Unravel the Impact of Anchoring Groups on the Photovoltaic Performances of Diketopyrrolopyrrole Sensitizers for Dye-Sensitized Solar Cells

Two D-pi-A dyes, one with a strong electron withdrawing cyanoacrylic acid DPP 20 and one with a weak acceptor carboxylic acid DPP 21 have been synthesized and characterized for their application in dye-sensitized solar cells. This allows us to understand the role of electron withdrawing strength of the acceptor anchoring groups on the optoelectronic properties of chromophoric pi spacer molecules, e.g., the diketopyrrolopyrrole (DPP) moiety. The low energy absorption maxima of DPP 20 is red-shifted by only 7 nm compared to DPP 21, implying a minimal role for the strong acceptor in the light harvesting properties of colored pi spacer molecules. Because of this small red shift, J(SC) of DPP 20 is marginally higher than that of DPP 21. However, because of the higher fill factor of DPP 21, the overall power conversion efficiency (PCE = 7.65%) is higher than that of DPP20 (7.34%). The data reveal that a weak acceptor is adequate to achieve good light harvesting as well as provide good photovoltaic efficiencies for colored pi spacer sensitizers.