We report efficient polymer photodiodes fabricated on flexible polyethyleneterephthalate (PET) substrates. The PET substrates were coated with a layer of poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) that was lithographically patterned to define the anode structure. A blend of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C61 (PCBM) was then spin-coated from a 1 : 1 mixture by weight of the two components in dichlorobenzene, and the device was completed by vacuum deposition of an aluminium electrode in vacuum. The resulting photodiodes had short-circuit quantum efficiencies of 45% and peak power efficiencies of 3%, which compare favourably with values for similar devices fabricated on rigid indium tin oxide (ITO) coated glass substrates. © the Owner Societies 2006.