Unexpected Opposite Influences of Para vs Ortho Backbone Fluorination on the Photovoltaic Performance of a Wide-Bandgap Conjugated Polymer.

Fluorination density and regioregularity are known factors that have high impact on the performance of organic solar cells; however, due to the limited available fluorination positions, the influence of backbone fluorination positions (such as ortho, para, and meta) has not been well studied. Here we disclose that the fluorination position on a conjugated polymer backbone may have completely opposite effects on its performance. Specifically, compared to the nonfluorinated control, Devices fabricated with the conjugated polymer based on para-fluorinated dibenzo[c,h][2,6]naphthyridine-5,11-(6H,12H)-dione (DBND) block exhibit improved power conversion efficiencies (PCEs) up to 6.55%, while devices fabricated with the conjugated polymer based on ortho-fluorinated DBND block exhibit much worse PCEs as low as 1.44%, although both polymers have similar HOMO/LUMO levels, bandgaps, and backbone torsion angles. It is found that different fluorination positions result in different dipole moments, intermolecular binding energies, and syn/anti conformer ratios, which eventually lead to the distinct phase-separation behaviors of the corresponding solar cells. [ABSTRACT FROM AUTHOR]