Origin of Intrinsic Operational Instability in Organic Field-Effect Transistors with Aligned High-Mobility Donor-Acceptor Copolymer Active Layers

The origin of intrinsic operational instability in organic field-effect transistors (OFETs) is discussed by comparing bias stress effects of OFETs with unidirectionally aligned and unaligned active layers of two different semiconducting polymers under vacuum environment. By forming hydrophobic nano-groove structures on gate dielectric surfaces, a high-mobility donor-acceptor copolymer, poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b ']-dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT) and a benchmark semicrystalline polymer, poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C16), are successfully aligned. The mobilities of the PCDTPT-OFETs are higher than those of the PBTTT-OFETs, but the operational stabilities are lower. Moreover, for both semiconducting polymers, the aligned OFETs exhibit higher mobility but lower operational stability than the unaligned OFETs. These results indicate that an increase in mobility does not necessarily lead to an increase in operational stability. The interface trap density of states analysis reveals that the lower operational stability of PCDTPT-OFETs is not due to the tail state change but primarily due to the much larger turn-on voltage shift (Delta Von). The major mechanism causing Delta Von should be the charge carrier transfer from the channel to the gate dielectric. Since the energy barrier limiting the charge carrier transfer decreases with increasing ionization potential of the active layer, the lower operational stability of PCDTPT-OFETs is attributed to the higher ionization potential of PCDTPT.