The role of excitons and interfaces in molecular organic devices

Publication Type:

Thesis

Source:

Princeton University (1998)

Keywords:

1998, 2013 and earlier

Abstract:

Operating characteristics of molecular organic devices are largely governed by the formation of excitons, and exciton interactions with
interfaces between dissimilar materials. The study of these excitonic
processes and their implementation in practical optoelectronic
applications is the focus of this work. We demonstrate a number of novel
molecular organic devices by utilizing unique optical and electronic
properties of this class of solids. Excitons dominate the fundamental
processes in organic materials determining their absorption,
photoconduction, luminescence, and lasing characteristics. The spatial
extent of the exciton governs its dynamics and depends on the strength of
intermolecular interaction. Different types of excitons are illustrated in
our spectroscopic study of the archetype molecular compound
3,4,9,10-perylineteracarboxylic dianhydride. Through our discussion of
exciton self-trapping, line broadening, diffusion, and inter-level
transitions in this archetype molecular organic crystal, we access a rich
array of excitonic processes. The exciton-interface interaction influences
the luminescence and photogeneration efficiency, energy quenching, exciton
quantum confinement, and exciton lifetime. After examining these
phenomena, we tailor our molecular organic structures to demonstrate
photovoltaic cells and efficient organic light emitting devices (OLEDs).
We demonstrate transparent OLEDs that can be used in lightweight,
conformable, head-up displays, and inverted OLEDs that can be integrated
with conventional electronics. We also demonstrate a stacked OLED that
integrates three transparent OLEDs to generate a color-tunable, true color
device. Bright and efficient electroluminescence (EL) is a general
property of many organic thin films. It is generated by radiative
recombination of an exciton formed by electrically injected carriers. We
investigate the formation of excitons in the EL process by analyzing our
measurements in terms of trap-limited conduction in amorphous materials.
We infer that the traps are due to molecular polarons, which also
determine the energy distribution of excitons, and hence the EL emission
spectrum. We also show that spectral emission can be modified when the
luminescent center is in the vicinity of a strong electric dipole, where
by adjusting the strength of the dipole field the EL spectrum can be
altered. The radiative recombination of Frenkel excitons in luminescent
devices is also influenced by the presence of multilayer structures which
introduce microcavity effects. We develop a comprehensive theoretical
description of microcavity effects in OLEDs which accounts for the
spectral shape and intensity as a function of the emission angle, treating
both radiative and waveguided modes. We finally show that optically
excited organic material in a microcavity can undergo population inversion
and lase.