Steric Engineering of Alkylthiolation Side Chains to Finely Tune Miscibility in Nonfullerene Polymer Solar Cells

Morphology and miscibility control are still a great challenge in polymer solar cells. Despite physical tools being applied, chemical strategies are still limited and complex. To finely tune blend miscibility to obtain optimized morphology, chemical steric engineering is proposed to systemically investigate its effects on optical and electronic properties, especially on a balance between crystallinity and miscibility. By changing the alkylthiol side chain orientation different steric effects are realized in three different polymers. Surprisingly, the photovoltaic device of the polymerPTBB-m with middle steric structure affords a better power conversion efficiency, over 12%, compared to those of the polymers PTBB-o and PTBB-p with large or small steric structures, which could be attributed to a more balanced blend miscibility without sacrificing charge-carrier transport. Space charge-limited current, atomic force microscopy, grazing incidence wide angle X-ray scattering, and resonant soft X-ray scattering measurements show that the steric engineering of alkylthiol side chains can have significant impacts on polymer aggregation properties, blend miscibility, and photovoltaic performances. More important, the control of miscibility via the simple chemical approach has preliminarily proved its great potential and will pave a new avenue for optimizing the blend morphology.