Enhancing performance of non-fullerene organic solar cells via side chain engineering of fused-ring electron acceptors

A series of fused-ring electron acceptors with indacenodithiophene as core, 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile as end groups, thiophene with different side chains as π bridge are designed, synthesized and applied in non-fullerene organic solar cells (OSCs). The effects of alkyl side chains on absorption, energy level, molecular packing, domain size, domain purity, charge transport and photovoltaic performance are systematically investigated. Via the steric hindrance effect, increase in carbon number of side chains slightly downshifts HOMO energy levels, upshifts LUMO energy levels and thereby increases open circuit voltage from 0.84 to 0.95 V gradually. Short circuit current, fill factor and efficiency vary non-monotonically with aliphatic carbon number of alkyl chains and reach maximum value simultaneously when carbon number is six.