Non-planar zinc(II) di(naphthylethynyl)tetraphenylazadipyrromethene (Zn(L2)₂) is a promising low-cost non-fullerene acceptors for organic photovoltaic applications, but it has a relatively low electron mobility that may limit its performance. Here, we explored fluorination of peripheral aryl groups to increase intermolecular face-on π- π stacking interactions, which are desirable for electron transport. Complexes with fluorine on the distal phenyls (Zn(1F-L2)₂, on the naphthyls (Zn(2F-L2)₂) and on both (Zn(3F-L2)₂ were synthesized and characterized. All three complexes had similar optical and electrochemical properties. Crystal packing of Zn(2F-L2)₂ and Zn(3F-L2)₂ revealed co-facial parallel-displaced π- π stacking between the fluorinated naphthylethynyl groups. Hole mobility increasing from 10^-4 cm² V^-1 s^-1for Zn(L2)₂ to 10^-3 cm² V^-1 s^-1for the fluorinated complexes. Fluorination on the naphthyl groups increased electron mobility from 10^-5 cm² V^-1 s^-1 for Zn(L2)₂ and Zn(1F-L2)₂, to 2× 10^-4 cm² V^-1 s^-1for Zn(2F-L2)₂ and Zn(3F-L2)₂, consistent with co-facial π- π stacking being favorable for electron transport. The three complexes were tested in OPVs using regioregular poly(3-hexylthiophene) (P3HT) as the p-type material, and the best PCE were 5.2%, 5.4% and 5.8% for Zn(2F-L2)₂, Zn(1F-L2)₂ and Zn(3F-L2)₂, respectively, compared to 5.5% for Zn(L2)₂. The fluorination combination found in Zn(3F-L2)₂ resulted in the most favorable blend morphology and device performance. This study points to a viable strategy to increase electron mobility and performance of non-planar zinc complexes of azadipyrromethene.

Jayvic Cristian Jimenez

Graduate Student, Case Western Reserve University

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