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-1 for Zn(L2)2 to 10-3 cm2 V-1 s-1 for 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-1 for 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.

Monofluorination of the naphthyls promotes co-facial π- π stacking and increase electron mobility of non-planar zinc(II) di(naphthylethynyl)aza-dipyrromethene non-fullerene electron acceptors

ACS Spring 2022

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