Multilayer organic device

1. A multilayer organic device comprising a substrate bearing an anode and a cathode, wherein located between the anode and the cathode is at least a first layer comprising a first organic semiconductor material; and a second layer over the first layer comprising a second organic semiconductor material wherein the first and the second layers are distinct and characterized in that the first organic semiconductor material is a polymer having a number average molecular weight of at least 1,100,000 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, wherein the first and the second organic semiconductor material have the same HOMO level and the first and second organic semiconductor material comprise the same monomer species, wherein an average length of the polymer of the first layer is at least 5,000 monomers and is larger than an average length of the polymer of the second layer, wherein the first and second organic semiconductor material is poly (vinyl carbazole).

2. A multilayer organic device as claimed in claim 1 , wherein more than two layers of different organic semiconductor materials are located between the anode and the cathode, the average molecular weight of one of the layers being higher than the average molecular weight of the following layer.

3. A multilayer organic device as claimed in claim 1 , wherein the first and second organic semiconductor material comprise the same polymer, the average length of the polymer of the first layer being higher than the average length of the polymer of the second layer.

4. A multilayer organic device as claimed in claim 1 , wherein the first organic semiconductor material is provided in a layer on its own.

5. A multilayer organic device as claimed in claim 1 , wherein the second organic semiconductor material is doped.

6. A multilayer organic device as claimed in claim 1 , wherein the second organic semiconductor material is provided in an electroluminescent layer.

7. A multilayer organic device as claimed in claim 6 , wherein the second organic semiconductor material is provided in an electroluminescent layer, the material acting as a host for an emitting species.

8. A multilayer organic device as claimed in claim 6 , wherein the electroluminescent layer comprises one or more phosphorescent moieties, the second organic semiconductor material acting as a host for the one or more phosphorescent moieties.

9. A multilayer organic device as claimed in claim 6 , wherein also the first organic semiconductor material acts as a host for a light emissive material.

10. A multilayer organic device as claimed in claim 9 , wherein the first organic semiconductor material acts as a host for a yellow emissive material and the second organic semiconductor acts as a host for a blue or blue-green emissive material.

11. A multilayer organic device as claimed in claim 1 , wherein the second organic semiconductor material is provided in a blended layer with other polymers, dendrimers or small molecules.

12. A multilayer organic device as claimed in claim 1 , wherein the first organic semiconductor polymer is provided in a charge transporting layer.

13. A multilayer organic device as claimed in claim 12 , wherein the charge transporting layer is a hole transport layer.

14. A multilayer organic device as claimed in claim 1 , wherein the average molecular weights of the organic semiconductor materials are a high weight average molecular weight and a low weight average molecular weight.

15. A method of fabricating a multilayer organic device comprising providing a substrate, depositing an anode over the substrate and by a solution processing step depositing over the anode at least a first layer comprising a first organic semiconductor material and a second layer over the first layer comprising a second organic semiconductor material wherein the first and the second layers are distinct; characterized in that the first organic semiconductor material is a polymer having a number average molecular weight of at least 1,100,000 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, wherein the first and the second organic semiconductor material have the same HOMO level and the first and second organic semiconductor material comprise the same monomer species, wherein an average length of the polymer of the first layer is at least 5,000 monomers and is larger than an average length of the polymer or oligomer of the second layer, wherein the first and second organic semiconductor material is poly (vinyl carbazole).

16. A method as claimed in claim 15 , wherein the first layer is thermally backed after being deposited.

17. A method as claimed in claim 16 , wherein the second layer is thermally backed after being deposited.

18. A method as claimed in claim 15 , wherein the solution processing step is spin coating or ink jet printing.

19. The multilayer organic device as claimed in claim 1 , wherein the first and the second layers are separated by an intermixed layer formed from the first and second organic semiconductor materials.

20. The multilayer organic device as claimed in claim 19 , wherein the intermixed layer has a thickness of less than 5 nm.