Graded organic photovoltaic device

1. A photovoltaic device comprising:

a first heterojunction layer, the first heterojunction layer comprising a mixture of a first donor type organic material and a first acceptor type organic material; and

an interlayer forming an interface with the first heterojunction layer,

wherein each of the first donor type organic material and the first acceptor type organic material is associated with a corresponding predefined concentration profile, a concentration of the first donor type organic material increasing continuously throughout the first heterojunction layer from a first side of the first heterojunction layer to a second side of the first heterojunction layer, and a concentration of the first acceptor type organic material decreasing continuously throughout the first heterojunction layer from the first side of the first heterojunction layer to the second side of the first heterojunction layer, and

wherein the concentration of the first donor type organic material is non-zero at the first side of the first heterojunction layer and the interlayer forms the interface with the first side of the first heterojunction layer, or the concentration of the first acceptor type organic material is non-zero at the second side of the first heterojunction layer and the interlayer forms the interface with the second side of the first heterojunction layer, and

wherein the interlayer has a material composition that is different from the first heterojunction layer and different from a pure first acceptor type organic material or a pure first donor type organic material.

2. The photovoltaic device of claim 1 wherein the concentrations of the first donor type organic material and the first acceptor type organic material are non-zero at both the first side and the second side of the first heterojunction layer.

3. The photovoltaic device of claim 1 wherein the predefined concentration profile of the first donor type organic material, the first acceptor type organic material, or both the first donor type organic material and the first acceptor type organic material comprises a linear graded profile.

4. The photovoltaic device of claim 1 further comprising:

an anode layer; and

a cathode layer,

wherein the first heterojunction layer is between the anode layer and the cathode layer.

5. The photovoltaic device of claim 4 further comprising a charge blocking layer between the anode layer and the first heterojunction layer.

6. The photovoltaic device of claim 5 wherein the charge blocking layer is molybdenum oxide.

7. The photovoltaic device of claim 1 wherein the first heterojunction layer further comprises a second donor type organic material.

8. The photovoltaic device of claim 7 wherein a concentration of the second donor type organic material is graded continuously from the first side of the first heterojunction layer to the second side of the first heterojunction layer.

9. The photovoltaic device of claim 1 further comprising a second acceptor type organic material.

10. The photovoltaic device of claim 9 wherein a concentration of the second acceptor type organic material is graded continuously from the first side of the first heterojunction layer to the second side of the first heterojunction layer.

11. The photovoltaic device of claim 1 further comprising a second heterojunction layer adjacent to the first heterojunction layer, the second heterojunction layer comprising a mixture of a second donor type organic material and a second acceptor type organic material,

wherein each of the second donor type organic material and the second acceptor type organic material is arranged according to a corresponding predefined concentration profile, a concentration of the second donor type organic material increasing continuously from the first side of the second heterojunction layer to the second side of the second heterojunction layer, and a concentration of the second acceptor type organic material decreasing continuously from the first side of the second heterojunction layer to the second side of the second heterojunction layer.

12. The photovoltaic device of claim 11 further comprising a charge coupling layer between the first heterojunction layer and the second heterojunction layer.

13. The photovoltaic device of claim 11 wherein the first heterojunction layer and the second heterojunction layer are arranged in a stack and are electrically coupled in series.

14. The photovoltaic device of claim 11 wherein the first heterojunction layer and the second heterojunction layer are arranged side by side and are electrically coupled in parallel.

15. The photovoltaic device of claim 1 wherein the predefined concentration profile of the first donor type organic material, the first acceptor type organic material, or both the first donor type organic material and the first acceptor type organic material comprises a nonlinear graded concentration profile.

16. The photovoltaic device of claim 1 wherein the predefined concentration profile of the first donor type organic material, the first acceptor type organic material, or both the first donor type organic material and the first acceptor type organic material comprises an exponential graded concentration profile.

17. The photovoltaic device of claim 1 , wherein the interlayer comprises a material selected from the group consisting of molybdenum oxide (MoO 3 ), bathocuproine (BCP), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), copper phthalocyanine, 2-TNATA (4,4′,4″-tris(2-naphthylphenylamino)triphenylamine), polyaniline, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (poly-TPD), spiro-TPD, 4-4′-N,N′-dicarbazolyl-biphenyl (CBP), 4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPD), polypyrrole, poly(phenylene vinylene), an aromatic tertiary amine, a polynuclear aromatic tertiary amine, a 4,4′-bis(p-carbazolyl)-1,1′-biphenyl compound, and N,N,N′,N′-tetraarylbenzidine.

18. The photovoltaic device of claim 1 , wherein the interlayer is doped.

19. The photovoltaic device of claim 18 , wherein the dopant is selected from the group consisting of Na, Li, and Cs.

20. A method of fabricating a photovoltaic device comprising:

depositing a mixture of a first donor type organic material and a first acceptor type organic material on a substrate to provide a first heterojunction layer depositing an interlayer wherein the interlayer forms an interface with the first heterojunction layer,

wherein each of the first donor type organic material and the first acceptor type organic material is associated with a corresponding predefined concentration profile, wherein a concentration of the first donor type organic material increasing continuously throughout the first heterojunction layer from a first side of the first heterojunction layer to a second side of the first heterojunction layer, and a concentration of the first acceptor type organic material decreasing continuously throughout the first heterojunction layer from the first side of the first heterojunction layer to the second side of the first heterojunction layer, and wherein the concentration of the first donor type organic material is non-zero at the first side of the first heterojunction layer and the interlayer forms the interface with the first side of the first heterojunction layer, or the concentration of the first acceptor type organic material is non-zero at the second side of the first heterojunction layer and the interlayer forms the interface with the second side of the first heterojunction layer, and wherein the interlayer has a material composition that is different from the first heterojunction layer and different from a pure first acceptor type organic material or a pure first donor type organic material.

21. The method of claim 20 further comprising depositing a second donor type organic material and a second acceptor type organic material on the first heterojunction layer to provide a second heterojunction layer.

22. The method of claim 20 further comprising annealing the photovoltaic device.