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

NIR-Absorbing Merocyanine Dyes for BHJ Solar Cells

We have synthesized a series of new, polymethine chain extended merocyanine dyes 1-4 bearing varied acceptor units and an aminothiophene donor moiety. The optical and electronic properties of these new merocyanines have been studied in comparison with their corresponding lower homologues 5-8, which contain two methine groups less, by UV-vis and electro-optical absorption (EOA) spectroscopy and cyclic voltammetry. The absorption spectra of pi-extended merocyanines are markedly red-shifted, and their extinction coefficients are significantly increased compared to those of their lower homologues. The photovoltaic characteristics of these dyes have been explored in devices using them as donor and PC61BM fullerene as acceptor materials. Our detailed studies reveal that, despite more favorable absorption properties, the it-extended merocyanines exhibit lower short-circuit current densities (J(SC)) as well as decreased open-circuit voltages (V-OC) and power conversion efficiencies (PCE) compared with those of their respective lower homologues. The unexpected decreased J(SC) values could be explained in terms of looser packing features of pi-extended chromophores in the solid state as revealed by single-crystal X-ray analysis of two pairs (1/S and 4/8) of these dyes. By optimization of device setup PCE of 2.3% has been achieved with the Jr-extended donor material 4