The benefits of non-silicon photovoltaic materials are many and varied. Some are transparent, permitting the face of a wristwatch to generate power, the power source having zero footprints. Some, such as Dye Sensitised Solar Cells printed by G24i in the UK, generate electricity at narrow angles of incidence and even with polarised eg reflected light. DSSC designs can use light of all visible frequencies, not just sunlight. G24i's initial target markets for its DSSC product include mobile phone chargers, particularly in the developing world.
Lightweight and efficient
Most types of photovoltaic film are unusually lightweight compared to silicon because silicon has to be protected and it is usually on glass. Weight saving is of benefit in many ways. It reduces shipping and installation costs, lets the cells be fitted in more places and it makes vehicles more energy efficient when carrying them as auxiliary power sources. Copper Indium Gallium Diselenide CIGS cells are more efficient than most and Nanosolar knows how to print them. They are being commercialised in Japan, Europe and the USA, for example.
Organics late to the party
Organic alternatives to the inorganic compounds in these photovoltaic devices have been the least efficient and there are, therefore, no major plans to commercialise them. Inherently they are not broad spectrum for instance, nor are they necessarily tolerant of an extreme angle of incident light. Work has continued with them partly because they may eventually be lower in cost, where space is not a problem. Very high speed printing of very thin flexible organic layers - wide area but cheaper nonetheless - that has been one dream.
In 2004, Professor Jiangeng Xue, Soichi Uchida, Barry P Rand and Professor Stephen R Forrest of the Department of Electrical Engineering, and Princeton Institute for the Science and Technology of Materials (PRISM) at Princeton University in the USA demonstrated organic photovoltaic cells of improved efficiency by stacking two hybrid, planar, heterojunction organic cells in series. Absorption of incident light was maximized by locating a sub cell tuned to absorb long-wavelength light nearest to the transparent anode, and placing a second sub cell closest to the reflecting metal cathode, this one being designed to preferentially absorb short-wavelength solar energy. Using the donor, copper phthalocyanine, and the acceptor, C60, they achieved a maximum power conversion efficiency of 5.7% under 1 sun simulated AM1.5G solar illumination. An open-circuit voltage of 1.2 V is obtained, doubling that of a single cell. At the time, their analytical models suggested that power conversion efficiencies exceeding 6.5% could be obtained by this architecture.
They were right. In 2007, a team led by Professor Kwang-hee Lee at the Gwangju Institute of Science and Technology (GIST) in Korea and Professor Alan Heeger of the University of California, Santa Barbara, announced that the tandem solar cell they had created, the better to utilize sunlight, has an efficiency of 6.5% and can even utilize infrared. This is achieved by one cell on top of the other and nano titanium oxide in between. The upper layer absorbs luminous light, while the lower layer makes use of infrared.
Professor Lee envisages that, using a special encapsulation process, the lifespan of their plastic-based organic photovoltaic solar cell could be extended considerably, overcoming the lifetime problems of most organic photovoltaics to date. It can be thin and flexible and he sees low manufacturing costs being achieved by adopting a form of spin coating.
He notes that conventional solar cells cost $2.3 to generate one watt of electricity compared to $0.1 for the latest plastic cells. He believes that the new design could take a large percentage of the global solar energy market that is expected to reach $34 billion in 2010 and $100 billion in 2050. Designs using various materials he has developed could make it feasible for such photovoltaics to be placed over windows and roofs, and allow mobile phones and electronic appliances to be recharged by sunshine, he argues.
Others have made dramatic improvements in single cell layers. Recently, Andy Hannah, CEO of Plextronics, reported to IDTechEx that, "the number we are publicly stating is that we have achieved 5.5% efficiency with a single layer, simple structure polymeric cell based on internal calibrated testing. We currently share the efficiency record of 5.2% at NREL for such a cell. We also expect to see higher values for both internal and NREL-tested cells in the third quarter. We believe that this proprietary ink set is commercializable and are working on process technology for scale-up and identifying and pushing forward with manufacturing partners."
Overview and new applications
At the forthcoming conference Printed Electronics USA, Dr Mark E Thompson, Professor of Chemistry, Dept Chair, University of Southern California, will assess recent advances in the development of organic photovoltaic devices. Another speaker, Professor Bernard Kippelen of organic photovoltaic start-up LumoFlex and the Georgia Institute of Technology says, "The efficiency at which organic photovoltaics can be commercialised has been variously quoted between 5% and 10%. We feel it depends on the products offered and we shall share our work on flexible power for wireless sensor networks, a major market need."
Clearly organic photovoltaics will cease to be the Cinderella technology some time soon and someone will build major production facilities for such devices. At the end of the day, the various photovoltaic technologies may turn out to be complementary.
Huge market after a slow start
Raghu Das, CEO of analysts IDTechEx says, "We believe that the market for printed and potentially printed photovoltaics will mainly consist of devices in the form of flexible film. After a slow start, we see the total market for these to be $1 billion in 2012 and $6 billion in 2014. As for the split between DSSC, CIGS, organic and other options, it is too early to call. However, one thing is certain. These photovoltaic devices will typically be linked to batteries and that is one reason why we have presentations on printed batteries at our conference as well. Optimal coupling of different printed components is becoming a focus of attention as all these technologies move firmly into the marketplace."
For more on photovoltaics read Printed Photovoltaics and batteries.