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

Abstract

The conduction band edge potentials (ECB) and electron lifetimes (τ) of the TiO2 electrodes in dye-sensitized solar cells (DSSCs) are affected by ion concentrations (e.g., Li+ and I–/I3–) at the TiO2/dye/electrolyte interface. To control the local concentrations of these ions in the vicinity of the TiO2 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–/I3– and MK-70 were compared with those between Li+/I–/I3– and MK-1, an analogue lacking the crown ether. The crown ether did not affect the ECB level of TiO2, 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 I3– from the bulk electrolyte even though the crown ethers are located far from the TiO2 surface. After the cells were aged, negative shifts in the ECB levels of the TiO2 electrode and blueshifts of the MK-70 absorption spectra were observed with electrolytes that included I–/I3– and Li+. The aging behavior may be determined by the balance between two attractive forces, K1 (I–/I3– and Li+ at the TiO2 surface) and K2 (dye interactions with I–/I3– and Li+).