Ambipolar-transporting coaxial nanotubes with a tailored molecular graphene-fullerene heterojunction

Despite a large steric bulk of C₆₀, a molecular graphene with a covalently linked C₆₀ pendant [hexabenzocoronene (HBC)-C₆₀; 1] self-assembles into a coaxial nanotube whose wall consists of a graphite-like π-stacked HBC array, whereas the nanotube surface is fully covered by a molecular layer of clustering C₆₀. Because of this explicit coaxial configuration, the nanotube exhibits an ambipolar character in the field-effect transistor output [hole mobility (μh) = 9.7 x 10⁻⁷ cm² V⁻¹ s⁻¹; electron mobility (μe) = 1.1 x 10⁻⁵ cm² V⁻¹ s⁻¹] and displays a photovoltaic response upon light illumination. Successful coassembly of 1 and an HBC derivative without C₆₀ (2) allows for tailoring the p/n heterojunction in the nanotube, so that its ambipolar carrier transport property can be optimized for enhancing the open-circuit voltage in the photovoltaic output. As evaluated by an electrodeless method called flash-photolysis time-resolved microwave conductivity technique, the intratubular hole mobility (2.0 cm² V⁻¹ s⁻¹) of a coassembled nanotube containing 10 mol % of HBC-C₆₀ (1) is as large as the intersheet mobility in graphite. The homotropic nanotube of 2 blended with a soluble C₆₀ derivative [(6,6)-phenyl C₆₁ butyric acid methyl ester] displayed a photovoltaic response with a much different composition dependency, where the largest open-circuit voltage attained was obviously lower than that realized by the coassembly of 1 and 2.