1. Dielectric Surface Control
Physical or chemical properties characterizing a surface of gate dielectric have a huge impact on the electrical properties of organic field-effect transistors. Here, we applied various organic interlayers between an organic semiconductor and a gate dielectric to describe field-effect mobilities being a function of a certain macroscopic parameter associated with the surface energy of gate dielectric. The organic interlayers with various chemical moieties, that is, hydroxyl, methyl, octadecyl, polystyrene, and polymethylmetacrylate, are obtained using diverse organosilane compounds and hydroxyl-end-terminated polymer brushes. Two prototypical vapor-deposited p-type organic small molecules, dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene and pentacene, are used as semiconducting layers. We separate the surface energy of the organic interlayers into two terms, that is polar and dispersive terms, and define three parameters consisting of these two terms, so-called surface energy ratio, polar ratio, and polarity. The three parameters are plotted with the field-effect mobilities and it becomes apparent that the field-effect mobility is a function of polar ratio and polarity regardless of the semiconducting material as well as its morphology and crystallinity. In particular, the polarity that is the polar energy term divided by the total surface energy showed a clear exponential relationship, allowing a reliable prediction of field-effect mobilities.
A thermal gradient distribution was applied to a substrate during the growth of a vacuum-deposited n-type organic semiconductor (OSC) film prepared from N,N′-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4:9,10-bis(dicarboxyimide) (PDI-CN2), and the electrical performances of the films deployed in organic field-effect transistors (OFETs) were characterized. The temperature gradient at the surface was controlled by tilting the substrate, which varied the temperature one-dimensionally between the heated bottom substrate and the cooled upper substrate. The vacuum-deposited OSC molecules diffused and rearranged on the surface according to the substrate temperature gradient, producing directional crystalline and grain structures in the PDI-CN2 film. The morphological and crystalline structures of the PDI-CN2 thin films grown under a vertical temperature gradient were dramatically enhanced, comparing with the structures obtained from either uniformly heated films or films prepared under a horizontally applied temperature gradient. The field effect mobilities of the PDI-CN2-FETs prepared using the vertically applied temperature gradient were as high as 0.59 cm2 V–1 s–1, more than a factor of 2 higher than the mobility of 0.25 cm2 V–1 s–1 submitted to conventional thermal annealing and the mobility of 0.29 cm2 V–1 s–1from the horizontally applied temperature gradient.