Thin-film PV is a rising star of solar PV in general, thanks to its flexibility, low weight, economical use of scarce materials and ability to be integrated into other products. Yet, no sooner has thin-film established itself within the PV firmament, than its own house has been rocked by the emergence of a more recent market challenger, organic PV (OPV).
Organic PV could be seen as the holy grail, providing the grid parity costings that conventional solar PV has so far failed to do. But it has a disadvantage – low energy conversion. The technology currently struggles to deliver efficiencies much above 5%, compared with the 20%-plus achievable with conventional crystalline silicon; or with the 10% or more for better-established thin-film competitors (such as amorphous silicon, and compound semiconductors like gallium arsenide, cadmium telluride and copper indium gallium diselenide).
The conundrum, therefore, is whether the promised low costs (if actually delivered) can sufficiently compensate for lower (though improving) efficiencies, enabling organic PV to become commercially viable.
Organic PV was first noticed as long ago as 1906 when the PV effect was observed in the organic compound anthracine. Interest was rekindled from the late 1970s when conducting polymers were discovered, an achievement that earned scientists Dr Alan Heeger, Professor of Physics at the University of Santa Barbara, California, along with co-workers New Zealander Dr Alan MacDiarmid and Japanese researcher Hideki Shirakawa, a Nobel Prize in 2000. Certain plastics, typically long-chain polymers with double bonds, when brought together with a second ‘dopant’ material, behave like metals in some respects and permit electron flow. A number of these plastics are midway between conductors and insulators – i.e. they are semiconductors.
As with conventional silicon or heavy metal compound based semiconductors, particular material combinations where one material is doped with a second can create junctions of electron donor and acceptor materials. Electronics engineers refer to these as p-n junctions – materials being defined as p or n according to whether electrons (p) or electron vacancies or ‘holes’ (n) are the majority electrical charge carriers.
These junctions are fundamental to electronic and opto-electronic activity. Different organic and organic/inorganic material combinations provide different levels of activity. Some lend themselves to use as electronic devices, notably organic light emitting diodes (OLEDs) and their ‘reverse’, namely organic PV cells that convert light into electrical energy rather than electrical energy into light.
Combinations considered to show organic PV promise include blends of fullerines with polymers and dendrimers, various polymer-polymer composites and hybrid nanocrystalline oxide-polymer composites. Research is also focusing on organic/inorganic formulations, in which inorganic nanofibres, nanotubes, nanoparticles or quantum dots are brought into organic PV devices.
Polymers have particular advantages. Unlike crystalline silicon, which is stiff and brittle, many organic materials are flexible and can be deposited on flexible as well as stiff substrates. It should be possible, for instance, to develop organic PV products that are flexible in use and can be rolled up and put away when not required. Organic materials can be deposited in very thin layers having high transparency, so that organic PV can be integrated into windows and translucent facades. Organic PV tend to be photo-active over a wider bandwidth of incident light than conventional PV, so that materials work not only in direct sunlight but also in subdued light, indoors and even with heat (infra red). Some organic PV materials are active over wider angles of incidence than conventional materials.
Laying flexible PV on flexible substrate raises the possibility of continuous roll-to-roll manufacture, with consequent drastic cost reductions, compared with the necessarily slow production of silicon wafers in batches under clean room conditions. Organic PV avoids the use of scarce and expensive materials like silicon and the heavy metals indium, cadmium etc. that are used in other thin film materials. In addition to their expense, heavy metals can raise long-term issues of health, and whether they can be safe disposed of.
The price to be paid for these advantages, however, is their reduced energy conversion efficiency. A major focus over the last several years has therefore been to raise organic PV efficiency to useful levels. A wide range of organic p-n material combinations have been tried or are still under investigation, but efficiencies are still well below those achievable with conventional PV materials.
Four years ago, researchers at the Georgia Institute of Technology were trumpeting the achievement of 3.4% efficiency. Professor Bernard Kippelen had, with co-workers, recorded this figure by irradiating with a broadband xenon source a combination of pentacene with carbon in its C60 form. Pentacene has a polycrystalline structure that facilitates the passage of electrons and Dr Kippelen is still working with this combination, among others, having since raised efficiency to 3.8% with a better optimised process.
“We now have a better idea of what molecular structures are desirable,” Kippelen explains: “Our simulation modelling suggests that it will be possible, with appropriate material combinations, to reach 10% efficiency with single-junction OPV devices, and nearer 15% with tandem-junction devices.”
While not dismissing suggestions made by researchers at recent conferences that organic PV can theoretically deliver 20%, Kippelen doubts that such levels can be reached soon.
Single layer efficiencies of better than 5% have already been achieved. In 2005 David Carroll at Wake Forest University achieved close to 5% with a PV cell comprising copper phthalocyanine and C60, but went on to surpass this in 2007 with a cell comprising poly-3-hexylthiophene and phenyl-C61-butyric acid methylester (P3HT:PCBM), realising 6%. Carroll hopes to achieve 10%, widely regarded as a benchmark for commercial viability, within a couple of years. His research is funded by the US Air Force, which desires lighter and more efficient PV cells for spacecraft and satellites. Meanwhile, commercial firm Plextronics Inc, a significant organic PV developer, has claimed 5.4%, with a single-cell, a figure that was certified by NREL.
By operating two stacked cells in tandem, levels can be raised further. In 2007, Alan Heeger and his fellow Nobel Prize winners, together with Lee, a South Korean researcher, presented an organic solar cell which, by virtue of a double layer that absorbs a broader spectrum of solar radiation than single-layer cells, achieved a conversion efficiency of 6.5%. A polymer-fullerine composite and polythiophene composite were the primary organic constituents. Olle Inganas at Linkoping University suggests that the upper limits for a PV cell based on P3HT blended with a fullerine derivative could be around 9%. Higher efficiencies still might ultimately require more complex devices, incorporating multi junctions or more exotic third-generation mechanisms in their design.
The impressive slimness that is possible with organic PV film is well illustrated by a product of UK research. Scientists at Cambridge University's Cavendish Laboratory claim to have deposited two semi conducting polymers in a film a mere 100 nanometres thick, compared with around 200 microns of silicon in a conventional PV cell. They say their technology promises efficiencies of up to 5%.
Arguably the biggest benefit of organic PV is likely to be its affordability. Experts calculate that inexpensive plastic PV material deposited on substrate using continuous roll-to-roll processes could bring costs down to well below the crucial US$1.00 per watt threshold said to be required for grid parity. (Industry sources suggest that it may take another decade for silicon to fall to US$1.00/w, from its present level in the region of US$2.3/w. The thought is that organic PV could get there rather sooner. According to a spokesman for the Plextronics company, mentioned above, its technology promises solar PV costs four or five times lower than those of crystalline silicon-based systems.
Sentiment about the commercial prospects of organic PV ranges from cautious pessimism to determined optimism.
The Carbon Trust, which has funded the Cavendish Laboratory work mentioned above to the tune of £5m, is sufficiently confident to express an aspiration to see more than one gigawatt of organic PV deployed by 2017. This, it says, will demonstrate the potential for delivering large-scale solar PV power at radically reduced cost. While admitting that efficiencies of around 5%-15% are a far cry from the 30%-40% peak efficiencies that laboratories have demonstrated with advanced silicon-based PV technology, the Trust hopes that the low cost achievable with roll-to-roll deposition and printing techniques can more than compensate.
Professor Paul O’Brien at the University of Manchester agrees, arguing that cost reduction eclipses all other considerations in commercialising solar PV. “For me, the big driver is always cost reduction rather than efficiency,” says the professor, who has been involved with solar cells for more than 20 years. “Making cells in foundries the way silicon chips are produced is far too expensive.”
One avenue being explored by O’Brien, together with Prof. Jenny Nelson at Imperial College, London, is to produce a hybrid organic/inorganic cell in which the active layer would be sprayable or printable. Such cells, the two suggest, could deliver efficiencies of around 10%. A planned first laboratory prototype of a cell will contain lead sulphide in nanorod form as the inorganic ingredient. The co-workers hope that producing solar cells costing a mere hundredth of the cost of a silicon cell will stimulate a solar PV energy revolution. Even so, they see large-scale power generation as a medium to long-term prospect, and anticipate marketable devices on a much smaller scale initially. They expect that their work, which has attracted a £1.5m grant from the UK's Engineering and Physical Sciences Research Council, should result in solar chargers for today's power-hungry personal electronic devices.
Prospects for commercialisation depend on methods being developed to deposit thin organic material layers onto a range of substrates, fast and accurately. Vacuum deposition, spin coating, spray and print processes are the chief options. In 2000, Ghassan Jabbour at the University of Arizona demonstrated the use of screen printing to deposit a polymer layer less than 100 nanometres thick, and was able to fabricate thin PV films of less than the usual 0.5 micrometer thickness. The following year, he screen printed a smooth, thin active layer of a polymer/fullerine blend, 40nm thick on average, in cells showing 4.3% efficiency. On the other hand Heliatek, in a new organic PV venture supported by giants BASF and Bosch, aims to beat the €1/Wp barrier with a 7%-10% efficient material applied by vacuum deposition, while another commercial enterprise, Optomec, is working on an acoustically constrained aerosol spraying process for PV deposition.
Some of the highest hopes rest on techniques originating from the print industry, where accurate, large-scale, high-speed processes are taken for granted. Both screen printing and the more contemporary inkjet printing have been adapted for use with functional inks, the latter being made up of suspensions of the materials required. For example Plextronic, mentioned above, has developed its Plexcore ink system to enable photo-active and charge carrier layers to be printed onto glass or plastic substrates. DuPont is likewise actively developing PV inks. A number of companies, such as Nanosolar in the USA, are embarking on large-scale production of PV cells using ink jet-based print processes. Their work, even where directed to production of TF PV other than organic, will also benefit organic PV since it has proved possible to produce organic PV materials as inks.
Konarka Technologies, a noted organic PV player, recently claimed the first ever achievement of organic solar cell fabrication by inkjet printing. Its process is said to result in “little or no loss compared to clean-room semiconductor technologies such as spin coating”. Although it is not yet clear when an actual product will be available, Konarka's method will further extend the growing flexible OPV capability for which this company – Alan Heeger was a prime mover in its formation – is already well known. Its Power Plastic photo-active medium is the basis of thin flexible solar strips that resemble 35mm photographic film. Chief technology officer Dr Christopher Brabec, who previously worked on polymer PV at Siemens AG (Konarka subsequently acquired Siemens’ OPV interests) and helped develop the nanomaterials that are the basis of Power Plastic, says that Konarka's material reacts to all visible light sources, not just sunlight.
Electronic elements that can be ink-jetted using functional fluids include PV structures. Producers of ink jet printing systems have recognised the opportunity presented and companies such as Xaar in the UK and Fujifilm Dimatix Inc have stepped up to the mark. Fujifilm, for example, says that its Dimatix Materials Printer operates without the waste associated with screen-based application methods. The printer's piezo drop-on-demand inkjet heads can print at speeds of up to one and a half metres per second, placing valuable material precisely where it is wanted. Other organisations using similar Spectra printheads from the Fuji company include UK-based Xennia Technology, which incorporates them into its high-precision XenJet materials deposition system.
Droplet printers might also be able to print encapsulent layers needed to protect the solar surfaces, and even print a layered battery below a PV array so that some of the power generated can be stored. Indeed, one vision of the future is multi functional organic circuitry powered by integrated organic PV. Thus, in a related development, the Organic Electronics Association (OE-A) has sponsored the production of promotional demonstrators and give-aways that combine, for instance, organic sensors, logic circuits, displays, batteries and other elements.
Having a viable technology is only one part of the commercialisation battle. Equally important is having a market which, in turn, depends on developed applications. Here also, there are signs to justify optimism.
PV technology has progressed from costly bulk silicon to thin-film silicon and now to printed organics and polymers. Flexible substrates are gradually displacing rigid ones, enabling new applications while also reducing manufacturing and installation costs. While organic PV cannot yet compete with conventional PV and the better established thin-film technologies, it offers an intriguing low cost alternative in certain applications, where flexibility is more important than efficiency.
NanoMarkets LC suggests that the most likely applications will be in disposable electronics from 2010, then conventional electronics from 2015, and only some years after that BIPV and other power electronics. In a recent report on the prospects for organic PV , the market analyst identified a potential market for flexible solar panels used for power cooling, communications and wireless sensors. Organic PV devices could be produced either as light, mobile power packs, or integrated into field structures such as soldiers’ uniforms. Consumer electronics are cited as a lucrative segment for very low-cost printed organic PV. Konarka, for example, is developing its organic PV-based Power Plastic for cell phones and portable music players, as well as for battery charging on the battlefield, remote power for unmanned vehicles, and solar-powered sensor networks. Flexible organic PV elements could provide power for future equally flexible ‘roll-up’ electronics.
Smart fabrics incorporated into items ranging from wallpaper to curtains, tents to clothing, could be organic PV powered. Military combat apparel might embody flexible organic PV panels to power the ever more numerous sensors and portable communications devices carried by combatants today. This would avoid weighing down backpacks with spare batteries. An organic PV research programme carried out at the US Army's Natick Soldier Center, Massachusetts, has helped move the vision of solar plastics towards realisation with its discovery of ways to process a variety of low-cost photo-active polymers at relatively low temperatures. This subsequently led to the founding of leading-edge company Konarka Technologies in 2001.
SkyShades is working with Konarka to develop smart shades incorporating organic PV panels. Arrays integrated into the umbrellas used at pavement coffee shops and eateries around the world could be used to deliver energy for recharging personal electronic devices such as iPods, cell phones and lap-tops. The fact that this PV technology can also be used indoors extends scope still further. Indeed, this provides competitive advantage over other thin-film technologies, which can otherwise offer similar flexibility and cost benefits to the new PV contender.
Prof Bernard Kippelen, mentioned earlier, saw great prospects for printed electronics, including solar cells, based on his work at Georgia Tech, which was funded by the US National Science Foundation, the Office of Naval Research and the National Renewable Energy Laboratory (NREL). He foresaw that early applications would be on a limited scale in small devices such as solar chargers for portable electronic devices, with mainstream power generation being further off into the future. With others of like mind, he went on to form LumoFlex, a spin-off company intended to move the research forward into commercial application. LumoFlex already produces sustainable printed power solutions able to replace traditional batteries in, for example, smart tracking systems.
Analysts point to the fact that trying to sell new products to new markets is a high-risk marketing strategy. However, where the strategy succeeds, the rewards can be great - witness the iPod revolution for example. Moreover, the sector is now attracting significant investment while large corporations and even governments have become interested. As a case in point, the German Federal Government is supporting efforts by BASF, Bosch, Merck, Schott and others to produce a truly viable low-cost thin-film material by 2015. Scientists at the Free University of Berlin believe that cost-effective thin-layer production techniques such as printing, along with energy conversion efficiencies in the 5-10% range, will make organic PV a viable competitor to established TF PV technologies.
These and many other initiatives around the world reflect optimism that organic PV could become commercial, if not imminently at least in the medium term. Admittedly, there are still serious obstacles to overcome. Further work is needed to extend the stability and life span of active organic materials, which can degrade within a few years in the presence of water vapour, oxygen, heat and other environmental agents.
More progress in both thin film manufacturing and encapsulation techniques is needed to match the continuing focus on efficiency. In printed systems, organic inks may have to compete with inks made of silicon, still the best-understood semiconductor, and other inorganics.
But progress and prospects overall are real enough and, as NanoMarkets’ principal analyst Lawrence Gasman has neatly summarised, given current energy cost levels and environmental pressures, ‘this time the solar PV opportunity seems to have legs’.
NB. The author wishes to acknowledge the help provided by Robert Nolan of NanoMarkets LC in providing informtion for this article.