Alberto Vomiero

Structure and optical properties of Au-polyimide nanocomposite films prepared by ion implantation

Au-polyimide nanocomposites have been synthesized by implanting different doses of Au+ ions in 100nm thick films of pyromellitic dianhydride-4,4′ oxydianiline polyimide, prepared by glow discharge vapor deposition polymerization. Unambiguous evidence of Au nanoclusters growth is found only at the highest implantation doses (5×1016Au+∕cm2). Structural, compositional, and optical characterizations show that the implantation induces the compactation of the initial film due to H and C loss. The resulting structure is a composite containing 2–3nm gold nanoparticles arranged in a layer of about 40nm and, just beneath the sample surface, a 15nm thick carbon-rich layer. Optical simulations suggest the presence of a gold-carbon core-shell structure in the nanoparticles.

Graphene below the percolation threshold in TiO2 for dye-sensitized solar cells

We demonstrate a fast and large area-scalable methodology for the fabrication of efficient dye sensitized solar cells (DSSCs) by simple addition of graphene micro-platelets to TiO2 nanoparticulate ...

Enhanced photovoltaic properties in dye sensitized solar cells by surface treatment of SnO2 photoanodes

We report the fabrication and testing of dye sensitized solar cells (DSSC) based on tin oxide (SnO2) particles of average size ~20u2009nm. Fluorine-doped tin oxide (FTO) conducting glass substrates were treated with TiOx or TiCl4 precursor solutions to create a blocking layer before tape casting the SnO2 mesoporous anode. In addition, SnO2 photoelectrodes were treated with the same precursor solutions to deposit a TiO2 passivating layer covering the SnO2 particles. We found that the modification enhances the short circuit current, open-circuit voltage and fill factor, leading to nearly 2-fold increase in power conversion efficiency, from 1.48% without any treatment, to 2.85% achieved with TiCl4 treatment. The superior photovoltaic performance of the DSSCs assembled with modified photoanode is attributed to enhanced electron lifetime and suppression of electron recombination to the electrolyte, as confirmed by electrochemical impedance spectroscopy (EIS) carried out under dark condition. These results indicate that modification of the FTO and SnO2 anode by titania can play a major role in maximizing the photo conversion efficiency.

Metal-free organic dyes for TiO2 and ZnO dye-sensitized solar cells

We report the synthesis and characterization of new metal-free organic dyes (namely B18, BTD-R, and CPTD-R) which designed with D-π-A concept to extending the light absorption region by strong conjugation group of π-linker part and applied as light harvester in dye sensitized solar cells (DSSCs). We compared the photovoltaic performance of these dyes in two different photoanodes: a standard TiO2 mesoporous photoanode and a ZnO photoanode composed of hierarchically assembled nanostructures. The results demonstrated that B18 dye has better photovoltaic properties compared to other two dyes (BTD-R and CPTD-R) and each dye has higher current density (Jsc) when applied to hierarchical ZnO nanocrystallites than the standard TiO2 mesoporous film. Transient photocurrent and photovoltage decay measurements (TCD/TVD) were applied to systematically study the charge transport and recombination kinetics in these devices, showing the electron life time (τR) of B18 dye in ZnO and TiO2 based DSSCs is higher than CPTD-R and BTD-R based DSSCs, which is consistent with the photovoltaic performances. The conversion efficiency in ZnO based DSSCs can be further boosted by 35%, when a compact ZnO blocking layer (BL) is applied to inhibit electron back reaction.

Effect of blocking layer to boost photoconversion efficiency in ZnO dye-sensitized solar cells.

The effect of a ZnO compact blocking layer (BL) in dye-sensitized solar cells (DSSCs) based on ZnO photoanodes is investigated. BL is generated through spray deposition onto fluorine-doped tin oxide (FTO) conducting glass before the deposition of a ZnO active layer. The functional properties of dye-sensitized solar cells (DSSCs) are then investigated as a function of the thickness of the BL for two different kinds of ZnO active layer, i.e., hierarchically self-assembled nanoparticles and microcubes composed of closely packed ZnO sheets. Presence of BL leads to the improvement of photoconversion efficiency (PCE), by physically insulating the electrolyte and the FTO. This effect increases at increasing BL thickness up to around 800 nm, while thicker BL results in reduced cell performance. Remarkable increase in Jsc is recorded, which doubles as compared to cells without blocking layer, leading to PCE as high as 5.6% in the best cell under one sun irradiation (AM 1.5 G, 100 mW cm(-2)). Electrochemical impedance spectroscopy (EIS) elucidates the mechanism boosting the functional features of the cells with BL, which relies with enhanced chemical capacitance together with an almost unchanged recombination resistance, which are reflected in an increased electron lifetime. The results foresee a straightforward way to significantly improve the performance of ZnO-based DSSCs.

Modulating Exciton Dynamics in Composite Nanocrystals for Excitonic Solar Cells

Quantum dots (QDs) represent one of the most promising materials for third-generation solar cells due to their potential to boost the photoconversion efficiency beyond the Shockley-Queisser limit. Composite nanocrystals can challenge the current scenario by combining broad spectral response and tailored energy levels to favor charge extraction and reduce energy and charge recombination. We synthesized PbS/CdS QDs with different compositions at the surface of TiO2 nanoparticles assembled in a mesoporous film. The ultrafast photoinduced dynamics and the charge injection processes were investigated by pump-probe spectroscopy. We demonstrated good injection of photogenerated electrons from QDs to TiO2 in the PbS/CdS blend and used the QDs to fabricate solar cells. The fine-tuning of chemical composition and size of lead and cadmium chalcogenide QDs led to highly efficient PV devices (3% maximum photoconversion efficiency). This combined study paves the way to the full exploitation of QDs in next-generation photovoltaic (PV) devices.

Low-energy-channeling surface analysis on silicon crystals designed for high-energy-channeling in accelerators

Channeling of relativistic particles in bent Si crystals is a powerful technique for use with accelerators. Its efficiency can be found to be highly dependent on the state of the surface of the crystal steering the particles. We investigated the morphology and structure of the surface of the samples that have been used with high efficiency for channeling in accelerators. Low-energy channeling of 2MeVα particles or protons was used as a probe. We found that mechanical treatment of the samples leads to a superficial damaged layer, which is correlated to efficiency limitations of the crystal in accelerators. In contrast, chemical etching, which was used to treat the surface of the most efficient crystals, leaves a surface with superior perfection.

Hybrid thermal-field emission of ZnO nanowires

The electron emission properties of an array of ZnO nanowires were studied in the temperature range of 300-473 K. An almost doubling of the current density at 473 K under an electric field of 8 V/μm (j(T=473u2009K) = 190 μA/cm2, j(T=300u2009K)u2009=u2009114 μA/cm2) was observed together with a reduction of the turn-on field from 552u2009V/μm to 482u2009V/μm. Theoretical model that combines the thermal-field emission for high electric field and the Schottky emission for the low field can satisfactorily account for temperature dependence of current at low as well as at high applied bias. The obtained effect is particularly appealing for the application in micro-gun for THz vacuum tubes.

Deposition of Nanostructured CdS Thin Films by Thermal Evaporation Method: Effect of Substrate Temperature

Nanocrystalline CdS thin films were grown on glass substrates by a thermal evaporation method in a vacuum of about 2 × 10−5 Torr at substrate temperatures ranging between 25 °C and 250 °C. The physical properties of the layers were analyzed by transmittance spectra, XRD, SEM, and four-point probe measurements, and exhibited strong dependence on substrate temperature. The XRD patterns of the films indicated the presence of single-phase hexagonal CdS with (002) orientation. The structural parameters of CdS thin films (namely crystallite size, number of grains per unit area, dislocation density and the strain of the deposited films) were also calculated. The resistivity of the as-deposited films were found to vary in the range 3.11–2.2 × 104 Ω·cm, depending on the substrate temperature. The low resistivity with reasonable transmittance suggest that this is a reliable way to fine-tune the functional properties of CdS films according to the specific application.

Light harvester band gap engineering in excitonic solar cells: A case study on semiconducting quantum dots sensitized rainbow solar cells

Abstract A systematic study on the fabrication of quantum dots sensitized solar cells (QDSSCs) exploiting hybrid networks of semiconducting light harvesters is presented, which shows how the engineering of band gaps of the device components by a very simple technique allows improving the solar energy conversion performances. Panchromatic devices are fabricated and tested, and correspondent functional parameters analyzed in order to highlight both advantages and drawbacks of the most common (CdS, CdSe, PbS) quantum dots applied for light collection in QDSSCs. Judicious engineering of the light harvester layer is demonstrated as a simple and powerful strategy for boosting device performances, through the management of light collection in a rather broad range of solar spectrum and photogenerated charges injection and collection.