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Polymer stability

Martin Helgesen
Figure 1: Weak points in different polymers.

One of the major causes for the degradation of polymer solar cells is the diffusion of oxygen and water into the various layers of the solar cell that, in combination with sun light, leads to damaging reactions with the polymer material (e.g. photo-oxidation). Thus, the polymer will degrade over time when it is subjected to sun light and air but there are ways whereby the degradation can be slowed down significantly. Some of the general rules is, firstly, that the use of exocyclic double bonds in the main backbone (e.g. PPVs: MEH–PPV, MDMO–PPV) should be avoided because these are very prone to photo-oxidation, which leads to polymer chain scission, see figure 1. Secondly, a clear observation from many reports is that especially the side chains play a key role in conjugated polymer degradation and their elimination can significantly improve the photochemical stability. In particular, the αC next to the conjugated polymer backbone appears to be very vulnerable to radical formation. For example, when looking at P3HT, this radical formation on the hexyl sidechain can lead to cleavage and carboxylic acid functionalities, initializing with a hydroperoxide formation, while the photo-oxidation of polyflourenes rapidly leads to the formation of fluorenone with cleavage of the sidechains, see figure 1.

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