In 1974, future Nobel laureates Alan Heeger, Alan MacDiarmid and Hideki Shirakawa discovered a new type of plastic — conjugated conducting polymers.
“This polymer was a completely new type that acted more like a metal than like other plastics as it was an excellent conductor of electricity,” recalled Niyazi Serdar Sariciftci, who started working with the polymers as a doctoral student at the University of Vienna in the mid-1980s. “It became quite the rage and elicited great interest due to its unique behavior.”
Drawing on that breakthrough, Sariciftci would create the plastic solar cell, one of the most promising avenues yet discovered for popularizing the photovoltaic approach to solar energy.
Known as organic or plastic photovoltaics, the new solar material not only converts sunlight into electricity, as do its inorganic counterparts, but it can be mixed in solution to become an ink or paint. Plastic photovoltaics lend themselves to printing on fabric or film, as in a newspaper-printing plant, or brushed or sprayed onto a surface like paint. Either way, production becomes fast, and they can conform to the shape of the structure they are placed on.
But plastic cells currently require a trade-off, considering their low efficiency and lack of durability compared to less flexible (and more expensive) silicon cells.
The Turkish-born Sariciftci spent four years, 1992-1996, working alongside Heeger at the University of California, Santa Barbara, before accepting the chairmanship of the Institute for Physical Chemistry at the Johannes Kepler University in Linz, Austria. He is the founding director of the Linz Institute for Organic Solar Cells (Linzer Institut für organische Solarzellen, or LIOS). His interests are not confined to the work bench: In 2003, he was elected to the City Council of Linz, Austria’s third-largest city, and he also has studied classical piano. In 2006, he was awarded the Turkish National Science Prize.
Miller-McCune.com’s John Perlin, himself the author of “From Space to Earth: The Story of Solar Electricity,” spent time with Sariciftci this summer at UCSB, where the chemist spoke about the past, present and future of this approach to harvesting sunlight for electrical production.
John Perlin: When did you get interested in plastic solar cells?
Niyazi Serdar Sariciftci: When I joined the Institute of Polymers in January 1992 at UCSB. I had asked myself for many years that if conjugated conducting polymers had been famous for light-emitting diodes and for conducting polymers, why don’t we also use these materials for the conversion of solar energy into electricity like silicon does. This was my big dream and motivation that I brought with me when I arrived here.
Conjugated conducting polymers alone would not do the job because in their light-excited state, they don’t create enough charge carriers. To facilitate their photovoltaic capability, I needed a trick, which is called photo-electron transfer. For this we needed two components: One is an electron donor; the other is an electron acceptor. The electron donor was clearly the conjugated conducting polymer. The electron acceptor was chosen for me by my colleague Fred Wudl. By pure chance, Fred was working on fullerene molecules and fullerenes make very good electron acceptors. So he opened up his drawer and gave me this black powder, saying, “Take it! This is a great electron acceptor. You will like it. It looks just like a soccer ball.”
So I took these fullerenes as acceptors and by mixing them with conjugated polymers, I came up with a functioning solar cell. This was the basis of the [November 1992] paper in Science magazine titled, “Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene,” and the patent, which Alan Heeger, my chief and professor, and myself obtained.
The article and patent brought on great interest throughout the world, encouraging many to engage in work on the new plastic solar cell we developed. Currently, there is a great amount of work coming out from many laboratories. But our paper and patent was the basis of all the exciting new developments in plastic solar cells that have occurred since our original work.
But that’s just part of the story. The fullerenes I got from Fred weren’t very soluble. We needed a more soluble fullerene to make a solar cell. That’s when I went to a chemist colleague, Kees Hummelen, and asked for such a material. And not to bother himself too much, Kees [now part of the Dutch Polymer Solar Energy Initiative] handed me something his group had synthesized for something else, thinking that as a physicist, I wouldn’t know the difference anyhow! And that material he pawned off on me — PCVN — turned out to be just the right stuff and still the best material ever introduced as the acceptor in a plastic solar cell.
JP: Were you the first to work with organic solar cells?
NSS: No. Small molecules were already being used as solar cells developed by Dr. Ching Tang, the chief engineer at Kodak. Ching discovered, in the early 1980s, light emission and photovoltaic action of organic molecules. His work predates all other work in organic photovoltaics.
JP: Why is the use of conjugated polymers better?
NSS: Conjugated polymers have the great advantage of combining the semiconducting properties of a true semiconductor like silicon with the large scale-up capability of plastics — easily synthesized, easily manipulated — which you cannot achieve with any other material out there. You can see this in the plastic revolution of our daily life — its scalability, low price and large production potentiality. We are bringing plastic technology and semiconductor technology together. This is unique in the world, this special quality of conjugated polymers. It is no wonder that Heeger, MacDiarmid and Shirakawa won the Nobel Prize for discovering this new polymer.
JP: You have made plastic solar cells your life’s work. Could you elaborate on the odyssey you have taken?
NSS: The first solar cells we made were well below 1 percent efficient. The materials we used were not very well understood, not very well engineered, and not very well purified. The technology was in its infancy. During the following four years which I spent at UCSB, we mostly concentrated on the photo physics and material characterization to better understand the fundamental nature of these solar cells.
When I took over as chairman in 1996 of physical chemistry at the University of Linz, the Johannes Kepler University of Linz, I decided it was time to research the fabrication and production aspects, which are quite different from what we had previously done. This is much more difficult because it involves an infinite number of small, small steps, hard, hard steps, toward creating a product. That work is greatly underestimated by many people but it is the most difficult part.
People expect a discovery to immediately become a product. But this is not how it happens in real life. It is impossible to overcome the hurdles to production overnight because we hadn’t whatsoever a clue how these new materials behaved, especially considering the very tiny size we were working with. Imagine, these thin films we produce are on the order of 100 nanometers — 0.1 micrometers — where even the smallest piece of dust becomes Mount Everest in the landscape we are working in.
JP: Please list the significant milestones in the development of plastic solar cells.
NSS: The very significant milestones after the discovery came about by using materials and material combinations that have a very good nanomorphology. In 2000 and 2001, we reached an efficiency of 2.5 percent. This got tremendous attention to our work. When the scientific world saw that we could double or triple the light-to-energy conversion factor, interest grew dramatically. In the last three years, progress has accelerated and one group reported a plastic solar cell turning 8 percent of the incoming light into electricity.
JP: What are the basic issues that need to be overcome for improved performance?
NSS: For one thing, light harvesting has to be improved. We have the wrong color. We have to adjust the color to harvest more photons from the sun. This means adjusting absorption to include the full spectrum of the sun. Organic solar cells also suffer from being very disordered, causing terrible efficiencies. The better ordered the material, the higher efficiency will be achieved. To overcome these obstacles, we will need the help of many disciplines, including organic, polymer and super molecular chemistry, as well as biology, semiconductor physics and device engineering. We need people from all these fields on board to make the real breakthroughs we require.
JP: Please give me the road map of the development of practical plastic solar applications.
NSS: Plastic solar cells powering mobile electronics are already on the market. Their efficiencies of 5-8 percent are perfect for this application. Their one- to three-year lifetime poses no obstacle since most of mobile electronics become obsolete in this time frame. We estimate that for outdoor remote applications, plastic photovoltaics will be ready by 2012, requiring cell lifetimes between three to five years. Building-integrated photovoltaics, plastic solar cells used as building material, will need an efficiency of 10 percent and must last for at least 10 years to be feasible. Such systems should be on the market by 2013 or at the latest, 2015. Grid-connected plastic photovoltaics will come on line by 2020. Durability has to be enhanced. Price and cost advantages over silicon will definitely be ours.
Increasing efficiency and stability combined with lower cost are the triad that will lead us to each step of the way. Ten percent efficiency will probably be achieved quite soon as we have experienced a rise in efficiency of 1 to 2 percent per annum. Stability will be a little more challenging, but we will get there by using new encapsulation methods or new materials. As for cost, I am very optimistic because the technology lends itself to roll-to-roll production with enormously high throughput. On this score, we will beat silicon production by an order of magnitude.
To arrive at each of our road stops will rely on advances in material science and nanotechnology. Nano-engineering is the key. There is no way around it. We will not get to the Holy Grail by chance. But we know what we need and through interdisciplinary, interactive science we will find our way. Work in organic solar cells has become a worldwide pursuit. I always say to my students that the ultimate material has yet to be found. We will probably have to find it through a lot of sweat and toil. We know the physical properties we need but we don’t have the ideal material as of this moment. We probably will have to design and synthesize it piece by piece. And, of course, there will be surprises at all levels. Sometimes, what we believe is correct turns out to be wrong and we have to learn from these processes. The pursuit makes it great fun from a scientific perspective but, of course, tries our patience.
JP: What is the current scientific interest in your field?
NSS: Young scientists, whom I encounter in all my lectures throughout the world, are very enthusiastic in this new approach of ours. I think their enthusiasm stems from a great deal of frustration in current photovoltaic technology because of its slow penetration in the energy market. They remark if we have such good systems with silicon at 20 percent efficiency and 30-year lifetimes, why doesn’t everyone switch to photovoltaics?
The organic solar cell approach has brought a great deal of new activity in the field. Suddenly, even the silicon people have started to embrace silicon nano-inks, thanks to our discoveries. And I am very proud to have contributed to this new development. I always say, even if organic solar cells fail to become the dominant technology in the future, we still have created such an avalanche on the silicon side to bring about such a great leap forward that we all will profit from that.
But as shown by the recent revolution in the television business, perhaps plastic solar cells will prevail. Look back 10 years ago: TV tubes seemed to have become so refined as to be regarded as the ultimate piece of equipment in its field, being extremely rugged, long lasting and producing brilliant picture quality. The only problem was their size and weight. They were quite heavy to carry and took up a lot of space.
Now, you go to any electronic store and you will not find any of these TVs. A new technology has replaced them, an organic electronic technology, which we call liquid crystal displays. LCDs allow consumers to enjoy the same picture quality and product durability as before — with a television only one inch thick — completely wiping out the old technology which had reached perfection as far as performance. Perhaps, the same story will hold true in the photovoltaic world.
