Amparo Forneli

Catalysis of Recombination and Its Limitation on Open Circuit Voltage for Dye Sensitized Photovoltaic Cells Using Phthalocyanine Dyes

In order to increase the energy efficiency of dye-sensitized solar cells beyond 10%, an improved dye needs to be developed with greater light absorption in the red and near-infrared. Many dyes have been tested for this purpose; however, no dye with significant absorption beyond 750 nm has functioned properly. We have examined a series of ruthenium phthalocyanines, a dye class with large and tunable absorption in the red. For these dyes we observe a large reduction in the output voltage of the cells relative to the benchmark dye (N719). By examination of photovoltage transients and charge density measurements, we demonstrate that this reduction in voltage is caused by a 100-fold increase in the rate constant for recombination (iodine reduction) at the TiO2/electrolyte interface. N719, however, does not seem to catalyze this reaction. By examination of the literature, we propose that catalysis of the recombination reaction may be occurring for many other classes of potentially useful dyes including porphyri...

Stepwise Cosensitization of Nanocrystalline TiO2 Films Utilizing Al2O3 Layers in Dye‐Sensitized Solar Cells

Keywords: aluminum oxide ; electron transfer ; hole transfer ; sensitizers ; titanium dioxide ; Efficient Co-Sensitization ; Organic Sensitizers ; Recombination Dynamics ; Charge Recombination ; Highly Efficient ; Blocking Layers ; Electrodes ; Dendrimers ; Cyanine Reference EPFL-ARTICLE-160552doi:10.1002/anie.200802852View record in Web of Science Record created on 2010-11-30, modified on 2016-08-09

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.

Interfacial charge recombination between e(-)-TiO2 and the I(-)/I3(-) electrolyte in ruthenium heteroleptic complexes: dye molecular structure-open circuit voltage relationship.

A series of heteroleptic ruthenium(II) polypyridyl complexes containing phenanthroline ligands have been designed, synthesized, and characterized. The spectroscopic and electrochemical properties of the complexes have been studied in solution and adsorbed onto semiconductor nanocrystalline metal oxide particles. The results show that for two of the ruthenium complexes, bearing electron-donating (-NH2) or electron-withdrawing (-NO2) groups, the presence of the redox-active I(-)/I3(-) electrolyte produces important changes in the interfacial charge transfer processes that limit the device performance. For example, those dyes enhanced the electron recombination reaction between the photoinjected electrons at TiO2 and the oxidized redox electrolyte. In an effort to understand the details of such striking observations, we have monitored the charge transfer reactions taking place at the different interfaces of the devices using time-resolved single photon counting, laser transient spectroscopy, and light-induced photovoltage measurements.

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.

Co-sensitized DSCs: dye selection criteria for optimized device Voc and efficiency

Co-sensitization of nanocrystalline TiO2 with the organic dye D2 and the zinc phthalocyanine dye TT1 improves light harvesting, Jsc and efficiency of DSC devices. However, the Voc of the co-sensitized cell is markedly inferior (∼130 mV) when compared to the reference device made with D2 only. We discuss the implications of our results with regard to selection criteria for dyes for co-sensitized DSCs.

Dye structure–charge transfer process relationship in efficient ruthenium-dye based dye sensitized solar cells

The characterization of the interfacial charge transfer processes taking place in dye solar cells made using the most efficient ruthenium complexes, namely cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (N719), tris(isothiocyanato)-ruthenium(II)-2,2′:6′,2′′-terpyridine-4,4′,4′′- tricarboxylic acid, tris-tertrabutylammonium salt (Black Dye) and cis-bis(isothiocanate)(4,4′-bis(5-hexylthiophene-2-yl)-2,2′-bipyridine)(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)ruthenium(II) sodium (C101), has been carried out. The comparison between these devices shows that devices made using N719 have the slowest recombination dynamics between the photo-injected electrons and the oxidized electrolyte. Moreover, for devices made using Black Dye, the dye ground state regeneration dynamics are faster than for C101 and N719. The implications for future ruthenium dyes are discussed.

The role of para-alkyl substituents on meso-phenyl porphyrin sensitised TiO2 solar cells: control of the eTiO2/electrolyte+ recombination reaction

We aim to investigate the effect of adding hydrophobic alkyl chains substituents to unsymmetrical free base tetra-phenyl porphyrins used for the preparation of dye sensitised solar cells (DSSC). We have used two different unsymmetrical meso-tetraphenyl substituted free base porphyrins attending to two objectives: (1) to observe how the substitution of three para positions of the meso-phenyl groups with hydrophobic alkyl chains influences the formation of molecular aggregates onto the semiconductor nanoparticles and (b) to deduce the influence that the substitution exerts over the eTiO2/electrolyte+ recombination reaction in operating devices. To achieve these goals we have focussed on the study of the electron transfer processes that take place at the different interfaces of the photovoltaic device using electrochemistry, steady-state and time resolved spectroscopic techniques.

From Biomass Wastes to Highly Efficient CO2 Adsorbents: Graphitisation of Chitosan and Alginate Biopolymers

Carbon spheres from natural biopolymers (alginate and chitosan) are easily synthesised by thermal treatment between 400 and 800 °C under an inert atmosphere. All the samples, including the untreated natural biopolymers, as well as the resulting carbon materials, exhibit a remarkable CO₂-adsorption capacity. The sample that exhibits the highest adsorption capacity was that obtained by carbonisation of alginate at 800 °C and subsequent treatment with KOH at 800 °C. This material exhibits a specific surface area of 765 m²  g⁻¹, specific micropore volume of 0.367 cm³  g⁻¹, ultra-micropore volume of 0.185 cm³  g⁻¹, average ultra-micropore size of 0.7 nm and CO₂-adsorption capacity of 5 mmol g⁻¹ measured at 0 °C and atmospheric pressure. This value is close to the absolute record for CO₂ adsorption and, by far, the highest if we compare unit areas or consider the density of the material. The combination of the high N content already included in the chitosan structure and the elevated microporosity in the case of alginate are crucial factors to obtain these satisfactory values with an easy and green preparation procedure. Also, owing to the high conductivity of the alginate-derived carbon (better than graphite), it has been possible to develop a process of reversible adsorption-desorption by applying a voltage, which is a low-energy desorption method compared with the conventional method of vacuum and high temperatures. All these properties, together with the spherical shape of the material of 0.1 mm, which is the most suitable form to favour mass transfer in fluidised-bed reactors, make this material a highly promising adsorbent for industrial applications.

Interfacial photo-induced charge transfer reactions in perylene imide dye sensitised solar cells

The interfacial electron transfer reactions of a perylene imide based dye sensitised solar cell (DSSC) with an efficiency of 3.15% under standard conditions have been characterised. The observed interfacial charge transfer dynamics under different sensitisation conditions have been correlated with the device efficiency.