Structure–Property Study of Homoleptic Zinc(II) Complexes of Di(arylethynyl) Azadipyrromethene as Nonfullerene Acceptors for Organic Photovoltaics: Effect of the Aryl Group

Azadipyrromethene-based zinc­(II) complexes were demonstrated to be promising molecular organic semiconductors for electronic applications due to their easy preparation, tunable structures, and high electron affinity. The first successful such complex incorporated phenylethynyl groups at the pyrrolic positions, which red-shifted the absorption spectra of zinc­(II) bis­(tetraphenyl azadipyrromethene) and improved the morphology in blends with poly­(3-hexylthiophene) (P3HT). We recently discovered that replacing the phenyl group in the pyrrolic positions with the larger 1-naphthyl group [Zn­(L2)2] increases the crystallinity and improves the organic photovoltaic (OPV) performance. In this work, two more aryl groups were explored to further investigate the relationship between the aryl groups in the pyrrolic position and electronic properties: naphthyl with a different anchoring site, 2-naphthyl [Zn­(L3)2], and a larger aryl group, 9-phenanthrenyl [Zn­(L4)2]. The larger aryl group slightly improved the absorptivity, red-shifted the absorption spectra, and led to different packing modes in crystals with most intermolecular π–π stacking interactions being of T-shaped-type involving the pyrrolic aryl group of one complex. Of the series, 1-naphthyl gave the highest crystallinity. The organic photovoltaic (OPV) power conversion efficiency (PCE) of Zn­(L3)2 and Zn­(L4)2 when blended with P3HT was 3.7 and 3.4%, respectively, both lower than that of Zn­(L2)2 (PCE of 5.5%) due to the higher trap-assisted recombination and less favorable morphology. The charge carrier mobility in these complexes was also relatively low, also limiting the performance. Single-point energy calculations point to low overlap integrals as a cause for the low mobility. The aryl group anchoring position and size, therefore, have a large effect on the properties in these systems, but do not appear to significantly enhance intermolecular interactions.