Flexible energy: Research on organic solar cells has entered an exciting phase

Organic and dye-sensitized solar cells offer many intriguing possibilities, and although large-scale adoption is still some way off, the first commercial plants for off-grid and niche uses are due to come on-line in the next few years. By Iris Krampitz.

Research on organic solar cells is hotting up, with several projects in the pipeline or already begun. Many of them are funded by German Federal ministries or the European Union, says Dr Konstantinos Fostiropoulos of the Berlin-based research centre, the Hahn-Meitner-Institut (HMI). ‘The jump forward is huge. Industry is really engaged’, he adds.

Together with UK-based Merck Organic Materials and the Aachen-based Aixtron AG, HMI is currently developing organic solar cells that can be used for serial production. The project is funded by the German Federal Ministry of the Environment and will end in November 2007. Other projects have as their targets the co-ordination of research activities or increasing the stability of organic solar cells. The latter is one of the big problems: ‘Organic solar cells can dissociate if they come into contact with oxygen or humidity’, says Andreas Gombert from the Freiburg-based research institute Fraunhofer-ISE, who is working on organic solar cells as well.

‘Organic’ in this sense does not mean ‘bio’. Instead, it is a chemical expression for materials that consist of carbon and its compounds. All living material is composed of organic molecules: saccharides, proteins, DNA, many substances in our daily life.

‘Pure’ organic solar cells are solid and consist either of two organic layers or a homogenous mixture of two organic materials. One of them – either an organic dye or a semiconducting polymer – donates the electrons. The other component serves as the electron acceptor. Usually, a fullerene (a spherical-shaped carbon molecule) is used for this purpose. After the charge (the electron-hole pair) has been separated, the holes are transported to the anode, while the electrons are transported to the cathode, to supply a direct current for the consumer load.

Dye-sensitized solar cells, in contrast, are made up of a substrate with a layer of a transparent conductive oxide (TCO) that is coated with the semiconductor titanium dioxide as the negative electrode (cathode) of the solar cell. Another substrate, coated with a thin TCO layer and platinum, serves as positive electrode (anode). In contrast to pure organic cells, dye-sensitized solar cells need an electrolyte for the electron transfer. It is filled in the area between the electrodes.

While a conventional inorganic silicon solar cell converts the absorbed sun light to electricity through its semiconducting structure, an electrochemical dye-sensitized solar cell uses organic dyes to absorb the light. The dye is embedded within the nanocrystalline titanium dioxide of the cathode. Dye-sensitized solar cells are also called Grätzel cells as they were developed by the Swiss researcher Michael Grätzel in 1991.

Dye-sensitized solar cells sealed in glass solaronix

Regarding Grätzel cells, it is the dye that is the focus of the researchers’ attention: the chemically reactive liquid can evaporate or run dry or – with insufficient sealing – even leak from the cell. According to Heinz Ossenbrink, Head of the Unit Renewable Energies at the Joint Research Centre of the European Commission in Ispra, researchers have therefore started to switch to gellable electrolytes instead of liquids. A team from Fraunhofer-ISE, in addition, has been trying to improve the sealing techniques – with some success. Within the framework of projects financially sponsored by the German Federal Ministry of Education and Research, the European Union and the German State of North Rhine-Westphalia, the Fraunhofer researchers have developed a new glass frit (a type of bonding) technology: all of the materials for the cell construction are printed as paste on two glass plates, using screen printing technology. The two plates are then connected to each other in such a way that canals are formed. These canals are then filled with the dye and a gel electrolyte. The glass plates are hermetically sealed to the outside environment and thereby protect the sensitive material inside against degradation. According to a press release from Fraunhofer-ISE, accelerated aging tests carried out over many thousands of hours, and under widely varying conditions, demonstrated the good long-term stability of the cells. However, it still has to be proven that this stability can also be attained when serial production methods are used.

Efficiencies of up to 8%

While dye-sensitized solar cells with efficiencies of 8% already exist, the efficiency of pure organic solar cells is still far away from this figure. With a combination of Phthalocyanines and Buckminster fullerenes, the team of Konstantinos Fostiropoulos, for example, has reached only 2%-3%. Fostiropoulos says other groups have reported 5% on small areas. ‘To be cost-effective, organic solar cells need to have an efficiency of 8%’, he says. With all the research being done at the moment, he thinks this figure can be achieved by 2010.

Heinz Ossenbrink of Ispra thinks an efficiency of 20% is a realistic target. But he is convinced that, even with a much lower efficiency and a shorter lifetime than crystalline solar cells, organic solar cells will play an important role – especially in niche markets. Here they can be used for products that do not necessarily need a long lifetime and a high efficiency. Major benefits of organic cells are their high flexibility and the inexpensive production methods. Techniques such as spin coating and screen printing can be used, which keeps the production costs very low. ‘For the manufacturing, you do not need a vacuum or high temperatures. Organic solar cells can be printed like magazines. We talk about square kilometres per night. In principle, you can bake organic solar cells in your kitchen’, Ossenbrink says.

Flexibility is very high because the substrate does not necessarily have to be glass. Encapsulated in a foil, organic solar cells can also produce electricity in the housing of a notebook, a digital camera or a mobile phone. As part of a smart label, they can control the flow of goods. Another possibility is the integration into textiles – this is currently being pushed by the US company Konarka. And because the possible colour spectrum is very wide, organic solar cells will also become interesting for building-integrated photovoltaics.

An organic cell on a flexible substrate heliatek

The Dresden-based company Heliatek expects to release the first ‘pure’ organic products in by the end of 2010 or the beginning of 2011. Heliatek is a spin-off of the University of Dresden and was founded in 2006. With tailor-made organic materials, Heliatek wants to reach efficiencies of 8%-10% by that stage. Heliatek’s Stefan Hartung expects production costs of less than €1/watt. The company wants to focus on large-area, lightweight organic modules for solar home systems in emerging markets. Heliatek’s aim for the first three years until 2009 is to develop organic tandem or triple solar cells with an efficiency of 7%-10% and give the proof of principle for a roll-to-roll production process. In the second phase, the company plans to focus on the establishment of a reliable, efficient and rapid roll-to-roll fabrication process for large-area modules, including encapsulation.

The US company Konarka expects to go commercial by 2010, says press officer Tracy Wemett. Konarka is focused on the development of polymer photovoltaic materials that can be printed or coated onto flexible substrates. These so-called ‘light-activated power plastics’ can be used to provide power for consumer electronics such as cell phones or MP3 players, business products such as laptops or PDAs, as well as military applications, including battery charging on the battlefield or remote power for unmanned vehicles and soldiers.

First dye-sensitized products in 2008

Dye-sensitized products will be available a little earlier. G24i, a UK-based combined venture between Konarka, Renewable Capital and Ecole Polytechnique Fédéral de Lausanne, will be producing cells from a new production facility it is currently building in Wales. When complete, the factory will have a capacity of around 300 MW, with the first 30 MW line due to come on line in 2007.

Another company working hard in this area is Israel-based Orionsolar Photovoltaics Ltd. The company wants to improve the existing dye solar cell technology and commercialize it. Orionsolar’s products are intended mainly for non-grid applications, as well. ‘Batteries don’t last longer than three years’, says David Weimann, CEO of Orionsolar. The lifetime of these solar cells will therefore not be a problem, and he expects his products to last seven years.

Testing organic cells. Low efficiencies and short lifespans remain a barrier to use in buildings heliatek

Weimann estimates the production cost of his dye-sensitized solar modules to be €1.20/W. Production lines will be delivered to developing countries with growing power needs, especially in Asia, Weimann says. By 2009, he expects a production capacity of 20-30 MW, with a cell efficiency of 7%.

Toby Meyer, founder of the Swiss company Solaronix from Aubonne, expects first dye-sensitized products in two to three years. His company is specialized in the fabrication of chemicals for the production of dye-sensitized solar cells, such as Ruthenium complexes to be used as dye, and nanocrystalline titanium dioxides. In addition, Solaronix assembles modules. Prototypes with an efficiency of 6% and lifetimes of 5-6 years have already been produced. ‘To be cost-effective, dye-sensitized solar cells must have an efficiency of at least 6%, with a lifetime of 1.5 to 2 years’, he says.

Meyer sees applications for dye-sensitized solar cells mainly as portable electricity sources for consumer goods like mobile phones or MP3 players. Later, he thinks dye-sensitized cells could also be used for industrial applications, e.g. as smart labels that control the flow of goods.

‘If we manage to increase the lifetime, we hope to develop products for other markets as well’, Weimann says. He names low-cost products for do-it-yourself applications: the user can transport the module from the retail outlet and install it on site by himself for off-grid use, or – with minimal support from an electrician – for on-grid use. ‘The potential of organic solar cells is immense’, says Weimann.

Iris Krampitz is a freelance journalist
e-mail: rew@pennwell.com

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