For basic understanding of transistor properties of doped organic semiconductors, 3,3′,5,5′-tetramethylbenzidine charge-transfer complexes are investigated, which change from neutral to ionic by varying the acceptors. When going into the ionic state, the bulk conduction increases more rapidly than the mobility, but sufficiently thin devices exhibit transistor properties. The resulting ambipolar characteristics are analyzed in the linear regions at small drain voltages in analogy with graphene transistors. The model is further extended to include partial overlap of electron and hole transport regions. In the temperature dependence, the activation energy loses gate voltage dependence when the neutral–ionic transition takes place by 0.1–0.2 eV above the equal acceptor and donor levels; the difference comes from the typical trap (polaron) depth or the Madelung energy.

Recently, dimethylbenzoimidazole (DMBIH) derivatives have attracted considerable attention as an n-channel dopant. Crystal structures and conducting properties of several aryl-substituted benzoimidazolium (DMBI) salts of fluorinated tetracyanoquinodimethanes (F n TCNQ, n = 0, 2, and 4) are investigated. The aryl-groups are not coplanar to the benzoimidazolium part, while the 1:1 complexes form mixed stacks and show a resistivity as high as 107 Ω cm. The 2:3 and 1:2 salts have segregate stacks of TCNQ trimer and tetramer, respectively, and show a comparatively small resistivity of 102–103 Ω cm. The DMBI complexes exhibit a large variety of compositions, crystal structures, and conducting properties, but DMBI tends to induce significant local structural distortion, which may be an origin of the conductivity drop upon overdoping.