Hole-injecting Contact Research Articles

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

Studies on the long-term degradation of organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinoline) aluminum (AlQ3), the most widely used electroluminescent molecule, reveal that injection of holes in AlQ3 is the main cause of device degradation. The transport of holes into AlQ3 caused a decrease in its fluorescence quantum efficiency, thus showing that cationic AlQ3 species are unstable and that their degradation products are fluorescence quenchers. These findings explain the success of different approaches to stabilizing OLEDs, such as doping of the hole transport layer, introducing a buffer layer at the hole-injecting contact, and using mixed emitting layers of hole and electron transporting molecules.

Nonlinear emission and recombination in conjugated polymer waveguides

The emission dynamics in photoexcited planar conjugated polymer waveguides is investigated at high excitation densities. Using femtosecond pump/probe experiments and photoluminescence spectroscopy we investigate the interplay of nonlinear radiative and nonradiative recombination processes. Amplified spontaneous emission (ASE) leads to an ultrafast depletion of the excited state at excitation densities above 1018 cm−3 in an ladder-type poly(p-phenylene) film deposited on a glass substrate. Owing to the different waveguide structure ASE is not observed for the same polymer deposited on an indium–tin–oxide (ITO)-coated substrate. Instead, we observe nonradiative bimolecular annihilation with a coefficient γ=4.2×10−9 cm3 s−1. Our results demonstrate that even in the absence of a resonator collective stimulated emission can be much more efficient than nonradiative recombination. A mandatory prerequisite, however, is a suitable waveguide design. The use of ITO as a hole-injecting contact is problematic due to its high refractive index and its relatively high losses.

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

ABSTRACTThe 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.

Improved electroluminescence lifetime and efficiency of polymer light- emitting diodes with plasma-treated indium tin oxide anodes

ABSTRACTWe studied the influence of various surface treatments of indium-tin oxide anodes on the operational stability of high-efficiency green-emitting polymer light-emitting diodes, fabilicated with a doped poly(3,4-ethylene dioxythiophene) PEDOT hole transport layer, a polyfluorene-based emissive layer, and Ca-Al cathodes. The anodes were modified by physical (oxygen-plasma), chemical (aquaregia), or combined treatments. Oxygen-plasma improves the stability under constant current over all the other anodes, with half-brightness lifetimes (initial brightness, 200 cd/m2) two to five times longer than for untreated samples, and 1000 times longer than for aquaregia ones. We derive two major indications for optimisation of PLEDs. First, thermal management of the diode is of the uppermost importance. Second, the ITO anode and in general the electrical properties of the hole-injecting contact are crucial to device operation, even in the presence of a hole transport layer.

Control of impurities in PPV light-emitting devices

The influence of different substrates used for the fabrication of poly( p-phenylene vinylene) (PPV) light-emitting devices on the device characteristics is investigated with different experimental techniques, like current-voltage, brightness-voltage and capacitance-voltage measurements. Using thermally stimulated currents we determine the energetic depth and density of states created by doping of PPV during device fabrication. In devices prepared on indium-tin oxide (ITO) substrates doping with InCl 3 leads to states with a depth of about 0.15 eV and an ionized acceptor concentration in excess of 10 16 cm −3. These carriers are mobile and form a depletion layer of width 120 nm when a metal with low work function, like Al. is used as cathode. This doping is responsible for the observed Schottky diode behaviour in PPV devices on ITO. With fluorine-doped tin dioxide as transparent hole-injecting contact, trap energies increase slightly to 0.2 eV and the ionized acceptor concentration is lowered by a factor of five. The lower doping concentration leads to an increase of the depletion layer width to about 270 nm and thickness-dependent device characteristics. For PPV convened on gold no doping is detectable with capacitance-voltage measurements and thermally stimulated currents. Photoluminescence measurements show a significant quenching of fluorescence in PPV converted on ITO. We regard this as an important limiting factor for single and heterolayer devices with PPV as emissive material.

Polythienylenevinylene as promoter of hole injection from ITO into bilayer light emitting diodes

AbstractDue to the low oxidation potential of poly‐(2,5‐thienylenevinylene) (PTV) an ohmic contact for hole injection is established at an indium tin oxide (ITO)/PTV interface. It gives rise to space‐charge‐limited conduction in an ITO/PTV/Al diode. The ohmic nature of the ITO/PTV contact can be exploited to increase hole injection into multilayer organic light emitting diodes in which internal charge accumulation would otherwise lower majority carrier injection by virtue of electric field screening at the anode.

Light-emitting diodes based on poly-p-phenylene-vinylene: I. Charge-carrier injection and transport

Detailed investigations of the device characteristics of poly-p-phenylene-vinylene (PPV) light-emitting diodes are reported. We analyze the influence of various hole- and electron-injecting electrodes on the current–voltage (I–V) characteristics and electroluminescence behavior. Our studies reveal that thermal conversion of the prepolymer on indium–tin–oxide (ITO) substrates leads—in contrast to conversion on Au and other high-work-function metals—to a p-type doping of PPV and, additionally, to the formation of an ohmic hole-injecting contact at the ITO/PPV interface. Hence, devices fabricated with low-work-function metals acting as the electron injecting contact (for example, Al and Ca) display Schottky behavior. These Schottky diodes are distinguished by a high rectification ratio ρr of about 106 and display electroluminescence at bias voltages as low as 1.5 V for ITO/PPV/Ca light-emitting diodes. The I–V characteristics can be quantitatively described within the modified Shockley equation, taking into account the voltage drop on a serial resistance, yielding ideality factors n ranging from 1.6 to 2.4 for different devices. At high current densities, space-charge-limited currents determine the device characteristics. The hole mobility μh is found to be of the order of 10−5 cm2/V s.

Organic electroluminescent devices with improved stability

Highly stable organic electroluminescent devices based on vapor-deposited Alq thin films have been achieved. The improvement in stability is derived from several factors including: (1) a multilayer thin-film structure with a CuPc stabilized hole-injection contact, (2) a hole-transport diamine layer using a naphthyl-substituted benzidine derivative, and (3) an ac drive wave form. These emissive devices have shown an operational half-lifetime of about 4000 h from an initial luminance of 510 cd/m2.

Blue electroluminescent diodes utilizing blends of poly( p-phenylphenylene vinylene) in poly(9-vinylcarbazole)

We report blue light emission from diodes made from polymer blends composed of poly( p-phenylphenylene vinylene) (PPPV) in a hole-transporting polymer, poly(9-vinylcarbazole) (PVK). The soluble PPPV and PVK allow fabrication of light-emitting diodes (LEDs) by spin-casting the electroluminescent polymer blend from solution at room temperature with no subsequent processing or heat treatment required. The initial devices utilized calcium as the electron-injecting (rectifying) contact on the front surface of a PPPV/PVK film spin-cast onto a glass substrate partially coated with a layer of indium/tin oxide (ITO) as the hole-injecting contact. The LEDs turn on at ∼30 V and have a peak emission wavelength in the blue at 495 nm (at room temperature). The quantum efficiency is measured as a function of PPPV content in the blend; the maximum efficiency is approximately 0.16% photons/electron at a concentration of only 2% PPPV in PVK.

Electroluminescence in anthracene crystals under time-varying electric fields

Electroluminescence in anthracene crystals has been systematically measured under time varying electric fields at various temperatures using a sodium electrode as the electron-injecting contact and a silver electrode as the hole-injecting contact. The results show that the electroluminescent brightness decreases with increasing frequency over the temperature range from − 20 to 60°C; and that for a given frequency the brightness increases with increasing temperature, reaches a peak at a critical temperature, and then decreases with increasing temperature. The electroluminescence under a half-wave rectified sinusoidal a.c. field is brighter than that under a sinusoidal a.c. field of the same peak amplitude. There is a delay time between the application of the field and the appearance of electroluminescence, and this delay time decreases with increasing frequency and saturates at a value which may be associated with the transit time of the injected electrons across the specimen. All these results can be explained on the basis of the model that electroluminescence occurs when an electron space charge built up near the anode is sufficient to enhance hole injection from the anode.

Space-Charge-Limited Currents in Organic Crystals

Electrical conductivity measurements were performed with thin (50 μ) single crystals of p-terphenyl, p-quaterphenyl, and anthracene supplied with aqueous electrodes, one of which was an iodine-iodide solution (acceptor electrode), and the other an iodide solution. The results strongly indicate that the acceptor electrode can form ohmic contact for hole injection into these crystals and that space-charge-limited currents can be drawn through them. The crystals were found to contain hole-trapping states the location-in-energy of which can be approximated by a decreasing exponential distribution above the valence band. The measurements showed that the hole mobility in p-terphenyl is about 3×10−2 cm2/v sec, is independent of the field at least up to about 4×104 v/cm, and that the hole-trap concentration is at least 1013 cm−3. The acceptor electrode used does not form ohmic contact to crystals of naphthalene and diphenyl; an explanation for this is proposed. Some theoretical aspects of ohmic contact formation to organic crystals and space-charge-limited current flow in insulators are also discussed.