Nitrofluorene-based A-D-A electron acceptors for organic photovoltaics

We explored nitrofluorene derivatives as tunable electron-withdrawing groups for acceptor-donor-acceptor (A-D-A) type non-fullerene acceptors. Their optoelectronic properties and performance in bulk heterojunction solar cells with PBDB-T donor polymer are reported. Introducing up to four nitro groups to the fluorene unit results in deeper frontier orbital levels, reduced HOMO-LUMO gap and red-shifted absorption bands. Optimal device performance was achieved for trinitrofluorene acceptors with a solubilizing ester group in the trade-off between decreased voltage and increased photocurrent. 2D brickwork packing of the tetranitrofluorene derivative found by single crystal X-ray analysis is different from the interpenetrating 3D network found for high-efficiency A-D-A acceptors and might be the reason for the poor charge transport properties. Steric hindrance between benzothiadiazole and nitrofluorene units can be relieved by the use of furan linkers, which planarized the conjugated backbone but decreased solubility.