IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Organic Films and Devices Thursday Sessions
       Session OF+TF-ThM

Paper OF+TF-ThM8
Photoelectron Spectroscopy of Exciton Dynamics and Interactions in Organic Thin Films: C@sub60@ and Photopolymerized C@sub60@

Thursday, November 1, 2001, 10:40 am, Room 131

Session: Characterization of Organic Thin Films
Presenter: J.P. Long, Naval Research Laboratory
Authors: J.P. Long, Naval Research Laboratory
S.J. Chase, Naval Research Laboratory
M.N. Kabler, Naval Research Laboratory
Correspondent: Click to Email
Of fundamental importance to the operation of many organic electronic devices are the controlling nonequilbrium populations of singlet (S@sub 1@) and triplet (T@sub 1@) excitons and charged carriers. For example, optoelectronic device efficiencies may be depressed if dark T@sub 1@ excitons compete with fluorescent S@sub 1@ states or if annihilation interactions limit excited state population densities. Because pump-probe photoelectron spectroscopy is uniquely capable, in principle, of resolving in energy and time the electrons of all nonequilibrium species, it promises to provide useful insights into the densities, lifetimes, and mutual interactions of the various nonequilibrium populations. Using both synchrotron and laser-harmonic photoemission sources, we have applied this technique to study the dynamics of excitons pumped by visible laser radiation in the model systems of C@sub 60@ and photopolymerized C@sub60@ films prepared in ultrahigh vacuum and studied in situ. We report the unambiguous identification of the transient photoelectron spectra of both S@sub 1@ and T@sub 1@ excitons. Studies on time scales from 100 ps to 10 µs and of exciton concentrations from below 10@super 18@ cm@super -3@ to more than 10@super 19@ cm@super -3@ reveal a rich dynamics. For exciton densities above ~10@super 19@ cm@super -3@, excited-state interactions lead to a surprising excess of T@sub 1@ excitons at the expense of S@sub 1@ excitons for times much less than the intersystem crossing time of ~2.5 ns. Such excess T@sub 1@ generation suggests that interactions among excited species may interfere with the operation of potential devices that would operate at high nonequilibrium densities. A rate-equation model that couples interacting S@sub 1@, T@sub 1@, and carrier populations reproduces the complex dynamics, including non-Markovian decays, and provides evidence for the annihilation of excitons by charged carriers.