Blended dendrimers

1. A composition comprising a mixture of at least two different dendrimers A and B which possess the same core and the same repeating unit or units in the dendrons wherein either the generation of at least one of said dendrons in one of said dendrimers (A) is different from the generation of at least one of the dendrons in the other of said dendrimers (B), or the number of dendrons in one of said dendrimers is different from the number of dendrons in the other of said dendrimers, or both the number of dendrons and the generation of at least one of the dendrons in one said dendrimer (A) is different from the number and generation of dendrons in the other said dendrimer (B), and wherein (i) the dendrimers A and B are phosphorescent organometallic dendrimers which have at least one inherently at least partially conjugated dendron, and (ii) the composition is charge transporting and/or emissive.

2. A composition according to claim 1 wherein the generation of the, or at least one of the, dendrons, in one of said dendrimers is 1.

3. A composition according to claim 1 wherein the generation of the, or at least one of the, dendrons in one of said dendrimers is one greater than that of the, or at least one of the, dendrons in the other said dendrimer.

4. A composition according to claim 1 wherein the generation of the, or all the, dendrons in one of said dendrimers is one greater than that of the other said dendrimers.

5. A composition according to claim 1 wherein the molar ratio of one said dendrimer to the other dendrimer is from 1:1 to 1:50.

6. A composition according to claim 1 which comprises dendrimers of three different generations where the dendrimers are comprised of the same core, and dendron type and surface groups.

7. A composition according to claim 1 wherein the at least one dendron which is of different generation in A and B is inherently at least partially conjugated.

8. A composition according to claim 1 wherein at least one of said dendrimers A and B has two inherently at least partially conjugated dendrons.

9. A composition according to claim 1 wherein at least one of said dendrimers A and B has three inherently at least partially conjugated dendrons.

10. A composition according to claim 1 wherein all dendrons of said dendrimers A and B are inherently at least partially conjugated dendrons.

11. A composition according to claim 1 wherein the said dendrimers have the same surface groups.

12. An organic light emitting device comprising, in sequence, layers of: an optional substrate, an electrode, a first optional charge-transporting layer, a light emissive layer, a second optional charge-transporting layer and a counter electrode, wherein at least one of the emissive layer, first optional charge-transporting layer and second optional charge-transporting layer is a solid film comprising a composition as claimed in claim 1 which is capable of emitting visible light.

13. A device according to claim 12 which has at least one charge-transporting layer.

14. A device according to claim 12 wherein the emissive layer also contains an emissive dopant, as additional component.

15. A device according to claim 12 wherein the emissive layer also contains one or more charge-transporting species, as additional component.

16. A device according to claim 12 wherein the emissive layer also contains a molecular species, as additional component.

17. A device according to claim 12 wherein the emissive layer also contains a dendritic species, as additional component.

18. A device according to claim 12 wherein the emissive layer also contains a polymer, as additional component.

19. A device according to claim 14 wherein the additional component comprises 95 to 5 mol % of the total composition.

20. A photovoltaic device that comprises at least a composition as claimed in claim 1 .