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Studying photochemical degradation

Morten Vesterager Madsen

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

The most used polymer in polymer solar cells is arguably P3HT. Degradation of P3HT is well documented and can be facilitated by exposure to light and molecular oxygen that destroys the π–conjugation and consequently induces loss of absorption. P3HT is degraded under these conditions in solution as well as a solid (e.g. a film). The consequence of degradation is well established but the mechanism responsible for it has been subject to discussion. Whereas singlet oxygen is known to be the cause of degradation in solution,DOI:10.1021/jp808141u the degradation mechanism in the solid state is believed to be different. Manceau et al. have proposed a degradation mechanism based on a radical process beginning from an abstraction of an allylic hydrogen, leading to side-chain and sulfur oxidation.DOI:10.1002/marc.200800421DOI:10.1016/j.polymdegradstab.2009.03.005 This process is responsible for breaking the macromolecular backbone resulting in loss of conjugation and consequent bleaching of the sample. This mechanism occurs under both photo- and thermo-oxidation enforcing the notion that singlet oxygen is not the main intermediate in the degradation process. Hintz et al. have conjectured that the polymer is mainly attacked at the terminal thiophene rings under photo-oxidation.DOI:10.1016/j.polymdegradstab.2010.02.004 The authors concluded this from observing the kinetics of the blueshift in the optical absorption. They observed that the blueshift, indicating loss of conjugation (observed for oligomers with less than 20 thiophene units), is not observed until the end of the degradation of the polymer. Hintz et al. have also demonstrated that a strong increase in photon effectiveness is observed for photo-degradation of P3HT films for decreasing irradiation wavelengths.DOI:10.1021/cm102373k Changing the illumination wavelength from 554 to 335 nm lead to an increase by a factor of 50 in effectiveness of the P3HT photo-oxidation. This observation supports the radical chain mechanism driven by photo-generation of radicals by the photo-lysis of precursors absorbing in the ultraviolet region.

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