Electrical properties of single crystals of rigid rodlike conjugated molecules

Acceptor concentrations, carrier (hole) mobilities, and trap densities in s ingle crystals of alpha-quaterthiophene, alpha-hexathiophene, and pentacene have been determined by temperature-dependent measurements of Ohmic and sp ace-charge limited currents. Bulk mobilities of 0.06, 0.46, and 1.4 cm(2)/V s have been measured in these three materials along the crystallographic d irections with best orbital overlap between molecules. The influence of red ucing the oxidizing atmosphere during crystal growth and annealing was inve stigated. The trap density in these materials is minimized by growing the c rystals in reducing atmospheres and increased by post-growth oxygen treatme nt. Residual dopant densities as low as similar to 10(11) cm(-3) could be a chieved. Iodine, introduced during the crystal-growth process, enhances the conductivity up to 10(-3) S/cm. [S0163-1829(98)04443-9].