Daniel B. Roitman

Organic electroluminescent devices

Electroluminescence from organic materials has the potential to enable low-cost, full-color flat-panel displays, as well as other emissive products. Some materials have now demonstrated adequate efficiencies (1 to 15 lumens/watt) and lifetimes (>5000 hours) for practical use; however, the factors that govern lifetime remain poorly understood. This article provides a brief review of device principles and applications requirements and focuses on the understanding of reliability issues.

Failure modes in polymer-based light-emitting diodes

Polymeric electroluminescent devices are now being made that achieve lifetimes of several thousands of hours at room temperature, and are probably nearing commercial appearance. This article reviews what has been learned about how to achieve these lifetimes, and considers what must still be done in order to extend them. Degradation processes discussed include formation of non-emissive spots due to cathode reaction, photooxidation in air, anode reactions, shorts and current leakage due to film defects and ion migration in strong fields, and probable sources of the slow luminance decay that limits the lifetime of state-of-the-art devices.

Effects of hole carrier injection and transport in organic light-emitting diodes

In this paper, we examine the effects of hole carrier injection and mobility on both the electroluminescence (EL) quantum efficiency and the operating voltage of bilayer organic light-emitting diodes (OLEDs). We find that hole-injection is limited by the nature of the hole injecting interface and significantly affects the operating voltage, but not the quantum efficiency of the OLED. Hole mobility is found not to affect the device quantum efficiency. We demonstrate the characteristics of an ideal ohmic contact by measuring space-charge-limited currents in a trap-free hole transporting polymer layer.

Polymer thermosetting organic light-emitting devices

We report a systematic study of novel single- and double-layer thermosetting light-emitting devices (LEDs) based on triarytamines for hole transport layer and fluorenes for the emitting and electron transport layer. These devices possess high-thermal stability, high-quantum efficiency, and high-bandgap emission (blue and green). We have fabricated dot matrix displays based on analogs of these materials.

Microwave transmission line dielectric probe to detect biomolecular surface interactions

A probe was developed to detect biomolecular binding events at a liquid-solid interface in the microwave regime in real time and without using fluorescence labels. The probe consists of a coplanar transmission line (CTL) fabricated on a glass slide that can detect dielectric changes in close proximity of the interface. The CTL geometry concentrates the electric flux density in the gap region between the signal and ground electrodes and makes it very sensitive to permittivity changes at the liquid-solid interface. The probe operation was demonstrated by immobilizing protein A on the glass surface and detecting rabbit IgG molecules in a flow channel. The sensitivity was conservatively estimated to be 100 pg/mm/sup 2/.

Poly(p-phenylene vinylene) copolymer patterns prepared via photolithographic techniques

Abstract Standard microlithographic techniques can be used for the spatially controlled light-directed synthesis of poly( p -phenylene vinylene) (PPV) block copolymers. Microscalar linear and circular patterns can be prepared in the 25 μm range. The success of the overall procedure rests on the photogeneration of triflic acid from triphenylsulfonium trifluoromethanesulfonate in a poly(9-hydroxy-[2.2]paracyclophan-1-ene) matrix to catalyze the formation of PPV. Absorption and photoluminescence studies were performed to demonstrate the efficiency of the conversion of the precursor polymer to PPV throughout the photolithographic sequence. FT-IR and UV-Vis data are also presented that illustrate how the precursor poly (paracyclophenes) can be patterned for certain block copolymer ratios in the absence of a photoacid generator.

XGA resolution full-video microdisplay using light-emitting polymers on a silicon active matrix circuit

Capable self-emissive polymers are being developed for use as emitting materials for a variety of display applications. This paper describes the use of standard CMOS integrated circuit silicon wafer technology along with a spin-cast polyfluorene-base polymer emissive layer, to demonstrate an XGA resolution, full video microdisplay. The silicon chip drive circuitry (Analog Pixel-APIX) is described along with results from our efforts to optimize the reflective anode, the semitransparent cathode process, and emissive cell construction. The 1024 X 768 pixel display achieves 200 Cd/m2 brightness at low power (<50 mW) with fast 1 usec response times. In addition, we summarize future directions to achieve color and the need to incorporate a production- worthy seal layer on microdisplays manufactured on silicon wafers.

Supramolecular Assembly Strategies Using Alternate Adsorption of Polyelectrolytes: Applications in Pled And Lc Display Devices

We present work on the modification, processing, and analysis of ultrathin films for display devices primarily using the supramolecular assembly strategy. This involves the use of various molecular assembly techniques (organic, polymer, metal) in which layer order and functionality is achieved at defined length scales approaching that of ultrathin films (a few to several hundred nm thicknesses). The use of alternate polyelectrolyte deposition (APD) is primarily investigated in ultrathin films that have significance in the fabrication of display devices. The first application involves modifying a polymer light emitting diode (PLED) device fabricated using the ITO/MEH-PPV/Ca protocol with polyaniline derivatives. The second is the use of the “command layer” amplification concept and photo-induced alignment using polarized light with ultrathin films of azo dye/polyelectrolytes in a hybrid liquid crystal (LC) cell configuration. Both strategies rely on the use of surface sensitive spectroscopic and microscopic techniques to correlate device performance with layer ordering at the molecular level. The concept of functional ultrathin layers for device fabrication and modification is emphasized.

Supramolecular Thin Film Architectures for Photonic Applications

Abstract We report on the fabrication and characterization of electroluminescent devices based on a thin film preparation strategy that involves the alternate polyelectrolyte deposition from aqueous solution for the hole injection/transporting layer combined with a conventional spin-cast method for the luminescent layer (MEH-PPV). Surface plasmon optical techniques are used to monitor the deposition process in-situ, as well as to quantify structural parameters of the final multilayer assemblies. These data are complemented by UV-vis spectroscopy, and X-ray reflectivity measurements. The final devices are functionally characterized in terms of their current-voltage, as well as their luminance-voltage performance.

Controllable growth of single walled CNTs using nanotemplates from diblock copolymers

We use diblock copolymers as nanotemplates to produce various catalyst nanoclusters or catalyst-containing inorganic nanostructures with controlled size and spacing for carbon nanotube growth. We are able to generate periodically ordered catalytic nanostructures by spin coating polymer-based catalyst systems. As a result, uniformly distributed, low defect density single walled nanotubes(CNTs) have been obtained. CNTs with diameters of 1nm or less have been produced from iron-containing inorganic nanostructures using conventional chemical vapor deposition. The superior film forming ability of polymer-based catalyst systems enables selective growth of carbon nanotubes on lithographically predefined catalyst islands over a large surface area. The ability to control the density and location of CNTs offers great potential for practical applications. The initial MALDI-MS (Matrix Assisted Laser Desorption Ionization-Mass Spectrometry) results indicate that we can positively identify bovine serum albumin (BSA) at 500 attomoles using CNT surfaces produced by this method.