Andreas Haldi

RF Tag Antenna Performance on Various Materials Using Radio Link Budgets

Passive radio frequency (RF) tags in the UHF and microwave bands have drawn considerable attention because of their great potential for use in many radio frequency identification (RFID) applications. However, more basic research is needed to increase the range and reliability of a passive RF tags radio link, particularly when the RF tag is placed onto any lossy dielectric or metallic surface. This paper presents two new useful forms of the radio link budget that describe the power link of an RF tag system when the tag is attached to an object. These radio link budgets are dependent upon the gain penalty, a term which quantifies the reduction in RF tag antenna gain due to material attachment. A series of measurements, or radio assay, was used to measure the far-field gain pattern and gain penalty of several flexible 915 MHz antennas when attached to cardboard, pine plywood, acrylic, deionized water, ethylene glycol, ground beef, and an aluminum slab. It is shown that the gain penalty due to material attachment can result in more than 20 dB of excess loss in the backscatter communication link.

Effects of surface modification of indium tin oxide electrodes on the performance of molecular multilayer organic photovoltaic devices

We investigate the effects of surface modification of indium tin oxide (ITO) on the performance of organic multilayer molecular photovoltaic devices based on pentacene/C60 bi-layer heterojunctions. Values of the open-circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η) are found invariant of the work function and surface hydrophobicity of ITO. Insensitivity of these parameters to variations of work function in the range of 4.50 to 5.40 eV achieved through the use of surface modifiers are correlated with an invariance of the barrier height (∼0.6 eV) due to Fermi level pinning at the ITO/pentacene interface. Energy barrier heights are extracted independently from the analysis of the electrical characteristics of single-layer diodes based on modified ITO and pentacene using an equivalent circuit model.

Effect of phosphonic acid surface modifiers on the work function of indium tin oxide and on the charge injection barrier into organic single-layer diodes

We investigate the use of several phosphonic acid surface modifiers in order to increase the indium tin oxide (ITO) work function in the range of 4.90–5.40 eV. Single-layer diodes consisting of ITO/modifier/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′ biphenyl-4,4″ diamine (α-NPD)/Al and ITO/modifier/pentacene/Al were fabricated to see the influence of the modified ITO substrates with different work functions on the charge injection. To calculate the charge injection barrier with different surface modifiers, the experimentally measured current density-voltage (J-V) characteristics at different temperatures are fitted using an equivalent circuit model that assumes thermionic emission across the barrier between the ITO work function and the highest occupied molecular orbital of the organic material. The charge injection barrier height extracted from the model for various surface modifier-based diodes is independent of the ITO work function within the range of changes achieved through modifiers for both α-NPD and...

Fluorenyl-substituted silole molecules: geometric, electronic, optical, and device properties

A series of silole molecules with fluorenyl substituents at varying positions—1-(9,9-dimethylfluoren-2-yl)-1,2,3,4,5-pentaphenylsilole, 1-(fluoren-9-yl)-1,2,3,4,5-pentaphenylsilole, 1,1,3,4-tetraphenyl-2,5-bis(9,9-dimethylfluoren-2-yl)silole, and 1,1-diphenyl-2,3,4,5-tetrakis(9,9-dimethylfluoren-2-yl)silole—has been synthesized and compared to the previously reported compounds, 1,1,2,3,4,5-hexaphenylsilole and 1,1-bis(9,9-dimethylfluoren-2-yl)-2,3,4,5-tetraphenylsilole. The effect of fluorenyl substitution pattern on the geometric, thermal, electronic, optical, and electroluminescence properties was investigated both experimentally and theoretically. Analysis of the X-ray crystal packing diagrams for two new fluorenyl-substituted siloles indicates the presence of π–π stacking and CH⋯π interactions in the solid state. Across the series, excellent thermal and morphological stabilities are displayed. Photoelectron/inverse-photoelectron spectroscopy measurements and density functional theory (DFT) calculations suggest that increased conjugation length through substitution at the 2- and 5-positions plays a more significant role in tuning the ionization potentials and electron affinities of these siloles than do inductive effects through substitution of the silicon. The electronic structure (e.g., HOMO–LUMO gap) and, hence, the optical absorption and fluorescence properties are also sensitive to the positions at which the fluorenyl groups are introduced, with substitution at the 2,5-positions having the largest effect. Solution-processed electroluminescent devices fabricated with the fluorenyl-substituted siloles as the emissive layer have luminous efficiencies as high as 3.6 cd A−1.

Equivalent circuit model for organic single-layer diodes

A simple equivalent circuit is proposed to model single-layer organic diodes. The circuit is based on thermionic emission to describe carrier injection from the electrode into the organic semiconductor and on space-charge limited currents across the semiconductor. By fitting the electrical characteristics measured as a function of temperature with the model, intrinsic material and interface parameters such as the mobility and the injection barrier energy are extracted. The resulting parameters agree well with independently measured values in the literature.

Highly efficient green phosphorescent organic light-emitting diodes with simplified device geometry

We report on the performance of green phosphorescent organic light-emitting diodes based on the well-known host 4,4′-di(carbazol-9-yl)-biphenyl and the green phosphor emitter fac tris(2-phenylpyridinato-N,C2′) iridium. Using a spin-coated hole-injection/transport layer of poly(N-vinyl-carbazole) and a hole-blocking/electron-transport layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, devices with efficiencies of 21.2% and 72cd∕A at 100cd∕m2 were obtained in a simplified device geometry that requires the deposition of only two organic layers from the vapor phase.

Comparison of Pentacene and Amorphous Silicon AMOLED Display Driver Circuits

Organic light-emitting diode (OLED) displays offer distinct advantages over liquid crystal displays for portable electronics applications, including light weight, high brightness, low power consumption, wide viewing angle, and low processing costs. They also are attractive candidates for highly flexible substrates. In active-matrix OLED (AMOLED) displays, a small transistor circuit is used to drive each OLED device. This paper compares the simulated performance of two state-of-the-art AMOLED drivers with a proposed 5 thin-film-transistor (TFT) voltage programmed driver circuit which combines the advantages of the first two configurations. A competitive evaluation is also done between amorphous silicon (alpha-Si) and organic TFTs (OTFTs,) using comparable empirical device models for alpha-Si) and pentacene OTFTs. The 5-TFT circuit is found to match the speed of the 2-TFT while achieving a stability closer to the 4-TFT circuits and demonstrating a better speed-stability tradeoff.

How to construct a test bed for RFID antenna measurements

Passive, radio frequency (RF) tags that communicate using modulated backscatter are the subject of increasing research interest because of their potential uses in sensor and radio frequency identification (RFID) applications. Applications of RFID tags include toll collection, inventory management, package tracking, airport luggage security, and many others. The quality of an RF tag system is fundamentally linked to the performance of the RF tag and reader antennas and is affected by many environmental factors. This paper presents a suite of custom microwave devices necessary to measure the performance of RFID system antennas. The test bed allows a wide range of backscatter and conventional one-way link measurements to be performed including RF tag operating range tests, the effect of material attachment and antenna deformation on the performance of the RF tag antenna, signal fading, optimal reader receiver designs, and reader antenna array designs

Organic light-emitting diodes based on arylamine molecules and polymers with a fluorene core

A series of soluble arylamine-based hole transporting molecules with a fluorene core and with various ionization potentials have been synthesized. The transport properties of these molecules doped into polystyrene have been measured by time-of-flight experiments and compared to those of analogous compounds with a biphenyl core (TPD). Reorganization energies between the neutral molecules and their cations have been calculated using density functional theory. The effects of bond length and geometry relaxations on the overall reorganization energy in these two classes of molecules are discussed. Molecules from both classes have been doped into polystyrene and used as hole-transport layers (HTLs) in multi-layer light-emitting diodes with the structure ITO/HTL/AlQ3/Mg:Ag [ITO = indium tin oxide, AlQ3 = tris(8-hydroxyquinolinato)aluminum]. The electroluminescent properties and lifetime measurements at constant current have been evaluated. Significant variations in lifetime when using different substituents have been observed.