Laser technology in the production of organic solar cells

Fig.1: Utilisation of lasers for materials micro machining offers further capabilities of parameter variation

Conventional solar cells, characterised as heavy and expensive, yet powerful, have found a firm place in the market. But they are not appropriate for every conceivable application, for example on tents or car roofs. In such cases, a flexible, organic solar cell (OPV) printed on a plastic film is more suitable. They may have the advantage of working well in low light conditions, but their efficiency is poor and they cannot be produced at low enough cost and with a satisfactory life span. Both aspects are to be addressed in the EPIO joint research project (evaluation and exploration of concepts for the production and integration of OPV in the areas of application of architecture, life science and textiles, number 13N10315), started in 2009 and supported by German Federal Ministry for Education and Research (BMBF) and by a business and research consortium.

Fig.2: Reel to reel manufacturing system for processing of flexible substrates

Ambitious aims for organic PV

Before flexible OPVs can be used on clothing or bus shelters, there are two major problems remaining to be solved: the present efficiency of about 6 % must be increased to 8 to 10 %, with an intended life span of two to three years, and there is a need for a large-scale production process to bring the cost per watt or per square metre down to an acceptable level. The Organic Electronic Association (OE-A ) envisions efficiencies of 5 to 10 %, a life span of five years, and a cost of less than 3€ /W in 2017 to compete against other PV technologies. The goal is to harness the efficiencies developed by the project to reduce the price to below €1/Wp, and come close to €0.5/Wp by 2020.

In principle, the knowledge required to attain these goals is already available, yet nearly all work on these topics is still proceeding worldwide on a laboratory scale. “We want to build OPVs in EPIO, but the main focus of 3D-Micromac is on the production of the cells using modular systems engineering. By means of modularity, new elements can be integrated into the machine to produce other printed electronics,” is how Jens Hänel, Head of Research and Development at 3D-Micromac, outlines the objectives of the EPIO joint project. Of course, OLEDs are to be considered as such products, in addition to every other type of electronics that can be implemented in printed form on foil. The modular concept provides those involved with an opportunity to achieve better system utilisation, i.e. more cost-effective manufacturing, above and beyond the “OPV” application. “We do not wish to manufacture our own solar cells in future, but intend to make our modular systems engineering available to producers of organic solar cells for cost-effective production, ” said Jens Hänel.

Fig.3: Stack layout of an Organic Solar Cell

Deceptively simple

Those involved in the project wish to produce OPV flexibly on a large scale and in a manner which is suitable for volume production in a roll-to-roll process. The initial basis for OPV is a PET foil with a transparent, electrically conductive contact layer usually made of ITO (indium tin oxide). On top of this comes a layer of PEDOT:PSS (polystyrene sulfonate), which smoothes the ITO layer and optimises the electrical characteristics. Next comes the future active layer, where photons will be absorbed. This layer consists of a polymer mixture, such as of P3HT:PCBM (poly(3-hexylthiophene: phenyl C61-butyric acid methyl ester). On this a further adaptation layer is deposited, for example, LiF (lithium fluoride). Aluminium acts as the back contact.

This may sound simple, but there are several points which must be observed to ensure correct treatment of the ITO-coated foil. When the functional layers are printed, the challenge is to achieve an even distribution of a wet film of only a few microns thickness over the entire substrate surface. The next challenge is encapsulation of the cells, the quality of which significantly impacts the life span of OPV because moisture and oxygen degrade the active material. The layers should be structured using laser techniques, which stand for precision and flexibility.

To solve the problems, a consortium of companies and research institutes have joined forces. Here 3D-Micromac is responsible for the systems engineering and laser processes. The demonstration OPVs produced in the project will be integrated into textiles by Hexonia. Furthermore, the architectural firm ‘freiräumer’ develops bus shelters which boast new features resulting from the integration of OPV. The required accumulator technology, which must also be flexible, will be developed by Varta Microbattery. Fraunhofer IAP and the Institute for High Frequency Technology of Braunschweig University of Technology will provide 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 printing technology partner.

Unbeatable laser technology

Importantly, this process differs from others currently being researched by exploiting the unbeatable flexibility of the laser beam for patterning the organic solar cells. This will obviate the need for masks and environmentally harmful chemicals. The commercial availability of picosecond lasers and, more recently, industry-standard femtosecond lasers is another advantage of this approach. Cold ablation is the name given to the laser process. The laser pulses are so short that almost no energy transfer takes place in the material. The laser energy breaks down the chemical bonds only in the top plastic layer and in this way the laser can selectively remove material as precisely as an etchant in microelectronics and also process plastics without damaging them. Thus the ITO layer, the PEDOT:PSS, P3HT:PCBM layer, and the aluminium rear electrode can be laser processed. Moreover, the laser can also to be used for welding the foils during encapsulation.

Various ultra-short pulse lasers with a wavelength in the infrared range are currently used. Removal of the ITO layer is very readily accomplished in this way with both picosecond and femtosecond lasers. The foil substrate is hardly damaged, and the swelling of the ITO on the cutting edge is usually well in the range of 50 nm. Generally, the process window when using 1064 nm is large enough for stable ablation. This is particularly important for roll-to-roll laser structuring. And “from the OLED production, knowledge gained in the field of laser patterning of conductive substrates and thin layers in general can be used. In turn, the EPIO project has a positive influence on future OLED research,” adds Jens Hänel.

The laser structuring methods presented bring the extensive and efficient mass production of organic solar cells within reach. The results obtained in the project will enable the equipment supplier to offer its manufacturing lines for the production of OPVs and other elements for flexible electronics, from the raw materials to the finished encapsulated cell. And a study conducted by market research institute NanoMarkets confirms that this is the way to go. According to this study, sales currently amounting to US $25 million are expected to increase to US $342 million in 2015.

Pictures: 3D-Micromac

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