Carbon nanotube Schottky barrier photovoltaic cell
Carbon nanotube Schottky barrier photovoltaic cells and methods and apparatus for making the cells are provided. The photovoltaic cells include at least one contact made from a first contact material, at least one contact made from a second contact material and a plurality of photoconducting carbon nanotubes bridging the contacts. A Schottky barrier is formed at the interface between the first contact material and the carbon nanotubes while at the interface between the second contact material and the carbon nanotubes, a Schottky barrier for the opposite carrier is formed, or a small, or no Schottky barrier is formed. It is the Schottky barrier asymmetry that allows the photo-excited electron-hole pairs to escape from the carbon nanotube device.
1. A photovoltaic cell comprising:
(a) an insulating support having opposed front and rear surfaces;
(b) a first contact extending along the front surface and supported thereby, the first contact comprising a first electrically conductive contact material;
(c) a second contact extending along the front surface and supported thereby, the second contact comprising a second electrically conductive contact material wherein the first and second contacts are spaced apart along the front surface to be bridgeable by a single nanotube;
(d) a plurality of photoconductive carbon nanotubes lying along the front surface aligned in a plane of the front surface, each nanotube bridging between the first contact comprising a first electrically conductive contact material and the second contact comprising a second electrically conductive contact material, the carbon nanotubes configured to receive light directed at the front surface along a path substantially normal to the front surface;
wherein the carbon nanotubes directly contact the contacts;
wherein the work function of the first electrically conductive contact material differs from the work function of the carbon nanotubes,
further wherein the junctions between the first electrically conductive material and the carbon nanotubes form Schottky barriers for electrons or holes; and
whereby light illuminating the photovoltaic cell produces an electrical voltage across the first and second contacts.
2. The photovoltaic cell of claim 1 wherein the work function of the first electrically conductive contact material is lower than the work function of the carbon nanotubes and further wherein the junctions between the first electrically conductive contact material and the carbon nanotubes form Schottky barriers for electrons and the junctions between the second electrically conductive contact material and the carbon nanotubes do not form Schottky barriers for electrons.
3. The photovoltaic cell of claim 2 wherein the junctions between the second electrically conductive contact material and the carbon nanotubes form Schottky barriers for holes.
4. The photovoltaic cell of claim 2 wherein the junctions between the second electrically conductive contact material and the carbon nanotubes do not form Schottky barriers.
5. The photovoltaic cell of claim 2 wherein the first and second electrically conductive contact materials are independently selected from the group consisting of metals, conducting polymers and conducting oxides.
6. The photovoltaic cell of claim 2 wherein the first electrically conductive contact material comprises a first metal and the second electrically conductive contact material comprises a second metal.
7. The photovoltaic cell of claim 6 wherein the work function of the first metal is lower than the work function of the carbon nanotubes by at least 0.3 eV and the work function of the second metal and the work function of the carbon nanotubes differ by no more than 0.1 eV.
8. The photovoltaic cell of claim 6 wherein the first metal is titanium and the second metal is gold.
9. The photovoltaic cell of claim 2 wherein the nanotubes are disposed on a polymer film.
10. The photovoltaic cell of claim 9 wherein the polymer film comprises a polymer selected from the group consisting of polyamides, polyethylene and polyesters.
11. The photovoltaic cell of claim 2 wherein the plurality of nanotubes comprises carbon nanotubes that have different bandgaps.
12. The photovoltaic cell of claim 2 wherein the surface density of carbon nanotubes on the first and second contacts is at least about 10 9 nanotubes/cm 2 .
13. The photovoltaic cell of claim 2 wherein the surface density of carbon nanotubes on the first and second contacts is at least about 10 10 nanotubes/cm 2 .
14. The photovoltaic cell of claim 2 wherein the photovoltaic cell has a current density of at least about 50 mA/cm 2 .
15. The photovoltaic cell of claim 2 wherein the photovoltaic cell has a current density of at least about 100 mA/cm 2 .
16. The apparatus of claim 1 wherein single carbon nanotubes have an aerial density of greater than 10 9 nanotubes per square centimeter.