Deciphering the Role of Fluorination: Morphological Manipulation Prompts Charge Separation and Reduces Carrier Recombination in All-Small-Molecule Photovoltaics

Fluorination has proven effective in increasing the absorption, downshifting the energy levels, and enhancing the crystallinity of high-performance fused-ring electron acceptors (FREAs). However, an in-depth understanding of the effects of fluorination is still lacking, as research efforts have mainly focused on increasing the power conversion efficiency (PCE). In addition, fluorination on FREAs has rarely been reported in all-small-molecule organic solar cells (ASM OSCs). Herein, fluorination on FREAs is systematically studied in ASM OSCs using the popular FREA 2,2 '-((2Z,2 ' Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b ']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile and its fluorinated analog paired with DRCN5T, an oligothiophene donor seldom investigated in ASM OSCs to date. (Photo)physical studies are conducted on both systems and it is identified that, along with the aforementioned ones, fluorination exerts several additional effects, including the following. First, it optimizes the morphology, thereby accelerating charge separation and reducing geminate recombination charge pairs; second, it suppresses energetic disorder; and third, it prolongs the carrier lifetime and thus aids charge extraction. Consequently, the short-circuit current density and fill factor are significantly enhanced, and in turn, the PCE yields a 36% improvement, climbing to 9.25% and rivaling that of the current state-of-the-art oligothiophene-donor/nonfullerene ASM OSCs. The insights decipher the working mechanism of ASM OSCs that use fluorinated FREAs, paving the way toward high-performance ASM OSCs.