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Why do polymer solar cells degrade

Morten Vesterager Madsen

As a generalization organic materials are not inert. They are naturally more susceptible to chemical degradation from e.g. oxygen and water than inorganic materials.DOI:10.1016/j.solmat.2005.03.004 DOI:10.1021/am800039w DOI:10.1117/1.3544010 In contrast the pn-junction of the monocrystalline silicon solar cells is very stable and 25 years of lifetime is the norm. Polymer materials are dynamic and reactive, and prone to attack by a range of agents. When organic matter is illuminated, the materials react via photochemical and photolytic processes.DOI:10.1016/j.solmat.2010.09.021 DOI:10.1016/j.polymdegradstab.2009.03.005 DOI:10.1039/c2jm16340c DOI:10.1016/j.polymdegradstab.2012.07.021 The materials are also not heat stable and can suffer heat or sunlight-induced morphology changes or interfacial degradation over their operating lifetime. The bulk heterojunction with a specific morphology of interconnected domains do not necessarily represent the most thermodynamically stable configuration and growth of PCBM acceptor crystallites altering the optimal morphology has been observed.DOI:10.1021/cm9009282 DOI:10.1002/adfm.200500812 DOI:10.1016/j.solmat.2008.01.006 DOI:10.1039/b505361g Polymer solar cells can also sustain damage to the top electrode, often made from a low-work-function metal that is reactive and easily oxidized in ambient air.DOI:10.1016/j.solmat.2008.01.020DOI:10.1149/1.2716553 All in all the stability of organic solar cells are on a different playing field compared to inorganic solar cells.

There are measures that can be taken to improve stability of polymer solar cells. Encapsulation technology can protect materials from environmental factors; minimize the availability of oxygen and water, thereby prolonging the life of the device. Organic semiconductors can be made more resistant to oxidation by appropriate tuning of their electronic levels, and they can be made "harder" by tuning their glass-transition temperature or with cross-linking. Using thermo-cleavable polymers is an approach in which side chains are eliminated post deposition, making otherwise unprocessable but stable materials available.DOI:10.1021/ja047452mDOI:10.1021/cm051320qDOI:10.1039/c1py00323b Electrodes can be made more stable by capping the reactive metal with a less reactive one.DOI:10.1063/1.2841067 There is no doubt that an improved understanding of degradation mechanisms will result in better solar cells if the constituent materials can be improved. A stable device prime for production will result from a compromise of optimizing the individual materials and by utilizing all available methods of improvement on device level including encapsulation.



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