Hybrid Dihalogenation on the End Group of Indacenodithieno[3,2-b]thiophene-Based Small-Molecule Acceptors Enables Efficient Polymer Solar Cells Processed from Nonhalogenated Solvents and Additives

Subtle modification of the electron-withdrawing end group (A) of small-molecule acceptors (SMAs) plays an important role in regulating structure, optoelectronic properties, and device performance. To obtain SMAs for nonhalogenated solvent-processing devices, we develop two A–D–A SMAs (IT-ClBr and IT-FBr) based on indacenodithieno[3,2-b]thiophene (D) by employing hybrid dihalogenated 1,1-dicyanomethylene-3-indanone (IC-ClBr and IC-FBr) as A groups. The effects of hybrid dihalogenated end groups on the solubility, photoelectrochemical properties, morphology, and device performance were investigated by comprehensively comparing with similar SMAs (IT-4F and IT-4Cl) using nonhybrid dihalogenated IC as A groups. Absorption spectra of IT-ClBr and IT-FBr are similar to that of IT-4Cl but red-shifted relative to that of IT-4F. Hybrid dihalogenation results in enhancing absorption ability and elevating the lowest unoccupied molecular orbitals (LUMOs) of the corresponding SMAs, which is beneficial to increasing short-circuit current density (Jsc) and open-circuit voltage (Voc), respectively. Furthermore, the solubility of IT-ClBr and IT-FBr in nonhalogenated solvents o-xylene (o-XY) can be improved, which makes it possible to fabricate devices with environmentally friendly nonhalogenated solvents. Using polymer PM6 as donor material, IT-FBr-based polymer solar cells (PSCs) present a higher power conversion efficiency (PCE) of 12.02% compared to IT-ClBr (PCE = 10.79%) with o-XY as the solvent and 0.5 vol % 1,8-diiodoactane (DIO) as the additive, owing to the increased Voc and fill factor (FF), which is also comparable to that of IT-4Cl (PCE = 12.14%). More importantly, the PCE can be further improved up to 12.35% when 1 vol % 2-methylnaphthalene (2-MN) replaced DIO as the additive, which is obviously superior to that of IT-4F (PCE = 10.11%). The enhanced efficiency could be attributed to the improved solubility in the nonhalogenated solvent and optimal miscibility between SMAs and the polymer donor (PM6). This finding suggests that hybrid dihalogenation on end groups is a feasible strategy to tailor the solubility, crystallinity, and miscibility of SMAs, thereby improving the morphology and device performance of PSCs fabricated with eco-friendly solvents and additives.