Azadipyrromethene (ADP, see image) is an aromatic monoanionic bidentate ligand whose core resembles half of a phthalocyanine ligand. Complexes of ADP have strong visible to near-IR absorption, high electron affinity and easy synthesis, making them attractive for a variety of applications. We have demonstrated that homoleptic zinc(II) complexes of ADP with phenylethynyl pyrrolic substituents (Zn(WS3)2, see image), are promising candidates as non-fullerene acceptors for solution-processed organic photovoltaics (OPVs). The complexe’s distorted tetrahedral structure (non-planar) facilitates favorable phase separation from the common conjugated polymer donor, poly(3-hexylthiophene) (P3HT), yielding good performance in OPVs. To further understand their structure-properties and increase OPV performance, we investigated replacing the phenyl in the phenylethynyl pyrrolic substituents with larger aromatic groups such as naphthyl and phenanthrenyl groups. Because these larger aromatic groups reduced solubility in organic solvents, we added hexyl solubilizing groups at the para position of the proximal phenyls. We found that the hexyl and the larger aromatic groups both contribute to improving crystallinity of the zinc(II) complexes, electron mobility and OPV performance. The best performance was obtained with 1-naphthyl substitution, called Zn(L2)2. OPVs using P3HT as the donor gave a high power conversion efficiency (PCE) of 5.5% and the PCE was not very sensitive to the P3HT:Zn(L2)2 weight ratio. Fluorination increased the PCE to 5.8%. Additionally, Zn(L2)2 can be synthesized on the gram scale using inexpensive starting materials without chromatography column purification, increasing its industrial accessibility. Due to its non-planar structure, Zn(L2)2 shows isotropic charge transport. Interestingly, it had high hole mobility, making it a good candidate as donor to be paired with a conjugated polymer acceptor. This combination of properties demonstrates the potential of these types of molecules for optoelectronic devices.

Structure-property studies of non-planar azadipyrromethene-based macromolecules for organic photovoltaic applications

ACS Spring Meeting 2021

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