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Why polymer solar cells

Morten Vesterager Madsen

The potential advantages of polymer solar cells (e.g. from infinityPV) are numerous including flexibility, processability, low material cost, and independence on scarce resources. The flexibility as an advantage, is shared with thin-film photovoltaics, and is a feature allowing the solar cells to be incorporated into applications where flexibility is an advantage. Solar panels that can be rolled out onto a roof or other surfaces are one option. Processability is another major selling point of polymer solar cells. Both first and second generation solar cells depend on vacuum deposition methods requiring massive amounts of energy; with polymer based organic solar cells, on the other hand, layers are processed from solution and complete solution processed cells are an posibility.DOI:10.1016/j.orgel.2009.03.009 This allows for up-scaling the production and thus reducing the cost per area of polymer solar cells. Large rolls of substrate can be used on which the layers are deposited using printing or coating techniques, generally referred to as roll-to-roll coating. The promise of low material cost and minimal use of scarce materials can be realized with polymer solar cells. Many second generation solar cells utilize materials that are scarce in nature. With polymer solar cells this can be avoided. Indium is, however, still used in most polymer solar cells, but demonstrations of indium free solar cells are available.DOI:10.1016/j.orgel.2009.03.009DOI:10.1016/j.solmat.2006.10.005DOI:10.1016/j.solmat.2008.12.022DOI:10.1016/j.orgel.2011.01.009

Figure 1: The unification challenge is defined as the challenge of unifying efficiency, stability and process for the same material.

Listing the advantages of polymer solar cells reveals a very enticing selling point; however, polymer solar cells have a number of drawbacks. Firstly, while inorganic silicon-based solar cells may last on the order of 25 years; polymer based devices struggle to last a year. Efficiency has long remained the other major drawback of the technology. With polymer solar cell the efficiency is still behind more traditional technologies, but recent records exceeding 10% have been reported.DOI:10.1002/pip.2163 For polymer solar cells to mature to the market, the strong points of the technology will have to match the weak points. However, it is still vital to optimize the weak points. Professor Fredrik C. Krebs have defined the unification challenge of polymer solar cells by stating that three issues share the same importance, see Figure 1.DOI:10.1016/j.solmat.2008.01.005DOI:10.1016/j.solmat.2008.10.004 These three issues were defined as; process, stability, and efficiency. The concept is very similar to the critical triangle for photovoltaics as presented by Professor Christoph J. Brabec, however substituting processability for cost.DOI:10.1016/j.solmat.2004.02.030 While no issue can be argued more important than another, the efficiency of solar cells have long been given special attention. As an area of focus, the power conversion efficiency is important in order to compete with the more mature silicon technology and to justify research in the field of polymer solar cells. As long as focus of research is not on all of the areas, progress towards application of the technology will remain slow. Within recent years the number of reports on both processability and stability has increased significantly. Roll-to-roll production is becoming an established technique for producing polymer solar cells. And more and more work has been published on the stability and degradation including guiding standards for testing OPV devices with respect to stability and operational lifetime.

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