properties for prevention of ingress of moisture and oxygen into the device the substrate is commonly glass however alternative substrates may be used in particular where flexibility of the device is desirable for example the substrate may comprise a plastic as in u s pat no 6 268 695 which discloses a substrate of alternating plastic and barrier layers or a laminate of thin glass and plastic as disclosed in ep 0949850 the device is preferably encapsulated with an encapsulant not shown to prevent ingress of moisture and oxygen suitable encapsulants include a sheet of glass films having suitable barrier properties such as alternating stacks of polymer and dielectric as disclosed in for example wo 01 81649 or an airtight container as disclosed in for example wo 01 19142 a getter material for absorption of any atmospheric moisture and or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant in a practical device at least one of the electrodes is semi transparent in order that light may be emitted where the anode is transparent it typically comprises indium tin oxide examples of transparent cathodes are disclosed in for example gb 2348316 the embodiment of fig1 illustrates a device wherein the device is formed by firstly forming an anode on a substrate followed by deposition of an electroluminescent layer and a cathode however it will be appreciated that the device of the invention could also be formed by firstly forming a cathode on a substrate followed by deposition of an electroluminescent layer and an anode referring to fig2 in the first step host polymers having a distribution of lengths are made for example according to the method described in wo 03 091355 each.