Optical, Electrochemical, and Photovoltaic Effects of an Electron-Withdrawing Tetrafluorophenylene Bridge in a Push–Pull Porphyrin Sensitizer Used for Dye-Sensitized Solar Cells

Abstract

We have prepared a novel pushÀpull porphyrin with an electrondonating diarylamino group at the meso-position and an electron-withdrawing 4-carboxy-2,3,5,6-tetrafluorophenylethynyl anchoring group at the opposite mesoposition to address the substitution effects of the electron-withdrawing fluorine atoms on photovoltaic properties for the first time. The fluoro-substituted porphyrin showed slightly improved light-harvesting properties when compared to the reference porphyrin in solution, due to the enhancement of charge-transfer character. The fluoro-substituted porphyrin-sensitized TiO 2 cell exhibited a moderate power conversion efficiency of 4.6% but was lower than the corresponding value (6.9%) of the reference porphyrin-sensitized TiO 2 cell. The carboxylic group in the reference porphyrin binds to the TiO 2 surface with bidentate coordination, leading to formation of the densely packed monolayer on the TiO 2 surface, whereas both of the electronwithdrawing fluorine atoms and the carboxylic group of the fluoro-substituted porphyrin interact with the TiO 2 surface, adopting relatively parallel orientation to the TiO 2 surface with monodentate binding of the carboxylic acid to the TiO 2 , eventually yielding to the small surface coverage and low cell performance due to the fast charge recombination and small amount of the long-lived porphyrin radical cation. Such fundamental information may be useful for the molecular design of highly efficient dye-sensitized solar cells based on pushÀpull porphyrins.

Meso-Substituted Porphyrins for Dye-Sensitized Solar Cells

“…On this scope, Imahori and co-workers reported in 2011 two similarstructured push−pull porphyrins ZnPF and ZnPH, differing only by the nature of the phenylene spacer of the acceptor moiety (i.e., an ethynylcarboxytetrafluorophenyl or ethynylcarboxyphenyl anchoring group, respectively) and studied the electron-withdrawing effect on the performances of these dyes in TiO 2 -DSSCs ( Figure 60 and Table 33). 297 The introduction of a diarylamino donor group on one hand, and electron-withdrawing fluorine atoms in the phenylene bridge of the acceptor moiety in ZnPF on the other hand, enhances the CT character ("push−pull effect") of this dye. Consequently, the fluoro-substituted porphyrin ZnPF displays slightly improved light-harvesting properties in solution compared to ZnPH, due to a stronger charge-transfer effect.…”

Zinc–Porphyrin Dyes with Different meso‐Aryl Substituents for Dye‐Sensitized Solar Cells: Experimental and Theoretical Studies

“…The coadsorption of CDCA thus fails to improve the cell performance of ZnPCPA, unlike for ZnPPA and other organic sensitizers. [35] The overall performance of a porphyrin-based DSSC is thus a subtle balance between these factors. It is noticeable that the efficiency of the ZnPPA/CDCA-based solar cell (h = 3.50 %) is still lower than that of ZnPCPA-and ZnPCTAbased cells (h = 5.09-6.24 %), which suggests that the higher efficiencies of the latter devices might not only result from lower dye aggregations on TiO 2 owing to bulky carbazole substituents but also come from an electron-donating property of the carbazole.…”

Porphyrins as Active Components for Electrochemical and Photoelectrochemical Devices

“…219 The authors prepared a push-pull zinc porphyrin bearing a diarylamino group as donor moiety and a 4-carboxy-2,3,5,6-tetrafluorophenylethynyl group as electronwithdrawing anchoring group (ZnPF), which would facilitate the electron injection process ( Figure 54). However, even though ZnPF showed superior performance in terms of light harvesting capacity in solution, the power conversion efficiency of the DSSC realized with ZnPF was lower (4.6%) than that of the DSSC functionalized with ZnPH (6.9%).…”

“…270 By and large, chemisorption is the most used method because of the basic character of the ZnO surface. Thus, porphyrins containing functional groups with acidic OH (carboxylic acid, 219,271,272,274,[276][277][278]281,[285][286][287]291,[293][294][295]297,298 sulfonic acid 279,283,284 and cathecol 275,282 ) were easily grafted onto ZnO 2 .…”