Synthesis and photovoltaic application of donor–acceptor type copolymers containing 9-cyanomethylenefluorene derivatives with chemically tunable electronic properties

Four 9-cyanomethylenefluorene derivatives were synthesized as a new class of electron-deficient building blocks for donor–acceptor (D–A) type alternating copolymers. UV–vis absorption spectroscopy, cyclic voltammetry and density functional theory calculations show that the substituent X at the cyanomethylene unit, depending on its electron-withdrawing ability (X = –H, –CONMe2, –COOiBu and –CN), can continuously control the LUMO energy level of the monomer. D–A type copolymers were synthesized using the derivatives and an oligothiophene as the electron-donating block. The photovoltaic performance of D–A copolymer:PCBM bulk heterojunction devices shows the importance of LUMO level alignment to achieve efficient charge separation at the donor/acceptor interface. After optimization, the power conversion efficiency of the device with X = –H reached 1.14% with VOC of 0.61 V, JSC of 3.24 mA cm−2 and FF of 0.58 under the irradiation of AM1.5 simulated solar light. The electronic properties of the copolymers correlated well with the electron-withdrawing ability of the monomers while the structure of the π-conjugated backbone was preserved, demonstrating the advantage of the chemically tunable monomer design.