Microstructure and Temperature Stability of APFO-3:PCBM Organic Photovoltaic Blends
In this thesis, the microstructure of organic photovoltaic APFO-3:PC61BM bulk-heterojunction blends was examined. Earlier studies have focused on the microstructure after spin coating. This thesis aims to give a better insight into microstructural degradation as the films are annealed above the glass transition temperature, Tg, and the mixture approaches thermodynamic equilibrium. Electro- and photoluminescence studies indicate that the polymer and PC61BM are intermixed on a scale shorter than the exciton diffusion length of 10 nm, even when annealed above Tg. The temperature stability of APFO-3:PC61BM was also investigated with respect to the molecular weight of the polymer. The photovoltaic performance of these blends was found to be stable up to temperatures approaching the glass transition temperature, especially if a high molecular-weight APFO-3 grade was used.
The crystallization of PC61BM was also investigated. Above Tg, PC61BM crystallization was found to commence, albeit slowly at temperatures close to Tg. At elevated temperatures instead, micrometer sized crystals were observed to form. It was also noted that illumination while annealing APFO-3:PC61BM thin films above Tg affected PC61BM crystallization, the origin of which is so far unclear although chemical degradation could be largely excluded.
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