Evolution and control of the three dimensional functional microstructure of Polymer:Fullerene photovoltaic blends

Abstract: Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells. However, their low thermal stability is a hindrance that complicates the manufacturing and influences the lifetime of devices. It is thus of high importance to understand what factors that a↵ect the thermal stability of polymer:fullerene blends and explore di↵erent strate- gies to increase the thermal stability. This work is focused on using various electron microscopy techniques to investigate how the nanostructure of poly- mer:fullerene films degrades when exposed to thermal stress and how the ther- mal stability of polymer:fullerene films can be increased. As a model system we have worked with the polymer poly(2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8- diyl-alt-thiophene-2,5-diyl) (TQ1) and the fullerene derivative (6,6)-phenyl- C61-butric acid methyl ester, (PCBM). One of the degradation pathways for polymer:fullerene films exposed to ther- mal stress is nucleation and growth of fullerene crystals. We have investi- gated the kinetics of fullerene crystal nucleation and growth in TQ1:PCBM films, and shown that the formation of fullerene crystals is strongly nucleation limited. We have also explored how transmission electron tomography can be used to visualize the three dimensional distribution of nucleation sites for fullerene crystals in polymer:fullerene films. The results show for our investi- gated TQ1:PCBM films that the fullerene crystals do not start to randomly nucleate in the bulk but have a preference to start nucleating in the upper part of the films. When it comes to increasing the thermal stability of polymer:fullerene films we have shown that the thermal stability of a TQ1:PCBM film can be increased by including additives such as C60, using mixtures of di↵erenet fullerenes, and exposing a film to electron radiation. By adding C60 or using mixtures of fullerenes the nucleation and growth rate of PCBM crystals can be influenced, avoiding large micron sized crystals and preserving the photovoltaic perfor- mance of a film. In a simliar way, exposure to electron radiation will quenche the nucleation of fullerene crystals and the nanostructure of a film will remain stable upon thermal exposure, with a greater e↵ect seen for larger doses.