Long-lived charge carrier generation in ordered films of a covalent perylenediimide–diketopyrrolopyrrole–perylenediimide molecule

• Published in: Chemical science (Cambridge) Vol. 6; no. 1; pp. 402 - 411
• Main Authors: Hartnett, Patrick E., Dyar, Scott M., Margulies, Eric A., Shoer, Leah E., Cook, Andrew W., Eaton, Samuel W., Marks, Tobin J., Wasielewski, Michael R.
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 01.01.2015
• Subjects: Annealing; Charge; Charge carriers; Chemistry; Covalence; Dynamical systems; Dynamics; Formations; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Separation; SOLAR ENERGY
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/C4SC02551B

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 τ = 3.1 ± 0.2 ps, with charge recombination in τ = 340 ± 10 ps, while in unannealed/disordered films of 1 , charge separation occurs in τ < 250 fs, while charge recombination displays a multiexponential decay in ∼6 ns. The absence of long-lived, charge separation in the disordered film suggests that few free charge carriers are generated. In contrast, upon CH 2 Cl 2 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 ∼4 μs. 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.