Long touted as the solution to pollution and energy problems the world over, photovoltaic solar cells have never made a major impact on large scale electricity production. In fact, in 2000, commercial electricity production from solar, geothermal and natural heat sources only accounted for 0.5% of global supply. The reason for this is simple – cost.
Cost of Solar Cells
The reason solar cells are currently so expensive is due to the materials they are made from. At present solar cells are made using semiconductor materials that require high temperatures, expensive equipment, high purity and controlled environments in the production process. The process is also extremely wasteful. The net result is that solar electricity can be up to 5 times more expensive to produce when compared to electricity generated from coal fired power stations (ignoring costs to the environment).
Solar Cell Efficiency
Semiconductor solar cells have an efficiency of around 6%, with the very best being 33% efficient. Depending upon the technology being investigated, researchers are reporting nanotechnology based solar cells having efficiencies between 0.15% and an astonishing 65%.
Nanotechnology in Solar Cells
The promise of nanotechnology for solar cells is a massive reduction in cost by using common materials and simple production processes. Research is currently investigating solar cells made from nanowires, hybrid polymers, coated thin films, spray-on coatings, Grätzel cells and quantum dots.
Hybrid polymer solar cells use semiconducting nanorods made from materials like Cadmium Selenide mixed with a conducting semicrystalline polymer like poly(3-hexylthiophene). Although this method produces cells with low efficiencies (currently below 2%) they can be produced using standard plastic forming techniques at costs that can be measured in cents per unit.
Grätzel solar cells use a thin film of titanium dioxide nanoparticles with an organic dye filling the pores between the nanoparticles. Transparent conducting electrodes and a catalyst complete the circuit. Athough 15% efficiencies are being aimed for the Grätzel cell is currently around 7-8% efficient. It has the advantage of being able to be used in low light conditions due to the organic dye incorporated in the system. Grätzel cells are already being used in some applications.
Solar cells made from nanowires are similar to Grätzel cells. Nanowires of zinc oxide are grown on a glass substrate, electrodes and an organic dye are added and the system bathed in an electrolyte. Electron transport is more difficult in the zinc oxide system so efficiencies are poor. Successful nanowire growth in a system using the more efficient titanium dioxide is a much more complex method.
Solar cells made from nanocrystals known as quantum dots are under investigation with theoretical research indicating they could be 65% efficient. This research makes use of an effect called “multiple electron generation” or MEG, that produces as many as 3 electrons per photon of light hitting the solar cell. The quantum dots are made from semiconductor materials like lead selenide and lead sulphide.
Spray-On Coatings and Coated Thin Films
Researchers have combined quantum dots in a liquid polymer that can be sprayed onto any material to create a solar cell. By controlling the quantum dots used this type of solar cell, it can be made sensitive to the infrared end of the spectrum. In this way electricity can be generated not just through the power of the sun but also from heat. As an example, clothing coated in this material could use body heat to provide power to a mobile telephone even in a darkened room.
A similar material can be printed directly onto thin plastic films to make solar cells of virtually any size or shape that are highly flexible.