Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.041
Publication date: 21st February 2018
The main application challenge of organic photovoltaics (OPV) is their rapid performance degradation, as compared to their inorganic counterparts. This rapid degradation is either caused by mesoscale phase segregation of donor and acceptor molecules or by compositional changes, mainly caused by uptake of oxygen and water. Although mesoscale effects on the bulk performance of OPV are well documented,1 a detailed understanding of the effects of oxygen and water uptake in the donor and acceptor phases, and most importantly across interfaces, are lacking.
Energy Filtered Transmission Electron Microscopy (EFTEM) is a technique that enables mapping of the local chemical composition of OPV devices,2 which is limited by electron beam-sample interactions, i.e., electron beam damage. The careful analysis of this beam damage enables the calculation of an analysis limit,3 which is subsequently used to enable the determination of chemical changes at the nanoscale, caused by uptake of oxygen and water during OPV degradation.
To resolve oxygen and water uptake in separate phases and across interfaces with EFTEM, a columnar model systems was employed. The phase separation of poly(3-hexylthiophene) (P3HT) and polystyrene is used to create a P3HT matrix, which is (after removing PS) filled with phenyl-C61-butyric acid methyl ester (PCBM) molecules to create PCBM columns in a P3HT matrix. The oxygen content throughout the three phases is subsequently quantified at different stages of degradation using EFTEM maps. EFTEM maps were acquired from samples that were either prepared in a glovebox (non-degraded reference) or prepared outside the glovebox (containing physisorbed water and oxygen) to measure oxygen and water diffusion into the OPV active layer. Furthermore, samples that were degraded in UV-light were used to measure chemical changes due to oxidation reactions. Our results indicate that physisorbed oxygen and water are mainly taken up in the PCBM phase, while UV-irradiation causes an increase in oxygen content in both the P3HT and PCBM phase, with still an excess of oxygen in the PCBM phase. In the future, we envision low-dose cryogenic EFTEM to be a critical technique in further resolving degradation issues in OPV research.
1. Jorgensen, M., Norrman, K., Krebs, F.C. Sol. Energy Mater. Sol. Cells 2008, 92, 686-714
2. Kozub, D. R., Vakhshouri, K., Orme, L. M., Wang, C., Hexemer, A., Gomez, E. D. Macromolecules 2011, 44, 5722-5726
3. Leijten, Z.J.W.A., Keizer, A.D.A., de With, G., Friedrich, H. J. Phys. Chem C 2017, 121, 10552-10561
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