While large efforts are put into describing the degradation and stability of solar cells, focusing purely on the stability of the polymer itself can yield valuable insight into the degradation mechanisms. Using UV-visible spectroscopy to study the rate of polymer degradation is a straight forward technique and a large number of publications exits in literature with this exact approach. DOI:10.1039/C0JM03105D DOI:10.1016/j.polymdegradstab.2010.02.004 DOI:10.1021/cm102373k DOI:10.1016/j.polymdegradstab.2010.08.004 DOI:10.1117/12.780564 DOI:10.1016/j.solmat.2010.03.012 DOI:10.1016/j.polymdegradstab.2009.11.021 DOI:10.1002/marc.200800421 It makes sense since the absorption is vital for solar cell operation as only absorbed photons can generate excitons. The technique of monitoring the gradual absorbance loss was first presented by Holdcroft in 1991, studying photo-chemical stability of P3HT.DOI:10.1021/ma00017a017 Describing photo-stability can be done either in solution DOI:10.1021/jp109900m or as thin films.DOI:10.1039/C0JM03105DDOI:10.1021/cm102373kChemical properties such as conjugation length and crystallinity can be qualitatively discussed based on the absorption measurements. Using degradation rates based on loss of absorbance directly allows for correlating the degradation state to the number of intact monomer units. The number of monomers scales directly with the absorbance, and thus the degradation state can be written as $$D_{state} = \frac{N_{monomer}}{N_{initial}}=\frac{A}{A_{initial}}$$ where $N_{initial}$ is the initial number of monomers, $A_{initial}$ and $A$ is the initial and current absorbance respectively. The number of monomers at a given time during degradation can be expressed by $$N_{monomer} = \frac{N_A\rho} {M}t \cdot D_{state}$$ where $N_A$ is Avogadro’s number, $\rho$ is the polymer density, $M$ is the molar mass, and $t$ is the film thickness. According to the Beer-Lambert law, the thickness of the film scales with the absorbance. The reciprocal rate of monomer loss yields the degradation event interval, $$\tau = \left( \frac{dN_{monomer}}{dt} \right)^{-1}$$ It is hereby evident that the use of UV-visible spectroscopy is a direct approach for obtaining information on the rate of photo-degradation. This can directly be used to compare polymers, but also to compare effects of barrier materials, temperature, atmosphere and more. Using this technique Manceau et al. has created a rule of thumb for photo-stability of a range of polymers.DOI:10.1039/C0JM03105D