Z. B. Wang

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.

Band alignment at metal/organic and metal/oxide/organic interfaces

Charge injection at metal/organic interfaces dictates the performance, lifetime, and stability of organic electronic devices. We demonstrate that interface dipole theory, originally developed to describe Schottky contacts at metal/semiconductor interfaces, can also accurately describe the injection barriers in real organic electronic devices. It is found that theoretically predicted hole injection barriers for various archetype metal/organic and metal/oxide/organic structures are in excellent agreement with values extracted from experimental transport measurements. Injection barriers at metal/organic and metal/oxide/organic interfaces can therefore be accurately predicted based on the knowledge of only a few fundamental material properties of the oxide and organic layers.

Pt(II) complex based phosphorescent organic light emitting diodes with external quantum efficiencies above 20

Phosphorescent organic light emitting diodes with >20% external quantum efficiency have been demonstrated for the first time using a cyclometalated Pt(II) complex with a triarylboron group, i.e., acetylacetonato(5-dimesitylboryl-2-(phenyl)pyridyl)platinum(II), or Pt-BppyA. This unprecedented device performance is mainly attributed to the high quantum yield of the Pt(II) complex that is achieved by using a triarylboron moiety as well as to the highly optimized double emission zone device architecture.

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).

Efficient bilayer phosphorescent organic light-emitting diodes: Direct hole injection into triplet dopants

In phosphorescent organic light-emitting diodes (OLEDs), a hole transporting layer is traditionally thought to be required to facilitate hole injection into the host molecule. It is found that fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] doped into 4,4′-N,N′-dicarbazole-biphenyl can be used to directly inject and transport holes from an indium tin oxide anode, and thus simplify the device structure and selection of materials. The efficiencies of the simplified bilayer OLEDs exceed 41 lm/W and 57 cd/A at a brightness of 100u2002cd/m2. We attribute the excellent performance to direct hole injection from the anode to Ir(ppy)3 dopant.

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.

Self-affine silver films and surface-enhanced Raman scattering: Linking spectroscopy to morphology

The relationship between the self-affine structure of cold-deposited films and the surface-enhanced Raman (SERS) intensity of benzene adsorbed on the films is examined. Based on variable temperature STM studies the structure of cold-deposited silver films is shown to be self-affine with a fractal dimension ∼2.6, more or less independent of temperature for T less than ∼270 K. The fractal structure is shown to collapse to a more or less compact structure when the films are annealed to ∼280 K. SERS activity ceases at a somewhat lower temperature (∼250 K) for all the films examined. SERS enhancements rise by factors as great as 8 as a function of annealing temperature over and above their values at the lowest deposition temperatures used (24 K). The rise reaches a maximum at a temperature that depends both on the deposition temperature of the films and the excitation wavelength. (Such an annealing effect on the SERS intensity has been known for many years.) We suggest that the observations are consistent with...

The effect of UV ozone treatment on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

The interface between ultraviolet (UV) ozone treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′-diamine (α-NPD) was investigated using single carrier hole-only devices and in situ ultraviolet and x-ray photoelectron spectroscopy to elucidate the implications for device applications. It is found that although the work function of PEDOT:PSS is increased by UV ozone treatment, the injection barrier to α-NPD is in fact increased, resulting in lower current density in devices. The apparent increase in work function is attributed to a metastable surface dipole as a result of UV ozone treatment, which does not significantly influence the energy-level alignment.

Rough silver films studied by surface enhanced raman spectroscopy and low temperature scanning tunnelling microscopy

The surface topography of Ag films and surface enhanced Raman scattering (SERS) from benzene on Ag films have been simultaneously recorded. The Ag films were formed by vacuum deposition at temperatures ranging from 100 K to 500 K. Analysis of scanning tunnelling microscopy (STM) images shows that films formed below 250 K are fractal structures with Hausdorff-Besicovitch dimension 2.55 < D < 2.72, while for those formed above 250 K, D≈2. The lower temperature, rough films exhibit strong surface enhanced Raman scattering but the higher temperature, smooth films do not. We consider the consequences of fractal character and the possible correlation between this and the SERS activity of these films.