Large Crystalline Domains and an Enhanced Exciton Diffusion Length Enable Efficient Organic Solar Cells

We studied crystallinity and exciton harvesting in bulk heterojunctions of the semiconducting polymer PffBT4T-2OD and electron acceptor PC71BM that are used to make highly efficient organic solar cells. Grazing incidence wide-angle X-ray scattering shows that the size of crystalline domains of PffBT4T-2OD increases to ∼18 nm in photovoltaic blends upon thermal annealing at 100 °C for 5 min. These domains are larger than the typical exciton diffusion lengths in conjugated polymers. Time-resolved fluorescence measurements show that the exciton diffusion length in PffBT4T-2OD increases from ∼14 to ∼24 nm upon thermal annealing, which enables efficient charge generation in blends with large domains. Solar cells prepared using thermally annealed blends show higher photocurrents, open circuit voltages, and fill factors compared to unannealed blends, which indicates reduced recombination losses. Our results demonstrate the advantages of large crystalline domains in organic photovoltaics, providing exciton diffusion is sufficient.