Miguel García-Iglesias

Structure-function relationships in unsymmetrical zinc phthalocyanines for dye-sensitized solar cells.

A series of unsymmetrical zinc phthalocyanines bearing an anchoring carboxylic function linked to the phthalocyanine ring through different spacers were designed for dye-sensitised solar cells (DSSC). The modification of the spacer group allows not only a variable distance between the dye and the nanocrystalline TiO(2), but also a distinct orientation of the phthalocyanine on the semiconductor surface. The photovoltaic data show that the nature of the spacer group plays a significant role in the electron injection from the photo-excited dye into the nanocrystalline TiO(2) semiconductor, the recombination rates and the efficiency of the cells. The incident monochromatic photon-to-current conversion efficiency (IPCE) for phthalocyanines bearing an insulating spacer is as low as 9%, whereas for those with a conducting spacer an outstanding IPCE 80% was obtained.

Increasing the efficiency of zinc-phthalocyanine based solar cells through modification of the anchoring ligand

Several zinc-based phthalocyanines have been synthesized and used in Dye-Sensitized Solar Cells (DSSC). The results have been compared with the standard TT1 phthalocyanine, which shows good light-to-energy conversion efficiencies in comparison with other IR sensitizers used in DSSC. We show herein that the anchoring moiety is critical for both achieving high injection yields and slow back electron transfer dynamics that affect the overall device efficiency. Moreover, based on these results, we have synthesized a new phthalocyanine with a superior performance, when compared to the TT1 dye, with a subtle change on the anchoring moiety, thus leading to a higher photocurrent response.

Effect of anchoring groups in zinc phthalocyanine on the dye-sensitized solar cell performance and stability

We have designed and developed an unsymmetrical zinc phthalocyanine (TT9) sensitizer that consists of three tert-butyl and two carboxylic acid groups that act as “push” and “pull”, respectively. The two carboxylic acid groups graft the sensitizer onto the semiconductor surface resulting in enhanced stability under heat and light compared to the similar unsymmetrical zinc phthalocyanine (TT1) sensitizer that consists of three tert-butyl and only one carboxylic acid groups. The solar cells containing the TT9 and TT1 sensitizers with non-volatile electrolyte were subjected to light soaking conditions at 60 °C. Under these conditions, the short circuit current of the TT1 sensitized solar cell after 1000 h decreases to half of its initial value where as the TT9 sensitized solar cell remained the same demonstrating the influence of number of anchoring groups on the stability of zinc phthalocyanine sensitized solar cells.

Light-harvesting with panchromatically absorbing BODIPY–porphyrazine conjugates to power electron transfer in supramolecular donor–acceptor ensembles

Prerequisites for the design of efficient organic solar light converting systems are intense absorptions across the visible region, the ability to funnel excited state energy by intramolecular energy transfer, and the option to partake in photoinduced electron transfer processes. We have established a versatile synthesis platform for functionalized porphyrazines and present the synthesis of a light harvesting dibenzoquinoxalinoporphyrazine that is peripherally substituted with eight alkynyl-linked BODIPY chromophores. Photophysical investigation by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy revealed efficient intramolecular energy transfer from the photoexcited BODIPY to the porphyrazine core. Coordination of a pyridyl-functionalized phenothiazine (PTZ) to the central metal ion of the porphyrazine generates a complex capable of an efficient electron transfer from the PTZ to the photoexcited porphyrazine core. The porphyrazine˙−/PTZ˙+ fingerprints in the visible and in the near-infrared regions as well as the electron transfer dynamics were determined using spectroelectrochemistry and femtosecond transient absorption spectroscopy. The findings form the basis for the development of supramolecular multichromophore ensembles as materials in solar light converting systems.

Effect of bulky groups in ruthenium heteroleptic sensitizers on dye sensitized solar cell performance

Two novel heteroleptic ruthenium sensitizers, TT204 and TT205, containing bulky substituents at the 4,4-positions of the ancillary 2,2-bipyridine ligand, were synthesized and characterized. They exhibit absorption maxima in the visible region at 520–530 nm due to metal-to-ligand charge transfer (MLCT) transitions. The EHOMO and ELUMO values indicate sufficient driving force for efficient dye regeneration by the iodide/tri-iodide redox electrolyte and efficient electron injection into the TiO2 conduction band following photoexcitation, respectively. The performance of these heteroleptic sensitizers in dye sensitized solar cells was investigated where TiO2 sensitization was carried out both in the presence and absence of chenodeoxycholic acid. The best efficiency among these sensitizers was recorded in the presence of chenodeoxycholic acid which generated a high short circuit current of 18.7 mA cm−2, an open circuit potential of 0.72 V and a fill factor of 73% with a resulting total power conversion efficiency of 9.81% under AM 1.5G 1 sun illumination. Comparison with the reference dye C101 indicates that though bulky groups can prevent aggregation resulting in high photocurrents even in the absence of chenodeoxycholic acid, they can also lead to lower cell voltages due to inefficient dye packing on the TiO2 surface.

Carboxy-1,4-phenylenevinylene- and carboxy-2, 6-naphthylene-vinylene unsymmetrical substituted zinc phthalocyanines for dye-sensitized solar cells

Two unsymmetrical Zn(II) phthalocyanines 1 and 2 bearing an anchoring carboxylic function linked to the phthalocyanine ring through different rigid arylenevinylene bridges have been designed for dye-sensitized solar cell (DSSC) applications. The phthalocyanines 1 and 2, when anchored onto nanocrystalline TiO2 films, yielded 30% incident monochromatic photon-to-current conversion efficiency (IPCE) and 2% power conversion efficiencies under AM1.5 sun.

Screening interactions of zinc phthalocyanine–PPV oligomers with single wall carbon nanotubes—a comparative study

In this paper, the ability to disperse single wall carbon nanotubes (SWNT) of several different-nature poly(p-phenylene vinylene) (PPV) oligomers having pendant zinc phthalocyanines (ZnPc) has been investigated. Based on the quenching of the ZnPc and SWNT fluorescence in the supramolecular assemblies, it has been shown that parameters such as p/n-type character of the oligomer, size and the distance between the ZnPc moiety and the conjugated backbone play an important role in the strength of the interactions. Important results suggest that n-type oligomers as well as certain flexibility in the phthalocyanine arrangement are breakthroughs for immobilizing SWNT in THF, affording stable and homogeneous suspensions. Transient absorption measurements confirm that upon photoexcitation the photoexcited ZnPc triggers an intraensemble charge transfer to yield oxidized ZnPc and reduced SWNT.

Supramolecular assembly of multicomponent photoactive systems via cooperatively coupled equilibria.

Here, we show that the synergistic interplay between two binding equilibria, acting at different sites of a (Zn)phthalocyanine-amidine molecule (Pc1), enables the dissociation of the photoinactive phthalocyanine dimer (Pc1)2 into a three-component system, in which a sequence of light harvesting, charge separation, and charge shift is successfully proven. The aforementioned dimer is assembled by dual amidine-Zn(II) coordination between neighboring Pc1 molecules and gives rise to high association constants (KD ≈ 10(11) M(-1)). Such extraordinary stability hampers the individual binding of either carboxylic acid ligands through the amidine group or pyridine-type ligands through the Zn(II) metal atom to (Pc1)2. However, the combined addition of both ligands, which cooperatively bind to different sites of Pc1 through distinct noncovalent interactions, efficiently shifts the overall equilibrium toward a photoactive tricomponent species. In particular, when a fullerene-carboxylic acid (C60A) and either a dimethylamino-pyridine (DMAP) or a phenothiazine-pyridine ligand (PTZP) are simultaneously present, the photoactivity is turned on and evidence is given for an electron transfer from photoexcited Pc1 to the electron-accepting C60A that affords the DMAP-Pc1(•+)-C60A(•-) or PTZP-Pc1(•+)-C60A(•-) radical ion pair states. Only in the latter case does a cascade of photoinduced electron transfer processes afford the PTZP(•+)-Pc1-C60A(•-) radical ion pair state. The latter is formed via a thermodynamically driven charge shift evolving from PTZP-Pc1(•+)-C60A(•-) and exhibits lifetimes that are notably longer than those of DMAP-Pc1(•+)-C60A(•-).

A Versatile Method for the Preparation of Ferroelectric Supramolecular Materials via Radical End-Functionalization of Vinylidene Fluoride Oligomers

A synthetic method for the end-functionalization of vinylidene fluoride oligomers (OVDF) via a radical reaction between terminal olefins and I-OVDF is described. The method shows a wide substrate scope and excellent conversions, and permits the preparation of different disc-shaped cores such as benzene-1,3,5-tricarboxamides (BTAs), perylenes bisimide (PBI), and phthalocyanines (Pc) bearing three to eight ferroelectric oligomers at their periphery. The formation, purity, OVDF conformation, and morphology of the final adducts has been assessed by a combination of techniques, such as NMR, size exclusion chromatography, differential scanning calorimetry, polarized optical microscopy, and atomic force microscopy. Finally, PBI-OVDF and Pc-OVDF materials show ferroelectric hysteresis behavior together with high remnant polarizations, with values as high as Pr ≈ 37 mC/m(2) for Pc-OVDF. This work demonstrates the potential of preparing a new set of ferroelectric materials simply by attaching OVDF oligomers to different small molecules. The use of carefully chosen small molecules paves the way to new functional materials in which ferroelectricity and electrical conductivity or light-harvesting properties coexist in a single compound.

Nanopatterned Superlattices in Self‐Assembled C2‐Symmetric Oligodimethylsiloxane‐Based Benzene‐1,3,5‐Tricarboxamides

The synthesis of C3 - and C2 -symmetric benzene-1,3,5-tricarboxamides (BTAs) containing well-defined oligodimethylsiloxane (oDMS) and/or alkyl side chains has been carried out. The influence of the bulkiness of the oDMS chains in the aggregation behavior of dilute solutions of the oDMS-BTAs in methylcyclohexane was studied by temperature-dependent UV spectroscopy. The formation of hierarchically self-assembled aggregates was observed at different BTA concentrations, the tendency of aggregation increases by shortening or removing oDMS chains. Chiral BTAs were investigated with circular dichroism (CD) spectroscopy, showing a stronger tendency to aggregate than the achiral ones. Majority rules experiments show a linear behavior consistent with the existence of a high mismatch penalty energy. The most efficient oDMS-BTAs organogelators have the ability to form stable organogels at 5 mg mL(-1) (0.75 wt %) in hexane. Solid-state characterization techniques indicate the formation of an intermolecular threefold hydrogen bonding between adjacent molecules forming thermotropic liquid crystals, exhibiting a hexagonal columnar organization from room temperature to above 150 °C. A decrease of the clearing temperatures was observed when increasing the number and length of the oligodimethylsiloxane chains. In addition to the three-fold hydrogen bonding that leads to columnar liquid crystalline phase, segregation between the oDMS and aliphatic chains takes place in the BTA functionalized with two alkyl and one oDMS chain leading to a superlattice within the hexagonal structure with potential applications in lithography.