Persistent photoexcited conducting states in functionalized pentacene

We report a study of long-lived photoexcited states in single crystals of functionalized forms of pentacene: the 6,13-bis(triisopropylsilylethynyl)pentacene and the 6,13-bis(triethylsilylethynyl)pentacene. These materials are organic semiconductors with band gaps in the range $\approx 1\mathrm{eV}$ . The relaxation rate of these states is thermally activated, as determined from time-and temperature-dependent measurements of the photoconductivity after illumination, and these states can be produced in a stable population at temperatures below $150\mathrm{K}$ . Consequently, after the low-temperature illumination, thermally stimulated current (TSC) is observed in the dark current with an increasing temperature. Analysis of the TSC signal allows estimation of energies associated with the excited states. Possible mechanisms for the current associated with the photoexcited states are discussed, and preliminary iodine-doped studies of the material are also presented.