Unraveling Structure-Performance Relationships in Porphyrin-Sensitized TiO2 Photocatalysts

Over the years, porphyrins have arisen as exceptional photosensitizers given their ability to act as chlorophyll-mimicking dyes, thus, transferring energy from the light-collecting areas to the reaction centers, as it happens in natural photosynthesis. For this reason, porphyrin-sensitized TiO₂-based nanocomposites have been widely exploited in the field of photovoltaics and photocatalysis in order to overcome the well-known limitations of these semiconductors. However, even though both areas of application share some common working principles, the development of solar cells has led the way in what is referred to the continuous improvement of these architectures, particularly regarding the molecular design of these photosynthetic pigments. Yet, those innovations have not been efficiently translated to the field of dye-sensitized photocatalysis. This review aims at filling this gap by performing an in-depth exploration of the most recent advances in the understanding of the role played by the different structural motifs of porphyrins as sensitizers in light-driven TiO₂-mediated catalysis. With this goal in mind, the chemical transformations, as well as the reaction conditions under which these dyes must operate, are taken in consideration. The conclusions drawn from this comprehensive analysis offer valuable hints for the implementation of novel porphyrin-TiO₂ composites, which may pave the way toward the fabrication of more efficient photocatalysts.

Synthesis and characterization of a conjugated porphyrin dyad entangled with carboxyl functionalized benzimidazolium: an efficient metal free sensitizer for DSSCs

Synthesis of a novel metal free porphyrin dyad with excellent electronic coupling and anti-aggregating stability via strong anchoring to semiconductor through six carboxyl groups exhibiting 6.9% PCE in a truly energy generating DSSC.

Co-sensitization of free-base and zinc porphyrins: An effective approach to improve the photon-to-current conversion efficiency of dye-sensitized solar cells

Co-sensitization of two or more sensitizers with complementary absorption profiles had been utilized as an imperative tool to achieve panchromatic sensitization in dye-sensitized solar cells. The highly efficient dye-sensitized solar cells consist of a co-sensitization system comprising of a mixture of porphyrin and organic dye. We have prepared four novel free-base and zinc porphyrins in minimum steps and examined their individual as well as co-sensitized performance in dye-sensitized solar cells. The UV-visible spectrum suggests that upon zinc insertion the absorption wavelength is red-shifted by 10 nm in both Soret and Q-band region. The density functional theory (DFT) calculations revealed that the presence of an electron withdrawing cyanoacrylic acid as the anchoring group has pronounced effect on the charge distribution. IPCE spectra suggest that the co-sensitization of a free-base porphyrin with a zinc porphyrin resulted in panchromatic absorption in whole visible region. The device made with Mix-2, which is composed from the free-base porphyrin N4CN and the zinc porphyrin N4ZnCN gave the best performance with an overall photon-to-current conversion efficiency of 4.18%, with Jsc of 10.4 mA.cm-2, Voc of 0.56 V and fill factor of 72%.

Effects of number and position of meta and para carboxyphenyl groups of zinc porphyrins in dye-sensitized solar cells: structure-performance relationship

Porphyrin sensitizers containing meta- and para-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 para-carboxyphenyl-substituted porphyrins on TiO2 show a broader Soret band compared to meta-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 VOC, JSC, 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 meta- and para-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 para-carboxyphenyl derivatives outperform meta-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.

Theoretical insight into a novel zinc di-corrole dye with excellent photoelectronic properties for solar cells

A new zinc di-corrole dye has been designed by substitution of Ga with Zn in a Ga di-corrole dye. Its optical and electronic properties were studied by extensive DFT calculations.

The cis-isomer performs better than the trans-isomer in porphyrin-sensitized solar cells: interfacial electron transport and charge recombination investigations

Distinctive differences in the performance of DSSC devices fabricated from cis- and trans-isomers of a zinc porphyrin have been rationalized.