Exploring the fluorination effect on photophysical and photovoltaic properties of Benzo[1,2-c:4,5-c′]dithiophene-4,8-dione copolymers

Fluorination exhibits multiple effects on the photophysical properties of conjugated copolymers and has attracted enormous attention in organic solar cells (OSCs). Herein, a series of structurally identical benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (T1) based polymers (named PPhF0-T1 to PPhF4-T1) with different numbers of fluorine substitutions have been designed and synthesized. With the number of fluorine substitution increases, the energy levels of the polymers decrease gradually. ITPTC non-fullerene acceptor was employed to fabricate OSCs with these polymers. We found that the introduction of fluorine can effectively improve the open-circuit voltage of the device. Upon one or two fluorine atoms are appended to the polymer, the corresponding short-circuit currents (Jscs) are obviously promoted. However, due to the excessive phase separation and hence suppressed charge carrier mobility, the Jsc decreases when there are four fluorine atoms in the polymer. Consequently, without any additives, the PPhF2-T1 based device gives the highest power conversion efficiency of 10.61%, which is much higher than that of nonfluorinated polymer based OSC (3.97%). This work presents a systematic study upon the effect of different numbers of fluorine atoms on the photovoltaic performance of OSCs, which may serve as a useful guideline in the design of efficient conjugated polymers.