Jerome R. Lenhard

High-efficiency, low-voltage phosphorescent organic light-emitting diode devices with mixed host

We report high-efficiency, low-voltage phosphorescent green and blue organic light-emitting diode (PHOLED) devices using mixed-host materials in the light-emitting layer (LEL) and various combinations of electron-injecting and electron-transporting layers. The low voltage does not rely on doping of the charge-transport layers. The mixed LEL architecture offers significantly improved efficiency and voltage compared to conventional PHOLEDs with neat hosts, in part by loosening the connection between the electrical band gap and the triplet energy. Bulk recombination in the LEL occurs within ∼10 nm of the interface with an electron-blocking layer. A “hole-blocking layer” need not have hole- or triplet-exciton-blocking properties. Optical microcavity effects on the spectrum and efficiency were used to locate the recombination zone. The effect of layer thickness on drive voltage was used to determine the voltage budget of a typical device. The behavior of undoped devices was investigated, and the electrolumines...

47.2: Luminescence Quenching in Blue Fluorescent OLEDs

The radical ion of the familiar hole-transport material NPB (i.e., NPB⋅+) absorbs strongly in the blue region of the spectrum. Therefore, it can quench blue luminescence by Forster energy transfer. A high concentration of NPB⋅+ at the interface between an NPB hole-transport layer (HTL) and a blue light-emitting layer (LEL) severely limits the luminescence efficiency of the emitting blue dopant. The efficiency can be improved dramatically by modifying either (a) the anode contact with a thin layer of CuPc, thereby increasing the field strength in the HTL, or (b) the cathode contact with a thin layer of Bphen, thereby reducing the field strength in the LEL. Both strategies reduce the difference in electric field strength across the interface, reduce the interfacial concentration of NPB⋅+, and suppress the quenching. Experimental evidence is provided by a spectrum of electrochemically generated NPB⋅+, simultaneous photo- and electroluminescence measurements on model OLEDs, and electrical characterization of the devices.

Novel electrochromic materials and devices

Electrochromic coatings have the ability to produce reversible and persistent changes of their optical properties. This paper summarizes the preparation of sol-gel derived electrochromic (EC) coatings in terms of processing chemistry, EC properties and device applications. Introduction The sol-gel process for synthesizing EC metal oxide thin films has attracted attention since it offers the possibility of producing large area coatings at low cost. Unfortunately the promise of sol-gel EC coatings has not materialized in any commercial EC device. This is due in part to the competition from lower cost EC components such as organic dyes [1] and to the lack of long-term durability data for sol-gel EC devices [2]. For large area applications such as displays and architectural glazing an all solid state inorganic system would be favored mainly for its UV stability and mechanical durability. For such applications sol-gel derived layers offer great promise. In this paper we review the most recent work over the past three years on the use of sol-gel technology to develop EC coatings and devices. Several reviews have been published on EC sol-gel materials and devices [2-6] relating that many sol-gel chemistries and materials were used to develop EC coatings and ion storage layers with no particular chemistry process being dominant. One material which excels for EC applications is tungsten trioxide because of its superior EC properties [7] and ease of fabrication by the sol-gel process [2]. WO3 in the reduced state is a deep blue color which for window applications may distort brightness perception and color rendition [8]. This has spurred studies by many authors to investigate doping WO3 to alter its color in the reduced state to neutral gray while maintaining a highly transparent colorless fully oxidized state. Sol-Gel Chemistry and Electrochromic Electrodes Sol-gel chemistry [5,9] applied to EC technology has led to the study of many precursor materials with a preference for the chlorides, oxychlorides and peroxide derivatives [2] because of their relative low cost and ability to produce stable coating solutions and EC coatings with comparable properties to physical vapor deposition processes. Desired properties of a sol-gel method to compete with conventional vapor phase processes are: 1. The precursor material should be easily available and stable in storage. 2. The coating solution should be stable in handling over a period of several weeks and coat the substrate uniformly with good wetting. 3. Preferably the coating should be deposited on the conductive side of the substrate only. It would be desirable to coat curved surfaces. 4. The curing temperature of the coating should be sufficiently low to avoid decomposition of the transparent conductor and/or deformation of the transparent substrate. 5. Large optical density changes are a requirement, which necessitate thick coatings in the range of 0.1 to 1.0 microns. The coating should be applied in a single deposition process. 6. Control over the stoichiometry and microstructure of the film. 7. The cured coating should meet the functional requirements of an electrochromic film. 8. All of the above should be achieved in a cost effective way to ensure that sol-gel deposition of electrochromic coatings is competitive in the market place. The above conditions are difficult to meet and have driven the synthesis of new precursor materials and coating solutions as well as the development of novel deposition methods. Precursor solutions for electrochromic coatings can be deposited by spray, spin, dip, roller and brush coating [9-11]. The most widely used method is dip coating, which offers the advantages of ease of use, low cost deposition equipment Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 337-342 doi:10.4028/www.scientific.net/KEM.264-268.337

Electrochromic materials and devices

This paper surveys materials, design concepts, and applications for electrochromic devices. Specific discussions are given on the electronic structure and optical properties of crystalline WO3, and of the Li+ dynamics in heavily disordered TiO2.The present invention relates to an electrochromic material, and a device utilizing the electrochromic material, comprising a substituted-1,1-dioxo-thiopyran of the general structure I: wherein: X is carbon, nitrogen, oxygen, or sulfur; n is 0, 1 or 2; R3 is independently an electron withdrawing group or a substituted or unsubstituted alky or aryl group; R1 and R5 each independently represent a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group; and R2 and R4 each independently represent hydrogen, or an electron withdrawing group, or a substituted or unsubstituted alkyl group.

New approach to silver halide photography using radical cation chemistry

A new mechanism for spectral sensitization of silver halide is described, which can potentially double the sensitivity of photographic emulsions. The photooxidized sensitizing dye is trapped using an organic donor molecule, which fragments to form a cation and a reducing radical, which injects an electron into the conduction band of the silver halide. In this way, two conduction-band electrons can be produced for each absorbed photon.