Probing the effects of photodegradation of acceptor materials in polymer solar cells: bulk, surface, and molecular level

Polymer solar cells (PSC) have reached record power conversion efficiencies of over 15%. The operational lifetime of PSCs, however, has to increase for their use in large area outdoor applications. In this work, a set of spectroscopic techniques (UV-vis, FTIR, NEXAFS, XPS) was used to study the impact of exposure to light and air (photo-oxidation) on the photoactive layer and its components. We focused on the electron acceptor components: the fullerene derivatives, PC60BM and PC70BM, and the polymer N2200. A comparative study of photo-oxidized PC60BM and PC70BM thin films by UV-vis and FTIR spectroscopy has shown that both materials undergo similar photochemical transformation, with the process being faster in PC60BM, due to the greater curvature of the C60 cage. Comparing experimental FTIR, XPS and NEXAFS spectra of the photo-oxidized PC60BM thin films with the calculated spectra for a large variety of photo-oxidation products, it was found that dicarbonyl and anhydride groups attach to the C60 cage during photo-oxidation. The study of photo-oxidized TQ1:PC70BM blend films by spectroscopic and J-V measurements shows that deterioration of the charge transport in PC70BM is the major contributor to the device performance degradation. Kelvin Probe measurements demonstrated that the charge transport deterioration was due to upward band bending and gap states being formed on the surface of photo-oxidized PC70BM. The TQ1:PC70BM blends films were further studied by AFM-IR in order to determine the lateral distribution of pristine components, as well as the photo-oxidation products. It was found that anhydride oxidation products of PC70BM are equally distributed over the blend film surface. The PC70BM is replaced with the polymer N2200 in the blend with TQ1. The photostability in air of the blend and its neat components was studied by UV-vis and FTIR spectroscopy. The spectra show that thermal annealing improves the photostability in air of both components.Increase of the global energy demand and the climate change are two factors motivating the study and use of renewable energy sources, such as the solar energy. Organic photovoltaics (OPV) is a technology that uses organic molecules to convert solar energy into electricity. These organic molecules can be kept in ink form, allowing OPV device manufacture via coating, and ultimately roll-to-roll printing techniques, resulting in inexpensive, light weight, portable, and mechanically flexible sources of electricity. OPV devices have reached over 15% in power conversion efficiency, but their operational lifetime has to increase. In this work, the photostability of the active layer in organic solar cells and its molecular components was studied by a variety of spectroscopy, microscopy and electrical characterization techniques, with focus on the chemical changes that these materials undergo during exposure to light and air. The aim was to determine the relation between materials’ degradation and the device performance degradation