Ambipolar transport and charge transfer at the interface between sexithiophene and N,N-bis(n-octyl)-dicyanoperylenediimide films

Heterostructures obtained by combining p-type and n-type organic semiconductors are attracting considerable attention for the interesting physical phenomena arising at organic/organic interfaces, as well as for their potential application in ambipolar field-effect transistors. Here we report on the observation of an interfacial charge transfer effect in high-quality sexithiophene (T6)/N,N-bis(n-octyl)-dicyanoperylenediimide (PDI-8CN2) heterostructures, fabricated in situ by a controlled sequential evaporation of T6 and PDI-8CN2 thin films on Si/SiO2 substrates. The electrical characterization of several heterostructures as a function of the thickness of each film shows that the hole and electron transport cannot be explained by invoking only the properties of the individual layers. Electrical characterization and nonlinear optical spectroscopy give clear indications of charge transfer between T6 and PDI-8CN2 layers, accompanied by band offset and interfacial charge accumulation (heterojunction effect) at the organic/organic interface. Signatures of such effect were the enhancement of drain-source current at zero gate voltage, a shift of gate threshold voltage in transfer curves, and an enhanced second-harmonic generation related to a built-in interfacial electric field. Gate-voltage-tunable negative transconductance has been also observed in heterostructures composed by thin T6 layers and explained in terms of gate-voltage-tunable recombination phenomena occurring in the hole/electron accumulation layer.