Extended pi-Bridge in Organic Dye-Sensitized Solar Cells: the Longer, the Better?

The elongation of pi-conjugated bridges between the donor (D) and the acceptor (A) represents a feasible strategy towards enhancement of light-harvesting in both breadth and depth of organic D-pi-A dyes suitable for nanocrystalline TiO2-based dye-sensitized solar cells (DSSCs). Here, a series of organic dyes with elongating conjugated bridges is synthesized and characterized. DSSC devices employing a cobalt (II/III) redox electrolyte are fabricated using these dyes as light-harvesting sensitizers. Compared to a dye with the 3,4-ethylenedioxythiophene (EDOT) linker (G188), the three counter-parts with further extended pi-bridges present gradually red-shifted electronic absorption spectra and a persistent decrease in oxidation potential. The photocurrent action spectra show that the extension of pi-conjugated bridges decreases the open-circuit photovoltage. The best performance is shown in G268 with a short-circuit photocurrent density (J(sc)) of 16.27 mA cm(2), an open-circuit photovoltage (V-oc) of 0.83 V, and a fill factor (FF) of 0.67, corresponding to an overall conversion efficiency of 9.24%. Unexpectedly, G270, which has with the longest pi-bridge, showed the lowest J(sc), V-oc, and efficiency.