Chlorophyll-a Derivatives with Various Hydrocarbon Ester Groups for Efficient Dye-Sensitized Solar Cells: Static and Ultrafast Evaluations on Electron Injection and Charge Collection Processes

Five chlorophyll-a derivatives, chlorins-1-5 possessing C3(2)-carboxy and 017(4)-esterified hydrocarbon groups including methyl, hexyl, dodecyl, 2-butyloctyl, and cholesteryl were synthesized. Their performance as sensitizers in dye-sensitized solar cells (DSSCs) was compared. These sensitizers have similar surface coverage on the unit surface of TiO2 film and their absorption spectra on transparent TiO2 films were identical. On the basis of DFT and TD-DFT calculations of these sensitizers in ethanol, a major difference between them was the geometry of the hydrocarbon ester group, to affect their electron injection and charge recombination with the TiO2 electrode rather than the energy level of their molecular orbitals. DSSC based on chlorin-3 with a dodecyl ester group gave a solar energy-to-electricity conversion efficiency of 8%, which was the highest among all the chlorophyllous sensitizers. The large photocurrent in the chlorin-3 sensitized solar cell can be explained by the least impedance in the electrolyte-dye-TiO2 interface in electrical impedance spectroscopy measurements. Subpicosecond time-resolved absorption spectroscopic studies have also been carried out to evaluate the electron injection and charge recombination dynamics in the dye-TiO2 interface. For the electron injection and charge recombination processes, a charge separated state of the dye-TiO2 complex has been found to be free from the type and concentration of dye sensitizer, reflecting the same type of electron transfer process for all the five chlorin sensitizers. A new quenching pathway of the dye excitation, which is probably from the exciton annihilation, in addition of the charge recombination has been observed for chlorin-1 and chlorin-5, but not for chlorin-3. The higher open-circuit photocurrent observed in the present dyes with larger ester groups can be attributed to the reduced leaking of charges in the TiO2-electrolyte interface, which was supported by the longer electron lifetimes.