Browsing by Author "Sequeira, S."
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- Assessment of dye distribution in sensitized solar cells by microprobe techniques [Comunicação oral]Publication . Barreiros, M. Alexandra; Corregidor, V.; Alves, L. C.; Guimarães, Fernanda; Sequeira, S.; Mascarenhas, João; Torres, Erica; Brites, Maria JoãoDye sensitized solar cells (DSC’s) have received considerable attention once this technology offers economic and environmental advantages over conventional photovoltaic (PV) devices. A DSC photoanode typically consists of a nanocrystalline porous TiO2 film, endowed with a large adsorptive surface area. Dye molecules that capture photons during device operation are attached to the film nanoparticles. The effective loading of the dye in the TiO2 electrode is of paramount relevance for controlling and optimizing solar cell parameters. In particular, the cell shortcircuit current density (Jsc) is directly proportional to the light harvesting ability of the photoanode, which in turn is strictly dependent on the dye concentration on the TiO2 adsorptive surface. In addition, the dye adsorption behavior affects the cell open circuit voltage (Voc). Relatively few methods are known today for quantitative evaluation of the total dye adsorbed in the film. In this context microprobe techniques come out as suitable tools to evaluate the dye distribution and dye depth profile in sensitized films. Electron Probe Microanalysis (EPMA) and Ion Beam Analytical (IBA) techniques using a micro-ion beam were used to quantify and to study the distribution of the ruthenium organometallic (N719) dye in TiO2 films, making use of their different penetration depth and beam sizes. Two different types of films were prepared and sensitized, mesoporous nanoparticles and 1D nanostructured TiO2 films (about 4 ìm thickness). The high sensitive analytical techniques used allowed to assess dye surface distribution and depth profile, by means of Ru signal, despite the low concentration of this element. X-ray mapping by EPMA/WDS technique made possible to visualise the dye distribution in sample cross-section. PIXE maps of Ru and Ti indicated an homogeneous surface distribution. The assessment of ruthenium depth profile by RBS showed that some films have homogeneous Ru depth distribution while others presented up to half of the Ru concentration in the top layer (2 ìm thickness) when compared to the lower one. Dye load evaluation in different TiO2 films by two different techniques (ìPIXE and EPMA/WDS) provided similar results of Ru/Ti. The assessment of the dye distribution and quantification across an oxide semiconductor film by microprobe techniques can lead to a better understanding of the device performance.
- Charge transport and recombination of dye sensitized 1D nanostructured-TiO2 films prepared by reactive sputteringPublication . Sequeira, S.; Lobato, K.; Torres, Erica; Brites, Maria João; Barreiros, M. Alexandra; Mascarenhas, JoãoDye sensitized solar cells (DSCs) are governed by light absorption, charge injection, electron transport and recombination and electrolyte diffusion. One way to improve the efficiency of these devices is by the design of highly ordered nanostructured semiconductor materials.The advantages can be two-fold: Firstly charge transport within the metal-oxide can be enhanced and hence thicker films can be employed and secondly, the complete permeation with a solid-state hole-transport medium of the sensitized metal-oxide can be facilitated. Nanostructured materials should promote vectorial electron diffusion and have as few recombination sights as possible so as to further enhance electron lifetimes and electron collection efficiencies. These materials should also have a high surface area so as to allow for efficient dye-loading and hence light absorption. Highly ordered TiO2 nanostructured films were prepared by reactive sputtering and their charge transport characteristics evaluated in DSCs. These were compared to DSCs employing mesoporous TiO2 films prepared by doctor blade technique using commercial paste. Charge transport characteristics were evaluated by impedance spectroscopy (IS), incident photon to current conversion efficiencies (IPCE) and current-voltage (iV) curves under simulated AM1.5G irradiation. Film morphology and structural properties were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively.
- Coumarin dye with ethynyl group as p-spacer unit for dye sensitized solar cellsPublication . Torres, Erica; Sequeira, S.; Parreira, P.; Mendes, Paulo; Silva, Tiago J. L.; Lobato, Killian; Brites, Maria JoãoC1-LEN coumarin dye, containing an ethynyl conjugated bridge was tested as a new chromophore for dye-sensitized solar cells (DSSCs). Investigation on the relationship of dye structure, optical properties, electrochemical properties and performance of DSSCs is described. The C1-LEN-sensitized solar cells showed an overall conversion efficiency of 2.2% (JSC = 6.11 mA/cm2, VOC = 547 mV, FF = 0.66), which corresponds to 68% of N719-based device efficiency, fabricated under similar conditions. Electrochemical impedance spectroscopy (EIS) analysis reveals that charge recombination at the nanocrystalline TiO2/dye/redox electrolyte interface is similar for C1-LEN and N719 sensitized solar cells, i.e., charge recombination is not the factor limiting the performance of C1-LEN device.
- Microscopy techniques for dye distribution in DSCs nanocrystalline TiO2 filmsPublication . Barreiros, M. Alexandra; Sequeira, S.; Alves, L. C.; Corregidor, V.; Guimarães, Fernanda; Mascarenhas, João; Brites, Maria João
- Microscopy techniques for dye distribution in DSCs nanocrystalline TiO2 filmsPublication . Barreiros, M. Alexandra; Sequeira, S.; Alves, L. C.; Corregidor, V.; Guimarães, Fernanda; Mascarenhas, João; Brites, Maria JoãoCapture of sunlight has attracted an increasing interest in the scientific community and triggered the development of efficient and cheap photovoltaic devices. Amongst recent generation technologies for solar energy conversion, dye-sensitized solar cells (DSCs) show an optimal trade-off between high-conversion efficiency and low-cost manufacturing. For the last two decades, significant progress has been made and best energy conversion efficiency of the DSC at the laboratory scale has surpassed 12% [1]. A lot of work has focused on the enlargement of surface areas to enhance the amount of adsorbed dyes by reduction of nanoparticle sizes or utilization of novel structures. Nevertheless there remain some crucial details of DSC operation for which limited information is available, namely dye diffusion and adsorption, surface coverage and dye distribution throughout the nc-TiO2 film. Microprobe techniques can be powerful tools to evaluate the dye load, the dye distribution and dye depth profile in sensitized films. Electron Probe Microanalysis (EPMA) and Ion Beam Analytical (IBA) techniques using a micro-ion beam, namely micro-Particle Induced X-ray Emission ( PIXE) and Rutherford Backscattering Spectrometry (RBS), were used to quantify and to study the distribution of the ruthenium organometallic (N719) dye in TiO2 films, profiting from the different penetration depth and beam sizes of each technique. Two different types of films were prepared and sensitized, mesoporous nanoparticles and 1D nanostructured TiO2 films (figure 1). Despite the low concentration of Ru, the high sensitive analytical techniques used allowed to assess the Ru surface distribution and depth profile. Fig. 2 shows the PIXE maps of Ru and Ti indicating an homogeneous surface distribution. The same figure presents the RBS spectra obtained with a 2 MeV proton beam of the same sample showing that a good spectra fit is obtained considering only two sample layers: the first one with a 1.7 ìm thickness; the second one being the SiO2 substrate. The Ru RBS signal also shows that the dye has an homogeneous depth distribution. Due to the fine spatial resolution of the EPMA/WDS (Wavelength Dispersive Spectroscopy) technique it was possible to visualise the dye distribution in sample cross-section (with micrometer or submicrometer dimensions) as presented in Fig. 3 for the elemental mapping of a mesoporous nanoparticle TiO2 film. Dye load evaluation by two different techniques (ìPIXE and EPMA/WDS) provided similar results (Ru/Ti values around 0.5 %). The distribution analysis of the organometallic dye (N719) was done through ruthenium distribution via X-ray mapping. RBS was used to assess the ruthenium depth profile. This assessment can lead to a better understanding of the device performance.
- Synthesis, optical and electrochemical properties of perylenes dyes for solar cells applicationsPublication . Torres, Erica; Sequeira, S.; Berberan-Santos, Mário N.; Brites, Maria JoãoPerylene monoimides or monoanhydrides are being intensively investigated as sensitizers in DSSCs. Keeping only one acceptor group, i.e. imide or anhydride, and introducing a donor group in the 9-position (such as diarylamine) proved to be important in order to obtain a favorable orbital partitioning strength and dipole moment of perylene compounds for DSSCs [1]. One way to control optical and electrochemical properties of perylenes dyes is achieved by functionalizing periand bay positions of perylene core with different substituents. In a simplified view, the peri groups coarsely tune the spectroscopic and electrochemical properties whereas the bay functional groups provide an additional fine tuning [2,3]. Here we report the design and synthesis of new perylene dyes (Figure 1) comprising: (1) a 4-alkoxyphenylamino moiety in the 9-position as a strong donating group, (2) a cyanoacrylic acid as electron acceptor and anchoring group and (3) a triple bond as short and rigid linker between perylene core and the acceptor group [4]. The photophysical properties (i.e. absorption and emission spectra, absorption extinction coefficients, fluorescence quantum yields and lifetime measurements) and electrochemical properties of the new perylene dyes were investigated and all results will be presented and discussed.