Introducing Phenyl End Group in The Inner Side Chains of A-Da'd-A Acceptors Enables High-Efficiency Organic Solar Cells Processed with Nonhalogenated Solvent.

Power conversion efficiency (PCE) of organic solar cells (OSCs) has reached the threshold for application. However, the high PCEs are mostly obtained from the small-area OSCs processed by the halogenated solvent, such as chloroform (CF), because the high-performance A-DA'D-A type small molecule acceptors (SMAs) need the CF processing for suitable solubility and rapid solvent evaporation to get appropriate aggregation morphology. Herein, we synthesized two new A-DA'D-A type SMAs of Y6-Ph and L8-Ph by introducing a phenyl end group in the branched alkyl inner side chains of the representative SMAs Y6 and L8-BO respectively, for overcoming the excessive aggregation of SMAs in the long-time film-forming process in the nonhalogenated solvent processing. Firstly, we studied the effect of the processing solvent boiling point on the aggregation property and photovoltaic performance of the four SMAs of Y6, L8-BO, Y6-Ph and L8-Ph, by using the most commonly used halogenated processing solvents: low-boiling-point CF (rapid film forming process) and high-boiling-point chlorobenzene (CB) (slow film forming process) for device fabrication of the OSCs with PM6 as polymer donor. It was found that the Y6- and L8-BO-based OSCs exhibit a dramatic drop in PCEs from CF- to CB-processed devices owing to the large phase separation in the CB-processed OSCs. On the contrary, the devices based on Y6-Ph and L8-Ph with the phenyl ending inner side chains show obviously increased PCEs for the OSCs processed by high-boiling-point CB. The L8-Ph-based devices processed by CB demonstrated an PCE of 18.83% with a high FF of 80.55%, which is even better than the L8-BO-based OSCs processed by CF. Furthermore, the L8-Ph-based OSCs processed by nonhalogenated solvent o-xylene (o-XY) also achieved a high PCE of 18.40% with an FF of 80.11%, which is the best-performed OSC fabricated from the nonhalogenated solvents so far. In addition, the large-area (1 cm ) OSCs based on PM6:L8-Ph processed by o-XY also demonstrated a higher PCE of 16.66%. The results indicate that introducing phenyl end group in the inner side chains is an effective strategy to improve photovoltaic performance of the A-DA'D-A SMAs for future application of the OSCs with nonhalogenated solvent processing. This article is protected by copyright. All rights reserved.