At first sight solar cells, i.e. photovoltaics, are a poor bet for printed electronics because most applications have the device spend a great deal of time in the dark or near darkness. However, the power from photovoltaics can be stored for hours by capacitors and for days by batteries. To take one example, a cheaper, smaller battery may suffice for RFID Smart Active Labels SALs even where range is enhanced over passive RFID i.e. in semi-active and fully active modes of RFID. That is why we have solar powered torches. The inefficiency of cheap flexible photovoltaic structures such as those employing organic semiconductors can, to a degree, be compensated for by the large area available on many packages.
Photovoltaics PV is progressing from bulk silicon to thin film silicon and now printed polymer. Rigid substrates are being replaced with flexible ones to reduce cost of manufacture and installation and permit installation in new places. For example, solar tape can conform to the side of a ship or be stuck along the concrete barriers between motorway lanes and be many kilometres in length without taking up any space.
"Third generation PV would be a complete breakthrough", says Tom Delay of the Carbon Trust, a UK Government funded company that stimulates the development of low-carbon technologies.
Researchers first created organic PV films in 1986. In late 2003, engineers at Princeton University reported asignificant advance. Using new polymers to get 3% efficiency. They said these cells would become commercially viable if 5% efficiency could be obtained. Making solar cells would become as simple as spraying inks onto a sheet of plastic, giving a step function reduction in cost.
Powering electronics and electrics
One use for cheap PV would be to power billboards down to smart packaging but another would be power production. The New Economics Foundation says, "All of non-electrified Sub Saharan Africa could be provided with energy from small scale solar facilities for less than 70% of what OECD countries spend on subsidising dirty energy each year." Already, in some countries, the roof under which an electric vehicle is kept cool in summer also recharges that vehicle thanks to solar cells.
There is further steady progress in improving the very low efficiency of today's flexible polymer photovoltaics. In 2004, researchers at Siemens AG reported increasing the efficiency of printed organic solar cells from 3% to over 5%. These solar cells, made from organic semiconducting polymers, are manufactured in a printing process. The photoactive layer of the cell has a thickness of approximately 100nm. As the polymers are printed onto foil they result in a lightweight and flexible solar module.
This figure of over 5% is claimed to be the highest obtained for printed organic solar cells to date. The researchers believe that at these levels of efficiency the technology has the potential to be commercially viable. It is thought possible that efficiencies of 7% can be achieved with the current state-of-the-art technology.
Christoph Brabec, development leader, says: "Our efforts are partly concentrated on replacing non-organic technology." He adds: "We mainly aim at new markets that we can open up because of the great benefits of our solar cells."
This Siemens activity was acquired by Konarka in September 2004.