M. T. Greiner

Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000 cd/m2

A simplified trilayer green phosphorescent organic light emitting diode with high efficiency and an ultralow efficiency roll-off has been demonstrated. In particular, the external quantum efficiency drops <1% from 100 to 5,000u2002cd/m2 and remains as high as ∼21.9% at 10,000u2002cd/m2. The power efficiency is also significantly improved, reaching 78.0 lm/W at 100u2002cd/m2, 50.5 lm/W at 5,000u2002cd/m2, and 42.8 lm/W at 10,000u2002cd/m2. The working mechanism of this simple device structure with an unprecedented high efficiency is also discussed.

Work function of fluorine doped tin oxide

Fluorine doped tin oxide (FTO) is a commonly used transparent conducting oxide in optoelectronic device applications. The work function of FTO is commonly cited as 4.4 eV, which is incommensurate with recent device performance results. Using x-ray photoelectron spectroscopy, the authors measured the work function of commercial FTO to be 5.0±0.1u2002eV. UV ozone treatment was found to increase the work function by ∼0.1u2002eV due to surface band bending. The origins of the much lower work function previously reported are also discussed and are found to be a result of carbon contamination and UV induced work function lowering.

A metallic molybdenum suboxide buffer layer for organic electronic devices

Molybdenum trioxide (MoO3) is commonly used as a buffer layer in organic electronic devices to improve hole-injection. However, stoichiometric MoO3 is an insulator, and adds a series resistance. Here it is shown that a MoO3 buffer layer can be reduced to form a metallic oxide buffer that exhibits more favorable energy-level alignment with N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′-diamine (α-NPD) than does MoO3. This buffer layer thus provides the conductivity of a metal with the favorable energy alignment of an oxide. Photoemission shows the reduced oxide contains Mo4+ and Mo5+, with a metallic valence band structure similar to MoO2.

Carrier mobility of organic semiconductors based on current-voltage characteristics

Carrier mobility is one of the most critical parameters in organic electronics. There is a strong interest in measuring the mobility of thin-film organic semiconductors using simple techniques, such as from current-voltage (I-V) measurements. This paper discusses how to extract mobility from I-V characteristics, ranging from space charge limited current (SCLC) to injection limited current (ILC). It is found that the mobility extracted from SCLC may significantly deviate from the value measured by time-of-flight depending on the contacting nature at the injection interface. Therefore, the SCLC cannot in general be used to accurately measure mobility. However, the mobility extracted from ILC, which incorporates the injection barrier height measured by ultraviolet photoelectron spectroscopy, is found to be more reliable for unknown materials systems.

Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes

The conventional carrier-blocking design of the exciton formation zone used in nearly all organic light emitting diodes is shown to be problematic, due to exciton quenching from accumulated radical cations. To reduce exciton quenching, a single layer of 4,4′-N,N′-dicarbazole-biphenyl (CBP) is used as hole transport layer, resulting in a dramatically improved device efficiency even at high luminance (e.g., 20.5 cd/A at 100u2009000u2002cd/m2 for fluorescent green). Various high work function transition metal oxides (WO3, V2O5, and MoO3) coated on indium tin oxide anodes have been shown to enable direct hole injection into the deep highest occupied molecular orbital of CBP (6.1 eV).

Controlling carrier accumulation and exciton formation in organic light emitting diodes

It is found that the device performance of organic light emitting diodes (OLEDs) can be significantly improved by separating the carrier accumulation zone from the exciton formation interface. The improvement is explained by suppression of exciton quenching caused by accumulated carriers at the exciton formation interface. It is also found that the position of the exciton formation interface in OLEDs correlates well with the interfacial dipole measured using ultraviolet photoelectron spectroscopy at the interface between a hole transport layer and an electron transport layer. The findings of this work provide useful material selection guidelines in designing high performance OLEDs.

Optical design of organic light emitting diodes

Out-coupling of light from organic light emitting diodes (OLEDs) is a significant challenge for the application of OLEDs in solid state lighting. Most of the light is trapped in the stratified thin film structure and the glass substrate. In this study, an optical model is developed to simulate the optical electrical field for OLEDs with a stratified structure based on the dipole source term and transfer matrix approach. The exciton distribution is also considered in the proposed model. OLEDs with weak microcavity are selected to evaluate the model. Calculation of the electroluminescence spectrum, device efficiency as well as the angular dependence is shown to have a good agreement with the experimental data. Moreover, by using the weak microcavity design, an OLED of more than 70% improved efficiency is achieved.

A comparison of CuO and CU2O hole-injection layers for low voltage organic devices

Cu2O and CuO have been grown with an aim to reduce junction electrical resistance when interfaced with N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-diamine (NPB). Organic light-emitting diodes employing Cu/CuO anodes have equivalent driving voltages as devices made with indium tin oxide. Hole-injection barriers are calculated from current-voltage characteristics of CuO/NPB/Cu and Cu2O/NPB/Cu devices via theoretical simulation. Photoelectron spectroscopies are used to measure oxide valence band spectra, interfacial dipole formation, and band bending during in situ sequential deposition of NPB on each oxide. Calculated hole-injection barriers and those derived from photoemission results accord well, explaining the superior hole injection at the CuO-NPB interface.

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.

Energy-level alignment and charge injection at metal/C60/organic interfaces

The energy-level alignment and charge injection at metal/C60/organic interfaces have been studied by ultraviolet photoelectron spectroscopy and temperature dependent current-voltage (IV) measurements. It is found that the Fermi level at the interface is pinned to ∼4.7eV by adsorbed C60 molecules on the metal surface, resulting in more favorable energy level alignment for charge injection. The findings are in excellent agreement with interface dipole theory derived from traditional semiconductor physics.