No abstract

Nonfullerene acceptors are being actively explored for polymer solar cells (PSCs) with their advantages in extending the absorption range, reducing the energy loss, and therefore enhancing the power conversion efficiency (PCE). But to boost the PCE further, mobilities of these nonfullerene acceptors should be improved. For nonfullerene acceptors, the π-π stacking distance between cofacially stacked molecules significantly affects their mobility. Here, we demonstrate a strategy to increase the mobility of heteroheptacene-based nonfullerene acceptors by reducing their π-π stacking distances via control over the bulkiness of lateral side-chains. The incorporation of 2-butyloctyl substituents into the nonfullerene acceptor (M36) leads to its increased mobility with a reduced π-π stacking distance of 3.45 Å. Consequently, M36 affords an enhanced PCE of 16% which is the highest among all acceptor-donor-acceptor-type nonfullerene acceptors to date. The strategy of control over the bulkiness of side-chains on nonfullerene acceptors should aid the development of more efficient PSCs.

The design, synthesis,a nd characterization of two novel nonfullerene acceptors (M8 and M34) based on laddertype heteroheptacenes with different heterocycles are reported. Replacing the furan heterocycles with the thiophene heterocycles in the heteroheptacene backbone leads to ahypsochromically shifted absorption band and greatly improved carrier transport for the resulting nonfullerene acceptor (M34) although the p-p-stacking distances are barely affected. Bulk-heterojunction polymer solar cells fabricated from M34 and aw ide band gap polymer (PM6) as the donor showed ab est power conversion efficiency (PCE) of 15.24 %w ith an open circuit voltage (V OC)of0.91 V, muchhigher than aPCE of 4.21 %a nd aV OC of 0.83 Vf or the counterparts based on M8:PM6. These results highlight the importance of key atoms in the construction of high-performance nonfullerene acceptors.