Organic solar cell (OSC) technology has recently achieved over 13% efficiency through the synthesis of novel non-fullerene small molecule acceptors (NFAs), which can be processed from benign solvents as low-cost third generation photovoltaics[1,2]. Of critical importance to OSCs is understanding the morphological and thermal stability of the active layers governed by thermodynamics and kinetics as an intrinsic stability process which cannot be controlled by encapsulation[3,4]. Here we highlight the importance of ductility of donor polymers on nucleation and growth of micro-size small molecule crystals which leads to the catastrophic failure of the solar cells in the long-term operating condition We consider three high performance polymers P3HT, PBnDBT-FTAZ, and PffBT4T-C9C13 blended with EH-IDTBR as the model systems to investigate the thermal stability of state of the art non-fullerene OSCs, where elevated temperatures were used to accelerate the crystal formation and imitate the long-term operation conditions of OCSs. We also propose an easy accessible method using differential scanning calorimetry (DSC) to investigate the thermal behavior of NFA in the blends. Although non-fullerene solar cells have shown to have better overall morphological stability compared to their fullerene counterparts, our results suggest that catastrophic failure due to micro-size crystal formation in non-fullerene systems can happen at a rate similar to fullerene systems unless the right donor polymer is chosen to suppress the crystallization of small molecule. It is also shown and argued that the growth rate of small molecule crystals can be reduced upon mixing of NFAs with semi-crystalline polymers, such as P3HT with a higher overall density compared to amorphous donor polymers, i.e. PBnDT-FTAZ. Our findings may pave a way to understand and predict the morphological stability of non-fullerene OSCs.
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