Timothy M. Miller

Design and application of electron-transporting organic materials

Operating lifetime is the main problem that complicates the use of polymeric light-emitting diodes (LEDs). A class of electron transport (ET) polymers [poly(aryl acrylate) and poly(aryl ether)s] is reported in which moieties with high electron affinities are covalently attached to stable polymer backbones. Devices based on poly(p-phenylenevinylene) (PPV) prepared with these materials exhibited a 30-fold improvement in stability and, in one case, dramatically lower (10 volts versus about 30 volts) operating voltage relative to those having conventional ET layers. The current-carrying capacity of indium tin oxide-PPV-polymeric ET layer-aluminum LEDs was also increased by a factor of 30. These improvements lead to an enhancement in power efficiency of nearly an order of magnitude. Choosing polymers with high glass transition temperatures increases device lifetime.

Intrinsic and extrinsic constraints on phenylenevinylene polymer electroluminescence

Studies of phenylenevinylene polymer photophysics show that luminescence yield is reduced by interchain charge transfer quenching of excitons. We present evidence that non-emissive interchain excitons also play a role in electroluminescent devices and describe one synthetic strategy to reduce their importance. Addition of oxidation defects can further reduce luminescence yield and the mechanism for this is clarified by studies of model oligomers.

Resonant cavity organic electroluminescent devices

Encapsulation of organic light-emitting diodes in Fabry-Perot microcavities permits us to fabricate red, green and blue emissive devices using a single broadband emitter. The procedure simply requires one to construct a standard LED on a prepatterned substrate, thereby eliminating the need to integrate three distinct emissive materials for full color display applications. The assembly of these devices and their properties are documented.<<ETX>>

Limits to Quantum Efficiency in Electroluminescent Devices Based on Conjugated Polymers

Abstract Excited state dynamics in poly(p-phenylenevinylene) (PPV) have been studied by transient absorption and photoluminescence measurements. The quantum yield of emissive singlet exciton photogeneration is estimated not to exceed 20 %. This is mostly limited by efficient formation of non-emissive interchain “spatially indirect” singlet excitons analogous to those in type II semiconductor heterostructures which decay nonradiatively. We also show that exciton quenching by oxidation defects is an additional process which can dramatically reduce photoluminescence quantum yield.

White and unsaturated color organic light emitting diodes

We describe the principles of operation and device characteristics of novel organic light emitting diodes in which the emission originates in a number of optically active layers. The effective emission color can be controlled by adjusting the thicknesses of the individual layers, and in this manner white and other unsaturated color LEDs with external quantum efficiency > 0.5% have been fabricated. The maximum luminance that has been achieved is ∼4,700 Cd/m 2 .