A UNIVERSAL RELATION BETWEEN CONDUCTIVITY AND FIELD-EFFECT MOBILITY IN DOPED AMORPHOUS ORGANIC SEMICONDUCTORS

We have fabricated metal-insulator-semiconductor field-effect transistors (MISFETs) from tetracyanoquinodimethane (TCNQ) doped with tetrathiofulvalene (TTF) and poly(beta'-dodecyloxy(-alpha,alpha'-alpha',alpha''-)terthienyl) (polyDOT(3)) doped with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Experimentally we find that the measured field-effect mobility strongly increases with increasing conductivity of the organic semiconductor. Moreover, by comparison with literature data on a variety of amorphous organic semiconductors, we propose that these two quantities and the dopant concentration may be linked by a universal empirical relationship. A tentative explanation based on the notion of electrical transport being dominated by hopping between localized states is given. A consequence is that large on/off ratios and high mobilities are not to be expected simultaneously in conventional MISFETs constructed from amorphous organic semiconductors.