Electron transport along semiconducting polymer heterojunctions

Most conjugated polymer field-effect transistors rely on charge accumulation between an organic semiconductor and a gate dielectric. Much less use has been made of devices based on heterojunctions between two semiconducting polymers. Here we have realized ambipolar polymer field-effect transistors based on bilayers of two polyfluorene-based semiconducting polymers, poly(9,9-dioctylfluorene-alt-N-(4-butylphenyl)diphenylamine) and poly(9,9-dioctylfluorene-alt-benzothiadiazole). We show that the energy offset between the lowest unoccupied molecular orbitals of the two polymers of 1–1.5 eV is sufficient to confine electrons at the bi-layer interface for n-type operation. However, the transfer current–voltage characteristics exhibit an unusual plateau. This is shown to be due to hole charge trapping in donor-like states in the TFB layer during p-type operation.