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

Abstract

The photophysics of a covalently linked perylenediimide-diketopyrrolopyrrole-perylenediimide acceptor-donor-acceptor molecule (PDI-DPP-PDI, <b>1</b>) were investigated and found to be markedly different in solution <i>versus</i> in unannealed and solvent annealed films. Photoexcitation of <b>1</b> in toluene results in quantitative charge separation in <i>τ</i> = 3.1 ± 0.2 ps, with charge recombination in <i>τ</i> = 340 ± 10 ps, while in unannealed/disordered films of <b>1</b>, charge separation occurs in <i>τ</i> < 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 CH₂Cl₂ vapor annealing films of <b>1</b>, 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.