Efficient and Stable Nonfullerene Polymer Solar Cell Enabled by a Narrow-Bandgap, Nonplanar, Small-Molecule Acceptor with an Extended Main Chain

In the past several years, the small-molecule electron-acceptors with proper twisted main chain geometry have achieved promising performances in polymer solar cells (PSCs) not only because of the significantly reduced overaggregation in the active layer but also due to their enhanced molar extinction coefficients, raised molecular energy levels, and better charge transfer characteristics. However, due to large steric hindrances, the reported main chain twisted electron-acceptors usually possess blue-shifted absorption spectra compared to their planar counterparts, which limits their light-harvesting properties. To broaden the absorption range of main chain twisted acceptors, a small molecule acceptor (i-V-4F) having two additional ethylene bonds in the main chain was designed and synthesized. Having the extended conjugated length and the suppressed twisted degree of the molecule, i-V-4F shows a largely bathochromic-shifted absorption spectrum ranging from 550 to 930 nm, resulting in a very low optical bandgap of 1.32 eV. Two wide-bandgap polymer donors (J52 and J52-2F) with similar absorption performances but differing energy levels were used to match with this low bandgap twisted small molecule acceptor in PSCs. Due to the better charge transfer properties, improved exciton dissociation properties, and simultaneously suppressed bimolecular and trap recombination properties, the J52:i-V-4F-based device successfully achieved a high short-circuit current density (J(SC)) of 19.83 mA cm(-1) and a resultant power conversion efficiency (PCE) of 8.61% compared to that of 8.63 mA cm(-1) and 3.71% for the J52-2F:i-V-4F-based device. Besides, the J52:i-V-4F-based device maintains over 92% PCE after 1000 h, indicating its good performance stability. This work, on the one side, illustrates the effective structural modification to largely expand the absorptive regions of main chain twisted acceptors, and on the other side, reveals the performances and stabilities of low bandgap main chain twisted small molecule acceptors with different polymer donors in polymer solar cells.