Organic photovoltaic cells

1. An article, comprising:

first and second electrodes;

a photoactive layer between the first and second electrodes; and

a hole blocking layer between the first electrode and the photoactive layer, the hole blocking layer comprising a polyethylenimine;

wherein the article is configured as a photovoltaic cell.

2. The article of claim 1 , wherein the polyethylenimine comprises at least one primary amino group or at least one secondary amino group.

3. The article of claim 1 , wherein the polyethylenimine comprises a primary amino group, a secondary amino group, or a tertiary amino group.

4. The article of claim 3 , wherein the polyethylenimine comprises NH 2 or N(CH 3 ) 2 .

5. The article of claim 2 , wherein the polyethylenimine further comprises an alkoxylsilyl group, a chlorosilyl group, a carboxylic acid group, a carbonyl chloride group, a phosphonic acid group, a phosphoryl chloride group, or a thiol group.

6. The article of claim 1 , wherein at least some molecules of the polyethylenimine are cross-linked.

7. The article of claim 1 , wherein at least some molecules of the polyethylenimine are adsorbed to a surface of the first electrode.

8. The article of claim 6 , wherein at least some molecules of the polyethylenimine are cross-linked by a cross-linking agent.

9. The article of claim 8 , wherein the cross-linking agent comprises an epoxy-containing compound.

10. The article of claim 9 , wherein the cross-linking agent comprises glycerol propoxylate triglycidyl ether or glycerol diglycidyl ether.

11. The article of claim 1 , wherein the hole blocking layer has a thickness of at most about 50 nm.

12. The article of claim 1 , wherein the hole blocking layer has a thickness of at most about 30 nm.

13. The article of claim 1 , wherein the hole blocking layer has a thickness of at most about 10 nm.

14. The article of claim 1 , wherein the photoactive layer comprises an electron donor material and an electron acceptor material.

15. The article of claim 14 , wherein the electron donor material comprises a polymer.

16. The article of claim 15 , wherein the polymer is selected from the group consisting of polythiophenes, polyanilines, polycarbazoles, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxaline, polybenzoisothiazole, polybenzothiazole, polythienothiophene, poly(thienothiophene oxide), polydithienothiophene, poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and copolymers thereof.

17. The article of claim 16 , wherein the electron donor material comprises a polymer selected from the group consisting of polythiophenes, polycyclopentadithiophenes, and copolymers thereof.

18. The article of claim 14 , wherein the electron donor material comprises poly(3-hexylthiophene) or poly(cyclopentadithiophene-co-benzothiadiazole).

19. The article of claim 14 , wherein the electron acceptor material comprises a material selected from the group consisting of fullerenes, inorganic nanoparticles, oxadiazoles, discotic liquid crystals, carbon nanorods, inorganic nanorods, polymers containing CN groups, polymers containing CF 3 groups, and combinations thereof.

20. The article of claim 19 , wherein the electron acceptor material comprises a substituted fullerene.

21. The article of claim 20 , wherein the substituted fullerene comprises C61-PCBM or C71-PCBM.

22. The article of claim 1 , further comprising a hole carrier layer between the second electrode and the photoactive layer.

23. The article of claim 22 , wherein the hole carrier layer comprises a polymer.

24. The article of claim 23 , wherein the polymer is selected from the group consisting of polythiophenes, polyanilines, polycarbazoles, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, and copolymers thereof.

25. The article of claim 24 , wherein the polymer comprises poly(3,4-ethylene dioxythiophene).

26. The article of claim 1 , wherein the first electrode is a cathode.