NSRRC Activity Report 2022

Chemical Science 027 Highly Efficent and Stable Non-Fullerene Organic Solar Cells Improved efficiency and thermal stability of organic solar cells enabled by an additive material using supramolecular fluorinated interactions I n recent years, organic-based non-fullerene acceptors (NFA) with a multifused-ring central core end-capped with two acceptors are revolutionary n-type materials for organic solar cells (OSCs) because of their enhanced light-harvesting ability, better matched energy levels, and better molecular packing properties. In 2019, a new nitrogen-bridged NFA, known as Y6, was developed. When it was paired with a p-type polymer donor PM6, the device achieved an unprecedented power conversion efficiency (PCE) of 15.7% ( Fig. 1 ). 1 Since then, tremendous efforts have been devoted to methods of molecular modification on the basis of the Y6 structure, including aliphatic side- chain, conjugated main-chain, and end-group engineering. To date, a series of Y6-based NFA molecules have increased the PCE to over 18%. Approaching the ultimate goal of a PCE over 20% is foreseeable in the near future. Despite the great success of Y6 in molecular architectures, maintaining the thermal stability of the Y6-based device performance remains a challenge. Fig. 2 : (a−c) Time-resolved GIWAXS patterns of the PM6:Y6:BF7-0.5% thin films measured before, during, and after 100 °C annealing for 12 min. (d) Out-of-plane and (e) in-plane profiles extracted from the corresponding 2D GIWAXS patterns. (f) Azimuthal-angle dependent GIWAXS profiles at 1.778 Å -1 each taken from the 2D GIWAXS patterns of the two films after annealing. [Reproduced from Ref. 3] Fig. 1 : Schematic of PM6:Y6 morphology in the presence of BF7 to induce F-π interaction with Y6. [Reproduced from Ref. 3] In 2017, Yen-Ju Cheng (National Yang Ming Chiao Tung University) developed a nonvolatile additive material bis(perfluorophenyl)pimelate, denoted as BF7, with two perfluorophenyl (C 6 F 5 ) groups linked to a linear aliphatic chain through an ester linkage ( Fig. 1 ). 2 They found that the C 6 F 5 moieties of BF7 retained in the film after