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Organic Photovoltaic Cells with an Electric Field Integrally-Formed at the Heterojunction Interface

National Renewable Energy Laboratory

Brookhaven National Laboratory

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Figure 4
Figure 4

Figure 6
Figure 6

Technology Marketing SummaryStandard solar cells made from inorganic semiconductors, such as silicon cells, have dominated the photovoltaic market since their inception in the 1950’s. First generation solar cells are a proven source for renewable electric power generation and currently possess the highest solar conversion efficiencies available for commercial sale. Unfortunately, these efficiencies are offset by high economic and environmental costs. Traditional Multi and Mono Crystalline Silicon solar cells command substantial manufacturing costs as a result of the extreme price of substrate inputs coupled with the tremendous amount of energy required for bulk production. Second generation solar cells offer reduced manufacturing costs through the introduction of a more flexible, less expensive thin film substrate. However, while the introduction of a cost-effective manufacturing process makes thin film a very promising technology, current thin film solar cells contain toxic elements such as arsenic, cadmium, and titanium which cause problems in both the manufacturing and disposal phases of the life of these cells. Organic Photovoltaic cells (OPV) offer hope for a relatively inexpensive, environmentally friendly solar cell that can be fabricated on a large, flexible substrate. Currently, the solar conversion efficiencies obtainable in existing OPV cells are much less than the efficiencies obtainable through traditional IPV cells. There is a need for a method of producing OPV cells that have higher efficiencies, and are inexpensive, easy to manufacture, and environmentally friendly. NREL scientists have developed a method that improves the efficiency of an OPV cell by generating an electric field at the p-n heterojunction interface of the cell rather than uniformly across the cell.DescriptionThe relative importance of interfacial processes between layers of semiconductor materials is a key difference between OPV and conventional IPV. The main difference is in the charge generation mechanism needed to produce a useful PV cell. In IPV cells, electron-hole pairs are generated immediately upon light absorption throughout the bulk of the material according to the exponential decrease of the light intensity. Since the electrons and holes are distributed spatially within the same material, both the electrons and holes are driven in the same direction and recombination must be controlled throughout the bulk. In contrast, light absorption in OPV cells results in the production of a mobile excited state, i.e., a tightly bound electron-hole pair often referred to as an exciton. Dissociation of the excitons occurs at the heterointerface between two dissimilar organic semiconductor materials or layers. (See FIG. 4) Thus, light absorption results in free electrons in one of the two semiconductor materials and free holes on the other side of the interface in the other semiconductor material, with the free electrons and holes being driven in opposite directions away from the interface. Recombination of the free electrons with the holes is a bane of both IPV and OPV cells. However, due to the differences in the dissociation mechanisms of OPV and IPV cells, recombination is a particular problem at the interface in OPV cells due to the much larger concentration of carriers at the interface when compared to IPV cells in which the carriers are distributed in bulk. In current OPV manufacturing practice, a uniform electric field is applied across the PV cell to cause the electrons to separate from the paired holes. This is effective for IPV cells in which the electrons can only be separated from the holes by applying such an electrical potential-energy gradient. However, the application of a uniform electric field across the bulk has not proven effective in OPV cells where a critical efficiency limitation is recombination at the interface of the two semiconductor layers. The present invention provides a method which uses nanocrystalline semiconductors that are dispersed in a polymeric binder, such as epoxy cement, to create a bi-layer OPV cell with an electric field applied at the p-n heterojunction interface. The large increase in efficiency produced by generating an electric field at the p-n interface rather than uniformly across the cell is pictured in FIG 6.Benefits
  • Increase in efficiency produced by generating an electric field at the p-n heterojunction interface.
  • The method utilizes polymers that are readily available, easily processed with known polymer fabrication techniques, and create a flexible finished product.
  • The nanocrystalline organic semiconductors can be selected to achieve both a desired effective bandgap, such as about 1.4eV, and a desired thickness which allows the cells to absorb light without adding unnecessary resistance, wasting energy, or adding material costs
  • Cells manufactured according to the method of the invention can absorb all or nearly all sunlight with wavelength in the range of 350 to 900nm.
Applications and Industries
  • This method is applicable to organic photovoltaic cell manufacturers
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
U.S. patent 7,157,641DevelopmentAvailable07/28/201007/28/2010

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