Preaggregation of Porphyrin Induced by Phenyl ortho-Alkyl Chains Provides a Platform for the Regulation of Morphology to Enhance the Performance of All-Small-Molecule Organic Solar Cells

Side-chain engineering plays important roles in organic solar cells (OSCs) as it impacts the solubilities and crystallinities of the active materials. Excessive aggregation of large π-active systems usually makes it difficult to dissolve and optimize the morphology of the blended film, which is unfavorable for organic solar cells (OSCs). Herein, the three porphyrins o-ZnPorDPP, m-ZnPorDPP, and p-ZnPorDPP with alkyl chains at the ortho, meta, and para positions of phenyl groups, respectively, are synthesized. The initial aggregation of m-ZnPorDPP and p-ZnPorDPP is too tight to change the morphology of their blends with Y6 even under 1,8-diiodooctane (DIO) additive and thermal annealing (TA) post-treatments, and thus, the power conversion efficiencies (PCEs) of m-ZnPorDPP:Y6 and p-ZnPorDPP:Y6 devices are only 1.62% and 1.61%, respectively. In contrast, the PCE of the o-ZnPorDPP:Y6 device is improved significantly by 396% from 2.03% to 10.06%, which is the highest for the OSCs based on porphyrin and Y-series acceptors to date. The significantly enhanced performance is mainly contributed by the more ordered molecular stacking of o-ZnPorDPP under DIO + TA treatment and demonstrates that the ortho-alkyl chains of the phenyl groups will provide a platform for further morphology modulation for enhancing the photovoltaic performance. The findings proposed an efficient and practical aggregation regulation strategy for the problem that large π-planar molecules are easy to aggregate but disordered and difficult to further optimize.