Boosting the photovoltaic performance of ladder-type heteroheptacene-based nonfullerene acceptors by incorporating auxochromic groups in the electron-rich core

Nonfullerene acceptors (NFAs) that efficiently harvest sunlight and transport the consequent charge are key to advancing the performance of polymer solar cells (PSCs). Herein we demonstrate that the NFA with alkylthio side-chains incorporated in the heteroheptacene core (MS1) shows a significantly increased molar extinction coefficient and enhanced molecular ordering in comparison with the counterpart with alkyl side-chains (MC1). Therefore, in combination with a benchmark donor polymer (PM6), the resulting MS1-based PSC shows an enhanced power conversion efficiency (PCE) of 14.13% with a fill factor (FF) of 71.66% and a short-circuit current density (Jsc) of 21.70 mA cm-2 comparing with the MC1-based PSC which shows a PCE of 11.87% with a FF of 62.55% and a Jsc of 20.39 mA cm-2. The enhanced photovoltaic performance for the MS1-based PSC can be in part attributed to the increased light-harvesting ability of the MS1-based active layer and in part attributed to its boosted carrier mobilities and reduced charge recombination. Moreover, with an electrode optimization, the best-performing MS1-based PSC shows a further enhanced PCE of 15.01% which is an impressive efficiency among all acceptor-donor-acceptor type NFAs reported to date. This work suggests the important role of the auxochromic groups in the electron-rich core in determining the photovoltaic performance of NFAs.