General observation of n-type field-effect behaviour in organic semiconductors

Organic semiconductors have been the subject of active research for over a decade now, with applications emerging in light-emitting displays and printable electronic circuits. One characteristic feature of these materials is the strong trapping of electrons but not holes¹: organic field-effect transistors (FETs) typically show p-type, but not n-type, conduction even with the appropriate low-work-function electrodes, except for a few special high-electron-affinity^2-4 or low-bandgap⁵ organic semiconductors. Here we demonstrate that the use of an appropriate hydroxyl-free gate dielectric-such as a divinyltetramethylsiloxane-bis(benzocyclobutene) derivative (BCB; ref. 6) - can yield n-channel FET conduction in most conjugated polymers. The FET electron mobilities thus obtained reveal that electrons are considerably more mobile in these materials than previously thought. Electron mobilities of the order of 10^-3 to 10^-2 cm² V^-1 s^-1 have been measured in a number of polyfluorene copolymers and in a dialkyl-substituted poly(p-phenylene-vinylene), all in the unaligned state. We further show that the reason why n-type behaviour has previously been so elusive is the trapping of electrons at the semiconductor-dielectric interface by hydroxyl groups, present in the form of silanols in the case of the commonly used SiO₂ dielectric. These findings should therefore open up new opportunities for organic complementary metal-oxide semiconductor (CMOS) circuits, in which both p-type and n-type behaviours are harnessed.