Photo-oxidation of a non-fullerene acceptor polymer
Vanja Blazinic a, André Johansson a, Cleber Marchiori a, Leif K.E. Ericsson a, C. Moyses Araujo a, Ellen Moons a
a Department of Engineering and Physics, Karlstad University, Sweden
Online Conference
Proceedings of NFA-Based Organic Solar Cells: Materials, Morphology and Fundamentals (NFASC)
Online, Spain, 2021 February 3rd - 4th
Organizers: Natalie Banerji and Feng Gao
Invited Speaker, Ellen Moons, presentation 009
Publication date: 25th January 2021

Solution-processed solar modules demand semiconducting materials with good photochemical stability. For the pi-conjugated molecules used in the active layers of organic solar cells, the simultaneous exposure to light and in-diffusing air components is particularly challenging for their stability. Our earlier studies of the photooxidation of fullerene derivatives have shown that fullerenes photo-oxidize readily, forming a variety of oxidation products.(1) This photooxidation occurs by reaction of molecular oxygen with the excited triplet states of fullerene, forming oxygen radicals that in their turn break double bonds and attach oxygen to the carbon cage. This vulnerability to photooxidation makes the fullerene derivatives the less stable component in bulk heterojunction blends with many electron donor polymers, and hence problematic as electron acceptor in organic solar cells.

In this study we present a joint experimental-theoretical effort to unveil the underlying effects of the photo-oxidation on the electronic structure, chemical composition and absorption spectrum of the non-fullerene electron acceptor polymer N2200. This is achieved by intentionally exposing thin films of pure acceptor and donor components to simulated sunlight in air. We have found that as-coated N2200 films photobleach faster than films of the TQ1 donor polymer, and that the rate of photobleaching is strongly reduced for pre-annealed films. By infrared spectroscopy we observed a lower tendency to form photo-oxidation products upon light exposure in air for the acceptor polymer N2200 than for the donor polymer TQ1, in contrast to PC60BM and PC70BM. The observed changes in the IR spectra have been assigned primarily to the C=C stretching from both bithiophene and naphthalene diimide moieties of the N2200 molecule. Molecular modelling within the density functional theory framework has further supported such experimental findings. Additionally, the theoretical assessment of the IR spectra shed light on the specific chemical bonds that are more prone to be broken during the photo-oxidation process, providing valuable guidelines to understanding the degradation processes.

From Near-edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, we learn that the effect of photooxidation on the electronic structure of the unoccupied molecular states of the non-fullerene acceptor polymer N2200 is significantly milder than for fullerene derivatives. From this we could expect that photooxidation has lesser deteriorating effects on the electron transport through the LUMO for the NFA polymer when compared to the fullerenes. Such enhanced photostability of the acceptor polymer N2200 is an additional advantage over fullerenes, and promising for increased device stability.

The authors are grateful for the technical support of PEEM/XAS beamline staff at the SOLARIS National Synchrotron Raduation Centre in Kraków, Poland and the Pollux beamline at the Swiss Light Source in Villigen, Switzerland. We acknowledge research funding from the Swedish Energy Council (Contract 38327-1), the Knut and Alice Wallenberg Foundation (Contract 2016.0059), and the Swedish Research Council (Contract 2015-03378). 

© Fundació Scito
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