The Photooxidation of PC60BM: New Insights from Spectroscopy

In the quest towards more durable solution-processed solar cells, the stability of the active layer materials under operation conditions is important. While lifetimes of several years have been demonstrated for encapsulated organic solar cells, it is generally known that degradation events can be accounted for by air components (O2 and/or water vapour) leaking into the cell through a non-ideal sealing. Here we present a fundamental study of intentional photo-degradation of the electron-acceptor PC60BM ([6,6]-phenyl-C61-butyric acid methyl ester) in air, with the purpose of improving the understanding of the electronic effects of fullerene photo-oxidation. We have studied spincoated thin films of PC60BM by X-ray Photoelectron Spectroscopy, Near-edge X-ray Absorption Fine Structure spectroscopy, and Fourier Transform Infrared Spectroscopy, before and after exposing them to simulated sunlight in air. The changes observed in the spectra obtained by these complementary methods were compared with calculated spectra of a large set of possible oxidation products of PC60BM where oxygen atoms have been attached to the C60 cage. The best fit with experimental IR spectra of photodegraded PC60BM films was obtained for a linear combination of calculated spectra for two degradation products, a dicarbonyl and an anhydride, both with open cages with 58 carbon atoms, and the pristine PC60BM molecule. From this comparison, we conclude that the conjugation of the fullerene cage is disturbed by the formation of several carbonyl-based derivatives on the C60 cage, accompanied by a transition from sp2 to sp3-hybridized carbon. The π* resonance in the C1s NEXAFS spectrum was found to be a very sensitive probe for small changes to the fullerene cage, and FT-IR was needed in combination with O1s NEXAFS, to identify the oxidation products.Article part of Blazinic's (2019) doctoral thesis Probing the effects of photodegradation of acceptor materials in polymer solar cells: bulk, surface, and molecular level as manuscript.