Long-range Exciton Diffusion in Non-Fullerene Acceptors and Coarse Bulk Heterojunctions Enable Highly Efficient Organic Photovoltaics

We investigated singlet exciton diffusion in three ITIC derivatives used as electron acceptors in organic photovoltaics (OPVs) by time-resolved photoluminescence volume quenching and exciton-exciton annihilation. The exciton diffusion coefficient was found to be in the range of 0.003-0.007 cm2s-1 which leads to a three-dimensional exciton diffusion length of up to 31 nm. We fabricated solar cells using bulk heterojunctions of these acceptors with the electron donor PTB7-Th which showed ~7% efficiency and large scale phase separation. Our results show that long range exciton diffusion and large domains offer significant advantages for OPVs.