Fullerene-linked tetrabenzoporphyrins for solution-processed organic photovoltaics: flexible vs. rigid linkers

Abstract

Solution processing of a layer-by-layer structure is possible with a precursor approach, in which the soluble precursors of the insoluble or low-solubility target materials are spin-coated on the substrate and converted to insoluble semiconducting films by heating. We attempted to construct a p–i–n structure (p: p-layer; i: interlayer, a mixture of p- and n-materials; n: n-layer) using a solution process. To increase the donor–acceptor interface area, we synthesized covalently linked donor–acceptor compounds, tetrabenzoporphyrin (BP)–fullerene (C₆₀) dyad. To investigate the effect of the covalent connection of the donor and acceptor units on the film morphology and on the OPV performances, the BP and fullerene units were connected by a flexible or rigid linker (f-BP–C₆₀ or r-BP–C₆₀). To synthesize these dyads, new synthetic routes with mono-meso-substituted BPs using [2 + 2] and [2 + 1 + 1] condensation reactions were performed. The intramolecular fluorescence quenching efficiency of BP by the C₆₀ unit of f-BP–C₆₀ was higher than that of r-BP–C₆₀ in CH₂Cl₂ because of the flexibility of the linker and the close location of the C₆₀ unit to the BP unit. The carrier mobility in the r-BP–C₆₀ film measured by the space-charge-limited-current (SCLC) technique was better than that of the f-BP–C₆₀ film, and the bulk heterojunction (BHJ)-type OPV performance of the r-BP–C₆₀ film was better than that of the f-BP–C₆₀ film. In spite of the superior OPV performance of the r-BP–C₆₀ device compared to the f-BP–C₆₀, the short circuit current density (J_SC) and FF of the p–i–n device with BP, f-BP–C₆₀, and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) for p-, i- and n-layers was better than the device with r-BP–C₆₀ as the i-layer. AFM images of the layers and OPV performances suggested that the flexible f-BP–C₆₀ is suitable for the i-layer on the highly crystalline BP film because the roughness of the BP film is leveled by the f-BP–C₆₀, and the electric properties of the device are improved by the smooth interface.