Photoresponsive Organic Field-Effect Transistors (photOFETs)

The photoresponsivity of organic field effect transistors (photOFETs) is interesting since it is the basis for light sensitive transistors. PhotOFETs can be used e.g. for light induced switches, light triggered amplification, and, in photOFET arrays, for highly sensitive image sensors. In this work, results on photOFETs based on MDMO-PPV: PCBM solid-state mixtures as active semiconductor layer and poly-vinyl-alcohol (PVA) or divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) as gate dielectrics are presented. With LiF/Al top source-drain contacts all devices show dominantly n-type transistor behaviour. An increase of the drain-source current by more than two orders of magnitude upon illumination is explained by the generation of a large carrier concentration due to photo-induced charge transfer at the conjugated polymer/fullerene bulk heterojunction (photodoping). In devices fabricated on top of cross-linked BCB as dielectric a shift of the dark transfer characteristics with respect to the initial values were observed, presumably due to charge trapping at the interface. Recovery of the initial dark state in these devices was achieved either by annealing or by applying a high negative gate voltage. It is proposed to exploit this effect in applications such as light activated memory elements ("light memory device").