Electrical properties of single crystals of rigid rodlike conjugated molecules

Acceptor concentrations, carrier (hole) mobilities, and trap densities in single crystals of α-quaterthiophene, α-hexathiophene, and pentacene have been determined by temperature-dependent measurements of Ohmic and space-charge limited currents. Bulk mobilities of 0.06, 0.46, and 1.4 cm²/V s have been measured in these three materials along the crystallographic directions with best orbital overlap between molecules. The influence of reducing the oxidizing atmosphere during crystal growth and annealing was investigated. The trap density in these materials is minimized by growing the crystals in reducing atmospheres and increased by post-growth oxygen treatment. Residual dopant densities as low as ~10¹¹ cm^-3 could be achieved. Iodine, introduced during the crystal-growth process, enhances the conductivity up to 10^-3 S/cm.