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Tailoring exciton diffusion and domain size in photovoltaic small molecules by processing

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Muhammad Tariq Sajjad, Yiwei Zhang, Paul B. Geraghty, Valerie D. Mitchell, Arvydas Ruseckas, Oskar Blaszczyk, David J Jones, Ifor David William Samuel

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Exciton diffusion is an important part of light harvesting in organic photovoltaics (OPVs) because it enables excitons to reach the interface betweeen donor and acceptor and contribute to the photocurrent. Here we used simple and cost-effective techniques of thermal annealing and solvent vapour annealing to increase the exciton diffusion coefficient and exciton diffusion length in two liquid crystalline electron donor materials BQR and BTR. We found that the three-dimensional exciton diffusion length increased to ~40 nm upon annealing in both materials. Grazing-incidence wide angle X-ray scattering (GIWAXS) measurements show an increase of crystallite size to ~37 nm in both materials after thermal annealing. We determined an average domain size of these materials in the blends with PC71BM using diffusion-limited fluorescence quenching and found that it increased to 31 nm in BTR PC71BM blends and to 60 nm in BQR PC71BM blends. Our results provide understanding of how annealing improves device efficiency.


Original languageEnglish
Number of pages8
JournalJournal of Materials Chemistry C
VolumeIn press
Early online date19 Apr 2019
Publication statusE-pub ahead of print - 19 Apr 2019

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