Organic Electroluminescent Devices

Easy access to 3,8-Diaryldifurano[2,3-a:2',3'-f]naphthalenes. A new extended aromatic system

3,8-Diaryldifurano[2,3-a:2',3'-f]naphthalenes were prepared in two simple steps. First, 1,5-dihydroxynaphthalene and 1-aryl-2-bromodecan-1-ones were condensed to the corresponding naphthalene 1,5-diethers. Second, these intermediates were cyclized using methanesulfonic acid in methylene chloride. Seven examples are given, three of which are doubly substituted with octyl chains to enhance the solubility. This increased solubility allowed further modification of the 3,8-aryl groups to attach electron-withdrawing groups (formyl, nitrile, dicyanovinyl, and benzoyl).

Femtosecond studies of electron dynamics at interfaces

A delocalized electron at a metal-dielectric interface interacts with the adlayer and spatially localizes or self-traps on the femtosecond time scale into what is termed a small polaron. The dynamics can be studied by two-photon photoemission. Theoretical and experimental analyses reveal the interaction energy and the lattice vibrational mode that mediates electron localization. These results contribute to a fundamental understanding of electron behavior in weakly bonded solids and can lead to a better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

Molecular geometry fluctuation model for the mobility of conjugated polymers

We present a model to describe electrical transport in dense films of conjugated polymers. The essential physical features of the model are as follows: (i) thermal fluctuations in the molecular geometry of the polymer modify the energy levels of localized electronic states in the material, and (ii) the primary restoring force for these fluctuations is steric, which leads to spatial correlation in the energies of the localized electronic states. The model describes the electric field dependence of the mobility and explains the carrier density dependences of mobility observed in polymer diodes and field effect transistors.

High-fidelity electronic display of digital radiographs

A fully digital radiography system requires high-fidelity electronic display devices that preserve diagnostic quality. Current cathode-ray tube monitors do not meet desired performance criteria for displaying radiographs and have excessive size, weight, and power consumption. Recent developments in flat-panel display technology (in particular active-matrix liquid crystal displays, field-emission displays, and organic light-emitting displays) suggest that high-fidelity, lightweight displays will be available in the near future. Large-size active-matrix liquid crystal display devices have been demonstrated. High brightness can be easily achieved with bright back illumination. Further developments in optical design for monochrome displays should provide high fidelity and improve the angular dependence of the emitted light. Field-emission display devices have attractive emission distribution and potential for low veiling glare. This technology needs to be extended to a large area, and problems with cathode aging and nonuniformity have to be contemplated. Organic light-emitting displays represent a simple and potentially inexpensive display technology with the ability to achieve high image quality. However, extensive research and development is required to achieve large-area manufacturing methods.

Degradation mechanism of small molecule-based organic light-emitting devices

Studies on the long-term degradation of organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinoline) aluminum (AlQ3), the most widely used electroluminescent molecule, reveal that injection of holes in AlQ3 is the main cause of device degradation. The transport of holes into AlQ3 caused a decrease in its fluorescence quantum efficiency, thus showing that cationic AlQ3 species are unstable and that their degradation products are fluorescence quenchers. These findings explain the success of different approaches to stabilizing OLEDs, such as doping of the hole transport layer, introducing a buffer layer at the hole-injecting contact, and using mixed emitting layers of hole and electron transporting molecules.

Synthesis and Characterization of a New p-n Diblock Light-Emitting Copolymer

In this paper we present the synthesis and characterization of a new light-emitting copolymer constituted of regularly alternating segments of 3,3'-didecyl-2,2'-bithiophene and 2,6-bis(1,3,4-oxadiazolyl)toluene, which are p-dopable and n-dopable, respectively. The new polymer emits intense green light under the irradiation of UV light in its film states and shows strong solvatochromism both on absorption and on emission. The n-doping potential of the polymer is measured to be similar with that of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and other reported oxadiazole-containing polymers, the currently excellent electron-transporting materials, while its p-doping potential is much lower than those of other oxadiazole-containing light-emitting polymers and is comparable with that of poly(p-phenylenevinylene) (PPV) and polythiophenes (PT), the typical p-dope type materials. The presented synthesis reveals a possible approach to balance the rates of injection of electrons and holes from opposite contacts into an emissive layer in polymer light-emitting diodes by controlling the intrinsic properties of light-emitting materials.