Paul F. Seidler

Organic electronics: introduction

For the past forty years inorganic silicon and gallium arsenide semiconductors, silicon dioxide insulators, and metals such as aluminum and copper have been the backbone of the semiconductor industry. However, there has been a growing research effort in “organic electronics” to improve the semiconducting, conducting, and lightemitting properties of organics (polymers, oligomers) and hybrids (organic–inorganic composites) through novel synthesis and self-assembly techniques. Performance improvements, coupled with the ability to process these “active” materials at low temperatures over large areas on materials such as plastic or paper, may provide unique technologies and generate new applications and form factors to address the growing needs for pervasive computing and enhanced connectivity. If we review the growth of the electronics industry, it is clear that innovative organic materials have been essential to the unparalleled performance increase in semiconductors, storage, and displays at the consistently lower costs that we see today. However, the majority of these organic materials are either used as sacrificial stencils (photoresists) or passive insulators and take no active role in the electronic functioning of a device. They do not conduct current to act as switches or wires, and they do not emit light. For semiconductors, two major classes of passive organic materials have made possible the current cost/performance ratio of logic chips: photoresists and insulators. Photoresists are the key materials that define chip circuitry and enable the constant shrinking of device dimensions [1–3]. In the late 1960s, photoresist materials limited the obtainable resolution of the optical tools to ;5.0 mm (;500 transistors/cm). As optical tools continued to improve, owing to unique lens design and light sources, new resists had to be developed to continue lithographic scaling. Chemists created unique photosensitive polymers to satisfy the resolution, sensitivity, and processing needs of each successive chip generation, and now photoresist materials improve the resolution that could normally be provided on an optical exposure tool. The increased resolution capability of photoresists combined with optical tool enhancements has enabled the fabrication of 1.2 million transistors/cm with feature sizes of 180 nm, significantly smaller than the 248-nm exposure wavelength of the current optical exposure tool—an achievement that was not considered possible a few years ago. Polymeric insulators have also been essential to the performance and reliability of semiconductor devices. They were first used in the packaging of semiconductor chips, where low-cost epoxy materials found applications as insulation for wiring in the fabrication of printed wiring boards and as encapsulants to provide support/protection and hence reliability for the chips [4, 5]. Although the first polymeric dielectrics were used in the packaging of chips, IBM recently introduced a polymer that replaces the silicon dioxide dielectric typically used on-chip throughout the industry as an insulator. The seven levels of metal wiring required to connect the millions of transistors on a chip can significantly affect chip performance because of signal propagation delay and crosstalk between wiring. Improvement in interconnect performance requires reduction of the resistance (R) and capacitance (C). IBM was the first to use copper to replace aluminum wiring as a low-resistivity metal, and the first to use a low-k

Electron mobility in tris(8-hydroxy-quinoline)aluminum thin films determined via transient electroluminescence from single- and multilayer organic light-emitting diodes

Transient electroluminescence (EL) from single- and multilayer organic light-emitting diodes (OLEDs) was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide (ITO)/tris(8-hydroxy-quinoline)aluminum (Alq3)/magnesium (Mg):silver (Ag), whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine (CuPc)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/Alq3/Mg:Ag. Apparent model-dependent values of the electron mobility (μe) in Alq3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the ...

Atom-resolved electronic spectra for Alq3 from theory and experiment

The electronic structure of Alq3 is investigated using density functional theory-based calculations, photoemission and near-edge x-ray absorption fine structure. The distinct features of the observed spectra are understood in terms of contributions from the different atoms and molecular orbitals. Fingerprints of the molecular bonding and of the individual atoms are identified. These results are meant to be a reference for the monitoring of chemical processes that Alq3 may undergo during fabrication or degradation of light-emitting devices, and for the understanding of the effects of ligand or metal substitution.

Lithium–aluminum contacts for organic light-emitting devices

Organic light-emitting devices have been prepared with multilayer Al–Li–Al cathodes in an ultrahigh vacuum molecular beam deposition system. The optimum device characteristics are obtained when there is a single Al layer separating the Li layer from the organic materials. Ab initio molecular dynamics calculations of the Al–Li interaction clarify the role of Al as a blocking layer to Li diffusion into the organic films as well as the behavior of the device when the thickness of this Al interfacial layer is changed.

Influence of trapped and interfacial charges in organic multilayer light-emitting devices

Trapped and interfacial charges have significant impact on the performance of organic light-emitting devices (OLEDs). We have studied devices consisting of 20 nm copper phthalocyanine (CuPc) as the buffer and hole-injection layer, 50 nm N, N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) as the hole transport layer, and 65 nm tris(8-hydroxyquinolinato)aluminum (Alq3) as the electron transport and emitting layer sandwiched between a high-work-function metal and a semitransparent Ca electrode. Current-voltage measurements show that the device characteristics in the negative bias direction and at low positive bias below the built-in voltage are influenced by trapped charges within the organic layers. This is manifested by a strong dependence of the current in this range on the direction and speed of the voltage sweep. Low-frequency capacitance-voltage and static charge measurements reveal a voltage-independent capacitance in the negative bias direction and a significant increase between 0 and 2 V in the given device configuration, indicating the presence of negative interfacial charges at the NPB/Alq3 interface. Transient experiments show that the delay time of electroluminescence at low voltages in these multilayer devices is controlled by the buildup of internal space charges, which facilitates electron injection, rather than by charge-carrier transport through the organic layers. To summarize, our results clearly demonstrate that the tailoring of internal barriers in multilayer devices leads to a significant improvement in device performance.

Organic–inorganic multilayer structures: a novel route to highly efficient organic light-emitting diodes

Abstract A novel device structure for organic light-emitting diodes (OLEDs) is described, which consists in the most general case of an alternating sequence of thin inorganic and organic layers sandwiched between two electrodes. Compared to conventional OLEDs, these devices have a significantly enhanced current flow, increased brightness, and higher luminous efficiency at a given voltage. These improvements in performance can be attributed to increased and more balanced charge-carrier injection as well as charge-carrier confinement effects, which together lead to higher radiative recombination probability.

Slotted photonic crystal nanobeam cavity with an ultrahigh quality factor-to-mode volume ratio

We describe the design, fabrication, and characterization of a 1-dimensional silicon photonic crystal cavity with a quality factor-to-mode volume ratio greater than 10(7), which exceeds the highest previous values by an order of magnitude. The maximum of the electric field is outside the silicon in a void formed by a central slot. An extremely small calculated mode volume of 0.0096 (λvac/n)(3) is achieved through the abrupt change of the electric field in the slot, despite which a high quality factor of 8.2 × 10(5) is predicted by simulation. Quality factors up to 1.4 × 10(5) are measured in actual devices. The observation of pronounced thermo-optic bistability is consistent with the strong confinement of light in these cavities.

STM-excited electroluminescence and spectroscopy on organic materials for display applications

We present an overview of the current status of our work on scanning-tunneling-microscope-based (STM) spectroscopy and electroluminescence (EL) excitation to study the physical and electronic structure of organic materials used in organic light-emitting devices (OLEDs). By these means we probe the critical device parameters in charge-carrier injection and transport, i.e., the height of the barrier for charge-carrier injection at interfaces between different materials and the energy gap between positive and negative polaronic states. In combination with optical absorption measurements, we gauge the exciton binding energy, a parameter that determines energy transport and EL efficiency. In STM experiments involving organic EL excitation, the tip functions as an OLED electrode in a highly localized fashion, allowing one to map the spatial distribution of the EL intensity across thin-film samples with nanometer lateral resolution as well as to measure the local EL emission spectra and the influence of thin-film morphology.

HIGH RESOLUTION ATOMIC CORE LEVEL SPECTROSCOPY WITH LASER HARMONICS

High resolution atomic core level spectroscopy is carried out on condensed matter systems using tunable harmonics generated by focusing light from an amplified 150 femtosecond dye laser system operating at 610 nm, into a pulsed source of Ar gas. We show core level spectra collected with the 15th (30.54 eV), 17th (34.61 eV), and 19th (38.68 eV) harmonics of the dye laser light. Each harmonic is separated by 4.07 eV and possesses a narrow energy bandwidth which can be used to generate high resolution core level spectra.

ELECTROLUMINESCENCE FROM A SOLUBLE POLY(P-PHENYLENEVINYLENE) DERIVATIVE GENERATED USING A SCANNING TUNNELING MICROSCOPE

A scanning tunneling microscope was used to generate electroluminescence (EL) from thin films of the conjugated polymer poly(1,3-phenylenevinylene-co-2,5-dioctyloxy-1,4-phenylenevinylene). This allowed the spatial distribution of EL to be mapped across the film, and also measurements of local EL emission spectra to be recorded. It was observed that both the emission spectra and their intensity are highly nonuniform on length scales ≳2 nm. This is important in the context of light emitting diodes since it indicates that control of nanostructure can greatly improve device performance.