Polymerizing Ladder-type Heteroheptacene-Cored Small-Molecule Acceptors for Efficient All-Polymer Solar Cells

One important subject in the field of all-polymer solar cells (all-PSCs) is the exploration of electron-deficient building blocks with optimized physicochemical properties to promote the performance of polymer acceptors. Here, two ladder-type heteroheptacene-containing small-molecule acceptors with branched 2-octyldodecyl or 2-hexyldecyl side-chains are synthesized and polymerized with the thiophene co-monomer to afford polymer acceptors (PW-OD and PW-HD) with strong near-infrared absorption. Experimental results reveal that the alkyl chain length has a large impact on the molecular packing behavior of the resulting polymers, which in turn affects their light-absorbing and charge transport properties, and thus the photovoltaic performance of the final devices. When blended with the polymer donor PM6, PW-HD-based all-PSCs deliver a higher power conversion efficiency (PCE) of 9.12% compared to the PCE of 6.47% for the PW-OD-based all-PSCs, mainly due to its more ordered inter-chain packing and more favorable blend morphology. This work provides a promising building block for the development of high-performance narrow-bandgap polymer acceptors and highlights the importance of side-chain substitution in optimizing the photovoltaic performance of polymer acceptors.