5-Alkyloxy-6-fluorobenzo[c][1,2,5]thiadiazole- and Silafluorene-Based D-A Alternating Conjugated Polymers: Synthesis and Application in Polymer Photovoltaic Cells

Three donor acceptor (D-A) alternating conjugated polymers with silafluorene as the donor unit, 5-alkyloxy-6-fluorobenzothiadiazole as the acceptor unit, and thiophene as the spacer has been synthesized and used as donor materials for polymer solar cells (PSCs). The introduction of a fluorine atom on the benzothiadiazole unit can lower the HOMO and LUMO energy level of the resulted polymers to afford higher open circuit voltage 7 (V-oc); whereas the introduction of a flexible alkyloxy chain on benzothiadiazole unit can increase the solubility of the resulted polymers without interfering the close packing of polymer chains in the solid state. High molecular weight polymers P-la, P-1b, and P-1c, which are fully soluble in 1,2-dichorobenzene (DCB) at elevated temperature, have been prepared by Suzuki polycondensation. Among these polymers, P-1c exhibited the highest hole mobility up to 1.36 X 10(-2) cm(2) V-1 s(-1). PSCs based P-1b:PC71BM demonstrated the highest V-oc to 0.98 V.P-1a:PC71BM based PSCs gave the highest power conversion efficiency (PCE) of 6.4196, which is the highest value among solar cells with benzothiadiazole- and silafluorene-containing polymers as the donor material.