Supramolecular Hemicage Cobalt Mediators for Dye‐Sensitized Solar Cells

A new class of dye‐sensitized solar cells (DSSCs) using the hemicage cobalt‐based mediator [Co(ttb)] 2+/3+ with the highly preorganized hexadentate ligand 5,5′′,5′′′′‐((2,4,6‐triethyl benzene‐1,3,5‐triyl)tris(ethane‐2,1‐diyl))tri‐2,2′‐bipyridine (ttb) has been fully investigated. The performances of DSSCs sensitized with organic D –π–A dyes utilizing either [Co(ttb)] 2+/3+ or the conventional [Co(bpy) 3 ] 2+/3+ (bpy=2,2′‐bipyridine) redox mediator are comparable under 1000 W m −2 AM 1.5 G illumination. However, the hemicage complexes exhibit exceptional stability under thermal and light stress. In particular, a 120‐hour continuous light illumination stability test for DSSCs using [Co(ttb)] 2+/3+ resulted in a 10 % increase in the performance, whereas a 40 % decrease in performance was found for [Co(bpy) 3 ] 2+/3+ electrolyte‐based DSSCs under the same conditions. These results demonstrate the great promise of [Co(ttb)] 2+/3+ complexes as redox mediators for efficient, cost‐effective, large‐scale DSSC devices.