Charge Transfer and Percolation in C60/Pentacene Field-Effect Transistors

Charge transfer at organic-organic heterojunctions is fundamental to several organic electronics devices, including light-emitting diodes, photovoltaic cells, light-emitting transistors, and ambipolar field-effect transistors. Here, probe charge transfer (CT) processes during the formation of an organic-organic heterojunction are probed by performing in situ real-time electrical characterization during the growth of pentacene/C60 ambipolar transistors. N-type C60 channel formation follows two different percolation pathways, one being associated to linear C60 structures growing along pentacene grain boundaries and the other to C60 island percolation. Upon n-channel formation a shift of pentacene p-type transfer characteristics is observed, which is attributed to CT across the heterojunction interface as a result of Fermi level alignment. The findings allow estimation of the energy of thermally accessible CT-states and their mobility along the interface. Finally, a model is proposed that relates the amount of transferred charge to details in the bilayer morphology and layer thicknesses. The model relies on the capacitive coupling between C60 and pentacene in the accumulation layer, which is consistent with a single hole transport channel.