Charge separation and recombination in a photoconducting polymer with electron donor-acceptor complexes

The dynamics of electron-hole (e-h) pairs created by photoexcitation of charge-transfer (CT) states in films and solutions of oligomeric poly-N-epoxypropylcarbazole (PEPCz) doped with electron-accepting trinitrofluorenone were studied by investigating polarized transient absorption kinetics and spectra with picosecond and femtosecond time resolution. We observed very fast formation of the carbazolyl radical cation in PEPCz films, within the 100 fs excitation pulse. Transient anisotropy, which was calculated from polarized kinetics at the time delays of 100 fs and longer, is about 5 times lower in the films than in the solution of a monomeric analogue. This very fast loss of the excitation polarization memory in the films is attributed to hole transfer from the parent carbazolyl (Cz) moiety to the neighboring Czs. An asymptotic decay of the anisotropy is observed on the picosecond time scale, indicating that the fast charge separation is completed within a few picoseconds. The geminate recombination of e-h pairs in PEPCz films is nonexponential and extends into the nanosecond time domain. A dependence of the primary e-h separation efficiency on the excitation photon energy is concluded from recombination dynamics. After photoexcitation, limited hole transfer also occurs in PEPCz solution on the short oligomer chain, but recombination is much faster than in the films.