Manufacturing plastic solar cells by the meter

Dr. Barbara Stumpp

The conventional solar cell is powerful, but heavy and expensive. For applications such as tents or car roofs, a more suitable option would be flexible organic photovoltaics (OPVs) printed on plastic foil. OPVs work well in low-light conditions, but their efficiency is poor, and they cannot be produced cost-effectively, given their expected life span.

There are two major problems to solve. First, the OPV cell’s efficiency, currently at about 6 percent, must be increased to 8 to 10 percent, over a life span of two to three years. Also, there is a need for a large-scale production process to bring the cost per watt or per square meter down to an acceptable level. The project’s goal is to reduce the price to below €1 per watt peak (W/p), and to come close to €0.5/Wp by 2020.

This device consists of modules that can be recombined, depending on the product being manufactured.

A consortium of German companies and research institutes is working to solve these problems as part of a government initiative known as EPIO (evaluation and exploration of concepts for the production and integration of OPVs in the application areas of architecture, life sciences and textiles, No. 13N10315). The laser micromachining company 3D-Micromac of Chemnitz is responsible for the systems engineering and laser processes. Hexonia of Nettetal, which develops and fabricates high-technology textiles, will integrate the OPV prototypes created during the project into textiles. In Potsdam, the architectural firm Freiräumer is developing bus shelters that integrate the OPVs. Varta Micro Battery of Ellwangen will develop the necessary flexible accumulator technology. Fraunhofer Institute for Applied Polymer Research and the Institute for High-Frequency Technology of Braunschweig Technical University will develop the structure of the cells and the necessary processing steps. The Institute for Print and Media Technology at Chemnitz University of Technology will serve as a printing technology partner.

A flexible laser-structured organic solar cell. Images courtesy of 3D-Micromac.

The project partners want to produce flexible OPVs on a large scale and in a way that is suitable for mass production. The starting point for an OPV is a polyethylene terephthalate foil covered with a transparent, electrically conductive contact layer, usually made of indium tin oxide (ITO). On this is a layered stack of the polymer mixture polystyrene sulfonate, the future active layer. On top of this, an adaptation layer is deposited; for example, lithium fluoride and aluminum act as the back contact.

This sounds simple, but several things must be observed to ensure the correct treatment of the ITO-coated foil. The difficulty is achieving an even distribution of the printed functional layers over the complete substrate surface, because the wet films have a thickness of only a few microns. The next step is the encapsulation of the cells; the quality of the result significantly affects the life span of OPVs because moisture and oxygen degrade the active material. To structure the layers, laser radiation should be used because of its precision and flexibility.

The OPV production process requires only ambient – not vacuum – conditions.

Unlike other processes, this method will harness the great flexibility of the laser beam for patterning the organic solar cells. The availability of picosecond lasers and, more recently, industry-standard femtosecond lasers on the market offers the chance to use cold ablation for structuring the OPV. With the cold ablation process, the laser pulses are so short that, ideally, no energy transfer takes place in the material. The laser energy breaks up the chemical bonds only in the top plastic layer. In this way, the laser can selectively remove material as precisely as an etchant in microelectronics, and it can process plastics without damaging them. The layers and the aluminum rear electrode are processed in this way. Furthermore, the laser welds the foils during encapsulation.

During the project, various ultrashort-pulse lasers with wavelengths in the infrared range are being used. Both picosecond and femtosecond lasers can remove the ITO layer very effectively, and at the 1064-nm wavelength, the process window is large enough for stable ablation. This is particularly important in terms of roll-to-roll laser structuring. The process has also benefited organic LED (OLED) development: “From the OLED production, knowledge in the field of laser patterning of conductive substrates and thin layers in general can be used. And vice versa: EPIO has a positive influence on future OLED research,” noted Jens Hänel, head of research and development for 3D-Micromac.

Close-up of a MicroFlex module by 3D-Micromac. In the future, it will be used to manufacture OPVs.

In the future, this laser processing method will allow extensive and efficient mass production of organic solar cells. With the results developed over the course of the project, 3D-Micromac will be in a position to offer manufacturing lines for low-cost production of OPVs and other elements of flexible electronics, from the raw materials to the complete encapsulated cell. A study by market research institute NanoMarkets confirms that this approach is the right way to go, with sales currently amounting to $25 million and an expected increase to $342 million in 2015.

The project, which began in 2009, is supported by the BMBF – the German Federal Ministry of Education and Research. The closing date for the project is February 2012.

Meet the author

Dr. Barbara Stumpp is a freelance journalist in Freiburg, Germany; e-mail: bstumpp@gmx.de.