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

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-pi-A dyes, including a fluorene core in the pi-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 mu m-thick transparent TiO2 and iodine-based electrolyte). The dyes were afterwards employed in thin-film devices (2 mu m-thick transparent TiO2) and tested in the presence of three different electrolytes including I-3(-)/I- [co(II)(bpy)(3)(PF6)(2)]/[Co-III(bPY)(3)(PF6)(3)] or [Co-II(bpy-pz)(2)(PF6)(2)]/[Co-III(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 V-OC above 800 mV, even reaching as high as 919 mV, thus compensating the slight decrease in photo-current arising from the low-mobility of the Co-complexes. (C) 2015 Elsevier Ltd. All rights reserved.