Design, synthesis and photovoltaic properties of two π-bridged cyclopentadithiophene-based polymers

Two fluorinated D–A type conjugated polymers, PCPDT-DTFBT (P1) and PCPDT-DTDFBT (P2) with an extended π-bridge, were synthesized through the palladium-catalyzed Stille coupling reaction. Both P1 and P2 exhibit a narrow band gap (1.63 eV for P1 and 1.60 eV for P2) and low lying energy level with the highest-occupied molecular orbital (HOMO) of −5.16 and −5.19 eV, respectively. Because of the insertion of the 4-hexylthiophene π-bridge between the donor and acceptor units, P1 and P2 exhibit excellent solubility in common organic solvents. Particularly for P2, the improved solubility was conducive to the film forming ability with a root-mean-square roughness (RMS) value of 3.60 nm and a nanoscale bicontinuous interpenetrating network in the active layer. As a result, a short-circuit current (JSC) of 13.58 mA cm−2, an open circuit voltage (VOC) of 0.70 V, and a fill factor (FF) of 61.6% were obtained, giving a high energy conversion efficiency (PCE) of 5.85% after device optimization.