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

Abstract

Five chlorophyll-a derivatives, chlorins-1-5 possessing C3(2)-carboxy and O17(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 TiO(2) film and their absorption spectra on transparent TiO(2) 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 TiO(2) 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-TiO(2) 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-TiO(2) interface. For the electron injection and charge recombination processes, a charge separated state of the dye-TiO(2) 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 TiO(2)-electrolyte interface, which was supported by the longer electron lifetimes.

Triazoloisoquinoline-based dual functional dyestuff for dye-sensitized solar cells

“…This indicates that dyestuff 5 has considerable superiority at 1-1 molar ratio, with some N719 molecules having lost their positions on TiO 2 and being replaced by dyestuff 5. This increases the opportunities for p-p stacking of organic dyestuff 5, and decreases the contribution of N719 to the incident photon-toelectron conversion efficiency [20,21]. This is the reason why device B has lower conversion efficiency than device C in Table 2.…”

Investigating New Materials and Architectures for Grätzel Cells

“…The structure of YD-2 is shown in figure 5. Moreover, chlorophyll itself is relatively simple to modify, which allows researchers to create highly biomimetic devices while manipulating properties such as excited-state lifetime and LUMO energy level as well as enhance adsorption to TiO 2 (Wang et al, 2010b). Chlorophyll (Chl) comes in several distinct forms, most commonly Chl a, Chl b, and Chl c; the latter two are typically found as accessory pigments in light-harvesting complexes, which serve to absorb additional solar radiation and transport excited electrons to the reaction center, which contains Chl a.…”

“…DSSCs with chlorin-3 obtained a conversion efficiency of 8% with an AcCN redox electrolyte HTL, the highest efficiency for any chlorophyllous sensitizer to date. (Wang et al, 2010b). www.intechopen.com…”

Chlorophylls and xanthophylls of crop plants as dyes for Dye-Sensitized Solar Cells (DSSC)

“…Nevertheless the presence of alkyl groups in the molecule hinders the ef icient binding of these pigments on the nanoporous TiO 2 surface [9]. Therefore, most of the literature reports in this ield are concerning the use of pure or modi ied chl molecules [7,10,11]. Beside chl, carotenoids are terpenoid pigments that adsorb light for photosynthesis and protect the photosystems from photo-oxidation.…”