Organic photovoltaics offer greener benefits to provide solar cell
January 10, 2012 // Paul Buckley
In this news analysis article EE Times Europe Power Management's editor, Paul Buckley quizzes Dr. Martin Pfeiffer, co-founder and CTO of Heliatek GmbH, a global leader and Heliatek's CEO, Thibaud Le Seguillon, to learn more about the high-end solar PV technology company's pioneering developments which continue to break world efficiency records in the field of organic photovoltaics and recently earned some of the company's technologists to clinch the 2011 Deutscher Zukunftspreis (German Prize for Innovation and Technology).
Last month Heliatek GmbH claimed a new world record for organic solar cells after Fraunhofer ISE CalLab (Freiburg, Germany) measured and certified under standard test conditions a cell efficiency of 9.8 % for a 1.1 cm² tandem cell manufactured with a low temperature deposition process. The new record is the third time in a row that Heliatek has set a world record for efficiency in the field of organic photovoltaics.
EE Times Europe: Can you explain more about the recent efficiency record and the benefits of the OPV technology?
Dr. Pfeiffer: Heliatek is the only solar company in the world focused on the deposition of small molecules with low temperature processes, a method that has already been widely adopted by the market for use with organic LEDs (OLEDs). Heliatek develops and synthesizes in-house the molecules responsible for capturing light in solar cells and converting this light into electricity.
Our latest record is 9.8 percent with a completely low temperature process organic solar cell. The process is below 120 degrees C which is to say that it is completely compatible with cost effective PET- foil. We don't require any highly temperature stable foils like PEN or polyimide.
The technology does not make use of any solvents. It is a dry process in vacuum and requires the minimum consumption of organic materials as the semiconductor. We only require a gram per square meter. The technology uses an abundantly available non-toxic material.
Our cells do not lose efficiency when you go up in temperature and they also do not lose efficiency when you go down in light intensity which contrasts favorably with typical silicon or Cd-Te solar cells. These factors translate into an improved 'harvesting factor' which is how much energy you expect to gain from a nominal power. Our 'harvesting factor' is some 10 to 20 percent better than the competition depending where you put your solar cells.
Our benchmark is Cd-Te because this is not a highly efficient technology like crystalline silicon but still they are extremely competitive in the market with module efficiencies of 10 or 11 percent.
We don't need top efficiency to be able to compete but our primary competitive factor is flexibility. There is no flexible Cd-Te solar cell. We can be lightweight and flexible. As an option we can also be transparent rather like a tinted window so this means we can address markets where you cannot use crystalline silicon or Cd-Te.
EE Times Europe: Will PET-foil close the efficiency gap compared with glass?
Dr. Pfeiffer: The gap is getting closer and closer. We are at 8.5 percent on the PET-foil. We believe that in future the distance between PET and glass will reduce to just one percent. The difference is optical losses. The cell itself has the same performance and the same photo-voltage only less current because there are higher reflections at the interfaces.
On the PET-foil we are still a little behind the results of glass but to the best of our knowledge we are by far the highest efficiency of PET-foil of any technology. With PET-foil there is nothing organic or inorganic that compares with the efficiency that we can currently achieve.
EE Times Europe: Is there any key step that will take you from 10 percent efficiency to 15 percent?
Dr. Pfeiffer: It is all about the chemistry. The most obvious potential for further improvement is that we still cannot make use of deep blue light and near infra-red light. Currently the absorbers have a limited bandwidth and we have a synthesis group at Heliatek which is integrating the physics and chemistry. We are working on similar materials with complementary absorption spectrums to harvest everything between deep blue and near infra-red. By changing these factors without changing anything else in the set-up we might already be able to reach about 12 or 13 percent. Everything else is just fine tuning.
EE Times Europe: Is there a practical upper limit to this technology?
Dr. Pfeiffer: I think the practical upper limit is around 15 percent. It could be 20 or 22 percent depending on who you asked. But for the timeframe we can see it is about 15 percent. The practical limit for Cd-Te is pretty similar.
Le Seguillon: So 15 percent is our target. It is obvious we want to get above 10 percent as soon as possible and we think we know how to do that. Our flexible, thin and light panels open up new fields of applications that were so far impossible to address with conventional PV technologies.EE Times Europe: How easy is it going to be to scale-up the technology to production volumes?
Dr. Pfeiffer: The other important achievement is our upscaling developments. We have produced modules of 15 x 15 cm in area with nine percent efficiency on the active area. With structuring losses we are down at 7.5 but this is by some distance the highest module efficiency ever reported for an organic solar cell. In that respect we are even further ahead than our polymer competitors. Our technology approach is pretty robust with respect to upscaling.
We expect that in the future production lines the losses will be even lower because we can benefit from optimized linear evaporation forces being available from the OLED industry. They have worked on that for many years to be able to generate full color OLED displays so they know exactly how to prepare a source with excellent homogeneity. We find in the lab that our process is robust enough that we can basically keep the performance when we are going to larger areas. Sputtering losses will be further reduced as well.
Fullerene C60, which contains 60 carbon atoms and nothing else and is the shape of a soccer ball, is used in our cell to transport electrodes. The Fullerene is prepared in a plant with a thiophene derivative which is an oligomer that is small enough so you can do a vacuum deposition to bring it into the gas phase just by heating and without decomposition. It is a well defined material where you can do physical purification and where you can bring it up to semiconductor quality. The big advantage of this oligomer approach is that we can prepare multilayer structures to optimize our cells.
For polymers you always work with solvents and coating processes and with multilayers the coating process is difficult because you always tend to dissolve the underlying layer when you try to prepare the next one.
We don't need anything like annealing steps because it is just vacuum deposition of one layer after the other and the layer is prepared. The layer is ready without the need for any post-production treatment.
For optimizing lifetime we can use dual interfaces to avoid degradation paths. For example, contact interfaces. This is one of the reasons why we have shown that our cells with the highest efficiency have very good long term stability when exposed to intense light or exposed to temperatures as high as 85 degrees C.
Although the device architecture might look pretty complicated we still only use about one gram of organic materials per square meter because all these layers are ultra-thin.
EE Times Europe: Is the market ready for this technology?
Le Seguillon: We are working with early adopters in Europe to integrate our product in materials for the building industry to get BIPV. We are looking at windows and glass. We are also looking at concrete facades or aluminium facades. We also have development agreements in place with companies where we put very thin, very light and possibly semi-transparent products into their products so that they can deliver to the industrial building and residential building applications.
We are not going to compete with Chinese made silicon PV which is a commodity now and which they are selling cheaply. We want to create new markets. It is a blue ocean type of strategy where we are going to go in places where today people cannot use PV. Where it is too heavy or too bulky or too thick or where they need transparency.
We want to go at a premium price in places where PV is not currently used. The sluggish nature of the current PV market should not affect us firstly because we are aiming for new markets and secondly because it is a little early for us to be impacted seeing as we will only start small production runs during the middle of 2012 and will be in volume production in 2014.
Le Seguillon: It is very difficult to find investors in the EuroZone or in Europe in general. We are going to extend the search in North America because they have always had something of an interest about new technology and start-ups. We will also be looking to Asia because they now have deep pockets and they are very interested in investing in Western technology. We recognise it is going to be a difficult investment environment.
The solar industry is going to go through a massive reconstruction in 2012 with consolidation and bankruptcies because there is too much capacity a not enough demand and that is going to impact on our ability to find an investor but the last time we went for a financing round was in 2009 and it was the year of the largest recession and then we managed to raise Euros 18 million.
In 2012 we are looking at about raising Euros 50m. I think we have a good story because we have proven time and again that we are making progress and it is a solid technology which can change the solar industry.
Dr. Pfeiffer: It is true that the solar industry is going through some kind of crisis and is restructuring but the mega trend is very much on our side. We can currently see that even China is starting to move towards cleaner technologies. The prices are going down which is why we are having a crisis but the demand and total installation of new PVs is still going up and every forecast says they will go up for the decade to come. There is a massive potential in there.
EE Times Europe: The Heliatek technology allows for different transparency levels?
Le Seguillon: The transparency capability is a property of the material.
It is not a gimmick like some people are offering such as those who are using laser scribing or are omitting some material so that you can see through the device. Our solution is more like sun glasses where it is embedded in the material. We can see through and we can harvest light and produce electricity at the same time.
This does affect the efficiency of the cell because you have to leave some photons to see through but we have shown that so far we have about seven percent efficiency with 25 percent transmission light and we are working on that because we think it is a clear differentiator of our technology.
Our solution also removes the reddish appearance that is associated with rival technologies that use tinted see-through materials. Using tinted windows with red colors is not a very attractive proposition. For tinted windows blues, greens or greys are what consumers seem to want.
EE Times Europe: Is there any other key differentiator of Heliatek technology?
Le Seguillon: I think we are the only truly 'green' technology. We don't use any toxic materials in our products. For example we don't feature any heavy metals. We also don't use any toxic materials in our processing. We also use low processing temperatures so we use less energy.
The majority of our cell is actually PET and essentially it is very like a plastic bottle which can be recycled. There is also aluminium inside which can be recycled as well. Everything else in terms of the chemistry is very much what you are used to regarding the color of your jeans. It doesn't create any trouble.
EE Times Europe: You also claim a shorter energy payback time?
Le Seguillon: We estimate that it is half a year or it could be even less depending where you apply your solar cell. For example, whether the cell is located in southern Europe or America compared with northern areas. The energy pay back must be better than the competitors because we use less material and use lower process temperatures.EE Times Europe: What would you say are the three key benefits of the OPV technology?
Le Seguillon: The three key selling benefits of OPV are:
3) Real life performance
Because the cells are thin and very light then smart phones could be an application but we will need to work on our efficiency performance so that they don't have to have one square meter to power a telephone.
We are looking at working with the GSMA Green Charger initiative.
Dr. Pfeiffer: A combination of a smart phone with an OLED display and an OPV to power the device would be a perfect fit. We have been profiting from all the experience and advances being made in the OLED world in the past couple of years and it has helped us to bring costs down. They have shown that good lifetimes, stability and perfect appearance can all be achieved by vacuum deposition of small molecules.
EE Times Europe: Are the new markets you envisage for your PV technology aimed at the electronics sector?
Le Seguillon: The first volume market will be BIPV but closely behind that we are looking at automotive applications. We have already been approached by two large European car manufacturers to work with them on the development of a glass rooftop using OPV.In the electronics sector the first market is going to be focused on mobile electronics with chargers and charging stations for the consumer electronics market. The chargers will not be disposable but they can be considered more disposable compared with the BIPV or automotive applications.
The applications we are looking at are those where people need to charge their devices more regularly. People with smart phones are people I consider are consumers and are early adopters willing to try new technologies. Smart phones would fit perfectly with what we are trying to do.
EE Times Europe: How far are you from a working product that could be used in the portable device sector?
Le Seguillon: In the mobile market we are about a year away to get a product on the market.
Scientists from Heliatek were recipients of the 2011 Deutscher Zukunftspreis (German Prize for Innovation and Technology) in recognition for their outstanding innovation, "Organic electronics - More light and energy from wafer thin molecule layers". In Germany the President's Award for Technology and Innovation honors innovative and economically promising research developments.
From left: Thibaud Le Seguillon (CEO Heliatek), Dr. Martin Pfeiffer (CTO Heliatek), and Hans-Peter Metzler (Chairman of the Supervisory Board of Heliatek) celebrate receiving the 2011 Deutscher Zukunftspreis
Visit Heliatek at www.heliatek.com
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