Publication date: 8th January 2019
A major breakthrough in organic photovoltaics was recently accomplished with the synthesis of the A-D-A type non-fullerene acceptor (NFA) so-called ITIC [1], some derivatives of which have delivered power conversion efficiencies of 14% [2] when blended with the suitable donor polymers. Nevertheless, ITIC-based devices are too frequently processed applying an Edisonian trial-and-error approach that hinders the development of even more refined molecular architectures and thus slows down the progress of the field. The application of these processing approaches is – at least in part – due to a clear lack of knowledge of the most fundamental structural features of ITIC-based materials, e.g. its stable crystalline forms, its phase transition temperatures, etc., as this information is key for the design of knowledge-based processing protocols that allow control over the microstructure and morphology of the active layers.
Employing a rational combination of ex situ and in situ GIWAXS, polarized optical microscopy and spectroscopy as well as fast scanning calorimetry experiments, we herein unravel the inner microstructure and the thermotropic phase behavior (i.e. the evolution of microstructure with temperature, the phase transitions, etc.) of spin coated and drop casted ITIC films. Interestingly, our results evidence a very complex phase behavior even for the simplest ITIC derivative. For example, we identify two polymorphic transitions before the melting of ITIC – thus three crystalline forms – and a different thermotropic behavior for glassy ITIC compared to its crystalline counterpart. Moreover, applying the suitable thermal history, we are able to prepare ITIC films in the three crystalline forms and the glassy form at room temperature, opening up the possibility to fully characterize their optical and electronic properties.
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