In a multilayer organic device (10) comprising a substrate bearing an anode (12) and a cathode (20), wherein located between the anode (12) and the cathode (20) is at least a first layer (16) comprising a first organic semiconductor material; and a second layer (18) over the first layer (16) comprising a second organic semiconductor material; the first organic semiconductor material is a polymer and the second organic material is a polymer or an oligomer, the average molecular weight of the first organic semiconductor material being higher than the average molecular weight of the second organic semiconductor material.