Crown Ether-Substituted Carbazole Dye for Dye-Sensitized Solar Cells: Controlling the Local Ion Concentration at the TiO₂/Dye/Electrolyte Interface

The conduction band edge potentials ( E CB ) and electron lifetimes (τ) of the TiO 2 electrodes in dye-sensitized solar cells (DSSCs) are affected by ion concentrations (e.g., Li + and I – /I 3 – ) at the TiO 2 /dye/electrolyte interface. To control the local concentrations of these ions in the vicinity of the TiO 2 surface, a novel carbazole-based dye incorporating a 12-crown-4 ether on the carbazole donor (MK-70) was synthesized as a DSSC sensitizer. The interactions between Li + /I – /I 3 – and MK-70 were compared with those between Li + /I – /I 3 – and MK-1, an analogue lacking the crown ether. The crown ether did not affect the E CB level of TiO 2, but it did decrease τ at a high electrolytic Li + concentration. Results suggest that localized Li + ions associated with the crown ethers electrostatically attract surplus I 3 – from the bulk electrolyte even though the crown ethers are located far from the TiO 2 surface. After the cells were aged, negative shifts in the E CB levels of the TiO 2 electrode and blueshifts of the MK-70 absorption spectra were observed with electrolytes that included I – /I 3 – and Li + . The aging behavior may be determined by the balance between two attractive forces, K 1 (I – /I 3 – and Li + at the TiO 2 surface) and K 2 (dye interactions with I – /I 3 – and Li + ).