Isothermal Crystallization of PC61BM in Thin Layers Far below the Glass Transition Temperature

An isothermal study was performed on 1.25 μm thin layers of PC61BM, a benchmark acceptor for organic photovoltaics. Using fast scanning chip calorimetry allowed for a scanning rate of 30 000 K·s–1, ensuring that all non-isothermal effects were avoided. The effect of isothermal treatments above and far below the glass transition was investigated. It was proven that PC61BM can crystallize below its glass transition, a behavior seen before for several organic glasses. Isothermal treatments as far as ca. 100 K below the glass transition lead to an initial enthalpic relaxation process around the glass transition, which over time gives rise to crystals melting around 300 °C. These observations show a remarkable similarity with the two-step nucleation mechanism documented in the literature. A standard crystallization process was observed for treatments above the glass transition. By combining chip calorimetry pretreatments with atomic force microscopy, different crystal morphologies were visualized for these two...