Femtosecond study of exciton dynamics in 9,9-di-n-hexylfluorene/anthracene random copolymers
- Department of Chemistry, University of California at Santa Cruz, California 95064 (United States)
- IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120 (United States)
- Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
Exciton dynamics of 9,9-di-n-hexylfluorene/anthracene (DHF/ANT) statistical copolymers have been measured using femtosecond transient absorption spectroscopy. An investigation of the excitation intensity dependence over the range of 0.1-1.0 mJ/(pulse cm2) for solutions and 1.0-17 {mu}J/(pulse cm2) for thin films has been conducted to explore exciton relaxation mechanisms below excitation densities where exciton-exciton interaction is important. Intrachain relaxation of photoexcited singlet excitons is observed in dilute solutions. In contrast, interchain relaxation mechanisms become predominant in thin films. Decay dynamics are independent of excitation intensity for dilute solutions and thin films of DHF/ANT when probed at 790 and 750 nm. In addition, time-resolved measurements for a DHF homopolymer and two copolymer thin films have been carried out as a function of probe wavelength. A stimulated emission (SE) feature and a photoinduced absorption (PA) feature are observed in the visible region. The SE and PA dynamics are similar for the copolymers, suggesting that the same excited state species, the singlet exciton, is responsible for both the SE and PA. There is a significant difference between the SE and PA dynamics for DHF thin films on the 0-3-ps timescale. The SE dynamics show a pulse-width limited rise and a subsequent decay. In contrast, both the 600 and 750 nm PA dynamics show a ''double'' rise that represents contributions from two separate photophysical processes. These results, in combination with the steady-state photoluminescence spectrum, which indicates excimer emission, lead to the conclusion that interchain species, such as excimers, are formed in <1 ps in DHF homopolymer films following photoexcitation. That the copolymer dynamics show no evidence of excited state species other than the singlet, emissive exciton, is consistent with the interpretation that anthracene substituents in the polymer backbone prevent interchain interactions in films. (c) 2000 The American Physical Society.
- OSTI ID:
- 20215761
- Journal Information:
- Physical Review. B, Condensed Matter and Materials Physics, Vol. 61, Issue 12; Other Information: PBD: 15 Mar 2000; ISSN 1098-0121
- Country of Publication:
- United States
- Language:
- English
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