Azadipyrromethene-Based Materials: From Synthesis to Electronic and Photovoltaic Properties
Authors: Sauve Genevieve 
Published: May 2020

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. The phenylethynyl groups also contribute to increase the electron affinity of the complex. To further increase electron affinity, we installed electronegative fluorine at various positions of the ADP ligands. To our surprise, the electrochemical properties are relatively unaffected by the substitutions, but the electron mobility and performance in OPVs improves. On the other hand, substitution with nitrile groups anodically shifts the redox potentials and thus increases electronegativity. The substitutions also improve electron mobility in diodes as compared to the un-substituted analogue. However, OPV performance is poor, possibly due to unfavorable blend morphology with P3HT, demonstrating the difficulties in optimizing all properties simultaneously.
To further enhance charge transport and 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, Zn(L2)2 shown in the image. 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. 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 for OPVs. This combination of properties demonstrates the potential of these types of molecules for optoelectronic devices.

Selected references:

Z. Mao; W. Senevirathna; J. Y. Liao; J. Gu; S. V. Kesava; C. Guo; E. D. Gomez; G. Sauvé, “Azadipyrromethene-Based Zn(II) Complexes as Nonplanar Conjugated Electron Acceptors for Organic Photovoltaics.” Adv. Mater.
26 (2014), 6290-6294.

S. Pejić; A. M. Thomsen; F. S. Etheridge; R. Fernando; C. Wang; G. Sauvé, “Fluorination Increases the Electron Mobility of Zinc Azadipyrromethene-Based Electron Acceptors and Enhances the Performance of Fullerene-Free Organic Solar Cells.” J. Mater. Chem. C
6 (2018), 3990-3998.

G. Sauvé, “Designing Alternative Non-Fullerene Molecular Electron Acceptors for Solution-Processable Organic Photovoltaics.” Chem. Rec.
19 (2019), 1-16.

C. Wang; P. Wei; J. H. L. Ngai; A. L. Rheingold; T. G. Gray; Y. Li; E. Pentzer; R. Li; L. Zhu; G. Sauvé, “A Zinc(Ii) Complex of Di(Naphthylethynyl)Azadipyrromethene with Low Synthetic Complexity Leads to OPV with High Industrial Accessibility.” J. Mater. Chem. A
7 (2019), 24614-24625.

C. Wang, C. Daddario, S. Pejić, G. Sauvé, “Synthesis and Properties of Azadipyrromethene-based Complexes with Nitrile Substitution” Submitted to Eur. JOC.

Figure 1