Porphyrin-Sensitized Solar Cells: Effect of Carboxyl Anchor Group Orientation on the Cell Performance

Abstract

The effect of the orientation of the porphyrin sensitizer onto the TiO2 surface on the performance of dye-sensitized solar cells (DSSCs) is reported. Free-base and zinc porphyrins bearing a carboxyl anchoring group at the para, meta, or ortho positions of one of the meso-phenyl rings were synthesized for application in Grätzel-type photoelectrochemical cells. The remainder of the meso-phenyl rings was substituted with alkyl chains of different length to visualize any aggregation effects. Absorption and fluorescence studies were performed to characterize and observe spectral coverage of the thirteen newly synthesized porphyrin derivatives. Photoelectrochemical studies were performed after immobilization of porphyrins onto nanocrystalline TiO2 and compared with DSSC constructed using N719 dye as reference. The performance of DSSCs with the porphyrin anchoring at the para or meta position were found to greatly exceed those with the anchoring group in the ortho position. Additionally, cells constructed using zinc porphyrin derivatives outperformed the free-base porphyrin analogs. Better dye regeneration efficiency for the zinc porphyrin derivatives compared to their free-base porphyrin analogs, and for the meta and para derivatives over the ortho derivatives was evaluated from electrochemical impedance spectroscopy studies. Femtosecond transient absorption spectroscopy studies were performed to probe the kinetics of charge injection and charge recombination with respect to the orientation of porphyrin macrocycle on TiO2 surface. The ortho porphyrin derivative with an almost flat orientation to the TiO2 surface revealed fast charge recombination and suggested occurrence of through-space charge transfer. The overall structure-performance trends observed for the present porphyrin DSSCs have been rationalized based on spectral, electrochemical, electrochemical impedance spectroscopy, and transient spectroscopy results.

Combining Electron‐Accepting Phthalocyanines and Nanorod‐like CuO Electrodes for p‐Type Dye‐Sensitized Solar Cells

“…In general, the nature of the linker determines both the dye-electrode distance and the orientation of the dye relative to the surface. [26] Both seem to govern the injection and recombination kinetics, while thermodynamic differences between ZnPc1 and ZnPc2 are ruled out -vide supra. Considering the CuO electrode opaqueness we turned to electrochemical impedance spectroscopy (EIS).…”

Chlorin e6 sensitized photovoltaic cells: effect of co-adsorbents on cell performance, charge transfer resistance, and charge recombination dynamics

“…3(b), strong peak at 582 nm, that was absent for pristine chlorin e6 was observed providing evidence for charge injection from the singlet excited state of chlorin e6 to the conduction band of TiO 2 , an observation similar to that reported earlier for FTO∕TiO 2 ∕porphyrin and phthalocyanine-based solar cells. 23,31,32 The time profile of this peak could be fitted satisfactorily to a biexponential decay fit resulting in time constants of 0.88 and 24.7 ps. The rate of charge injection, k CI , and charge recombination, k CR , evaluated from these time constants were found to be 1.14 × 10 12 and 4.05 × 10 10 s −1 , respectively.…”

“…The computational calculations were performed by DFT B3LYP/6-31G* methods with Gaussian 09 software package 23 on high-speed computers. The HOMO and LUMO orbitals were generated using the GuessView program.…”

“…The oxidation process of chlorin e6 was found to be lower than that of free-base tetraphenylporphyrin, commonly used in building DSSCs. 23 Furthermore, the excited state reduction potential and free-energy change for electron injection were calculated according to Rehm-Weller approximation, 24 using Eqs. (1) and (2), since these values determine the thermodynamic feasibility of photoinduced electron transfer from the excited chlorin e6 into the TiO 2 conduction band …”

“…The magnitude of these values was close to that reported for porphyrin-based solar cells. 23,[31][32][33] Figures 8(c) and 8(d) show transient spectra of FTO∕TiO 2 ∕chlorin e6þcholic acid and FTO∕TiO 2 ∕chlorin e6þdeoxycholic acid electrodes. The spectral features were quite similar to that gathered for the FTO∕TiO 2 ∕chlorin e6 electrode suggesting that the co-adsorbent does not directly interact with the sensitizer to change the spectral features.…”