Design of a New Small-Molecule Electron Acceptor Enables Efficient Polymer Solar Cells with High Fill Factor

Improving the fill factor (FF) is known as a challenging issue in organic solar cells (OSCs). Herein, a strategy of extending the conjugated area of end-group is proposed for the molecular design of acceptor-donor-acceptor (A-D-A)-type small molecule acceptor (SMA), and an indaceno[1,2-b: 5,6-b'] dithiophene- based SMA, namely IDTN, by end-capping with the naphthyl fused 2-(3-oxocyclopentylidene) malononitrile is synthesized. Benefiting from the pi-conjugation extension by fusing two phenyls, IDTN shows stronger molecular aggregation, more ordered packing structure, thus over one order of magnitude higher electron mobility relative to its counterpart. By utilizing the fluorinated polymer (PBDB-TF) as the electron donor, the corresponding device exhibits a high efficiency of 12.2% with a record-high FF of 0.78, which is approaching the theoretical limit of OSCs. Compared with the reference molecule, such a high FF in the IDTN system can be mainly attributed to the more ordered pi-pi packing of acceptor aggregates, higher domain purity and symmetric carrier transport in the blend. Hence, enlarging the conjugated area of the terminal-group in these A-D-A-type SMAs is a promising approach not only for enhancing the electron mobility, but also for improving the blend morphology, and both of them are conducive to the fill-factor breakthrough.