Picosecond Transient Photoconductivity in Functionalized Pentacene Molecular Crystals Probed by Terahertz Pulse Spectroscopy

We have measured transient photoconductivity in functionalized pentacene molecular crystals using ultrafast optical pump–terahertz probe techniques. The single crystal samples were excited using 800 nm, 100 fs pulses, and the change in transmission of time-delayed, subpicosecond terahertz pulses was used to probe the photoconducting state over a temperature range from 10 to 300 K. A subpicosecond rise in photoconductivity is observed, suggesting that mobile carriers are a primary photoexcitation. At times longer than 4 ps, a power-law decay is observed consistent with dispersive transport.

Figure 1

(a) Molecular structure of FPc. (b) Stacking of FPC in the $a\mathrm{\text{−}}b$ plane of crystal (side groups not shown).

Figure 2

(a) THz pulses transmitted through a FPc crystal at 10 K at various probe delay times. The 800 nm, 100 fs pump pulse excites the sample at $t=0$, and the probe delay time is the arrival time of the positive peak of the THz probe pulse. The inset shows the amplitude spectrum of the THz probe pulse transmitted through the unexcited sample. (b) Change in amplitude transmission of the peak of the THz probe pulse as a function of delay time at 10 K. (Pump fluence is $2.5\mathrm{m}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{\mathrm{2}}$.)

Figure 3

(a) Percent differential transmission of the peak of the THz probe pulse as a function of delay time and temperature for sample A. The photoconductivity transients increase as the temperature decreases. The inset shows a 10%–90% rise time of 0.5 ps for the transients. (b) Log-log plot of the data shown in (a). Lines labeled $m=-1$ and $m=-0.5$ are guides to the eye for power-law decay rates of $t^{-1}$ and $t^{-0.5}$, respectively. (Pump fluence is $2.5\mathrm{m}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{\mathrm{2}}$.)

Figure 4

Temperature dependence of (a) the peak differential transmission and (b) the exponent $\beta$ for a power-law fit $t^{-\beta }$ at long times for sample A (squares) and sample B (circles). The error bars are smaller than the size of the symbols.