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.