Carbon-based organic photovoltaic (OPV) cells, which use organic conductive polymers as semiconductors for light absorption and electric charge transport, are significantly thinner and cheaper than their inorganic silicon-based counterparts. Unfortunately, they are also much less efficient at converting sunlight into electricity.
Though they may never reach the power efficiency of traditional materials, their lower cost of OPVs do give them a niche. Flexible and lightweight OPV cells are considered as the next generation of PVs because they can be made so thin that they are printable and cheaper to produce. The trick is to figure out how to make OPVs as efficient as inorganic PVs.
Fabricating OPVs
Being made from carbon compounds, OPV cells are also known as “plastic” solar cells. The center of an OPV is a layer only a hundred millionth of a millimeter thick, made of two components mixed together, polymers and fullerenes.
When light strikes this layer, the polymer transforms into an excited state, known as an exciton. When an exciton bumps the fullerene, an electron jumps over to the fullerene molecule leaving a "hole" in the polymer. The current flows when the electrons and “hole,” called polarons, travel to opposite contacts, with the electrons traveling via the fullerenes while the polarons travel via the polymer. But the polarons can obstruct one another along this path, which substantially reduces the efficiency of the OPV cell.
The fabrication of organic PVs was previously done using two types of organic semiconductors: n-type fullerene and p-type phthalocyanine. But research by the Institute for Molecular Science of the National Institute of Natural Sciences inJapan announced on March 3, 2011, indicates that organic PVs can be fabricated using just fullerene by doping the semiconductor with molybdenum oxide and rendering the need for phthalocyanine unnecessary. The researchers, led by Professor Masahiro Hiramoto, were able to convert the fullerene from n- to p-type by molybdenum oxide doping.
Less Energy to Manufacture
In September 2010, researchers from the Rochester Institute of Technology’s Golisano Institute for Sustainability and NanoPower Research Lab confirmed that manufacturing organic PVs is actually more energy efficient than that of conventional silicon PVs, one of the first lifecycle assessments done for OPVs. The analysis included a breakdown of the individual materials used to produce OPVs and calculated the total energy payback - the energy produced versus the energy required for manufacturing. The researchers discovered that both the total energy required for fabrication and the energy payback time for OPV cells was substantially lower than inorganic PVs.
Closing the Efficiency Gap
Optimizing the power conversion efficiency (PCE) for OPVs is one of the main focuses of current research and requires manipulation of many factors, including the structure and dynamics of the OPV materials.
Earlier attempts had been made to discover the potential of OPVs in January 2009, when scientists at the University of Toronto found that conjugated polymers could be used to manufacture more efficient OPVs. But it wasn’t until the summer of 2011 that real advances began to surface.
On August 17, 2011, researchers from the Institute of Materials Research and Engineering of the Agency for Science, Technology and Research in Singapore announced they had created a new polymer with a PCE of 6.3 percent that could also yield cheaper OPVs. According to the researchers, their new polymer has the same high charge mobility as convential PV materials.
"Current polymers are usually good in one aspect or another, either as a good conductor for use in electronics or endowed with high power conversion efficiency - but not both," said lead researcher Dr. Chen Zhi Kuan in a written statement.
The researchers believe that the new polymer can be used by manufacturers in plastic electronics and OPVs that would save costs by using a single bulk resource for making both printed electronics and the OPVs.
Also in August 2011,an international team of researchers reported boosting the efficiency of existing OPVs by 20 percent using gold nanoparticles. The team was led by University of California Los Angeles (UCLA) Professor Yang Yang, who worked with Xing Wang Zhang from Beijing's Chinese Academy of Science and Ziruo Hong from Japan's Yamagata University. Yang is also director of the Nano Renewable Energy Center at UCLA's California NanoSystems Institute.
In their experiments, the scientists sandwiched a layer of gold nanoparticles between two light-absorbing layers in a “tandem” polymer OPV. This caused a “plasmonic effect,” in which the particles created an electromagnetic field that actually concentrate the light so that more of it could be absorbed, leading to an improvement in the PCE of 5.22 percent to 6.24 percent. The team is the first to create a plasmonic-enhanced tandem OPV cell and prove that the plasmonic effect has potential for the future development of OPV cells.
Konarka Still Has thePCE Record
Konarka Technologies is headquartered in Lowell, Massachusetts, with European headquarters in Nürnberg, Germany, a business development office in Japan and an R&D facility in Austria. The company has been producing its OPVs under the name of Power Plastic for a number of years, which are comprised of several thin layers; a photo-reactive printed layer, a transparent electrode layer, a plastic substrate and a protective packaging layer.
The U.S. government’s National Energy Renewable Laboratory announced in December 2010 that Konarka’s OPV cells had achieved a record breaking 8.3 percent efficiency, the highest performance recorded by NREL for an OPV.
Power Plasticis marketed to other companies to incorporate into their consumer products, such as lanterns, backpacks, briefcases, tents and café umbrellas, as well as a component for PV windows and curtain walls. In June 2011,Konarka announced its largest OPV installation when the semi-transparent Power Plastic was installed at the company's Massachusetts facility for further testing as a BIPV curtain wall.
Viability of OPVs Today
Many are optimistic that OPV cells will reach an efficiency level where they will be commercially viable. The challenge also remains to mass produce OPVs in a way that is as energy and cost efficient as the current silicon or other thin-film technology.
The main OPV developers, such as Plextronics, Solarmer Energy, Dyesol, AGFA Materials and Konarka are not exactly household names within the “mainstream” solar industry but theseare the companies to watch.
While Konarka's 8.3 percent power conversion efficiency and the 6.24 percent achieved by adding gold nanoparticles falls far below the overall efficiency of conventional inorganic PVs, this is actually offset by other factors that may make OPVs viable now. The much lower cost of organic cells needs to be taken into account, as does the fact that OPV cells can be multi-stacked for a higher combined output.
The low conversion efficiency of OPVs may also be offset by the applications available now where high conversion rates are not a factor, such as in defense, BIPV materials,
signage, and developing world uses such as the current solar development boom in India.
Moreover, OPVs may be used in designing new plastic devices that needed both energy harvesting and electronics in a single unit. Konarka, like other thin-film inorganic PV manufacturers, markets itsOPVsto other consumer companies that make such consumer products as backpacks, briefcases, tents and even clothing that double as mini recharging stations for personal electronic devices where high power output is not a concern.
According to some recent reported forecasts, the market for OPVs by 2020 could range between $159 million and $470 million. This future of OPVs will probably include a combining of technologies and ideas, such as using UCLA’sgold nanoparticle layering technique to further boost the 8.3 percent efficiency of Konarka's Power Plastic OPVs.
The recent OPV advances that have surfaced over the summer of 2011 will no doubt be the subject of much discussion at the 5th Annual IntertechPira Organic Photovoltaics 2011 Conference, taking place September 20 - 21, 2011, in Philadelphia.
