Ring-Expanded Phthalocyanines Fused with Fluorene Skeleton for Light-Harvesting in the Near-IR Region

Organic photovoltaic devices (OPVs) based on thin films of organic semiconductors have been intensely investigated as a promising candidate for low-cost, lightweight, and scalable solar cells. While OPVs have achieved high conversion efficiencies, their organic semiconductors mainly absorb the light in visible range. To enhance their efficiency, the expansion of light-harvesting range is a key issue in this field.

Phthalocyanines (Pcs) and their metal complexes (MPcs), synthetic porphyrin analogues, have been widely investigated as near-IR absorbing dyes because of their intense Q band (600-700 nm), high molar extinction coefficients (e > 100000 M^-1 cm^-1), and excellent thermal, chemical, and photolytic stabilities. The electronic structure and properties of MPcs can be tailored by modifying the macrocyclic ligands and changing the central metals [1].

We present here designed and synthesized novel ring-expanded zinc phthalocyanine (ZnFc , Figure1), in which four fluorene rings were fused with porphyrazine ring. Fusing fluorene rings with the porphyrazine rings lead to red-shifting the Q band and tailoring of the HOMO and LUMO energy levels as well as an expansion of p-system. We demonstrated the expansion of the light-harvesting area in BHJ solar cells based on poly(3-hexylthiophen) (P3HT) and 1-(3-methoxycalbonyl)-propyl-1-phenyl-(6,6)C₆₁ (PCBM) by co-sensitizing with ZnFc. When the ZnFc decorated with eight alkyl chains was added to P3HT/PCBM BHJ solar cells, the solar cells showed a higher device efficiency with 16 % enhancement of J_SC as compared with that of the reference P3HT/PCBM BHJ solar cells due to the expansion of light-harvesting range [2].

We also synthesized an asymmetrical fluorene-fused phthalocyanine (FcS1, Figure1), which containing three fused fluorene rings and one carboxylic acid. FcS1 indicated a potential for near-IR sensitizer in DSSCs. The absorption spectrum of FcS1 adsorbed onto a nano-crystalline TiO₂ film suggested the formation of J-aggregation with tilted orientation of macrocyclic ligands. While the DSSC cells sensitized by FcS1 showed a modest efficiency of 3.2 % under simulated air mass 1.5 global sunlight, FcS1 could convert the red and near-IR light regions between 600 and 860 nm [3].

Reference

[1] J. Mack, N. Kobayashi, Chem. Rev. 2011, 111, 281-321.

[2] S. Yamamoto, M. Kimura, ACS Appl. Mater. Interfaces 2013, 5, 4367-4373.

[3] S. Yamamoto, T. Ikeuchi, S. Mori, M. Kimura, Asian J. Org. Chem. 2014, 10, 1083-1088.