Enhanced efficiency in nonfullerene organic solar cells by tuning molecular order and domain characteristics

Despite significant advances in the design and synthesis of photovoltaic materials, halogenated solvent additives have been predominately used in the device fabrication of state-of-the-art materials. Also, little attention has been paid to replace these additives with halogen-free ones. Aimed at addressing this problem, this study shows the effectiveness of a hydrocarbon solvent additive, namely 1-phenylnaphthalene (PN) in two high-efficiency polymer:nonfullerene blends (PTB7-Th:IEICS-4F and PM6:Y6) and importantly presents a detailed understanding of the role of PN by combined X-ray scattering and calorimetric analysis. Tuning the concentration of PN allows us to control the film crystallinity and domain correlation characteristics of polymer:nonfullerene blend films, thereby resulting in distinct device performances. Inclusion of 0.5% volume amount of PN dramatically enhances the nonfullerene ordering in the blend films, as evidenced by sharp reflection peaks and high melting enthalpies. The promoted paracrystalline order together with the highest composition variation related to domain purity result in ~35% improvement in the device efficiency of PTB7-Th:IEICS-4F over reference devices. Applying the same treatment results in enhanced efficiency in the PM6:Y6 device, which outperforms its additive-free control device. This mechanistic understanding will be of great significance to the future development of OPVs towards eco-friendly fabrication.