Fluorenyl-based polyurethane efficiently improves the flexibility and photovoltaic performance of organic solar cells

Introducing proper insulating polymer in the active layer can improve the photovoltaic and mechanical properties of organic solar cells. Here, we design and synthesize a polyurethane insulating material K-BA20 (or 50) containing PDMS and 2D aryl block 9,9-diphenyl-9H-fluorene (DPF). Isophorone diisocyanate (IPDI) is selected as the connection unit and hydrogen bond is introduced to reduce the chain slippage caused by excessive one-dimensional structure. This strategy can enhance the adhesion between components, which can successfully improve the photovoltaic performance and mechanical properties of the devices. When 5 and 15 wt% K-BA20 was introduced into the PM6:Y6-based system, the short-circuit current density (JSC) of 26.71 and 26.65 mA cm−2, and the power conversion efficiency (PCE) of 16.67% and 16.05% were obtained. More interestingly, the active layer introduced K-BA20 with low DPF ratio has good stretchable properties, while K-BA50 with high DPF ratio has more stable flexibility. The breaking elongation of the blend film with introducing 15 wt% K-BA20 was increased 3.2-fold, from 5.81% to 18.46%, and the PM6:Y6-based flexible devices after doping 5 wt% K-BA50 can still maintain half of the initial efficiency under 600 bending fatigues. Hence, this work shows that introducing K-BA20 (or 50) into PM6:Y6 -based devices can bring out a series of positive feedbacks in terms of charge recombination, crystallinity and molecular interaction. It can be seen that constructing new insulating polyurethane materials with rigid and flexible segments is an effective strategy for development of flexible organic solar cells.