Javier Ortiz

Energetic factors governing injection, regeneration and recombination in dye solar cells with phthalocyanine sensitizers

Phthalocyanines (Pcs) are promising candidates for photon collectors for the near-infrared region of the solar spectrum in dye solar cells (DSC). Using two new Pc sensitizers, differing only in their Zn or metal-free center, we discuss the behaviour of the Pc dyes as sensitizers in DSC, and their influence on the solar cell performance, in comparison with standard N719 dye, at identical electrolyte conditions. Based on the separate identification of recombination resistance and the energy levels of titania by impedance spectroscopy, we determine the recombination rates and we also map the energies of the relevant components of the solar cells. An integration of the absorbance of the three sensitizers with the solar spectral distribution shows that electron injection should be similar for all the dyes (although 20% less in the case of the Pc with Zn center). However, since the LUMO level of these Pcs is low on the energy scale, it is necessary to use an electrolyte that keeps the conduction band of titania down to obtain some injection from the Pcs. Hence, the photovoltage is limited because the conduction band of titania is at a much lower position than in normal DSCs. We find that the conduction band of titania with the metal-free Pc is located on a similar energy level as for N719, and the recombination rate is not significantly different from the N719 cell. The main reason for the lower performance of this Pc is the lower injection than the ruthenium complex. In the case of the ZnPc, the conduction band is at higher energy than for N719. This allows that the Voc obtained in these two samples becomes nearly the same despite the lower electron injection in the Zn phthalocyanines.

Control of Photoinduced Electron Transfer in Zinc Phthalocyanine−Perylenediimide Dyad and Triad by the Magnesium Ion

Photoexcitation of a zinc phthalocyanine-perylenediimide (ZnPc-PDI) dyad and a bis(zinc phthalocyanine)-perylenediimide [(ZnPc) 2-PDI] triad results in formation of the triplet excited state of the PDI moiety without the fluorescence emission, whereas addition of Mg (2+) ions to the dyad and triad results in formation of long-lived charge-separated (CS) states (ZnPc (*+)-PDI (*-)/Mg (2+) and (ZnPc) 2 (*+)-PDI (*-)/Mg (2+)) in which PDI (*-) forms a complex with Mg (2+). Formation of the CS states in the presence of Mg (2+) was confirmed by appearance of the absorption bands due to ZnPc (*+) and PDI (*-)/Mg (2+) complex in the time-resolved transient absorption spectra of the dyad and triad. The one-electron reduction potential ( E red) of the PDI moiety in the presence of a metal ion is shifted to a positive direction due to the binding of Mg (2+) to PDI (*-), whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg (2+) to PDI (*-) was confirmed by the ESR spectrum, which is different from that of PDI (*-) without Mg (2+). The energy of the CS state (ZnPc (*+)-PDI (*-)/Mg (2+)) is determined to be 0.79 eV, which becomes lower that of the triplet excited state (ZnPc- (3)PDI*: 1.07 eV). This is the reason why the long-lived CS states were attained in the presence of Mg (2+) instead of the triplet excited state of the PDI moiety.

Submillisecond-lived photoinduced charge separation in a fully conjugated phthalocyanine–perylenebenzimidazole dyad

A fully electronically conjugated phthalocyanine–perylenemonoimidebenzimidazole system, ZnPc–PMIBI 2, where the conjugation goes through the imide position of the perylene has been synthesized. The preparation was made possible by the condensation of a new unsymmetrically substituted diaminophthalocyanine, ZnPc(NH2)2, with a perylene monoanhydride monoimide. Both the experimental and the computational (DFT) results indicate that ZnPc–PMIBI exhibits significant intramolecular electronic interactions. The lifetime of the charge-separated (CS) state was extended to 0.26 ms, corresponding to the longest value ever reported for a covalent phthalocyanine–peryleneimide system in solution, and is attributed to the synergy of an extremely low CS energy, lower than the triplet energy of each chromophore, together with the coupling between both units, allowing fast charge separation.

Fullerene acting as an electron donor in a donor–acceptor dyad to attain the long-lived charge-separated state by complexation with scandium ion

A long-lived charge-separated (CS) state of fullerene-trinitrofluorenone linked dyad in which fullerene acts as an electron donor is formed by photoinduced electron transfer from C60 to TNF in the presence of Sc(OTf)3; the CS lifetime is determined as 23 ms in PhCN at 298 K.

Determination of the glass transition temperature of photorefractive polymer composites from photoconductivity measurements

A method to determine the glass transition temperature (Tg) of photorefractive (PR) polymer composites, based on photoconductivity measurements, is proposed. The composite Tg is identified as the temperature at which a clear change in slope in the Arrhenius plot representation of the photoconductivity is obtained. We demonstrate that for PR polymers, this method is more appropriate than the one generally used, based on differential scanning calorimetry (DSC). For PR composites based on the hole transporting polymer poly(N-vinylcarbazole), the Tg value determined from photoconductivity data is around 30°C higher than that obtained by DSC.

Near-Infrared Photoelectrochemical Conversion via Photoinduced Charge Separation in Supramolecular Complexes of Anionic Phthalocyanines with Li+@C60

Two phthalocyanines possessing carboxylate groups ((TBA)4H2Pc·1 and (TBA)4H2Pc·2) form 1:2 supramolecular complexes with lithium cation-encapsulated C60 (Li(+)@C60) [H2Pc·1(4-)/(Li(+)@C60)2 and H2Pc·2(4-)/(Li(+)@C60)2] in a polar mixed solvent. From the UV-vis spectral changes, the binding constants (K) were estimated as ca. 10(12) M(-2). Upon the photoexcitation of constructed supramolecular complexes, photoinduced electron transfer occurred to form the charge-separated (CS) state. The lifetime of the CS state was determined to be 1.2 ms for H2Pc·2(4-)/(Li(+)@C60)2, which is the longest CS lifetime among the porphyrinoid/fullerene supramolecular complexes. H2Pc·1(4-)/(Li(+)@C60)2 also afforded the long-lived CS state of 1.0 ms. The spin state of the long-lived CS states was determined to be a triplet, as indicated by the EPR signal at g = 4. The reorganization energy (λ) and the electronic coupling term were determined to be λ = 1.70 eV, V = 0.15 cm(-1) from the temperature dependence of the rate constant for the charge recombination of the CS state of H2Pc·1(4-)/(Li(+)@C60)2. The energy of the CS state (0.49 eV) is much smaller than the reorganization energy, indicating that the back-electron-transfer process is located in the Marcus normal region. The small electronic coupling term results from the spin-forbidden back electron transfer due to the triplet CS state. Supramolecular complexes of anionic zinc phthalocyanines with Li(+)@C60 were also prepared and investigated. The ZnPc·4(4-)/Li(+)@C60 supramolecular nanoclusters were assembled on the optically transparent electrode (OTE) of nanostructured SnO2 (OTE/SnO2) to construct the dye-sensitized solar cell. The IPCE (incident photon-to-photocurrent efficiency) values of OTE/SnO2/(ZnPc·4(4-)/Li(+)@C60)n were much higher than the sum of the two IPCE values of the individual systems OTE/SnO2/(Li(+)@C60)n and OTE/SnO2/(ZnPc·4(4-))n, covering the near-infrared region.

Photorefractive properties of an unsensitized polymer composite based on a dicyanostyrene derivative as nonlinear optical chromophore

We report on the photorefractive (PR) properties at the 633nm laser wavelength of a polymer composite based on the polymer poly(n-vinyl carbazole) (PVK), doped with the dicyanostyrene derivative 4-piperidinobenzylidene-malonitrile (PDCST) as nonlinear optical chromophore and the liquid plasticizer butyl benzyl phthalate (BBP), without the presence of sensitizer. The PR-effect is observed only when samples are previously subjected to an electric field (i.e., 20V∕μm for 10min). Photoconductivity and birefringence of the composite become significant when the electric field treatment is performed at temperatures higher than room temperature (24°C). Gain coefficient and PR speed, determined from two-beam coupling experiments, are compared to those obtained with the PVK∕PDSCT∕BBP∕C60 standard sensitized composite.

Large-Size Star-Shaped Conjugated (Fused) Triphthalocyaninehexaazatriphenylene

Star-shaped triphthalocyaninehexaazatriphenylene 1 was synthesized via condensation between a new building block 1,2-diaminophthalocyanine and cyclohexanehexaone. Compound 1 represents the largest star-shaped phthalocyanine-fused hexaazatriphenylene reported so far. This largely expanded phthalocyanine shows good solubility and has a strong tendency to aggregate both in solution and on the surface, indicating its potential as an active component in organic electronic devices.

Synthesis and characterization of a sulfur-containing phtha locyanine-gold nanoparticle hybrid

This work reports on the synthesis and characterization of a new gold nanoparticle- zinc phthalocyanine system, AuNP-S( t Bu)3ZnPc, prepared by a ligand exchange reaction of tetraoctylammonium bromide with a novel unsymmetrically substituted zinc phthalocyanine which contains one thioester group in the peripheral position (AcS( t Bu)3ZnPc). The AuNp-S( t Bu)3ZnPc hybrid was characterized using UV-vis and 1 H NMR spectroscopies. Transmission electron microscopy allowed the estimation of the size, which was calculated to be ~5 nm. AuNPs-S( t Bu)3ZnPc conjugate showed much lower fluorescence quantum yield values than the AcS( t Bu)3ZnPc demonstrating either an energy or electron transfer from the ZnPc to the AuNP. The AuNP-S( t Bu)3ZnPc hybrid has been anchored to a TiO2 semiconducting layer using lipoic acid. A solid configuration of TiO 2-lipoic acid-AuNP-S( t Bu)3ZnPc has been prepared by anchoring lipoic acid to the TiO2 (TiO2-LA) and introducing later the TiO2-LA with free thiol groups in a toluene solution of AuNP-S( t Bu)3ZnPc. We have also observed by UV-vis and fluorescence measurements the importance of the ZnPc in avoiding AuNP aggregation on the TiO 2 surface.

Phthalocyanine-Gold Nanoparticle Hybrids: Modulating Quenching with a Silica Matrix Shell.

An asymmetrically substituted zinc phthalocyanine (ZnPc) with a terminal dithiolane group is anchored to Au nanoparticles (NPs) directly or through an interposed silica matrix. Transient absorption spectroscopy shows that the quenching of the ZnPc excited state by AuNPs occurs through a photoinduced electron transfer, the efficiency of which is modulated by the presence of the insulating silica matrix.