Porphyrin-sensitized solar cells: effect of carboxyl anchor group orientation on the cell performance

Synthesis and Photovoltaic Performance of β-Amino-Substituted Porphyrin Derivatives

New β-amino-substituted porphyrin derivatives bearing carboxy groups were synthesized and their performance as sensitizers in dye-sensitized solar cells (DSSC) was evaluated. The new compounds were obtained in good yields (63-74%) through nucleophilic aromatic substitution reactions with 3-sulfanyl- and 4-sulfanylbenzoic acids. Although the electrochemical studies indicated suitable HOMO and LUMO energy levels for use in DSSC, the devices fabricated with these compounds revealed a low power conversion efficiency (PCE) that is primarily due to the low open-circuit voltage (Voc) and short-circuit current density (Jsc) values.

Suppression of Charge Recombination by Vertical Arrangement of A Donor Moiety on Flat Planar Dyes for Efficient Dye-Sensitized Solar Cells

In dye-sensitized solar cells (DSSCs), flat planar dyes (e. g., highly light-harvesting porphyrins and corroles) with multiple anchoring groups are known to adopt a horizontal orientation on TiO2 through the multiple binding to TiO2. Due to the strong electronic coupling between the dye and TiO2, fast charge recombination between the oxidized dye and an electron in TiO2 occurs, lowering the power conversion efficiency (η). To overcome this situation, an additional donor moiety can be placed on top of the planar dye on TiO2 to slow down the undesirable charge recombination. Here we report the synthesis and photovoltaic properties of a triarylamine (TAA)-tethered gold(III) corrole (TAA-AuCor). The DSSC with TAA-AuCor using iodine redox shuttle exhibited the highest η-value among corrole-based DSSCs, which is much higher than that with the reference AuCor. The transient absorption spectroscopies clearly demonstrated that fast electron transfer from the TAA moiety to the corrole radical cation in TAA-AuCor competes with the undesirable charge recombination to generate long-lived charge separated state TAA⋅+-Cor/TiO2⋅- efficiently. Consequently, the introduction of the TAA moiety enhanced the η-value remarkably, demonstrating the usefulness of our new concept to manipulate charge-separated states toward highly efficient DSSCs.

Biomedical Application of Porphyrin-Based Amphiphiles and Their Self-Assembled Nanomaterials

Porphyrins have been vastly explored and applied in many cutting-edge fields with plenty of encouraging achievements because of their excellent properties. As important derivatives of porphyrins, porphyrin-based amphiphiles (PBAs) not only maintain the advanced properties of porphyrins (catalysis, imaging, and energy transfer) but also possess self-assembly and encapsulation capability in aqueous solution. Accordingly, PBAs and their self-assembles have had important roles in diagnosing and treating tumors and inflammation lesions in vivo, but not limited to these. In this article, we introduce the research progress of PBAs, including their constitution, structure design strategies, and performances in tumor and inflammation lesion diagnosis and treatments. On that basis, the defects of synthesized PBAs during their application and the possible effective strategies to overcome the limitations are also proposed. Finally, perspectives on PBAs exploration are updated based on our knowledge. We hope this review will bring researchers from various domains insights about PBAs.

Highly efficient light-driven hydrogen evolution utilizing porphyrin-based nanoparticles

We developed dye-sensitized photocatalytic systems (DSPs) by utilizing porphyrins as a photosensitizer (PS) or as a photosensitizer-catalyst (PS/CAT) upon their chemisorption onto platinum-doped titanium dioxide nanoparticles (Pt-TiO2 NPs). The DSPs coated with Pt-Tc3CP (PS/CAT entity) exhibited a record-high stability (25 500 TONs) and H2 evolution activity (707 mmol g-1 h-1) compared to similar DSPs in the literature.

Influence of the Polymeric Matrix on the Optical and Electrical Properties of Copper Porphine-Based Semiconductor Hybrid Films

In this study, we assessed the electrical and optical behavior of semiconductor hybrid films fabricated from octaethyl-21H,23H-porphine copper (CuP), embedded in polymethylmethacrylate (PMMA), and polystyrene (PS). The hybrid films were characterized structurally and morphologically using infrared spectroscopy (IR), atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Subsequently, the PMMA:CuP and PS:CuP hybrid films were evaluated optically by UV-vis spectroscopy, as well as electrically, with the four-point collinear method. Hybrid films present a homogeneous and low roughness morphology. In addition, the PS matrix allows the crystallization of the porphin, while PMMA promotes the amorphous structure in CuP. The polymeric matrix also affects the optical behavior of the films, since the smallest optical gap (2.16 eV) and onset gap (1.89 eV), and the highest transparency are obtained in the film with a PMMA matrix. Finally, the electrical behavior in hybrid films is also affected by the matrix: the largest amount of current carried is approximately 0.01 A for the PS:CuP film, and 0.0015 A for the PMMA:CuP film. Thanks to the above properties, hybrid films are promising candidates for use in optoelectronic devices.

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.

Molecular Orientation of -PO3H2 and -COOH Functionalized Dyes on TiO2, Al2O3, ZrO2, and ITO: A Comparative Study

Modification of transparent metal oxide (MO_x) surfaces with organic monolayers is widely employed to tailor the properties of interfaces in organic electronic devices, and MO_x substrates modified with light-absorbing chromophores are a key component of dye-sensitized solar cells (DSSCs). The effects of an organic modifier on the performance of a MO_x-based device are frequently assessed by performing experiments on model monolayer|MO_x interfaces, where an "inert" MO_x (e.g., Al₂O₃) is used as a control for an "active" MO_x (e.g., TiO₂). An underlying assumption in these studies is that the structure of the MO_x-monolayer complex is similar between different metal oxides. The validity of this assumption was examined in the present study. Using UV-Vis attenuated total reflection spectroscopy, we measured the mean dipole tilt angle of 4,4'-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonic acid (A1P) adsorbed on indium tin oxide (ITO), TiO₂, ZrO₂, and Al₂O₃. When the surface roughness of the MO_x substrate and the surface coverage (𝛤) of the A1P film were constant, the molecular orientation of A1P was the same on these substrates. The study was extended to 4,4'-(anthracene-9,10-diyl)bis(4,1-phenylene)dicarboxylic acid (A1C) adsorbed on the same group of MO_x substrates. The mean tilt angle of A1C and A1P films on ITO was the same, which is likely due the intermolecular interactions resulting from the high and approximately equal 𝛤 of both films. Comparing A1C films at the same 𝛤 on TiO₂ and Al₂O₃ having the same surface roughness, there was no difference in the mean tilt angle. MD simulations of A1C and A1P on TiO₂ produced nearly identical tilt angle distributions, which supports the experimental findings. This study provides first experimental support for the assumption that the structure of the MO_x-modifer film is the same on an "active" substrate vs. a "inert" control substrate.

Single-Atom Catalysts: A Review of Synthesis Strategies and Their Potential for Biofuel Production

Biofuels have been derived from various feedstocks by using thermochemical or biochemical procedures. In order to synthesise liquid and gas biofuel efficiently, single-atom catalysts (SACs) and single-atom alloys (SAAs) have been used in the reaction to promote it. SACs are made up of single metal atoms that are anchored or confined to a suitable support to keep them stable, while SAAs are materials generated by bi- and multi-metallic complexes, where one of these metals is atomically distributed in such a material. The structure of SACs and SAAs influences their catalytic performance. The challenge to practically using SACs in biofuel production is to design SACs and SAAs that are stable and able to operate efficiently during reaction. Hence, the present study reviews the system and configuration of SACs and SAAs, stabilisation strategies such as mutual metal support interaction and geometric coordination, and the synthesis strategies. This paper aims to provide useful and informative knowledge about the current synthesis strategies of SACs and SAAs for future development in the field of biofuel production.

DFT and TD-DFT studies for optoelectronic properties of coumarin based donor-π-acceptor (D-π-A) dyes: applications in dye-sensitized solar cells (DSSCS)

A series of six coumarin based dye derivatives were investigated and their geometry and optoelectronic properties elucidated for suitability in dye-sensitized solar cells (DSSCs) using TD-DFT/B3LYP with 6-31G basis set. D-π-A schemes were developed by attaching various donors and acceptors to coumarin dye (CM) to calculate changes in their photovoltaic properties. D2-CM-A2 and D4-CM-A4 showed less dihedral angle because of the low steric effect between donor and connector. The D1-CM-A1 and D2-CM-A2 results of intramolecular charge transfer were higher because of low bond length and a strong group of electron donors. The results revealed that LUMO energies of D1-CM-A1, D2-CM-A2, D3-CM-A3 and D4-CM-A4 were higher than the conduction band edge of TiO2 electrode (-4.0 eV) suggesting that these dyes will inject the electrons into the conduction band of the semiconductor. In addition, the light harvest efficiency (LHE), open-circuit voltage (VOC) and band energy gap (Eg) values are calculated in the gas phase, as well as in the solvent phase. This study shows that D1-CM-D1 and D2-CM-A2 derivatives have better properties for application in the DSSCs.

Fluorenyl Indoline as an Efficient Electron Donor for Concerted Companion Dyes: Enhanced Light-Harvesting and Photocurrent

Concerted companion dyes (CC dyes) like XW61 have been demonstrated to be an effective platform for developing efficient DSSCs. However, the moderated phenothiazine-based electron donor in XW61 results in unsatisfactory J_sc. To address this problem, a stronger fluorenyl indoline-based electron donor has been used to construct porphyrin dye XW68 and organic dyes Y1-Y2. The stronger electron-donating character of the fluorenyl indoline unit leads to an enhanced J_sc value (20.48 mA·cm^-2) for the individual dye XW68. On this basis, CC dyes XW69-XW70-C8 have been designed and synthesized by combining the frameworks of Y1 and Y2 with XW68. The complementary absorption characters of the porphyrin and the organic dye moieties lead to panchromatic absorption with a strong light-harvesting capability from 350 to 700 nm and the onset wavelength extended to ca. 840 nm in the IPCE curves. As a result, excellent J_sc values have been achieved (>22 mA·cm^-2). In addition to the advantages of high J_sc, bulky octyl groups have been introduced into the donor of XW70-C8 to reduce dye aggregation and suppress charge recombination. Finally, a highest PCE of 11.1% with a satisfactory J_sc (22.25 mA·cm^-2) and an enhanced V_oc (750 mV) has been achieved upon coadsorption of XW70-C8 with CDCA. In addition, the CC dye XW70-C8-based solar cells exhibit excellent long-term photostability. These results provide an effective method for rationally improving the photovoltaic behavior, especially the J_sc of CC dyes, by introducing strong electron donor moieties with suitable substituents.

Construction of Ultrathin Layer-by-Layer Films of Oligothiophene Derivatives on an Electrode

Oligothiophene derivatives, which are known as p-type materials, have been synthesized, and their ultrathin layer-by-layer films have been constructed on an electrode using a simple and convenient dipping method. The stepwise deposition behavior of quaterthiophene and sexithiophene derivatives on the electrode via hydrogen bonding was monitored by electronic spectra measurement, and the constructed films were evaluated by X-ray photoelectron spectroscopy, grazing-incidence small-angle X-ray scattering, and cyclic voltammetry. It has been clarified that the constructed layer-by-layer films were electroactive and photoelectroactive.

Examining the influence of bilayer structure on energy transfer and molecular photon upconversion in metal ion linked multilayers

Metal ion linked multilayers is a unique motif to spatially control and geometrically restrict molecules on a metal oxide surface and is of interest in a number of promising applications. Here we use a bilayer composed of a metal oxide surface, an anthracene annihilator molecule, Zn(II) linking ion, and porphyrin sensitizers to probe the influence of the position of the metal ion binding site on energy transfer, photon upconversion, and photocurrent generation. Despite being energetically similar, varying the position of the carboxy metal ion binding group (i.e. ortho, meta, para) of the Pt(II) tetraphenyl porphyrin sensitizer had a large impact on energy transfer rates and upconverted photocurrent that can be attributed to differences in their geometries. From polarized attenuated total reflectance measurements of the bilayers on ITO, we found that the orientation of the first layer (anthracene) was largely unperturbed by subsequent layers. However, the tilt angle of the porphyrin plane varies dramatically from 41° to 64° to 57° for the para-, meta-, and ortho-COOH substituted porphyrin molecules, which is likely responsible for the variation in energy transfer rates. We go on to show using molecular dynamics simulations that there is considerable flexibility in porphyrin orientation, indicating that an average structure is insufficient to predict the ensemble behavior. Instead, even a small subset of the population with highly favorable energy transfer rates can be the primary driver in increasing the likelihood of energy transfer. Gaining control of the orientation and its distribution will be a critical step in maximizing the potential of the metal ion linked structures.

2-Thiohydantoin Moiety as a Novel Acceptor/Anchoring Group of Photosensitizers for Dye-Sensitized Solar Cells

Very recently, we have reported the synthesis and evaluation of biological properties of new merocyanine dyes composed of triphenylamine moiety, π-aromatic spacer, and rhodanine/2-thiohydantoin-based moiety. Interestingly, 2-thiohydantoin has never been studied before as an electron-accepting/anchoring group for the dye-sensitized solar cells (DSSCs). In the presented study, we examined the applicability of 2-thiohydantoin, an analog of rhodanine, in DSSC technology. The research included theoretical calculations, electrochemical measurements, optical characterization, and tests of the solar cells. As a result, we proved that 2-thiohydantoin might be considered as an acceptor/anchoring group since all the compounds examined in this study were active. The most efficient device showed power conversion efficiency of 2.59%, which is a promising value for molecules of such a simple structure. It was found that the cells' performances were mainly attributed to the dye loading and the ICT molecular absorption coefficients, both affected by the differences in the chemical structure of the dyes. Moreover, the effect of the aromatic spacer size and the introduction of carboxymethyl co-anchoring group on photovoltaic properties was observed and discussed.

A new type of multibenzyloxy-wrapped porphyrin sensitizers for developing efficient dye-sensitized solar cells

Porphyrin dyes have been widely used for the fabrication of efficient dye-sensitized solar cells (DSSCs). However, dye aggregation and charge recombination still exert negative effects on photovoltaic performance, resulting in unsatisfactory power conversion efficiencies (PCEs). Herein, we report a new class of porphyrin sensitizers, XW52 and XW53 employing four benzyloxy groups to wrap the porphyrin cores. As a result, an efficiency of 7.6% was obtained for XW52, with [Formula: see text] and [Formula: see text] of 668 mV and 16.63 mA cm[Formula: see text], respectively. Compared with XW52, an additional 2,6-dialkoxyphenyl group has been introduced to the N-atom of the phenothiazine donor to furnish XW53 with the aim to further improve the anti-aggregation character and the solubility, and thus the [Formula: see text] was improved to 674 mV, and a higher efficiency of 7.9% was achieved for XW53. Upon cosensitization with PT-C6, the[Formula: see text] and [Formula: see text] were synergistically enhanced to 727 mV and 18.67 mA cm[Formula: see text], respectively. As a result, a high efficiency of 9.6% was successfully achieved for the cosensitization system of XW53 + PT-C6. These results provide an effective novel strategy for designing efficient porphyrin dyes by introducing multiple benzyloxy groups to the meso-phenyl groups.

Synthesis, Spectral, Electrochemical and Photovoltaic Studies of A3 B Porphyrinic Dyes having Peripheral Donors

Three new 'push-pull' A₃ B Zn(II)porphyrin dyes having meso-pyrenyl, carbazolyl and phenothiazine as electron donors (A) and phenylcarboxylic acid as acceptor/anchor (B) were synthesized and utilized for DSSC application. The spectral and electrochemical redox properties of these new dyes were studied and compared with trans-A₂ BC Zn(II) porphyrin dyes under similar experimental conditions. Red-shifted, broadened absorption peaks, lower fluorescence quantum yields, and shortened lifetimes were observed for the A₃ B dyes as compared to zinc tetraphenylporphyrin control, ZnTPP. DFT optimized structures suggested effective charge separation related to enhanced charge injection efficiency. Driving force for electron injection (ΔG_inj ) and dye regeneration (ΔG_reg ) calculated from the spectral and electrochemical studies predicted facile electron injection from excited dye into semiconductor TiO₂ in the constructed solar cells. Phenothiazine appended dye (KP-TriPTZ-Zn) showed the highest η value of 7.3 % for PCE with greater J_sc and V_oc values due to its better light harvesting ability and reduced dye aggregation as compared to other dyes. Our studies demonstrate that the dyes having multiple electron-donating groups exhibit higher photon-to-current conversion efficiency.

Complexes between core-modified porphyrins ZnP(X)4 (X = P and S) and small semiconductor nanoparticle Zn6S6: are they possible?

The synthetic approach of the anchoring of porphyrins to the surface of semiconductor nanoparticles (NPs) has been realized to form very promising organic/inorganic nanocomposites. They have been of considerable scientific and a wide practical interest including such areas as material science, biomedical applications, and dye-sensitized solar cells (DSSCs). Macrocyclic pyrrole-containing compounds, such as phthalocyanines and porphyrins, can bind to the NP surface by a variety of modes: as monodentate ligands oriented perpendicular to the NP surface, parallel to the NP surface, or, alternatively, in a perpendicular orientation bridging two adjacent NPs. Also, non-covalent (coordination) interactions may be realized between the NP via its metal centers and appropriate meso-attached groups of porphyrins. Recently, we showed computationally that the prominent structural feature of the core-modified MP(X)4 porphyrins (X = P) is their significant distortion from planarity. Motivated by the phenomenon of numerous complexes formation between tetrapyrrols and NPs, we performed the density functional theory (DFT) studies of the complex formation between the core-modified ZnP(X)4 species (X = P and S) without any substituents or linkers and semiconductor NPs, exemplified by small NP Zn6S6. The complexes formation was investigated using the following theoretical approaches: (i) B3LYP/6-31G* and (ii) CAM-B3LYP/6-31G*, both in the gas phase and with implicit effects from C6H6 considered. The calculated binding energies of the complexes studied were found to be significant, varying from ca. 29 up to ca. 69 kcal/mol, depending on the complex and the approach employed.

Effect of donor modification on the photo-physical and photo-voltaic properties of N-alkyl/aryl amine based chromophores

Six push–pull sensitizers (MD1 to MD6) having rhodanine-3-acetic acid acting as an electron withdrawing group and N-alkyl/aryl amine as a donor moiety were planned and prepared to rationalize the influence of donor alteration on absorption/emission properties and photon to current conversion efficiency (η).

Efficient solar cells sensitized by a promising new type of porphyrin: dye-aggregation suppressed by double strapping

Doubly strapped porphyrin improved efficiency from 8.6% to 9.3% and finally to 10.6% through a combined approach of coadsorption and cosensitization.

Porphyrin sensitizers play essential roles in the development of efficient dye-sensitized solar cells (DSSCs). To further improve power conversion efficiency (PCE), it is vital to reduce undesirable dye aggregation that causes serious charge recombination and lowered open-circuit voltages (V_oc). To this end, we herein report a new class of porphyrin-based dyes XW40 and XW41, with the porphyrin cores strapped with two circle chains. Compared with the reference sensitizer XW10 which contains a porphyrin core wrapped in four dodecoxyl chains, the double strapping in XW40 not only effectively suppresses the dye aggregation but also improves the dye loading amount. As a result, the V_oc and photocurrent (J_sc) were improved by 19 mV and 0.8 mA cm^–2, respectively, compared with the corresponding values of XW10, and the efficiency was improved from 8.6% obtained for XW10 to 9.3% for XW40. To further extend the spectral response, an electron-withdrawing benzothiadiazole (BTD) unit was introduced as an auxiliary acceptor in XW41. Impressively, the onset wavelength of its IPCE spectrum was dramatically red-shifted to 830 nm. However, the extended π-conjugation framework results in aggravated dye aggregation, and thus a lowered efficiency of 8.2% was obtained for XW41. Through a combined approach of coadsorption and cosensitization, the efficiencies were dramatically enhanced to 10.6% and 10.2% for XW40 and XW41, respectively, as a result of simultaneously enhanced V_oc and J_sc. The results of this work provide a novel strategy for developing efficient DSSCs by employing strapped porphyrin dyes.

Effect of Carboxylic Acid-Doped Carbon Nanotube Catalyst on the Performance of Aqueous Organic Redox Flow Battery Using the Modified Alloxazine and Ferrocyanide Redox Couple

Alloxazine and ferrocyanide are suggested as the redox couple for an aqueous organic redox flow battery (AORFB). Alloxazine is further modified by carboxylic acid (COOH) groups (alloxazine-COOH) to increase the aqueous solubility and to pursue a desirable shift in the redox potential. For obtaining a better AORFB performance, the overall redox reactivity of AORFB should be improved by the enhancement of the rate-determining reaction of the redox couple. A carboxylic acid-doped carbon nanotube (CA-CNT) catalyst is considered for increasing the reactivity. The utilization of CA-CNT allows for the induction of a better redox reactivity of alloxazine-COOH because of the role of COOH within alloxazine-COOH as a proton donor, the fortified hydrophilic attribute of alloxazine-COOH, and the increased number of active sites. With the assistance of these attributes, the mass transfer of aqueous alloxazine-COOH molecules can be promoted. However, CA-CNT does not have an effect on the increase of the redox reactivity of ferrocyanide because the redox reaction is not affected by the same influence of protons that the redox reactivity of alloxazine-COOH is affected by. Such a behavior is proven by measuring the electron transfer rate constant and diffusivity. With regard to AORFB full cell testing, when CA-CNT is used as a catalyst for the negative electrode, the performance of the AORFB increases. Specifically, the charge-discharge overpotential and infrared drop potential are improved. As a result, the voltage efficiency affected by the potentials increases to 64%. Furthermore, the discharging capacity reaches 26.7 A h·L^-1, and the state of charge attains 83% even after 30 cycles.

Two novel self-assemblies of supramolecular solar cells using N-heterocyclic-anchoring porphyrins

Two novel N-substituted anchoring porphyrins (ZnPAtz and ZnPAim) have been devised and synthesized. Moreover, these two anchoring porphyrins were linked to the TiO₂ semiconductor through carboxyl groups and then a zinc porphyrin ZnP was bound to the anchoring porphyrin using a zinc-to-ligand axial coordination approach. The different performances of these assemblies were compared with single anchoring porphyrin devices ZnPAtz and ZnPAim. The photoelectric conversion efficiency of the new supramolecular solar cells sensitized by ZnP-ZnPAx (x=tz, im) has been improved. The ZnP-ZnPAtz-based DSSCs provided the highest photovoltaic efficiency (1.86%). Fundamental studies showed that incorporation of these assemblies promote light-harvesting efficiency.

Theoretical investigations on the unsymmetrical effect of β-link Zn-porphyrin sensitizers on the performance for dye-sensitized solar cells

Dye sensitizers play an important role in dye-sensitized solar cells (DSSCs). As a promising strategy for the design of novel porphyrin sensitizers, the asymmetric modification of the porphyrin ring to meso-link porphyrin sensitizer has emerged in recent years, which can improve the light-harvesting properties and enhance the electron distribution. In this work, in order to reveal the essence of the effect of unsymmetrical substitution on the performance of β-link porphyrin dyes in DSSCs, four kinds of common β-link porphyrin dyes with different structures are calculated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The electronic structures and optical properties of these studied dyes in dimethylformamide (DMF) are also investigated. The key parameters of the short-circuit current density (J_sc), including light harvesting efficiency (LHE), electron injection driving force (ΔG_inject), and intra-molecular charge transfer (ICT) are discussed in detail. In addition, the periodic DFT calculations in the dye-TiO₂ systems are also employed to investigate the geometrical and electronic injection process of the different connection types of these studied dyes adsorbed on the periodic TiO₂ model with an exposed anatase (101) surface. We expect the present study would deepen the understanding of the alternative function of unsymmetrical substitution and may contribute to future DSSC design.

Cosensitization of Structurally Simple Porphyrin and Anthracene-Based Dye for Dye-Sensitized Solar Cells

Since their introduction, dye-sensitized solar cells (DSCs) have achieved huge success at a laboratory level. Recently, research is concentrated to visualize large DSC modules at the commercial platform. In that aspect, we have tested structurally simple porphyrin-based dye SK6 and anthracene-based dye CW10 for DSCs application under simulated 1 sun (AM 1.5G) and indoor light sources. These two dyes can be easily synthesized and yet are efficient with cell performances of ca. 5.42% and ca. 5.75% (without coadsorbent/additive) for SK6 and CW10, respectively, under AM 1.5G illumination. The power conversion efficiency (PCE) of SK6 reported in this work is the highest ever reported; this is achieved by optimizing the adsorption of SK6 on TiO₂ photoanode using the most suitable solvent and immersion period. Cosensitization of SK6 with CW10 on TiO₂ surface has boosted cell performance further and achieved PCE of ca. 6.31% under AM 1.5G illumination. Charge-transfer properties of individual and cosensitized devices at TiO₂/dye/electrolyte interface were examined via electrochemical impedance spectroscopy. To understand the cell performances under ambient light conditions, we soaked individual and cosensitized devices under T5 and light-emitting diode light sources in the range of 300-6000 lx. The PCE of ca. 22.91% under T5 light (6000 lx) with J_SC = 0.883 mA cm^-2, V_OC = 0.646 V, and FF = 0.749 was noted for the cosensitized device, which equals a power output of 426 μW cm^-2. These results reveal that DSCs made of structurally simple dyes performed efficiently under both 1 sun (AM 1.5G) and indoor light conditions, which is undoubtedly a significant achievement when it comes to a choice of commercial application.

Near-Infrared-Absorbing Metal-Free Organic, Porphyrin, and Phthalocyanine Sensitizers for Panchromatic Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are a promising source of renewable energy. However, the power conversion efficiency (PCE) of devices has been limited largely by the difficulty of producing electricity using photons from the near-infrared (NIR) spectral region. Metal-free organic sensitizers frequently employ strong electron-donating or -withdrawing moieties to tune the optical band gap to allow the absorption of lower energy wavelengths in charge-transfer systems, whereas porphyrins and phthalocyanines use substituents to shift the Soret and Q bands toward lower energy absorption. Very few devices employing precious metal-free dyes have achieved panchromatic and NIR photon conversion for electricity generation at wavelengths >750 nm despite a tremendous number of sensitizers published over the last 25 years. This Minireview seeks to compile a summary of these sensitizers to encourage assimilation, analysis, and development of efficient future sensitizers with absorption extending into the NIR. Herein, we discuss common synthetic strategies, optical properties, and electronic properties of the most successful panchromatic organic sensitizers.

Efficient Solar Cells Based on Porphyrin Dyes with Flexible Chains Attached to the Auxiliary Benzothiadiazole Acceptor: Suppression of Dye Aggregation and the Effect of Distortion

Donor-π-acceptor-type porphyrin dyes have been widely used for the fabrication of efficient dye-sensitized solar cells (DSSCs) owing to their strong absorption in the visible region and the ease of modifying their chemical structures and photovoltaic behavior. On the basis of our previously reported efficient porphyrin dye XW11, which contains a phenothiazine-based electron donor, a π-extending ethynylene unit, and an auxiliary benzothiadiazole acceptor, we herein report the syntheses of novel porphyrin dyes XW26-XW28 by introducing one or two alkyl/alkoxy chains into the auxiliary acceptor. The introduced chains can effectively suppress dye aggregation. As a result, XW26-XW28 show excellent photovoltages of 700, 701, and 711 mV, respectively, obviously higher than 645 mV obtained for XW11. Nevertheless, the optimized structures of XW26 and XW27 exhibit severe distortion, showing large dihedral angles of 57.2° and 44.0°, respectively, between the benzothiadiazole and benzoic acid units, resulting from the steric hindrance between the benzoic acid unit and the neighboring alkyl/alkoxy chain on the benzothiadiazole unit, and thus blue-shifted absorption, decreased photocurrents. and low efficiencies of 5.19% and 6.42% were observed for XW26 and XW27, respectively. Interestingly, XW26 exhibits a more blue-shifted absorption spectrum relative to XW27, indicating that the steric hindrance of the alkyl/alkoxy chains has a more pronounced effect than the electronic effect. Different from XW26 and XW27, XW28 contains only one alkyl chain neighboring the ethynylene unit, which does not induce obvious steric hindrance with the benzoic acid unit, and thus distortion of the molecule is not seriously aggravated compared with XW11. Hence, its absorption spectrum and photocurrent are similar to those of XW11. As a result, a higher efficiency of 9.12% was achieved for XW28 because of its suppressed dye aggregation and higher photovoltage. It is worth noting that a high efficiency of 10.14% was successfully achieved for XW28 upon coadsorption with CDCA, which is also higher than the corresponding efficiency obtained for XW11. These results provide a novel approach for developing efficient porphyrin dyes by introducing chains into the suitable position of the auxiliary benzothiadiazolyl moiety to suppress dye aggregation, without seriously aggravating distortion of the dye molecules.

Interplay between π-Bridges and Positions of Branched Alkyl Groups of Unsymmetrical D-A-D-π-A Squaraines in Dye-Sensitized Solar Cells: Mode of Dye Anchoring and the Charge Transfer Process at the TiO2/Dye/Electrolyte Interface

Far-red-absorbing squaraines possessing high molar absorptivity (>10⁵ M^-1 cm^-1) are being attracted as high-efficiency chromophores in dye-sensitized solar cells (DSSCs). A series of donor-acceptor-donor-π spacer-acceptor (D-A-D-π-A) unsymmetrical squaraines, PSQ1-5, with indoline donor and squaric/cyanoacetic acid acceptor units, were designed for sensitized solar cells. For extending the absorption toward the near-infrared region (NIR) and controlling the orientation on the TiO₂ surface, benzene (PSQ1 and PSQ2) and thiophene (PSQ3-5) π-spacers and out-of-plane branched alkyl groups at the indoline that are away (PSQ1, PSQ3, and PSQ5) or near (PSQ2 and PSQ4) the anchoring group, respectively, were introduced. Dynamic aggregation tendency of PSQ1 and PSQ3 than that of their isomers systematically modulates the orientation on the TiO₂ surface, which in turn enhances photovoltaic performance. Absorptance on a thin transparent TiO₂ film shows a visible-to-NIR response with an onset around 800 nm for PSQ3-5. Although there is close resemblance in electrochemical redox levels, their high injection efficiency and recombination resistance differentiated their impact on the way of anchoring and the dihedral angle between D-A-D units and π-spacers. DSSCs sensitized with PSQ5 achieved a PCE of 8.15% under simulated AM 1.5G illumination (100 mW cm^-2), with the current density (J_sc) and open-circuit voltage (V_oc) of 19.73 mA cm^-2 and 630 mV, respectively. A clear comparison of the incident-photon-to-current conversion efficiency versus the light-harvesting efficiency correlates the structure-property relationship with J_sc obtained for PSQ dyes. Electrochemical impedance spectroscopy was carried out to examine the TiO₂/dye/electrolyte interface for further confirmation of the enhanced PCE of top-sp³-alkylated PSQ5 over that of other dyes.

Novel self-assembly with zinc porphyrin via axial coordination for dye-sensitized solar cells

Two zinc porphyrins substituted at the meso-positions with different donor units (denoted as ZnP1 and ZnP2) have been designed and synthesized to construct new self-assemblies (denoted as ZnPx-ZnPA) with anchor porphyrin (ZnPA) dyads formed by the coordination bonds of Zn-to-ligand approach. Then these assemblies were absorbed on the semiconducting TiO2 electrode surfaces by the carboxylic groups of anchor porphyrin (ZnPA) to build dye sensitized solar cells. Their spectral properties, electrochemical, theoretical calculations and photovoltaic properties were systematically investigated. Interestingly, these self-assemblies devices compared to monomer anchoring porphyrin device showed higher [Formula: see text] and [Formula: see text]. Especially, the dyad of ester-based zinc porphyrin (ZnP2) assembly device has the highest photoelectric conversion efficiency. The assembled modes of the assemblies immobilized on TiO2 electrode surfaces were also verified by transmission electron microscopy (TEM).

Theoretical and experimental analysis of porphyrin derivatives with suitable anchoring groups for DSSC applications

In this contribution, different porphyrin derivatives were experimentally synthesized and theoretically analyzed using several electronic structure methods to study the geometrical and electronic properties of A4, trans-A2B[Formula: see text]and A3B porphyrins bearing several functional groups (–OH, –COOH, -3,5-di-[Formula: see text]Bu, –OCH2CH2CH2COOEt and –OMe) suitable to be employed as dyes in Dye Sensitized Solar Cells (DSSC). A4 (R [Formula: see text] -H, -OMe, -OH, -3,5-di-tert-butyl, –OCH2CH2CH2COOEt) and A3B (R[Formula: see text][Formula: see text]R[Formula: see text][Formula: see text]R[Formula: see text][Formula: see text]–H; R[Formula: see text][Formula: see text]–OH and R[Formula: see text][Formula: see text]-3,5-di-[Formula: see text]Bu) porphyrins were synthesized and characterized by UV-vis and 1H NMR spectroscopies for comparison. The geometrical parameters were analyzed in the ground state and gas phase using the semiempirical method PM6 and the DFT functionals M06-2X and B3LYP, in combination with the 6-31G(d), DZVP and TZVP basis set. For calculations of the electronic and excited state properties, CAM-B3LYP, M06-2X and HSE06, using SMD as solvation model, were applied. This study revealed that HSE06/DZVP protocol is the best methodology to simulate electronic spectra in these porphyrin derivatives. Indeed, whereas substituent groups did not significantly affect the geometrical structure of the porphyrin derivatives studied, they do influence their electronic structures, mainly in the LUMO (lowest unoccupied molecular orbital) energy levels.

Synthesis and spectral evaluation of 5,10,15,20-tetrakis(3,4-dibenzyloxyphenyl)porphyrin

Porphyrins are of interest in many applications that involve electron transfer and absorption of light, such as solar energy and photodynamic cancer therapy. The newly synthesized 5,10,15,20-tetrakis(3,4-dibenzyloxyphenyl)porphyrin, TDBOPP, was characterized using 1H NMR, 13C NMR, UV-vis and fluorescence spectroscopy and MALDI-TOF/TOF high resolution mass spectrometry. Standard 1H NMR and 13C NMR experiments coupled with nuclear Overhauser effect (NOE) experiments confirmed the structure of the compound. The expected M+ and [M+H]+ ions are observed in the MALDI-TOF/TOF mass spectrum. The UV-vis absorption spectrum of TDBOPP shows a Soret band at 424 nm and three Q bands at 519 nm, 556 nm, and 650 nm with molar absorptivity 3.6×105 cm−1m−1, 1.6×104 cm−1m−1, 1.0×104 cm−1m−1 and 5.3×103 cm−1m−1, respectively. Excitation at 424 nm gives emission at 650 nm. The quantum yield of the porphyrin is 0.11.

Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations

Presented herein is a comprehensive account of the electronic structure of corrole derivatives. Our knowledge in this area derives from a broad range of methods, including UV-vis-NIR absorption and MCD spectroscopies, single-crystal X-ray structure determination, vibrational spectroscopy, NMR and EPR spectroscopies, electrochemistry, X-ray absorption spectroscopy, and quantum chemical calculations, the latter including both density functional theory and ab initio multiconfigurational methods. The review is organized according to the Periodic Table, describing free-base and main-group element corrole derivatives, then transition-metal corroles, and finally f-block element corroles. Like porphyrins, corrole derivatives with a redox-inactive coordinated atom follow the Gouterman four-orbital model. A key difference from porphyrins is the much wider prevalence of noninnocent electronic structures as well as full-fledged corrole^•2- radicals among corrole derivatives. The most common orbital pathways mediating ligand noninnocence in transition-metal corroles are the metal(d_z2)-corrole("a_2u") interaction (most commonly observed in Mn and Fe corroles) and the metal(d_x2-y2)-corrole(a_2u) interaction in coinage metal corroles. Less commonly encountered is the metal(d_π)-corrole("a_1u") interaction, a unique feature of formal d⁵ metallocorroles. Corrole derivatives exhibit a rich array of optical properties, including substituent-sensitive Soret maxima indicative of ligand noninnocence, strong fluorescence in the case of lighter main-group element complexes, and room-temperature near-IR phosphorescence in the case of several 5d metal complexes. The review concludes with an attempt at identifying gaps in our current knowledge and potential future directions of electronic-structural research on corrole derivatives.

Gold Tris(carboxyphenyl)corroles as Multifunctional Materials: Room Temperature Near-IR Phosphorescence and Applications to Photodynamic Therapy and Dye-Sensitized Solar Cells

Two amphiphilic corroles-5,10,15-tris(3-carboxyphenyl)corrole (H3[mTCPC]) and 5,10,15-tris(4-carboxyphenyl)corrole (H3[pTCPC])-and their gold complexes have been synthesized, and their photophysical properties and photovoltaic behavior have been investigated. Like other nonpolar gold corroles, Au[mTCPC] and Au[pTCPC] were both found to exhibit room temperature phosphorescence in deoxygenated solutions with quantum yields of ∼0.3% and triplet lifetimes of ∼75 μs. Both compounds exhibited significant activity as dyes in photodynamic therapy experiments and in dye-sensitized solar cells. Upon irradiation at 435 nm, both Au corroles exhibited significant phototoxicity against AY27 rat bladder cancer cells while the free-base corroles proved inactive. Dye-sensitized solar cells constructed using the free bases H3[mTCPC] and H3[pTCPC] exhibited low efficiencies (≪1%), well under that obtained with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, H2[pTCPP] (1.9%, cf. N719 9.5%). Likewise, Au[pTCPC] proved inefficient, with an efficiency of ∼0.2%. By contrast, Au[mTCPC] proved remarkably effective, exhibiting an open-circuit voltage (Voc) of 0.56 V, a short-circuit current of 8.7 mA cm(-2), a fill factor of 0.72, and an efficiency of 3.5%.

Effects of Bulky Substituents of Push-Pull Porphyrins on Photovoltaic Properties of Dye-Sensitized Solar Cells

To evaluate the effects of substituent bulkiness around a porphyrin core on the photovoltaic properties of porphyrin-sensitized solar cells, long alkoxy groups were introduced at the meso-phenyl group (ZnPBAT-o-C8) and the anchoring group (ZnPBAT-o-C8Cn, n = 4, 8) of an asymmetrically substituted push-pull porphyrin with double electron-donating diarylamino groups and a single electron-withdrawing carboxyphenylethynyl anchoring group. The spectroscopic and electrochemical properties of ZnPBAT-o-C8 and ZnPBAT-o-C8Cn were found to be superior to those of a push-pull porphyrin reference (YD2-o-C8), demonstrating their excellent light-harvesting and redox properties for dye-sensitized solar cells. A power conversion efficiency (η) of the ZnPBAT-o-C8-sensitized solar cell (η = 9.1%) is higher than that of the YD2-o-C8-sensitized solar cell (η = 8.6%) using iodine-based electrolyte due to the enhanced light-harvesting ability of ZnPBAT-o-C8. In contrast, the solar cells based on ZnPBAT-o-C8Cn, possessing the additional alkoxy chains in the anchoring group, revealed the lower η values of 7.3% (n = 4) and 7.0% (n = 8). Although ZnPBAT-o-C8Cn exhibited higher resistance at the TiO2-dye-electrolyte interface by virtue of the extra alkoxy chains, the reduced amount of the porphyrins on TiO2 by excessive addition of coadsorbent chenodeoxycholic acid (CDCA) for mitigating the aggregation on TiO2 resulted in the low η values. Meanwhile, the ZnPBAT-o-C8-sensitized solar cell showed the lower η value of 8.1% than the YD2-o-C8-sensitized solar cell (η = 9.8%) using cobalt-based electrolyte. The smaller η value of the ZnPBAT-o-C8-sensitized solar cell may be attributed to the insufficient blocking effect of the bulky substituents of ZnPBAT-o-C8 under the cobalt-based electrolyte conditions. Overall, the alkoxy chain length and substitution position around the porphyrin core are important factors to affect the cell performance.

Ordered heteromolecular overlayers formed by metal phthalocyanines and porphyrins on rutile titanium dioxide surface studied at room temperature

Molecular heterostructures are formed from meso-tetraphenyl porphyrins-Zn(II) (ZnTPP) and Cu(II)-phthalocyanines (CuPc) on the rutile TiO2(011) surface. We demonstrate that ZnTPP molecules form a quasi-ordered wetting layer with flat-lying molecules, which provides the support for growth of islands comprised of upright CuPc molecules. The incorporation of the ZnTPP layer and the growth of heterostructures increase the stability of the system and allow for room temperature scanning tunneling microscopy (STM) measurements, which is contrasted with unstable STM probing of only CuPc species on TiO2. We demonstrate that within the CuPc layer the molecules arrange in two phases and we identify molecular dimers as basic building blocks of the dominant structural phase.

Functionalized Porphyrins on an Atomically Defined Oxide Surface: Anchoring and Coverage-Dependent Reorientation of MCTPP on Co3O4(111)

We have studied the adsorption of tetraphenylporphyrin (2HTPP) and its carboxylated counterpart mono-para-carboxyphenyltriphenylporphyrin (MCTPP) on an atomically defined Co3O4(111) film under ultrahigh vacuum (UHV) conditions. Using time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we show that 2HTPP adsorbs molecularly in a flat-lying orientation, whereas MCTPP binds to the surface via formation of a chelating bidentate carboxylate upon deposition at 400 K. Combining TR-IRAS and density-functional theory (DFT), we determine the molecular tilting angle as a function of coverage. We show that the MCTPP adsorption geometry changes from a nearly flat-lying orientation (tilting angle <30°) at low coverage to a nearly perfectly upright-standing orientation (tilting angle of approximately 80°) in the full monolayer.

Fabrication and analysis of dye-sensitized solar cells (DSSCs) using porphyrin dyes with catechol anchoring groups

Zinc(ii) porphyrins with catechol anchoring groups used in dye-sensitized solar cells for the first time.

Can nitro groups really anchor onto TiO2? Case study of dye-to-TiO2adsorption using azo dyes with NO2substituents

Molecular rationalization of the photovoltaic performance of dye-sensitized solar cells that employ azo dyes bearing a NO₂anchoring group.