Equivalent circuit model for organic single-layer diodes

© 2008 American Institute of Physics. The electronic version of this article is the complete one and can be found at: http://dx.doi.org/10.1063/1.2980324DOI: 10.1063/1.2980324A 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

Comparison of pentacene and amorphous silicon AMOLED display driver circuits

© 2008 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

Patternable electrophosphorescent organic light-emitting diodes with solution-processed organic layers

Organic light-emitting diodes (OLEDs) have drawn much attention in the last two decades. In recent years, the power efficiency of OLEDs has been increased to exceed the efficiency of fluorescent light bulbs. However, such high-efficiency devices are typically based on small molecules that have to be evaporated in vacuum. A much higher fabrication throughput and therefore lowered costs are expected if high-efficiency OLEDs were processed from solution. This thesis shows how solution-processed electrophosphorescent multilayer OLEDs can be achieved by starting with an evaporated three-layer device structure and replacing layer by layer with a solution-processed layer. First, the hole-transport layer was replaced by a polymer and high efficiencies were observed when using a hole-transport polymer with a high ionization potential and a low hole mobility. Then, the emissive layer was replaced by a copolymer consisting of hole-transport groups and emissive complexes in its side-chains. OLEDs with four different colors are shown where the orange devices showed the highest efficiency. The orange copolymer was further optimized by making changes to the chemical nature of the polymer, such as different molecular weight, different concentrations of the emissive complex and different linkers between the side-chains and the polymer backbone. Finally, a three-layer solution-processed OLED was fabricated by crosslinking the hole-transport and the emissive layer, and by spin-coating an electron-transport polymer on top. Moreover, using the photocrosslinking properties of the emissive layer, solution-processed multilayer OLEDs of two different colors were patterned using photolithography to fabricate a white-light source with a tunable emission spectrum. Furthermore, with more and more organic semiconductors being integrated into the circuitry of commercial products, good electrical models are needed for a circuit design with predictive capabilities. Therefore, a model for the example of an organic single-layer diode is introduced in the last chapter of this thesis. The model has been implemented into SPICE and consists of an equivalent circuit that is mostly based on intrinsic material properties, which can be measured in independent experiments. The model has been tested on four different organic materials, and good agreement between model and experimental results is shown.Ph.D.Committee Chair: Kippelen Bernard; Committee Member: Brand Oliver; Committee Member: Bredas Jean-Luc; Committee Member: Dupuis Russell D.; Committee Member: Smith Glenn S

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

© 2009 American Institute of Physics. The electronic version of this article is the complete one and can be found at: http://dx.doi.org/10.1063/1.3095490DOI: 10.1063/1.3095490We 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 pentacene-based single-layer diodes

Radio link budgets for 915 MHz RFID antennas placed on various objects

Passive radio frequency (RF) tags in the UHF and microwave bands have recently drawn considerable attention with their great potential for use in inventory management, parcel and postal tracking, as remote sensors, and in a host of other radio frequency identification (RFID) applications [1, 2, 3]. However, much more basic research is needed to increase the range and reliability of a passive RF tag’s power and communication link, particularly when the RF tag is placed onto a lossy dielectric object or a metallic surface [4]. This paper presents the results of a radio assay, aseries of tests for measuring the excess link loss of an RF tag antenna when placed on a variety of common materials. Using radio assay measurements for thin, flexible 915 MHz antennas, the results document the far-field pattern and gain change as the RF tag antenna is attached to cardboard, wood, acrylic, de-ionized water, ethylene glycol, ground beef, and an aluminum slab. It is shown that more than 18 dB of excess loss on the backscatter link is added when RF tag antennas are placed onto lossy and metallic surfaces. 1

RF tag antenna performance on various materials using radio link budgets

© 2006 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.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 tag’s 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

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

© 2008 American Institute of Physics. The electronic version of this article is the complete one and can be found at: http://dx.doi.org/10.1063/1.2952452DOI: 10.1063/1.2952452We 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 72 cd/A at 100 cd/m² were obtained in a simplified device geometry that requires the deposition of only two organic layers from the vapor phase.(c) 2008 American Institute of Physics.DOI: 10.1063/1.295245

Fabrication of a blue M x N pixel organic light-emitting diode video display incorporating a thermally stable emitter

© 2009 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.DOI: 10.1109/JDT.2008.2004782A 7x11 pixel blue OLED display was fabricated using a patterned indium-tin-oxide (ITO) substrate. The fabrication process for an M x N pixel organic light-emitting diode (OLED) video display including an electrical insulating layer and a physical pixel separator layer is presented. An efficient and thermally stable blue fluorescent organic material, 6, 6'-bis((2-p-biphenyl)-4-phenylquinoline) (B2PPQ), was used in combination with an evaporated hole-transport small molecule with a high ionization potential