A Novel BODIPY-Based Low-Band-Gap Small-Molecule Acceptor for Efficient Non-fullerene Polymer Solar Cells

We report herein an efficient A(1)-CC-A(2)-CC-A(1) type small-molecule 4,4'-difluoro-4-bora-3a,4a-diaza-s- indacene (BODIPY) acceptor (A(1)=BODIPY and A(2)=diketopyrrolopyrrole (DPP)) by following the A-to-A excited electron delocalization via the BODIPY meso-position, the inherent directionality for the excited electron delocalization. The lowest unoccupied molecular orbital (LUMO) delocalizes across over whole the two flanking A(1) and the central A(2), and the highest occupied molecular orbital (HOMO) localizes dominantly on the -CC-DPP-CC- segment. The excited electron upon light excitation of the DPP segment delocalizes over both the BODIPY and DPP segments. The acceptor in chloroform shows an unprecedented plateau-like broad absorption between 550 and 700 nm with a large FWHM value of 195 nm. Upon transition into solid film, the acceptor shows absorption in the whole near ultraviolet-visible-near infrared wavelength region (300-830 nm) with a low band gap of 1.5 eV and a maximum absorptivity of 0.85x10(5) cm(-1). Introduction of the ethynyl spacer between the A(1) and A(2) and the close BODIPY-to-DPP LUMO energy levels are crucial for the excited -electron delocalization across over whole the conjugation backbone. A power conversion efficiency of 6.60% was obtained from the ternary non-fullerene solar cell with PTB7-Th:p-DTS(FBTTh2)(2) (0.5 : 0.5) as the donor materials, which is the highest value among the non-fullerene organic solar cells with BODIPY as the electron acceptor material.