Small-molecule acceptors with long alkyl chains for high-performance as-cast nonfullerene organic solar cells

Three fused-ring small-molecule electron acceptors, IDTC16-IC, IDTC16-Th, and IDTC16-4F, were designed and synthesized by introducing indacenodithiophene (IDT) as the electron-donating core and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC), fluorinated IC, and a thiophene-based unit as the electron-withdrawing end group. Here, instead of the commonly used n-hexyl or n-hexylphenyl side chains, n-hexadecyl peripheral substituents were employed at the IDT core to study the influence of alkyl groups on photovoltaic performance of the nonfullerene acceptors. The introduction of flexible n-hexadecyl group endowed the three acceptors with excellent solubility in common organic solvents. All the three acceptors presented strong absorption ranging from 450 nm to 720 nm in solution with high molar extinction coefficients. As a result, the as-cast organic solar cells (OSCs) based on IDTC16-IC and the wide bandgap polymer donor PM6 exhibited a power conversion efficiency (PCE) of 5.12%. The OSCs based on PM6:IDTC16-Th and PM6:IDTC16-4F showed much better photovoltaic performance with PCEs of 8.76% and 8.55%, respectively. The PCE values were improved to 5.89%, 9.09%, and 9.42% for the PM6:IDTC16-IC, PM6:IDTC16-Th, and PM6:IDTC16-4F OSCs, respectively, with the addition of the solvent additive 1,8-diiodooctane. These findings demonstrate that the combination of alkyl chains at the fused rings and fluorination or aromatic structure change of the terminal groups leads to greatly enhanced photovoltaic performance of nonfullerene acceptors through improving the photophysical, molecular orbital, and film morphological properties.