Journal
CHEMICAL SCIENCE
Volume 6, Issue 1, Pages 402-411Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c4sc02551b
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Funding
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division [DE-FG02-99ER14999]
- Materials Science and Engineering Division [DE-FG02-08ER46536]
- U.S. DOE [DE-AC02-06CH11357]
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The photophysics of a covalently linked perylenediimide-diketopyrrolopyrrole-perylenediimide acceptor-donor-acceptor molecule (PDI-DPP-PDI, 1) were investigated and found to be markedly different in solution versus in unannealed and solvent annealed films. Photoexcitation of 1 in toluene results in quantitative charge separation in tau = 3.1 +/- 0.2 ps, with charge recombination in tau = 340 +/- 10 ps, while in unannealed/disordered films of 1, charge separation occurs in tau < 250 fs, while charge recombination displays a multiexponential decay in similar to 6 ns. The absence of long-lived, charge separation in the disordered film suggests that few free charge carriers are generated. In contrast, upon CH2Cl2 vapor annealing films of 1, grazing-incidence X-ray scattering shows that the molecules form a more ordered structure. Photoexcitation of the ordered films results in initial formation of a spin-correlated radical ion pair (electron-hole pair) as indicated by magnetic field effects on the formation of free charge carriers which live for similar to 4 mu s. This result has significant implications for the design of organic solar cells based on covalent donor-acceptor systems and shows that long-lived, charge-separated states can be achieved by controlling intramolecular charge separation dynamics in well-ordered systems.
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