We observe bandlike transport in pentacene and functionalized pentacene thin films using time-resolved terahertz pulse spectroscopy. The measured transient photoconductivity exhibits fast ͑Ͻ400 fs͒ photogeneration of mobile charge carriers and reveals a transient carrier mobility that increases as the temperature decreases from 300 K down to 10 K, indicative of bandlike transport over subpicosecond time scales. A wavelength-independent photoconductive signal is observed. The transient photoconductivity in the thin-film samples exhibits a single-exponential decay, whereas a power-law decay is seen in single-crystal samples.

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 solution͒ 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 ϳ0.02-0.04 cm 2 / V s in the case of regular pentacene films and ϳ0.01-0.06 cm 2 / V s in the case of functionalized pentacene films, depending on the film morphology. The best functionalized pentacene films exhibited transient photoconductivity values reaching ϳ30%-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.

No abstract

A high-performance orthogonal time-of flight (TOF) mass spectrometer, in combination with the matrix assisted laser desorption/ionization (MALDI) source operating at elevated pressure ( approximately 1 torr in N(2)), was used to perform MALDI-TOF analyses of pentacene and some of its derivatives with and without an added matrix. These molecules are among the most interesting semiconductor materials for organic thin film transistor applications (OTFT). The observation of ion-molecule reactions between "cold" analyte ions and neutral analyte molecules in the gas phase has provided some insight into the mechanism of pentacene cluster formation and its functionalized derivatives. Furthermore, some of the matrices employed to assist the desorption/ionization process of these compounds were observed to influence the outcome via ion-molecule reactions of analyte ions and matrix molecules in the gas phase. The stability and reactivity of the compounds and their clusters in the MALDI plume during gas-phase expansion were evaluated; possible structures of the resulting clusters are discussed. The MALDI-TOF technique was also helpful in distinguishing between two isomeric forms of bis-[(triisopropylsilyl)-ethynyl]-pentacene.

No abstract