X. Chi

Field-effect transistor on pentacene single crystal

We report on the fabrication and characterization of field-effect transistors (FETs) on single-crystal pentacene. These FETs exhibit hole conductivity with room-temperature effective mobility (μeff) up to 0.30 cm2/V s and on/off ratios up to 5×106. A negative gate voltage of −50 V significantly decreases the activation energy (Ea) down to 0.143 eV near room temperature. Using this value of Ea and a simple model, we find the number of free carriers is only ∼0.4% of the total number of injected carriers. Along with μeff∼0.3 cm2/V s, this places the intrinsic mobility in the range of tens of cm2/V s.

Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

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 ∼2.2 eV, and the induced change in the far-infrared transmission was measured. At 30 K, the carrier mobility is μ≈0.4 cm2/V s and decreases to μ≈0.2 cm2/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.

Ultrafast conductivity dynamics in pentacene probed using terahertz time-domain spectroscopy

We present measurements of the transient photoconductivity in pentacene single crystals using optical pump THz-probe spectroscopy. The temperature and fluence dependence of both photoinduced mobility and its dynamics are characterized.