Insulating Polymers as Additives to Bulk‐Heterojunction Organic Solar Cells: The Effect of Miscibility

Adding insulating polymers to conjugated polymers is an efficient strategy to tailor their mechanical properties for flexible organic electronics. In this work, we selected two insulating polymers as additives for high‐performance photoactive layers and investigated the mechanical and photovoltaic properties in organic solar cells (OSCs). The insulating polymers were found to reduce the electron mobilities in the photoactive layers, and hence the power conversion efficiencies were significantly decreased. More importantly, we found that the insulating polymers exhibited negative effect on the mechanical properties of the photoactive layers, with reduced Young's modulus and low crack onset strains. Further studies revealed that the insulating polymers had poor miscibility with the photoactive layers, providing large domains and more cavities in blend thin films, which act as negative effect for the tensile test. The studies indicate that rational selection of insulating polymers, especially enhancing the non‐covalent interaction with the photoactive layers, will be critically important for the stretchable OSCs. Adding insulating polymers polystyrene and styrene‐butadiene‐styrene to the high‐performance photoactive layer reduces the crack onset strain in thin films due to their immiscibility. The miscibility between insulating polymers and the organic semiconductors is responsible for the reduced mechanical properties.