Molecular orientation-dependent bias stress stability in bottom-gate organic transistors based on an n-type semiconducting polymer

Despite remarkable advances in the performances of organic field-effect transistors (OFETs) in recent years, the bias stability of OFET devices remains a critical obstacle to their commercial use. The microstructural origins of charge traps inside OFET devices are not yet clearly understood, and investigating these origins presents an important challenge. The unique electrical properties of an n-type semiconducting polymer, poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)] (P(NDI2OD-T2)), are explored here to study the correlation between the microstructures of polymer semiconductor thin films and the bias stability of an OFET. It is demonstrated that although the charge carrier mobilities in a series of devices may be similar, the bias stress stabilities can differ significantly, depending on the molecular orientations of the semiconducting thin films. A higher degree of bias stress stability is attained in the P(NDI2OD-T2) FETs prepared with face-on thin-film structures compared to the bias stress stability attained in the edge-on film structures. Further experimental evidence suggests that the aliphatic alkyl chains in edge-on-oriented P(NDI2OD-T2) films present a hurdle to vertical charge transport and induce large numbers of bipolarons during bias stress, in contrast with the face-on structured thin films.