A novel naphthyl side-chained benzodithiophene polymer for efficient photovoltaic cells with a high fill factor of 75%

To study which strategy (extending side chain π-conjugation with single-bonded or fused aromatic rings) is more effective in improving the photovoltaic properties of benzodithiophene (BDT)-based polymers, naphthyl (NP) and biphenyl (BP) units were introduced to the BDT core as conjugated side chains. Two polymers PBDTNP-DTBO and PBDTBP-DTBO based on NP or BP-substituted BDT and 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]oxadiazole (DTBO) were designed. These two polymers only exhibited small differences in the chemical structure, but great differences in photovoltaic performance. PBDTNP-DTBO showed a high power conversion efficiency (PCE) of 8.79%, with an open-circuit voltage (Voc) of 0.82 V, a short-circuit current density (Jsc) of 14.34 mA cm−2, and a fill factor (FF) of 74.73%, while PBDTBP-DTBO only showed a maximum PCE of 6.69%, with a Voc of 0.75 V, a Jsc of 13.55 mA cm−2, and a FF of 65.86%. Hence, extending the side chain π-conjugation system of a BDT-based polymer with naphthyl is a more effective method to enhance the photovoltaic properties.