Design of Bodipy based organic dyes for high-efficient dye-sensitized solar cells employing double electron acceptors

• Bodipy-furan as the π-conjugated bridge and double cyanoacetic acid units as electron acceptor are favorable to improve the efficiency of DSSCs.
• The best reported conversion efficiency of 4.52% was achieved with the DSSCs based on UY11.
• EIS showed that the injected electron lifetime was enhanced for UY11-based cell with D–(–π–π–A)2 framework.

Three 2,6-conjugated Bodipy metal-free organic dyes, UY10, UY11, and UY1 with D–(–π–A)2, D–(–π–π–A)2, and D–π–A frameworks, have been synthesized which contain a rigid alkyl-functionalized phenothiazine (PTZ) core as the donor part and one or double cyanoacetic acid units as acceptor and anchoring part. Their photophysical and electrochemical properties as well as theoretical computation have been extensively investigated. Nanocrystalline TiO2-based dye sensitized solar cells (DSSCs) are fabricated using these dye molecules as light-harvesting sensitizers. Among these dyes, the dye UY11 with D–(–π–π–A)2 framework exhibits the best photovoltaic performance with a short-circuit photocurrent density (Jsc) of 11.82 mA cm−2, an open-circuit photovoltage (Voc) of 548 mV and a fill factor (ff) of 0.70, corresponding to an overall conversion efficiency (η) of 4.52% under AM 1.5 irradiation (100 mW cm−2). The structure-property relationship shows that both conjugated bridge and acceptor play key roles in increasing the efficiency of DSSCs. Electron-rich furan and two cyanoacetic acid-based dye appear to enhance light harvesting capacity and help convey the charge transfer from the excited dye molecules to the conduction band of TiO2, leading to a higher efficiency of the assembled devices using such a dye. Electrochemical impedance measurements also support the effect on enhancing charge transfer of TiO2 for dye UY11.

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