For solar power to be truly competitive with traditional energy sources, it has to approach cost parity with power generation using traditional sources. The two primary ways to do this are to increase the efficiency of converting sunlight to power (the approach most in the industry have taken) and to lower the manufacturing cost of solar cells. Nanosolar has focused on the latter, and has at least three patents that cover its manufacturing technique. According to this New York Times article, Nanosolar can profitably sell solar panels for less than $1 a watt, the price at which solar energy becomes cheaper than coal.
Solar cells typically are made by manufacturing methods borrowed from the semiconductor industry that consist of depositing photovoltaic film on silicon wafers. Nanosolar instead uses a printing technique to lay thin film photovoltaic material on a conductive metal foil. This method is possible because Nanosolar’s thin film solar cells are made of copper indium gallium selenide (CIGS) instead of silicon, which provides additional savings by giving Nanosolar immunity from silicon supply shortages and fluctuations in the price of the silicon raw material. Silicon solar cells are made from slices of solid silicon instead of the metal foil used by Nanosolar to make CIGS cells.
At the particle level, Nanosolar’s solar cells have a different structure from traditional silicon solar cells. In silicon cells, the absorption of photons of light results in the formation of a free electron and hole (the space remaining when an electron moves). In the Nanosolar cells, the photoexcitation leads to formation of a bound electron and hole pair called an exciton. To optimally convert light to electrical energy, the electron and hole comprising the exciton have to be spaced a certain distance from each other so that charges can be collected at different electrodes.
U.S. Patent No. 7,253,017 describes Nanosolar’s manufacturing process and claims a method of fabricating thin film solar cells having electron and hole-accepting materials at the right distance for optimal exciton activity. A film having a network of interconnecting nanoscale pores is formed on a substrate. Then semiconducting materials are deposited in the pores. After the pores in the template film have been filled, the template is removed using a chemical process that leaves the semiconducting material intact as a nanoscale grid network. The remaining spaces between the nanoscale grid network are filled with a complementary semiconducting material so the resulting interfaces have ideal conditions for electron travel and charge collection.
