Effect of regio-specific arylamine substitution on novel π-extended zinc salophen complexes: density functional and time-dependent density functional study on DSSC applications

A series of π-extended salophen-type Schiff-base zinc(ii) complexes, e.g., zinc-salophen complexes (ZSC), were investigated toward potential applications for dye-sensitized solar cells. The ZSC dyes adopt linear-, X-, or π-shaped geometries either with the functionalization of 1 donor/1 acceptor or 2 donors/2 acceptors to achieve a push-pull type molecular structure. The frontier molecular orbitals, light-harvesting properties as well as charge transfer characters against regio-specific substitution of donor/acceptor groups were studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results reveal that all ZSC dyes of D-ZnS-π-A geometry (where D, S, and A denote to donor, salophen ligand, and acceptor, respectively) exhibit relatively lower HOMO energy compared to the structurally resembled porphyrin dye YD2-o-C8. Natural transition orbital (NTO) and electron-hole separation (EHS) approaches clearly differentiate the linear type YD-series dyes from CL-, AJ1-, and AJ2-series dyes because of poor charge transfer (CT) properties. In contrast, the π-shaped AJ2-series and X-shaped AJ1-series dyes outperform the others in a manner of stronger CT characteristics, broadened UV-vis absorption as well as tunable bandgap simply via substitution of p-ethynylbenzoic acids (EBAs) and arylamine donors at salophen 7,8- and 2,3,12,13-positions, respectively. Both EHS and calculated exciton binding energies suggest the strength of CT character for ZSC dyes with an amino donor in the trend TPA > AN > DPA. This work has provided clear illustration toward molecular design of efficient dyes featuring a zinc-salophen backbone.

Pyridyl/hydroxyphenyl versus carboxyphenyl anchoring moieties in Zn - Thienyl porphyrins for dye sensitized solar cells

For developing efficient DSCs, anchoring moieties need to be understood in detail with respect to a particular dye. Herein, we have synthesized new Zn-thienyl porphyrin dyes based on pyridyl and hydroxyphenyl anchors and compared with traditional carboxyphenyl anchoring group. The effect of such variation in anchoring groups was investigated with UV-vis and fluorescence spectroscopy as well as electrochemical studies. The hydroxyphenyl appended dye showed higher molar extinction coefficient and red shifted emission bands. Both pyridyl and hydroxyphenyl dyes showed adsorption over TiO₂ semiconductor. The dye loading amount was found to be greater for carboxyphenyl dye due to stronger bidendate coordination of the carboxy group, as revealed by FT-IR studies. The electrochemical potentials were found to be strongly influenced by the anchoring functionality and all the dyes showed positive driving force for electron injection to the conduction band of TiO₂ and regeneration of oxidized dye by the redox electrolyte. DSC devices were fabricated for each of the dyes and characterized well. Hydroxyphenyl dye showed higher V_oc as a result of lower back reactions. Transient photovoltage decay measurments indicated lower recombination rate in hydroxyphenyl and pyridyl than the carboxyphenyl anchored thienyl porphyrins offering potential for further dye optimization.

Theoretical design of porphyrin sensitizers with different acceptors for application in dye-sensitized solar cells

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, three porphyrin dyes with different acceptors, such as carboxylic acid, cyanoacrylic acid, and 2-cyano-N-hydroxyacrylamide, have been designed. Compared to the best sensitizer (YD2-o-C8) so far, these designed dyes have small highest occupied orbital to lowest unoccupied orbital (HOMO–LUMO) band gaps, and wide absorptions with large oscillator strength at porphyrin Q bands. And the designed Dye1 is similar to YD2-o-C8 in electronic coupling with TiO₂, while improved Dye2 and Dye3 are better than YD2-o-C8, thus, Dye2 and Dye3 will be much faster for electron injection in dye-sensitized solar cell systems based on their long-term stable and efficient anchor groups. All these features show that our designed dyes, especially Dye2 and Dye3, have better absorption performance and faster electron injection. In addition, our results point out that 2-cyano-N-hydroxyacrylamide is a new promising acceptor. This study is expected to assist the molecular design of new efficient dyes for the advancement of dye-sensitized solar cells.

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, three porphyrin dyes with different acceptors, such as carboxylic acid, cyanoacrylic acid, and 2-cyano-N-hydroxyacrylamide, have been designed.

Dye-sensitized solar cell based on an inclusion complex of a cyclic porphyrin dimer bearing four 4-pyridyl groups and fullerene C60

Cyclic free-base porphyrin dimers linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes with fullerene C₆₀ have been applied to dye-sensitized solar cells as a new class of porphyrin dye sensitizers.

Effect of Substituents in Catechol Dye Sensitizers on Photovoltaic Performance of Type II Dye-Sensitized Solar Cells

In order to provide a direction in molecular design of catechol (Cat) dyes for type II dye-sensitized solar cells (DSSCs), the dye-to-TiO2 charge-transfer (DTCT) characteristics of Cat dyes with various substituents and their photovoltaic performance in DSSCs are investigated. The Cat dyes with electron-donating or moderately electron-withdrawing substituents exhibit a broad absorption band corresponding to DTCT upon binding to TiO2 films, whereas those with strongly electron-withdrawing substituents exhibit weak DTCT. This study indicates that the introduction of a moderately electron-withdrawing substituent on the Cat moiety leads to not only an increase in the DTCT efficiency, but also the retardation of back electron transfer. This results in favorable conditions for the type II electron-injection pathway from the ground state of the Cat dye to the conduction band of the TiO2 electrode by the photoexcitation of DTCT bands.

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.

Development of a functionally separated D–π-A fluorescent dye with a pyrazyl group as an electron-accepting group for dye-sensitized solar cells

A functionally separated D–π-A dye OUK-3 with a pyrazyl group as an electron-accepting group and a carboxyl group as an anchoring group has been newly developed as a photosensitizer for dye-sensitized solar cells.

Porphyrins as excellent dyes for dye-sensitized solar cells: recent developments and insights

Porphyrin sensitizers have exhibited power conversion efficiencies that are comparable to or even higher than those of well-established highly efficient DSSCs based on ruthenium complexes.

Enhancing the stability of porphyrin dye-sensitized solar cells by manipulation of electrolyte additives

The use of porphyrin-based photosensitizers with superior light-harvesting properties has enabled the power conversion efficiency of dye-sensitized solar cells (DSCs) to reach 13 % under full sun illumination. However, a major limitation of such devices corresponds to the volatility of the solvent used so far for the electrolyte, which prevents practical applications. In this work, we describe a porphyrin-ionic liquid DSC, which not only affords the highest efficiency reported to date, but is also stable for more than 300 h under continuous full sun illumination at 60 °C. Furthermore, we identify a previously unreported pathway for device degradation, and show that the addition of N-methylbenzimidazole and a thiocyanate salt to the electrolyte is critical to obtaining long-lived devices.

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.

Synthesis of carboxylate functionalized A3B and A2B2 thiaporphyrins and their application in dye-sensitized solar cells

Thiaporphyrins were effectively applied in dye-sensitized solar cells. Among them, N3S-ECN with an ethynylphenyl linker and cyanoacrylic acid anchor, achieved the highest efficiency of 1.69%.

The importance of various anchoring groups attached on porphyrins as potential dyes for DSSC applications

The influence of different anchoring groups in the efficiency of porphyrin dye sensitized solar cells.