Effects of Number and Position of Meta and Para Carboxyphenyl Groups of Zinc Porphyrins in Dye-Sensitized Solar Cells: Structure–Performance Relationship

Abstract

Porphyrin sensitizers containing <i>meta</i>- and <i>para</i>-carboxyphenyl groups in their meso positions have been synthesized and investigated for their performance in dye-sensitized solar cells (DSSCs). The superior performance of para-derivative compared to meta-derivative porphyrins was revealed by optical spectroscopy, electrochemical property measurements, density functional theory (DFT) calculations, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, incident photon-to-current conversion efficiency (IPCE), electrochemical impedance spectroscopy (EIS), and stability performance. Absorption spectra of <i>para</i>-carboxyphenyl-substituted porphyrins on TiO₂ show a broader Soret band compared to <i>meta</i>-carboxyphenyl-substituted porphyrins. ATR-FTIR spectra of the studied porphyrins on TiO₂ were applied to investigate the number and mode of carboxyl groups attached to TiO₂. The <i>V</i>_OC, <i>J</i>_SC, and IPCE values of para-series porphyrins were distinctly superior to those of meta-series porphyrins. The Nyquist plots of the studied porphyrins show that charge injection in para-series porphyrins is superior to that in meta-series porphyrins. The orthogonally positioned para derivatives have more efficient charge injection and charge transfer over charge recombination, whereas the efficiencies of flat-oriented meta derivatives are retarded by rapid charge recombination. Photovoltaic measurements of the studied <i>meta</i>- and <i>para</i>-carboxyphenyl-functionalized porphyrins show that the number and position of carboxyphenyl groups play a crucial role in the performance of the DSSC. Our results indicate that <i>para</i>-carboxyphenyl derivatives outperform <i>meta</i>-carboxyphenyl derivatives to give better device performance. This study will serve as a guideline for the design and development of organic, porphyrin, and ruthenium dyes in DSSCs