Asymmetric electron and hole transport in a high-mobility n-type conjugated polymer

Electron-and hole-transport properties of the n-type copolymer poly{[N, N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-dithiophene)} [P(NDI2OD-T2), PolyeraActivInk (TM) N2200] are investigated. Electron- and hole-only devices with Ohmic contacts are demonstrated, exhibiting trap-free space-charge-limited currents for both types of charge carriers. While hole and electron mobilities are frequently equal in organic semiconductors, room-temperature mobilities of 5 x 10(-8) m(2)/Vs for electrons and 3.4 x 10(-10) m(2)/Vs for holes are determined, both showing universal Arrhenius temperature scaling. The origin of the large difference between electron and hole mobility is explained by quantum-chemical calculations, which reveal that the internal reorganization energy for electrons is smaller than for holes, while the transfer integral is larger. As a result, electron transport is intrinsically superior to hole transport under the same injection and extraction conditions.