Bipolar charge transport in organic field-effect transistors: Enabling high mobilities and transport of photo-generated charge carriers by a molecular passivation layer

High mobility bipolar charge carrier transport in organic field-effect transistors (OFETs) can be enabled by a molecular passivation layer and selective electrode materials. Using tetratetracontane as passivation layer bipolar transport was realised in the organic semiconductors copper-phthalocyanine, diindenoperylene, pentacene, TIPS-pentacene and sexithiophene and mobilities of up to 0.1 cm²/V s were achieved for both electrons and holes. Furthermore, the trap and injection behaviour was analysed leading to a more general understanding of the transport levels of the used molecular semiconductors and their limitations for electron and hole transport in OFETs. With this knowledge the transistor operation can be further improved by applying two different electrode materials and a light-emitting transistor was demonstrated.Additionally, the effect of illumination on organic field-effect transistors was investigated for unipolar and bipolar devices. We find that the behaviour of photo-excited electrons and holes depends on the interface between the insulator and the semiconductor and the choice of contact materials. Whereas filling of electron traps by photo-generated charges and the related accumulation field are the reason for changes in charge carrier transport upon illumination without passivation layer, both types of charge carriers can be transported also in unipolar OFETs, if a passivation layer is present.