Dynamics of Excitation Transfer and Charge Separation in Polymeric Semiconductors

Abstract: Dynamics of optically excited electronic states have been studied in solid films and solutions of semiconducting polymers and model compounds. Transient absorption, monitored in the spectral range from 0.9 eV to 2.5 eV with a time resolution of about 100 fs, was the main experimental technique. Transient anisotropy, obtained from different polarizations of light, was employed to trace the dipole moment dynamics of the probed optical transitions. Dynamics of electron-hole pairs created by direct photoexcitation to the charge transfer state of electron donor-acceptor complexes was studied in sensitised films of a photoconducting polymer poly(epoxypropyl-carbazole) and in model solutions. Fast hole transfer (within 100 fs) from the parent carbazolyl moiety to the neighbouring carbazolyls in films is concluded from transient anisotropy measurements. This is supported by the charge recombination dependence on excitation photon energy and acceptor concentration. Comparison of charge recombination kinetics with Monte Carlo simulations suggests the separation distance between the thermalized charge pairs to be in the range of 10-20 Å. Correlation is observed between the charge separation distance and the probability of radiative recombination. Emissive intra-chain singlet states are found to be the predominant primary excitations in polythiophene conjugated polymers. A red - shift of emission spectra with time and a gradual decay of the anisotropy indicate excitation transfer to the lower energy sites in the bulk films and in the isolated polymer chains in solution. Very fast excitation transfer between adjacent chains on a time scale of 100 fs is observed in films with dense packing. Short inter-chain distance (about 4 Å) allows for the formation of inter-chain charge pairs, which are generated on a femtosecond time scale with about 20% efficiency. Mixed inter-chain exciton – charge transfer states are proposed to be precursors for charge pairs. Finally, electron transfer from the excited model molecule terthiophene into an electron accepting TiO2-SiO2 hybrid polymer matrix is found to proceed on a time scale of 1 ps. Charge separation is slower than in polymer systems. That result is explained by a fact that the primary excited state is localized on a single molecule, while it is considered delocalized in conjugated polymer systems at early time after excitation.