Comparative Studies on Optical, Redox, and Photovoltaic Properties of a Series of D-A-D and Analogous D-A Chromophores

A series of new symmetrical donor-acceptor-donor (D-A-D) dyes based on s-indacene-1,3,5,7(2H,6H)-tetraone as an acceptor unit containing varying electron donating moieties and analogous donor-acceptor (D-A) chromophores with indane-1,3-dione as an acceptor are synthesized. By employing these two sets of dyes, the influence of a scaffold change from unsymmetric push-pull (D-A) to symmetrical (D-A-D) systems on optical, electrochemical, and photovoltaic properties are explored. Detailed comparative studies reveal favorable optical characteristics and considerably decreased bandgaps for the D-A-D dyes compared to those of the reference D-A chromophores. Accordingly, the evaluation of the present dyes as donor materials in bulk heterojunction (BHJ) solar cells in combination with fullerene derivatives PC61BM or PC71BM as acceptors afforded significantly improved performance for devices based on D-A-D blends (up to a factor of 4 compared to the respective D-A reference) with power conversion efficiencies of up to 2.8%. In less polar solvents such as toluene, some of the novel D-A-D chromophores exhibit unexpectedly high fluorescence quantum yields phi(em) of up to unity, in striking contrast to their weakly fluorescent D-A counterparts.