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

We have fabricated metal-insulator-semiconductor field-effect transist ors (MISFETs) from tetracyanoquinodimethane (TCNQ) doped with tetrathi ofulvalene (TTF) and decyloxy(-alpha,alpha'-alpha',alpha''-)terthienyl ) (polyDOT(3)) doped with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (D DQ). Experimentally we find that the measured field-effect mobility st rongly increases with increasing conductivity of the organic semicondu ctor. Moreover, by comparison with literature data on a variety of amo rphous organic semiconductors, we propose that these two quantities an d the dopant concentration may be linked by a universal empirical rela tionship. A tentative explanation based on the notion of electrical tr ansport 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.