For a sensitizer with a strong π-conjugation system, a coadsorbent is needed to hinder dye aggregation. However, coadsorption always brings a decrease in dye coverage on the TiO2 surface. Organic ‘‘D–A–π–A’’ dyes, WS-6 and WS-11, are designed and synthesized based on the known WS-2 material for coadsorbent-free, dye-sensitized solar cells (DSSCs). Compared with the traditional D–π–A structure, these D–A–π–A indoline dyes, with the additional incorporated acceptor unit of benzothiadiazole in the π-conjugation, exhibit a broad photoresponse, high redox stability, and convenient energy-level tuning. The attached n-hexyl chains in both dyes are effective to suppress charge recombination, resulting in a decreased dark current and enhanced open-circuit voltage. Electrochemical impedance spectroscopy studies indicate that both the resistance for charge recombination and the electron lifetime are increased after the introduction of alkyl chains to the dye molecules. Without deoxycholic acid coadsorption, the power-conversion efficiency of WS-6 (7.76%) on a 16 μm-thick TiO2 film device is 45% higher than that of WS-2 (5.31%) under the same conditions. The additional n-hexylthiophene in WS-11 extends the photoresponse to a panchromatic spectrum but causes a low incident photon-to-current conversion efficiency.
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