Ultrafast conductivity dynamics in pentacene probed using terahertz spectroscopy

We present measurements of the transient photoconductivity in pentacene single crystals using optical-pump terahertz-probe spectroscopy. We have measured the temperature and fluence dependence of the mobility of the photoexcited charge carriers with picosecond resolution. The pentacene crystals were excited at 3.0 eV, which is above the bandgap of $\text{∼2.2\hspace{0.17em}}\mathrm{eV},$ and the induced change in the far-infrared transmission was measured. At 30 K, the carrier mobility is $\text{μ≈0.4\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V s}$ and decreases to $\text{μ≈0.2\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V s}$ at room temperature. The transient terahertz signal reveals the presence of free carriers that are trapped on the time scale of a few picoseconds or less, possibly through the formation of excitons, small polarons, or trapping by impurities.