Optical and transient photoconductive properties of pentacene and functionalized pentacene thin films: Dependence on film morphology

We present a comprehensive study of the optical and transient photoconductive properties of pentacene and functionalized pentacene thin films grown by evaporation or from solution onto a variety of substrates. The transient photoconductivity was studied over picosecond time scales using time-resolved terahertz pulse spectroscopy. The structure and morphology of the films were assessed using x-ray diffraction, atomic force microscopy, and scanning electron microscopy. Regular pentacene films grown by evaporation under similar conditions but on different substrates yielded polycrystalline films with similar morphology and similar optical and transient photoconductive properties. Single exponential or biexponential decay dynamics was observed in all of the regular pentacene films studied. Functionalized pentacene films grown by evaporation at two different substrate temperatures (as well as from solution) yielded significant variations in morphology, resulting in different optical-absorption spectra and transient photoconductivities that could be correlated with film structure. The lower limit of the charge-carrier mobility, estimated from the amplitude of the transient photoconductive response, was ∼0.02–0.04cm2∕Vs in the case of regular pentacene films and ∼0.01–0.06cm2∕Vs in the case of functionalized pentacene films, depending on the film morphology. The best functionalized pentacene films exhibited transient photoconductivity values reaching ∼30%–40% of those obtained in functionalized pentacene single-crystal samples, and showed similar power-law decay dynamics. We also report on terahertz pulse generation from voltage-biased pentacene thin films.