacrificing the power efficiency of the device, the device current efficiency and external quantum efficiency were significantly enhanced at an oblique deposition angle of θ=60° for MoO3.

Long-Term Degradation Mechanism of Organic Light Emitting Devices Based on Small Molecules

The intrinsic degradation of tris(8-hydroxyquinoline) aluminum (AlQ3)-based organic light emitting devices, that leads to the long-term decrease in the electroluminescence efficiency of the devices operated under constant current conditions, is studied. The injection of holes in A1Q3 is found to be the main factor responsible for device degradation. OLEDs with dual HTLs in different arrangements are also presented to demonstrate the proposed degradation mechanism. The role of various approaches to increase OLED lifetime, such as, doping the hole transport layer, introducing a buffer layer at the hole-injecting contact, or using a mixed emitting layer of hole and electron transporting molecules, is explained.

Degradation mechanisms in small-molecule and polymer organic light-emitting diodes

Degradation in organic light-emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small-molecule and polymer OLEDs is presented. For small-molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance-efficiency loss and operating-voltage rise. For polymer OLEDs, it is shown how the charge-transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.

White light emission from Mn2 + doped ZnS nanocrystals through the surface chelating of 8-hydroxyquinoline-5-sulfonic acid

White light emitting semiconductor nanocrystals (NCs) have been successfully synthesized from 8-hydroxyquinoline-5-sulfonic acid (HQS) decorated manganese doped ZnS NCs through fine tuning the surface-coordination emission and dopant emission of the NC host. The HQS functionalized manganese doped ZnS NCs (QS-ZnS:Mn), with a cubic crystal structure, have the same diameter of about 4.0 nm as ZnS:Mn NCs without HQS. The intensity of the surface-coordination emission peak increased with increasing HQS content or augmenting excited wavelength. The emission of white light was achieved by carefully controlling the dosage of HQS in NCs and appropriately tuning the excited wavelength. The color coordinates (0.35, 0.34) for the efficient white light emitting NCs were very close to the ideal Commission Internationale de l'Eclairage (CIE) chromaticity coordinates for pure white light (0.33, 0.33). The photoluminescence (PL) decay study revealed that the white light emitting NCs exhibited maximum lifetime values at different emission peaks for different NC samples. The study results also indicated that the HQS molecules were attached to the surface of ZnS:Mn NCs in a single coordination fashion due to the steric hindrance effect of the special spherical surface of NCs, which made the QS-ZnS:Mn NCs possess stable and high fluorescent properties in different organic solvents as compared with the conventional small molecule complexes.

Synthesis and Properties of the Bipolar Triphenylamine-benzimidazole Derivatives

A series of newly-developed bipolar triphenylamine benzimidazole derivatives, containing both hole-transporting and electron-transporting moieties, have been completely characterized by 1H and 13C NMR and HRMS, as well as their thermal, optical, and electrochemical properties. The results indicate that these compounds exhibit high fluorescence quantum yields, desirable HOMO levels and high thermal stabilities. Quantum chemical calculations were used to obtain the optimized ground-state geometries, spatial distributions of the HOMO, and the LUMO levels of these compounds. The agreement between experimental and calculated data suggests such compounds may have potential applications in organic light emitting diodes materials.

Experimental design for the determination of the injection barrier height at metal/organic interfaces using temperature dependent current-voltage measurements

Determination of the injection barrier height for holes or electrons at metal/organic interfaces is essential to understanding the device physics of organic electronics. Due to the disordered molecular packing of organic semiconductors, careful consideration is required in the design of both the device structure and the experimental measurement technique used to extract the barrier height. We report a methodology for extracting the injection barrier height at metal/organic interfaces from temperature dependent current-voltage measurements. This methodology includes the design of single carrier devices with specific consideration of the intrinsic properties of organic semiconductors, as well as the design of a variable temperature cryostat suited to the measurement of organic electronic device architectures. Experimental results for single carrier hole-only devices using two commonly studied hole transport materials, namely N,N(')-diphenyl-N,N(')-bis-(1-naphthyl)-1-1(')-biphenyl-4,4(')-diamine (alpha-NPD) and 4,4('),4(")-tris(N-3- methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) are also presented as examples.

Alq3 nanorods: promising building blocks for optical devices

Monodisperse Alq3 nanorods with hexagonal-prism-like morphology are produced via a facile, emulsion based synthesis route. The photoluminescence of individual nanorods differs from the bulk material. These nanorods are promising building blocks for novel optical devices.

Deposition Rate Effect of Alq3 Thin Film Growth: A Kinetic Monte Carlo Study

This paper is the winner of the Young Scientist Award at the Asian Chemical Congress in Kuala Lumpur, 2007. Applying the Kinetic Monte Carlo (KMC) technique, we successfully investigated the effect of deposition rate on the growth pattern of an Alq3 thin film. In good agreement with experimental results, our simulation results indicate that there exists a transition growth in terms of the deposition rate that corresponds to the transition between the island growth and random deposition growth. In the regions of island growth (where the deposition rate is lower than 1.1 Å s–1) and random deposition growth (where the deposition rate is higher than 3 Å s–1), the surface morphology is not suitable for luminant devices because of a high roughness, a larger inner vacancy ratio at higher deposition rate, and low homogeneity at lower deposition rate conditions. Within the transition growth region (deposition rate is between 1.1 and 3.0 Å s–1), the homogeneity of the film surface improves as the deposition rate increases. Not only does the pattern of the island structures become blurred, but the inner vacancy ratio and surface roughness also remain low as the deposition rate increases. From our results, there may exist a deposition rate to optimize the Alq3 film with a suitable surface morphology for luminant devices.

Synthesis and Characterization of Hemicage 8-Hydroxyquinoline Chelates with Enhanced Electrochemical and Photophysical Properties

A new hexadentate, tripodal 8-hydroxyquinoline ligand (QH3) and its trivalent metal chelates (MQ, M=Al3+, Ga3+, In3+) with hemicage structures have been prepared and the electrochemical and photophysical properties systematically studied. The hemicage structure of the metal complexes was characterized by 1H NMR, indicating a pure facial geometry, in contrast to their uncaged cousins with 8-hydroxyquinoline (Mq3) and 3-methyl-8-hydroxyquinoline (M(3Meq)3), which all exist only as the meridional form in fluid solutions at room temperature. The photoluminescence quantum efficiency for the three hemicage complexes is 1.48, 1.79, and 1.26 times higher for AlQ, GaQ, and InQ, respectively, than their corresponding 3-methyl-8-hydroxyquinoline complexes, likely due to the rigidity of the ligand system, which can efficiently decrease the nonradiative decay of the excited states. The improved electrochemical stability of the hemicage complexes has been demonstrated by cyclic voltammetry, showing an increasingly reversible behavior from InQ to GaQ to AlQ (Ered=-2.15, -2.17, and -2.22 V vs Fc/Fc+ in DMSO). We infer that the degree of reversibility and redox potential result from the metal-ligand bond strength, which is largest in the case of aluminum.

Facile solution synthesis of hexagonal Alq3 nanorods and their field emission properties

A facile self-assembly growth route assisted by surfactant has been developed to synthesize tris(8-hydroxyquinoline)aluminium (Alq(3)) nanorods with regular hexagonal shape and good crystallinity, which exhibit field-emission characteristics with a very low turn-on field of ca. 3.1 V microm(-1) and a high field-enhancement factor of ca. 1300.