Highly efficient near-infrared and semitransparent polymer solar cells based on an ultra-narrow bandgap nonfullerene acceptor

In this work, we design and synthesize an acceptor–donor–acceptor structured non-fullerene acceptor (ACS8) with an ultra-narrow bandgap (1.3 eV). It possesses a well-ordered molecular orientation and π–π stacking, and hence exhibits a high electron mobility of 2.65 × 10−4 cm2 V−1 s−1. Polymer solar cells (PSCs) based on PTB7-Th : ACS8 (1 : 2, w/w) processed using toluene and 0.5% PN (1-phenylnaphthalene) treatment exhibited an optimal power conversion efficiency (PCE) of 13.2% with an open circuit voltage (Voc) of 0.75 V, a short-circuit current density (Jsc) of 25.3 mA cm−2 and a fill factor (FF) of 69.3% under the illumination of AM 1.5G, 100 mW cm−2. This PCE is among the highest values reported in the literature to date for PSCs based on ultra-narrow bandgap acceptors (Eoptg ≤ 1.3 eV). Furthermore, semitransparent devices based on PTB7-Th : ACS8 exhibit an outstanding PCE up to 11.1% with an average visible transmittance of 28.6%.