Modifications of an unsymmetrical phthalocyanine: Towards stable blue dyes for dye-sensitized solar cells

A new copper phthalocyanine, namely 9(10),16(17),23(24)-tri-tert-butyl-2-[acetynyl-(4-carboxy)phenyl]phthalocyaninatocopper and its related free base have been synthesized as potential stable blue dyes for dye sensitized solar cells. The molecule structure consists on an unsymmetrically-substituted macrocycle bearing three tert-butyl groups and one phenylethynyl moiety as peripheral substituents and it is analogue to that of a previously published zinc derivative. Chemical and optical characterizations, as well as theoretical calculations of the frontier orbitals of both molecules are discussed. To evaluate the effect of the central metal on the photovoltaic behavior of this dye, the novel molecules have been both tested and confronted with the zinc derivative. This last one has shown efficiency values significantly higher than those previously published with a 2.10% maximum efficiency, while the other two dyes yielded 1.66% and 1.38% maximum efficiency respectively.

Scanning probe microscopy studies on the adsorption of selected molecular dyes on titania

Titanium dioxide, or titania, sensitized with organic dyes is a very attractive platform for photovoltaic applications. In this context, the knowledge of properties of the titania-sensitizer junction is essential for designing efficient devices. Consequently, studies on the adsorption of organic dyes on titania surfaces and on the influence of the adsorption geometry on the energy level alignment between the substrate and an organic adsorbate are necessary. The method of choice for investigating the local environment of a single dye molecule is high-resolution scanning probe microscopy. Microscopic results combined with the outcome of common spectroscopic methods provide a better understanding of the mechanism taking place at the titania-sensitizer interface. In the following paper, we review the recent scanning probe microscopic research of a certain group of molecular assemblies on rutile titania surfaces as it pertains to dye-sensitized solar cell applications. We focus on experiments on adsorption of three types of prototypical dye molecules, i.e., perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), phtalocyanines and porphyrins. Two interesting heteromolecular systems comprising molecules that are aligned with the given review are discussed as well.

N-Substituted Azacalixphyrins: Synthesis, Properties, and Self-Assembly

Pre- and postintroduction of substituents with respect to the macrocyclization step leads to previously unknown N-substituted azacalixphyrins. The stepwise synthetic approach has been studied in detail to highlight the key role of the N-substituents of the precursors and/or intermediates in terms of reactivity. Based on a combined experimental and theoretical investigation, the relationship between the properties of the macrocycles and their degree of substitution is rationalized. Depending on the nature of the N-substituents, the formation of supramolecular ribbon-like structures could also be observed, as demonstrated by combined TEM, SEM, AFM, and FTIR experiments.

Significant Influences of Elaborately Modulating Electron Donors on Light Absorption and Multichannel Charge-Transfer Dynamics for 4-(Benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic Acid Dyes

4-(Benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTEBA) as a promising electron acceptor has been used in the highly efficient organic dye-sensitized solar cells (DSCs) recently. Because of its strong electron-deficient character, BTEBA could bring forth a remarkable decline in the energy level of the lowest unoccupied molecular orbital (LUMO) and further reduce the energy gap of dye molecules significantly. In this contribution, two metal-free organic dyes WEF1 and WEF2 were synthesized by simply combining BTEBA with two slightly tailored electron-releasing moieties: 4-hexylphenyl substituted indaceno[1,2-b:5,6-b']dithiophene (IDT) and cyclopenta[1,2-b:5,4-b']dithiophene[2',1':4,5]thieno[2,3-d]thiophene (CPDTDT), which were screened rationally from an electron-donor pool via computational simulation. With respect to those of WEF1, WEF2-sensitized solar cells demonstrate a far better short-circuit photocurrent density (JSC) and open-circuit photovoltage (VOC), resulting in a ∼50% improved power conversion efficiency of 10.0% under irradiance of 100 mW cm(-2) AM1.5G sunlight. We resorted to theoretical calculations, electrical measurements, steady-state, and time-resolved spectroscopic methods to shed light on the fatal influences of elaborately modulating electron donors on light absorption, interfacial energetics, and multichannel charge-transfer dynamics.

The Effect of Donor Group Rigidification on the Electronic and Optical Properties of Arylamine-Based Metal-Free Dyes for Dye-Sensitized Solar Cells: A Computational Study

One of the most significant aspects in the development of dye-sensitized solar cells is the exploration and design of high-efficiency and low-cost dyes. This paper reports the theoretical design of various triphenylamine analogues, wherein the central nitrogen moiety establishes an sp(2)-hybridization, which endows a significant participation in the charge-transfer properties. Density functional theory (DFT) and time-dependent DFT methodologies were utilized to investigate the geometry, electronic structure, photochemical properties, and electrochemical properties of these dyes. Different exchange-correlation functionals were initially evaluated to establish a proper methodology for calculating the excited-state energy of the reference dye, known as DIA3. Consequently, TD-LC-ωPBE with a damping parameter of 0.175 Bohr(-1) best correlates with the experimental value. Four new dyes, namely, Dhk1, Dhk2, Dhk3, and Dhk4, were designed by modifying the rigidity of the donor moiety. According to the results, altering the type and position of binding in the donor group leads to distinct planarity of the dyes, which significantly affects their properties. The designed Dhk4 dye showed more red-shifted and broadened absorption spectra owing to the enhanced coplanarity between its donor and π-bridge moiety, which brings an advantage for its potential use as sensitizer for photovoltaic applications.

Unravelling the effect of anchoring groups on the ground and excited state properties of pyrene using computational and spectroscopic methods

Anchoring groups play an important role in dye sensitized solar cells (DSCs). In order to acquire a suitable anchoring group for DSCs, a deeper understanding of the effect of anchoring groups on the ground and excited state properties of the dye is significant. In this context, various anchoring group connected pyrene derivatives are successfully synthesized and well characterized by using (1)H, (13)C-NMR, FT-IR and EI-MS spectrometry. The anchoring groups employed are carboxylic acid, malonic acid, acrylic acid, malononitrile, cyanoacrylic acid, rhodanine and rhodanine-3-acetic acid. The optimized geometries, HOMO-LUMO energy gap, light harvesting efficiency (LHE) and electronic absorption spectra of these dyes are studied by using density functional theory (DFT) calculations. The results show that pyrene connected with anchoring groups with weak electron pulling strength (PC, PAC and PMC) has a larger HOMO-LUMO energy gap, whereas that connected with anchoring groups with strong electron pulling strength (PCC, PMN, PR and PRA) has a reduced HOMO-LUMO energy gap. These molecules with a reduced energy gap are primarily preferred for DSC applications. Moreover, P, PC, PAC and PMC molecules undergo π→π* transition, whereas PCC, PMN, PR and PRA molecules show significant charge transfer along with π→π* transition. UV-visible absorption spectral studies on these dyes reveal that connecting various anchoring groups with different electron pulling abilities enables the pyrene chromophore to absorb in the longer wavelength region. Notably, an efficient bathochromic shift is observed for PCC, PMN, PR and PRA molecules in both electronic absorption and fluorescence spectral measurements, which suggests that the excitation is delocalized throughout the entire π-system of the molecules. Both theoretical and spectral studies reveal that dyes with an ICT character (PCC, PMN, PR and PRA) are suitable for dye sensitized solar cell applications.

A triazine di(carboxy)porphyrin dyad versus a triazine di(carboxy)porphyrin triad for sensitizers in DSSCs

Two porphyrin-chromophores, i.e. triad PorZn-(PorCOOH)(2)-(piper)2 (GZ-T1) and dyad (PorZn)(2)-NMe2 (GZ-T1), have been synthesized and their photophysical and electrochemical properties have been investigated. The optical properties together with the appropriate electronic energy levels, i.e. the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, revealed that both porphyrin assemblies can function as sensitizers for dye sensitized solar cells (DSSCs). The and -based DSSCs have been prepared and studied using 20 mM CDCA as coadsorbent and were found to exhibit an overall power conversion efficiency (PCE) of 5.88% and 4.56%, respectively (under an illumination intensity of 100 mW cm(-2) with TiO(2) films of 12 μm). The higher PCE of the -sensitized DSSC, as revealed from the current-voltage characteristic under illumination and the incident photon to current conversion efficiency (IPCE) spectra of the two DSSCs, is mainly attributed to its enhanced short circuit current (J(sc)), although both the open circuit voltage (V(oc)) and the fill factor are improved too. The electrochemical impedance spectra (EIS) demonstrated a shorter electron transport time, longer electron lifetime and higher charge recombination resistance for the DSSC sensitized with the dye as well as a larger dye loading onto the TiO(2) surface.

A Systematic Study on the Influence of Electron-Acceptors in Phenanthrocarbazole Dye-Sensitized Solar Cells

In this work, by conjugating 2-cyanoacrylic acid (CA), 4-(benzo[c][1,2,5]thiadiazol-7-yl)benzoic acid (BTBA), 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (EBTBA), and 4-((7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)ethynyl)benzoic acid (EBTEBA) to a binary electron-donor diphenylamine-phenanthrocarbazole (DPA-PC), we systematically investigate the impacts of electron-acceptors upon energy level, energy gap, light-harvesting ability, photovoltaic parameter, and cell stability of donor-acceptor dyes in photoelectrochemical cells. In conjunction with an ionic liquid composite electrolyte, the DPA-PC dye with EBTEBA as electron-acceptor yields a high power conversion efficiency of 8% and an outstanding stability after a 1000 h aging test under the soaking of full sunlight at 60 °C in a dye-sensitized solar cell. Femtosecond fluorescence up-conversion measurements have suggested that energy relaxation and electron injection both occur to dye molecules in the nonequilibrium excited states. Moreover, the time constants of injecting electrons from dye molecules in the excited states to titania are very dispersive for over 1 order of magnitude, mainly owing to the broad energy distribution of excited states.

Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications

A series of phosphonate-derivatized, high redox potential porphyrins with mesityl, pentafluorophenyl, and heptafluoropropyl meso-substituents were synthesized by acid-catalyzed condensation reactions. Ground and excited state redox potentials in the series were varied systematically with the electron-donating or electron-accepting nature of the meso-substitutents. The extent of excitation and injection by porphyrin singlet excited states surface-bound to SnO2/TiO2 core/shell metal oxide nanoparticle films varies with the excited state reduction potential, E°(')(P(+)/P*). With the mesityl-substituted porphyrin, high current density and sustained photocurrents are observed at pH 7 with the addition of the electron transfer donor hydroquinone.

Optical Interference Enhances Nonlinear Spectroscopic Sensitivity: When Light Gives You Lemons, Model Lemonade

Optical interference effects can be a nuisance in spectroscopy, especially in nonlinear experiments in which multiple incoming and outgoing beams are present. Vibrational sum frequency generation is particularly susceptible to interference effects because it is often applied to planar, layered materials, driving many of its practitioners to great lengths to avoid signal generation from multiple interfaces. In this perspective, we take a positive view of this metaphorical "lemon" and demonstrate how optical interference can be used as a tool to extract subtle changes in interfacial vibrational spectra. Specifically, we use small frequency shifts at a buried interface in an organic field-effect transistor to determine the fractional charge per molecule during device operation. The transfer matrix approach to nonlinear signal modeling is general and readily applied to complex layered samples that are increasingly popular in modern studies. More importantly, we show that a failure to consider interference effects can lead to erroneous interpretations of nonlinear data.

Synthesis and characterization of simple cost-effective trans-A2BC porphyrins with various donor groups for dye-sensitized solar cells

Zn(ii) porphyrin dyes have been synthesized in three steps and exhibited power conversion efficiencies of 2.1 to 4.2% which depend on the electron donating ability of the R group.

Star-shaped and star-block polymers with a porphyrin core: from LCST–UCST thermoresponsive transition to tunable self-assembly behaviour and fluorescence performance

The micelles self-assembled from star-shaped and star-block copolymers present a transition of LCST–UCST thermoresponsive properties through a facile quaternization reaction.

π-Extended diketopyrrolopyrrole–porphyrin arrays: one- and two-photon photophysical investigations and theoretical studies

Diketopyrrolopyrrole–porphyrin conjugates show remarkable NIR emission properties, high two-photon absorption cross-sections and significant singlet oxygen production efficiency.

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.

Influence of alkoxy chain envelopes on the interfacial photoinduced processes in tetraarylporphyrin-sensitized solar cells

We have investigated the effect of the alkoxy chain length and nature on the reduction of dye-to-dye aggregation as well as on the enhancement of light harvesting capabilities.

Novel metal-free organic dyes possessing fused heterocyclic structural motifs for efficient molecular photovoltaics

Fused heterocyclic structural motifs possessing novel organic dyes have been synthesized as efficient sensitizers with a highest PCE of 8.08%.

Novel porphyrin-preparation, characterization, and applications in solar energy conversion

Accelerated inner charge transfer in porphyrins promotes a broad light-harvesting ability up to 840 nm and a conversion efficiency of 9.2%.

Are the orientation and bond strength of the RCO2(-)···M link key factors for ultrafast electron transfers?

The photo-induced electron transfers in the "straight up" ionic assemblies [Pd3(2+)]···MCP and [Pd3(2+)]···DCP···[Pd3(2+)] ([Pd3(2+)]* → MCP or DCP) are ultrafast (<85 fs) indicating that it is not necessary to have a strong coordination bond or a bent geometry to obtain fast electron injection in porphyrin-containing DSSCs.

The Role of Porphyrin-Free-Base in the Electronic Structures and Related Properties of N-Fused Carbazole-Zinc Porphyrin Dye Sensitizers

Dye sensitizers can significantly affect power conversion efficiency of dye-sensitized solar cells (DSSCs). Porphyrin-based dyes are promising sensitizers due to their performances in DSSCs. Here, based upon a N-fused carbazole-zinc porphyrin-free-base porphyrin triad containing an ethynyl-linkage (coded as DTBC), the novel porphyrin dyes named DTBC-MP and DTBC-TP were designed by varying the porphyrin-free-base units in the π conjugation of DTBC in order to study the effect of porphyrin-free-base in the modification of electronic structures and related properties. The calculated results indicate that, the extension of the conjugate bridge with the porphyrin-free-base unit results in elevation of the highest occupied molecular orbital (HOMO) energies, decrease of the lowest unoccupied molecular orbital (LUMO) energies, reduction of the HOMO-LUMO gap, red-shift of the absorption bands, and enhancement of the absorbance. The free energy changes demonstrate that introducing more porphyrin-free-base units in the conjugate bridge induces a faster rate of electron injection. The transition properties and molecular orbital characters suggest that the different transition properties might lead to a different electron injection mechanism. In terms of electronic structure, absorption spectra, light harvesting capability, and free energy changes, the designed DTBC-TP is a promising candidate dye sensitizer for DSSCs.

Carbazole Dye-Sensitized Solar Cells Studied from Femtoseconds to Seconds-Effect of Additives in Cobalt- and Iodide-Based Electrolytes

Comprehensive studies of all charge-separation processes in efficient carbazole dye-sensitized solar cells are correlated with their photovoltaic parameters. An important role of partial, fast electron recombination from the semiconductor nanoparticles to the oxidized dye is revealed; this takes place on the picosecond and sub-nanosecond timescales. The charge-transfer dynamics in cobalt tris(bipyridyl) based electrolytes and iodide-based electrolyte is observed to depend on potential-determining additives in a similar way. Upon addition of 0.5 M 4-tert-butylpiridine to both types of electrolytes, the stability of the cells is greatly improved; the cell photovoltage increases by 150-200 mV, the electron injection rate decreases about five times (from 5 to 1 ps(-1) ), and fast recombination slows down about two to three times. Dye regeneration proceeds at a rate of about 1 μs(-1) in all electrolytes. Electron recombination from titania to cobalt electrolytes is much faster than that to iodide ones.

Nanostructures for peroxidases

Peroxidases are enzymes catalyzing redox reactions that cleave peroxides. Their active redox centers have heme, cysteine thiols, selenium, manganese, and other chemical moieties. Peroxidases and their mimetic systems have several technological and biomedical applications such as environment protection, energy production, bioremediation, sensors and immunoassays design, and drug delivery devices. The combination of peroxidases or systems with peroxidase-like activity with nanostructures such as nanoparticles, nanotubes, thin films, liposomes, micelles, nanoflowers, nanorods and others is often an efficient strategy to improve catalytic activity, targeting, and reusability.

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%.

Synthesis and photophysical properties of β-triazole bridged porphyrin–coumarin dyads

Novel β-triazole bridged porphyrin–coumarin dyads are synthesized via a Cu(i)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction and evaluated for their photophysical properties.

Organic dyes with a fused segment comprising benzotriazole and thieno[3,2-b]pyrrole entities as the conjugated spacer for high performance dye-sensitized solar cells

Dyes with a fused segment comprising electron-deficient benzotriazole and electron-rich heteroaromatic entities as the conjugated spacer are promising sensitizers for DSSCs.

Synthesis of 5,10,15,20-tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin (TNBP) and its complexes with zinc and cobalt and an investigation of the photocatalytic activity of nanoFe3O4@ZrO2–TNBP

5,10,15,20-Tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin and its porphyrin complexes with zinc and cobalt and nanoFe₃O₄@ZrO₂–TNBP photocatalysts were synthesized with high yields and the structure and morphology were fully characterized.

BODIPY functionalized o-carborane dyads for low-energy photosensitization

Bodipy-o-carborane dyads undergo a photo-induced electron transfer (PET) process under low energy visible light sensitization and their PET efficiencies are in the 63–71% range.

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.

Enhanced dye-sensitized solar cell photocurrent and efficiency using a Y-shaped, pyrazine-containing heteroaromatic sensitizer linkage

A new approach to dye sensitized solar cells using a pyrazine-containing linkage to join two dyes to the TiO₂ photoanode enhances photovoltage, photocurrent and solar efficiency.

Saddle-shaped porphyrins for dye-sensitized solar cells: new insight into the relationship between nonplanarity and photovoltaic properties

We report on the theoretical and experimental studies of the new dye-sensitized solar cells functionalized with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin zinc(ii) complexes bearing 2- and 8-bromo substituents at the β positions.

Disorder engineering of undoped TiO2 nanotube arrays for highly efficient solar-driven oxygen evolution

Fine-tuning of the bandgap structure of undoped highly ordered TiO₂ nanotube arrays for efficient photo-electrochemical water oxidation.

Synthesis, characterization and electrochemical properties of meso-thiocarboxylate-substituted porphyrin derivatives

Considering the versatility of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPPF20) to react with nucleophiles we highlight here the synthesis and characterization of several mono- and tetra-thiocarboxylate derivatives. The selective displacement of the para-fluorine groups in TPPF20 by thiocarboxylic acids demonstrates that TPPF20 is an ideal platform for the rapid formation of thiocarboxylate porphyrins. The optical and electrochemical features of the thiocarboxylate derivatives were also examined thinking on their potential use in photovoltaic devices. From their electrochemical characterization the following parameters were taken into account: (i) electronegative induced effect of the thiocarboxylate dyes owing the presence of the fluorine and sulfur atoms on the molecular structure of the porphyrin; and (ii) the free rotation and flexibility features that such S atom gives to the porphyrin relatively to the semiconductor.

Influence of porphyrinic structure on electron transfer processes at the electrolyte/dye/TiO₂ interface in PSSCs: a comparison between meso push-pull and β-pyrrolic architectures

Time-resolved photophysical and photoelectrochemical investigations have been carried out to compare the electron transfer dynamics of a 2-β-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO2 interface in PSSCs. Although the meso push-pull structural arrangement has shown, up to now, to have the best performing architecture for solar cell applications, we have obtained superior energy conversion efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the I(-)/I3(-)-based electrolyte. To gain deeper insights about these unexpected results, we have investigated whether the intrinsic structural features of the two different porphyrinic dyes can play a key role on electron transfer processes occurring at the dye-sensitized TiO2 interface. We have found that charge injection yields into TiO2 are quite similar for both dyes and that the regeneration efficiencies by I(-), are also comparable and in the range of 75-85%. Moreover, besides injection quantum yields above 80%, identical dye loading, for both ZnB and ZnM, has been evidenced by spectrophotometric measurements on transparent thin TiO2 layers after the same adsorption period. Conversely, major differences have emerged by DC and AC (electrochemical impedance spectroscopy) photoelectrochemical investigations, pointing out a slower charge recombination rate when ZnB is adsorbed on TiO2. This may result from its more sterically hindered macrocyclic core which, besides guaranteeing a decrease of π-staking aggregation of the dye, promotes a superior shielding of the TiO2 surface against charge recombination involving oxidized species of the electrolyte.

Influence of structural variations in push-pull zinc porphyrins on photovoltaic performance of dye-sensitized solar cells

We designed and synthesized two new zinc porphyrin dyes for dye-sensitized solar cells (DSCs). Subtle molecular structural variation in the dyes significantly influenced the performance of the DSC devices. By utilizing these dyes in combination with a cobalt-based redox electrolyte using a photoanode made of mesoporous TiO2 , we achieved a power conversion efficiency (PCE) of up to 12.0 % under AM 1.5 G (100 mW cm(-2)) simulated solar light. Moreover, we obtained a high PCE of 6.4 % for solid-state dye-sensitized solar cells by using 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene as a hole-transporting material.

New solution processed bulk-heterojunction organic solar cells based on a triazine-bridged porphyrin dyad as electron donor

An unsymmetrical porphyrin dyad (ZnP)-[triazine-Npip]-(ZnPCOOH) consisting of two zinc-porphyrin units covalently linked to a peridine-containing triazine group has been used with PC₇₁BM as electron donor and acceptor, respectively, for the active layer of solution-processed BHJ organic solar cells.