Molecular engineering of carbazole-fluorene sensitizers for high open-circuit voltage DSSCs: Synthesis and performance comparison with iodine and cobalt electrolytes

• Novel organic dyes for DSSCs were designed through rational molecular engineering.
• The original carbazole-fluorene D–π–A dyes present strong light-harvesting capacity.
• The dyes were tested in DSSCs using different TiO2 thicknesses and electrolytes.
• The dyes yielded up to 6.5% PCE in standard devices with iodine-based electrolyte.
• Remarkably high open-circuit voltages were reached with Co(bpy-pz)2 redox mediator.

A rational molecular engineering strategy was set up to prepare an original series of efficient carbazole-based sensitizers for dye-sensitized solar cells. The new D–π–A dyes, including a fluorene core in the π-bridge, auxiliary thienyl groups on the donor and multiple anti-stacking chains, showed particularly favorable optoelectronic properties for DSSC application. Accordingly, the new dyes achieved up to 6.5% power conversion efficiency in standard devices (7.5 μm-thick transparent TiO2 and iodine-based electrolyte). The dyes were afterwards employed in thin-film devices (2 μm-thick transparent TiO2) and tested in the presence of three different electrolytes including I3−/I−, [CoII(bpy)3(PF6)2]/[CoIII(bpy)3(PF6)3] or [CoII(bpy-pz)2(PF6)2]/[CoIII(bpy-pz)2(PF6)3] as redox mediators. Overall performance under these conditions was around 4% PCE, whatever the electrolyte. However, remarkable open-circuit voltages were observed with cobalt-based electrolytes. In particular, with [Co(bpy-pz)2] the three dyes afforded VOC above 800 mV, even reaching as high as 919 mV, thus compensating the slight decrease in photocurrent arising from the low-mobility of the Co-complexes.

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