Nanowires of oligothiophene-functionalized naphthalene diimides: self assembly, morphology, and all-nanowire bulk heterojunction solar cells

We report the solution-phase self-assembly of nanowires of a series of six oligothiophene-functionalized naphthalene diimides (NDI-nTH and NDI-nT) and their use as electron acceptors in all-nanowire bulk heterojunction organic solar cells. The dimensions and detailed morphology of the nanowires varied greatly with molecular structure of the NDI molecules and the solution concentration and solvent composition. Nanowires of NDI-3TH have typical dimensions of 80-250 nm in width, 2-10 mu m in length, and 14.2-15.3 nm in thickness. Significant red-shift and enhancement of optical absorption in the low energy region (>600 nm) were observed in the nanowire suspension compared with solution and thin film, suggesting an increased supramolecular order in the nanowires. All-nanowire bulk heterojunction solar cells fabricated using NDI-3TH nanowires as electron acceptor and poly(3-hexylthiophene) nanowires as donor had a power conversion efficiency of 1.15%. External quantum efficiency of the photovoltaic cells showed that the n-type organic semiconductor nanowires contributed significantly to light harvesting. Our results represent the first demonstration of all-nanowire organic solar cells and show that non-fullerene n-type organic semiconductor nanowires can be effectively incorporated into bulk heterojunction solar cells.