Ultrafast relaxation of charge-transfer excitons in low-bandgap conjugated copolymers

摘要: A photophysical study, employing linear optical and femtosecond two-dimensional electronic spectroscopies, of two conjugated copolymers in solution is reported. The study focuses on the effect backbone conformation properties dynamics excitons. For predominantly planar copolymer, poly((4,8-diethylhexyloxyl) benzo([1,2-b:4,5-b′]dithiophene)-2,6-diyl)-alt-((5-octylthieno[3,4-c]pyrrole-4,6-dione)-1,3-diyl) (PBDTTPD), dipole moment excitons was found to be unchanged upon photoexcitation, no excitonic relaxation detected within 1 ps. poly[N-11′′-henicosanyl-2,7-carbazole-alt-5,5-(4′,7′-di-thienyl-2′.1′.3′-benzothiadiazole)] (PCDTBT), however, bathochromic shifts absorption fluorescence spectra as a function solvent polarity indicate change approximately 3 Debye photoexcitation at red-edge spectrum. Excitation PCDTBT maximum, where an ensemble polymer chains have twisted backbone, causes excited state with stronger charge-transfer character timescale ∼200 fs. This dynamic Stokes shift not observed when exciting red edge, which photo-selects more conformations. Ultrafast involving coupled torsional coordinates increases exciton. Although there concomitant loss energy, electron-hole correlation (binding) weakened, may contribute efficiency organic solar cells based electron donor.