Theoretical Study on Thin Film Dye Sensitized Photovoltaic Solar Cells

This thesis presents two models of a dye-sensitized solar cell (DSC): diffusion model and electrical model. The main purpose is to investigate interfacial charge transfer and charge transport within the semiconductor/electrolyte layer under illuminated conditions. These two interrelated models confirm that diffusion is the major driving force for electron and ion transport, while the drift of electrons is negligible. The diffusion model was utilized to simulate the temperature influence on the overall efficiency of DSC with a consideration of the voltage loss at titanium dioxide (TiO2)/ transparent conductive oxide (TCO) interface. It reveals that low temperature conditions have serious detrimental effects on the DSCs' performance. Further the electrical model was used to analyze the effect of diffusion/drift, dye loading, and electrode thickness on DSC performance. The predicted optimal electrode thickness ranges between 10-15 μm which is consistent with the thickness (10 μm) used in experimental studies published in the literature

Push-pull heterocyclic dyes based on pyrrole and thiophene: synthesis and evaluation of their optical, redox and photovoltaic properties

Three heterocyclic dyes were synthesized having in mind the changes in the photovoltaic, optical and redox properties by functionalization of 5-aryl-thieno[3,2-b]thiophene, 5-arylthiophene and bis-methylpyrrolylthiophene π-bridges with different donor, acceptor/anchoring groups. Knoevenagel condensation of the aldehyde precursors with 2-cyanoacetic acid was used to prepare the donor-acceptor functionalized heterocyclic molecules. These organic metal-free dyes are constituted by thieno[3,2-b]thiophene, arylthiophene, bis-methylpyrrolylthiophene, spacers and one or two cyanoacetic acid acceptor groups and different electron donor groups (alkoxyl, and pyrrole electron-rich heterocycle). The evaluation of the redox, optical and photovoltaic properties of these compounds indicate that 5-aryl-thieno[3,2-b]thiophene-based dye functionalized with an ethoxyl electron donor and a cyanoacetic acid electron acceptor group/anchoring moiety displays as sensitizer for DSSCs the best conversion efficiency (2.21%). It is mainly assigned to the higher molar extinction coefficient, long π-conjugation of the heterocyclic system, higher oxidation potential and strong electron donating capacity of the ethoxyl group compared to the pirrolyl moiety.Thanks are due to the Fundacao para a Ciencia e Tecnologia (FCT) for a PhD grant to S.S.M. Fernandes (SFRH/BD/87786/2012) and to FEDER-COMPETE for financial support at the research center CQUM (UID/QUI/0686/2016) and (UID/QUI/0686/2020). The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project No. 022161 (co-financed by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). The group in Porto would like to acknowledge the European Commission through the Seventh Framework Programme, the Specific Programme "Ideas" of the European Research Council for research and technological development as part of an Advanced Grant under Grant No. 321315, which also partially funded this work. Some of this work was also performed under the project "SunStorage-Harvesting and storage of solar energy", with reference POCI-01-0145-FEDER-016387, funded by the European Regional Development Fund (ERDF), through COMPETE 2020-Operational Programme for Competitiveness and Internationalization (OPCI), and by national funds, through FCT. The group in Porto is also thankful to POCI-01-0145-FEDER-006939 (Laboratory for Process Engineering, Environment, Biotechnology and Energy-UID/EQU/00511/2013) funded by the European Regional Development Fund (ERDF), through COMPETE2020-Programa Operacional Competitividade e Internacionalizacao (POCI) and by national funds, LEPABE-2-ECO-INNOVATION, supported by the North Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERD). Institutional Review Board Statement: Not applicable

Nanostructured semiconductor materials for dye-sensitized solar cells

Since O'Regan and Grätzel's first report in 1991, dye-sensitized solar cells (DSSCs) appeared immediately as a promising low-cost photovoltaic technology. In fact, though being far less efficient than conventional silicon-based photovoltaics (being the maximum, lab scale prototype reported efficiency around 13%), the simple design of the device and the absence of the strict and expensive manufacturing processes needed for conventional photovoltaics make them attractive in small-power applications especially in low-light conditions, where they outperform their silicon counterparts. Nanomaterials are at the very heart of DSSC, as the success of its design is due to the use of nanostructures at both the anode and the cathode. In this review, we present the state of the art for both n-type and p-type semiconductors used in the photoelectrodes of DSSCs, showing the evolution of the materials during the 25 years of history of this kind of devices. In the case of p-type semiconductors, also some other energy conversion applications are touched upon. © 2017 Carmen Cavallo et al

Organic dyes containing coplanar dihexyl-substituted dithienosilole groups for efficient dye-sensitised solar cells

peer-reviewedA chromophore containing a coplanar dihexyl-substituted dithienosilole (CL1) synthesised for use in dye-sensitised solar cells displayed an energy conversion efficiency of 6.90% under AM 1.5 sunlight irradiation. The new sensitiser showed a similar fill factor and open-circuit voltage when compared with N719. Impedance measurements showed that, in the dark, the charge-transfer resistance of a cell using CL1 in the intermediate-frequency region was higher compared to N719 (69.8 versus 41.3 Omega). Under illumination at AM 1.5G-simulated conditions, the charge-transfer resistances were comparable, indicative of similar recombination rates by the oxidised form of the redox couple. The dye showed instability in ethanol solution, but excellent stability when attached to TiO2. Classical molecular dynamics indicated that interactions between ethanol and the dye are likely to reduce the stability of CL1 in solution form. Time-dependent density functional theory studies were performed to ascertain the absorption spectrum of the dye and assess the contribution of various transitions to optical excitation, which showed good agreement with experimental results.PUBLISHEDpeer-reviewe

Visualization and Quantification of the Laser-Induced Art of TiO2 by Photoexcitation of Adsorbed Dyes

Dye-pretreated anatase TiO2 films, commonly used as photoanodes in dye sensitized solar cells (DSSC), were utilized as a model system to investigate the laser-induced anatase to rutile phase transformation (ART), using N719 dye, N749 dye, D149 dye, and MC540 dye as photo-sensitizers. The visible lasers (532 and 785 nm) used for Raman spectroscopy were able to transform pure anatase into rutile at the laser spot when excitation of the dye sensitizer caused an electron injection from the excited state of the dye molecule into the conduction band of the TiO2. The three dyes with carboxylic acid anchor groups (N719, N749 and D149 dyes) experienced ART upon dye excitation; diffuse reflectance infrared Fourier transform (DRIFTS) and Raman spectra validated that these dyes were chemisorbed to the semiconductor surface. The MC540 dye with a sulfonic acid anchor group did not experience ART and the DRIFTS and Raman spectra were inconclusive about the chemisorption of this dye to the TiO2. A TiO2 calibration curve and percent rutile contour plots developed for this project are able to quantify the amount of rutile created at the surface of the samples. These improved chemical images which map rutile concentration help to visualize how ART propagates from the center of the laser spot to the surroundings. Factors such as visible light absorption and anchor groups that covalently bind to the semiconductor play a key role in effective laser induced ART

Quantifying Regeneration in Dye Sensitized Solar Cells: A Step Toward Red Absorbing Dyes having Lower Energy Loss

A limiting factor on DSSC efficiency is the lower fraction of the solar spectrum that is absorbed by the dye molecules developed to this point. Dye molecules that function well in DSSCs tend to have poor or no absorption to the red of 750 nm. Extending this absorption to the red by 100 nm, without losing efficiency in other ways, would result in a significant improvement in photocurrent. This challenge has proven difficult, in large part because of one slow reaction in the electron transfer cycle of DSSCs, the regeneration reaction. Better understanding of this reaction is thus critical. The kinetics of regeneration is understudied relative to the other processes in DSSCs, this is in part because the regeneration reaction produces no, as yet detected, measurable electrical signal. It must be studied by more difficult transient absorbance (TA) techniques. The first step of this thesis focuses on isolating a reliable transient signal that reflects the regeneration reaction. This is made by upgrading the conventional TA system to also acquire transient electrical (TE) signals simultaneously (TA-TE). The system is used to characterize dye-sensitized solar cells (DSSCs) under 1 sun illumination whilst the cells are fully operational and their stability is monitored. The second step of the work consists of the development of a methodology and a kinetic model which uses the isolated regeneration signal and a range of complimentary measurements on operating cells, to determine the quantum yield and the associated intrinsic rate constants and orders of the regeneration reaction. This enabled understanding of the regeneration mechanism and its optional rate limiting steps. Finally, the use of steady state photoinduced absorption (SSPA), as a complementary or alternative tool to assess regeneration, is also questioned. SSPA is compared with the regeneration TA –TE and charge extraction measurements

Solar Cells

The second book of the four-volume edition of "Solar cells" is devoted to dye-sensitized solar cells (DSSCs), which are considered to be extremely promising because they are made of low-cost materials with simple inexpensive manufacturing procedures and can be engineered into flexible sheets. DSSCs are emerged as a truly new class of energy conversion devices, which are representatives of the third generation solar technology. Mechanism of conversion of solar energy into electricity in these devices is quite peculiar. The achieved energy conversion efficiency in DSSCs is low, however, it has improved quickly in the last years. It is believed that DSSCs are still at the start of their development stage and will take a worthy place in the large-scale production for the future

Push-Pull Heterocyclic Dyes Based on Pyrrole and Thiophene: Synthesis and Evaluation of Their Optical, Redox and Photovoltaic Properties

Three heterocyclic dyes were synthesized having in mind the changes in the photovoltaic, optical and redox properties by functionalization of 5-aryl-thieno[3,2-b]thiophene, 5-arylthiophene and bis-methylpyrrolylthiophene π-bridges with different donor, acceptor/anchoring groups. Kno-evenagel condensation of the aldehyde precursors with 2-cyanoacetic acid was used to prepare the donor-acceptor functionalized heterocyclic molecules. These organic metal-free dyes are con-stituted by thieno[3,2-b]thiophene, arylthiophene, bis-methylpyrrolylthiophene, spacers and one or two cyanoacetic acid acceptor groups and different electron donor groups (alkoxyl, and pyrrole electron-rich heterocycle). The evaluation of the redox, optical and photovoltaic properties of these compounds indicate that 5-aryl-thieno[3,2-b]thiophene-based dye functionalized with an ethoxyl electron donor and a cyanoacetic acid electron acceptor group/anchoring moiety displays as sensitizer for DSSCs the best conversion efficiency (2.21%). It is mainly assigned to the higher molar extinction coefficient, long π-conjugation of the heterocyclic system, higher oxidation potential and strong electron donating capacity of the ethoxyl group compared to the pirrolyl moiety

Photoelectrochemical cells employing molecular light-harvesting materials for the capture and conversion of solar energy

2017 Spring.Includes bibliographical references.Solar light has the potential to be a substantial contributor to global renewable energy production. The diffuse nature of solar energy requires that commercially viable devices used to capture, convert, and store that energy be inexpensive relative to other energy-producing technologies. Towards this end, photoelectrochemical cells have been the subject of study for several decades. Particularly interesting to chemists, molecular light-harvesting materials can be employed in photoelectrochemical cells. For example, a dye-sensitized solar cell (DSSC) is a type of photoelectrochemical cell designed to capture solar energy and convert it to electricity. Alternatively, molecular light-harvesting materials have also been employed in water-splitting photoelectrolysis cells (PECs), which capture solar energy and store it in the form of chemical bonds such as H2 and O2. The work presented in this dissertation falls into two major projects. The first involves fundamental studies of water-oxidizing PECs employing a novel perylene diimide molecule as the light-harvesting unit. Background is provided in Chapter II, composed of a comprehensive literature review of water-oxidizing PEC systems that employ light-harvesting materials composed of earth-abundant elements. Chapter III describes preliminary studies of a water oxidizing PEC composed of a perylene diimide organic thin-film (OTF) and cobalt oxide catalyst, the first of its kind in the literature. Characterization of this novel device provided knowledge of the efficiency-limiting processes that would need to be addressed in order to improve device performance. Subsequently, Chapter IV describes preliminary studies of the same perylene diimide molecule in an alternative, literature-precedented, dye-sensitized photoelectrolysis cell (DS-PEC) architecture aimed at improving the efficiency-limiting processes of the first OTF-PEC. Characterization of this DS-PEC architecture reveals that the efficiency-limiting processes of the OTF-PEC were indeed improved. However, deposition of the cobalt oxide catalyst onto the DS-PEC did not successfully result in water oxidation. Alternative catalyst-deposition strategies from the literature are described as direction for future studies. The second project of this dissertation involves the study of novel high-redox-potential organometallic cobalt complexes as redox mediators in DSSCs, and is presented in Chapter V. Therein, it was found that the use of electron-withdrawing functional groups on cobalt coordinating ligands not only increased the redox potential, but also increased the lability of the ligands. The resulting complex instability caused performance-limiting electron-recombination reactions in assembled DSSCs. These results point future researchers towards the study of higher-chelating ligands for enhanced stability in high-potential cobalt complexes