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Proceedings Paper

Femtosecond study of exciton dynamics in polyfluorene statistical copolymers in solutions and thin films
Author(s): Jin Z. Zhang; Melissa A. Kreger; Gerrit Klaerner; M. Kreyenschmidt; Robert D. Miller; J. Campbell Scott
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Paper Abstract

The formation and decay dynamics of photogenerated excitons in polyfluorene statistical co-polymers in solutions and in thin films have been studied using femtosecond transient absorption spectroscopy. In solution photoexcitation of the polymer generates primarily intrachain singlet excitons which are initially hot and then relax quickly (< 200 fs) towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 30 ps and 330 ps in toluene. The fast decay is attributable to vibrational relaxation, spectral diffusion, or internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and non-radiative pathways. In films the exciton dynamics are found to depend strongly on excitation intensity. At low intensity, the dynamics are similar to that in solutions, with a double exponential decay with time constants of 15 ps and 300 ps. At high intensities, a fast decay component with a time constant of 0.8 ps appears, which becomes more dominant at higher intensities. This fast decay is attributed to exciton- exciton annihilation due to high density of excitons created. The signal in films at both low and high excitation intensities is attributable to intrachain singlet excitons, as in solution. There is no evidence for formation of interchain bound polaron pairs in films at low intensities. At high intensities, the possibility cannot be ruled out completely, especially in relation to the fast decay. If bound polaron pairs are formed as indicated by the fast decay, they must be generated as a result of interaction between excitons on different chains since they are absent at low power, an they must be created and then decay within about 1 ps.

Paper Details

Date Published: 1 December 1997
PDF: 12 pages
Proc. SPIE 3145, Optical Probes of Conjugated Polymers, (1 December 1997); doi: 10.1117/12.284150
Show Author Affiliations
Jin Z. Zhang, Univ. of California/Santa Cruz (United States)
Melissa A. Kreger, Univ. of California/Santa Cruz (United States)
Gerrit Klaerner, IBM Almaden Research Ctr. (United States)
M. Kreyenschmidt, IBM Almaden Research Ctr. (United States)
Robert D. Miller, IBM Almaden Research Ctr. (United States)
J. Campbell Scott, IBM Almaden Research Ctr. (United States)

Published in SPIE Proceedings Vol. 3145:
Optical Probes of Conjugated Polymers
Z. Valy Vardeny; Lewis J. Rothberg, Editor(s)

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