Alessandra Tacca

Photoanodes Based on Nanostructured WO3 for Water Splitting

Anodically grown WO(3) photoelectrodes prepared in an N-methylformamide (NMF) electrolyte have been investigated with the aim of exploring the effects induced by anodization time and water concentration in the electrochemical bath on the properties of the resulting photoanodes. An n-type WO(3) semiconductor is one of the most promising photoanodes for hydrogen production from water splitting and the electrochemical anodization of tungsten allows very good photoelectrodes, which are characterized by a low charge-transfer resistance and an increased spectral response in the visible region, to be obtained. These photoanodes were investigated by a combination of steady state and transient photoelectrochemical techniques and a correlation between photocurrent produced, morphology, and charge transport has been evaluated.

Quasi-1D hyperbranched WO3 nanostructures for low-voltage photoelectrochemical water splitting

Arrays of hyperbranched high aspect ratio mesostructures are grown by pulsed laser deposition through a self-assembly process from the gas phase. These hierarchical arrays are successfully used as photoanodes for the photoelectrochemical splitting of water, exhibiting an onset potential as low as 0.4 V vs. RHE and saturation current densities up to 1.85 mA cm−2 at 0.8 V vs. RHE. While the latter is similar to the state of the art values for WO3 nanostructured photoanodes, both the onset and saturation voltages are significantly lower than any other reported values. This peculiar behavior is attributed to the hyperbranched structure and to its excellent optical and electronic properties.

Hematite photoanodes modified with an Fe(III) water oxidation catalyst.

Hematite photoelectrodes prepared via a hydrothermal route are functionalized with a water oxidation catalyst consisting of amorphous Fe(III) oxide, obtained by successive ionic layer adsorption and reaction. The performances of the catalyst-modified photoanodes are considerably higher than those of the parent electrodes, resulting in a nearly doubled photoanodic current in all the basic aqueous electrolytes explored in this study. The combination of electrochemical impedance spectroscopy and laser flash photolysis indicates that the presence of the catalyst results in enhanced hole trapping in surface reactive states exposed to the electrolyte, allowing for a more successful competition between charge transfer and recombination.

Double acceptor D–A copolymers containing benzotriazole and benzothiadiazole units: chemical tailoring towards efficient photovoltaic properties

This study reports the design and synthesis of a series of D–A copolymers alternating benzothiadiazole and benzotriazole acceptors to a donor co-unit, and investigates their photovoltaic properties in bulk heterojunction solar cells with PC71BM. Successive modifications to the copolymers are carried out, passing from thiophene to benzodithiophene (BDT) donor co-units and from regular to random alternation of the accepting units. A copolymer containing a thiophene co-unit has reached a power conversion efficiency (PCE) of 1.88% in optimised devices. Moving from thiophene to BDT leads to some advantages, such as a lower optical energy gap and a lower confinement of the photo-generated charges, that positively affect the spectral coverage to solar radiation and the fill factor (FF) parameter in the devices. Passing from regular to random distribution of the accepting units, the BDT copolymer solar cells have reached a PCE of 5%. Such encouraging photovoltaic performances are combined with high solubility, high molecular weight and with an easy preparation procedure. These characteristics are necessary requirements to envisage the scale-up to industrial applications.

Novel Terthiophene-Substituted Fullerene Derivatives as Easily Accessible Acceptor Molecules for Bulk-Heterojunction Polymer Solar Cells

Five fulleropyrrolidines and methanofullerenes, bearing one or two terthiophene moieties, have been prepared in a convenient way and well characterized. These novel fullerene derivatives are characterized by good solubility and by better harvesting of the solar radiation with respect to traditional PCBM. In addition, they have a relatively high LUMO level and a low band gap that can be easily tuned by an adequate design of the link between the fullerene and the terthiophene. Preliminary results show that they are potential acceptors for the creation of efficient bulk-heterojunction solar cells based on donor polymers containing thiophene units.

Reactivity of decafluorobenzophenone and decafluoroazobenzene towards aromatic diamines: a practical entry to donor–acceptor systems

A series of Donor–Acceptor–Donor (D–A–D) and Acceptor–Donor–Acceptor (A–D–A) compounds have been prepared exploiting the relative ability of polyfluorinated azobenzenes and benzophenone to undergo aromatic nucleophilic substitution reactions with aromatic amines. A high para-regioselectivity is obtained when fluorene and carbazole-based diamines have been used in a high Donor Number solvent environment such as DMSO. The prepared triads have been employed in the synthesis of oligomers with the aim of evaluating them as photovoltaic material additives in optoelectronic applications.

Hyperbranched TiO2–CdS nano-heterostructures for highly efficient photoelectrochemical photoanodes

Quasi-1D-hyperbranched TiO2 nanostructures are grown via pulsed laser deposition and sensitized with thin layers of CdS to act as a highly efficient photoelectrochemical photoanode. The device properties are systematically investigated by optimizing the height of TiO2 scaffold structure and thickness of the CdS sensitizing layer, achieving photocurrent values up to 6.6 mA cm-2 and reaching saturation with applied biases as low as 0.35 VRHE. The high internal conversion efficiency of these devices is to be found in the efficient charge generation and injection of the thin CdS photoactive film and in the enhanced charge transport properties of the hyperbranched TiO2 scaffold. Hence, the proposed device represents a promising architecture for heterostructures capable of achieving high solar-to-hydrogen efficiency.

Design, Synthesis, Characterization and Use of Random Conjugated Copolymers for Optoelectronic Applications

We report the synthesis and the optoelectronic characterization of a new family of random conjugated copolymers based on 9, 9-bisalkylfluorene, thiophene and benzothiadiazole monomers unit synthesized by a palladium-catalyzed Suzuki cross-coupling reaction. The photophysical, thermal, electrochemical properties were investigated. The electronic structures of the copolymers were simulated via quantum chemical calculations. Bulk heterojunction solar cells based on these copolymers blended with fullerene, exhibited power conversion efficiency as high as 1% under illumination of 97 mWcm− 2. One of the synthesized copolymers has been successfully tested as active layer in simple light-emitting diode, working in the green spectral region and exhibiting promising optical and electrical properties. This study suggests that these random copolymers are versatile and are promising in a wide range of optoelectronic devices.