Polymer Side-Chain Variation Induces Microstructural Disparity in Nonfullerene Solar Cells

In addition to the innovation of nonfullerene acceptors, the development of highly efficient nonfullerene organic solar cells requires the design of new polymer donors and fundamental understanding of their structural and morphological properties. Utilizing meta-alkoxy-phenyl-substituted benzodithiophene and benzodithiophene-4,8-dione building blocks, we designed and prepared a new class of structurally similar photovoltaic polymers named PBDx (x = 1–4), which are capable of being processed from nonchlorinated solvents. From PBD1 to PBD4, the total carbon number of the alkyl side chains in each repeat unit increased by four in turn. The effect of side chain structure variation on the molecular aggregation, molecular arrangement, mesoscale phase separation, charge transport, and nonfullerene solar cell performance was systematically studied. Our hard and soft X-ray scattering results indicate that small side chain variation yields vastly different molecular packing and mesoscale morphology for these analogues. It was found that PBD1 with the shortest alkyl side chain exhibited the strongest molecular aggregation, most attractive interaction with solvent additive, highest composition variation at a small length scale of 30 nm, and best photovoltaic performance of over 12% efficiency among the four polymers. Moreover, the structure–performance connections were discussed in the context of polymer thermodynamics, and the composition of the mixed phase was most likely quenched closer to the percolation threshold for the PBD1:IDIC system according to solubility limit measurements. This work thus elaborates the origin of such disparity in morphology and performance of nonfullerene solar cells.