Mechanically Robust All-Polymer Solar Cells from Narrow Band Gap Acceptors with Hetero-Bridging Atoms

We developed three narrow band-gap polymer acceptors PF2-DTC, PF2-DTSi, and PF2-DTGe with different bridging atoms (i.e., C, Si, and Ge). Studies found that such different bridging atoms significantly affect the crystallinity, extinction coefficient, electron mobility of the polymer acceptors, and the morphology and mechanical robustness of related active layers. In all-polymer solar cells (all-PSCs), these polymer acceptors achieved high power conversion efficiencies (PCEs) over 8.0%, while PF2-DTSi obtained the highest PCE of 10.77% due to its improved exciton dissociation, charge transport, and optimized morphology. Moreover, the PF2-DTSi-based active layer showed excellent mechanical robustness with a high toughness value of 9.3 MJ m⁻³ and a large elongation at a break of 8.6%, which is a great advantage for the practical applications of flexible devices. As a result, the PF2-DTSi-based flexible all-PSC retained >90% of its initial PCE (6.37%) after bending and relaxing 1,200 times at a bending radius of ∼4 mm.