Pentacene is a well known organic semiconductor because of its high mobility in the field-effect transistor devices, and also because film conductivity can be dramatically increased by electron and hole doping. For chemical and structural identification of the conducting phase, we carried out an intercalation experiment of pentacene with iodine at various iodine/ pentacene ratios. X-ray powder diffraction measurements revealed that there are two stable phases with different iodine concentrations. In the intermediate intercalation phase with the concentration of (pentacene)I2 and high conductivity, I3 - ions are inserted between the neighboring layers of pentacene. The saturation phase with higher iodine concentration is formed mainly of I5 -, associated with a significant lattice expansion.
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