Naphtho[1,2-b:5,6-b′]dithiophene-based copolymers for applications to polymer solar cells

Two naphtho[1,2-b:5,6-b′]dithiophene-based copolymers, poly[4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b′]dithiophene-2, 7-diyl-alt-[4,6-{(1-thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one}] (P1) and poly[4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b′]dithiophene-2, 7-diyl-alt-[4,6-{(1-(3-fluorothieno[3,4-b]thiophen)-2-yl)-2-ethylhexan-1-one}] (P2), were designed and synthesized for use in polymer solar cells (PSCs). We systematically investigated the synthesis, thermal stability, optical and electrochemical properties, field-effect carrier mobilities, and photovoltaic characteristics of these polymers. The field-effect mobilities of P1 and P2 were determined to be 5.8 × 10^-4 and 1.2 × 10^-3 cm² V^-1 s^-1, respectively. Among the naphtho[1,2-b:5,6-b′]dithiophene-based copolymers studied here, P2 showed the best photovoltaic performance, with an open-circuit voltage (V_oc) of 0.71 V, a short-circuit current density (J_sc) of 13.52 mA cm ^-2, a fill factor (FF) of 0.51, and a power conversion efficiency of 4.88% under AM 1.5 irradiation (100 mW cm^-2).