Master of Science / Department of Chemistry / Jun Li / With the increasing consumption of energy and the depletion of fossil fuels, finding an alternative energy source is critical. Solar energy is one of the most promising energy sources and solar cells are the devices that convert solar radiation into electricity. Currently, the most widely used solar cell is based on p-n junction formed with crystalline silicon materials. While showing high efficiency, the high fabrication cost limits its broad applications. Dye sensitized solar cell (DSSC) is a promising low-cost alternative to the Si solar cell, but its efficiency is much lower. Improvements in materials and interfaces are needed to increase the DSSC efficiency while maintain the low cost. In this thesis, three projects were investigated to optimize the DSSC efficiency and reduce the cost. The first project is to optimize the TiO[subscript]2 barrier layers on Fluorine-doped Tin Dioxide (FTO) surface. Two preparation methods, i.e. TiCl[subscript]4 solution treatment and thermal oxidation of sputtered Ti metal films, were employed and systematically studied in order to minimize electron-hole recombination and electron backflow during photovoltaic processes of DSSCs. TiCl[subscript]4 solution treatment method was found to create a porous TiO[subscript]2 barrier layer. Ti sputtering method created a very compact TiO[subscript]2 blocking layer. Two methods showed different characteristics and may be used for different DSSC studies. The second project is to reduce the DSSC cost while maintaining the efficiency by replacing the expensive Pt counter electrode with a novel vertically aligned carbon nanofiber (VACNF) electrode. A large specific electrode surface area (~125 cm[superscript]2 over 1 cm[superscript]2 geometric area) was obtained by using VACNFs. The relatively high surface area, good electric conductivity and the large numbers of active graphitic edges existed in cone-like microstructure of VACNFs were employed to improve redox reaction rate of I[subscript]3[superscript]-/I[superscript]- mediators in the electrolyte. Faster electron transfer and good catalytic activities were obtained with such counter electrodes. The third project is to develop a metal organic chemical vapor deposition (MOCVD) method to coat TiO[subscript]2 shells on VACNF arrays as potential photoanodes in the DSSC system in order to improve the electron transfer. Fabrication processes were demonstrated and preliminary materials were characterized with scanning electron microscopy and transmission electron microscopy. MOCVD at 300 mTorr vapor pressure at 550° C for 120 min was found to be the optimal condition.

Dye-sensitized solar cells (DSSCs), based on use of a black counter electrode (BCE) and thin TiO2 electrode (photoelectrode), have been developed to reduce related manufacturing costs. Despite their effectiveness in lowering manufacturing cost, the above DSSCs have a low photovoltaic performance, owing to their insufficient light harvesting efficiency. This work presents a novel metal-based light scattering layer (MLSL), which can be formed either on a black counter electrode or on a thin TiO2 electrode, to reflect the light passing through the latter. The proposed MLSL increases the light harvesting efficiency from the interior of the cell, thus enhancing the photovoltaic performance of DSSC. Experimental results indicate that the proposed MLSL also reduces the internal resistance, as well as increases the electron collection efficiency of DSSC, subsequently increasing the power conversion efficiency by 116%. This work also designs a low-cost external lightguide (EL), which is disposed on the exterior of photoelectrode of DSSC, to direct light towards the dye-covered nanoporous TiO2 film (D-NTF) of the photoelectrode. Incorporating EL can increase the light harvesting efficiency from the exterior of the cell, thus enhancing the photovoltaic performance of DSSC. Furthermore, in addition to increasing the light harvesting efficiency by 30.69%, the proposed EL increases the photocurrent density by 38.12% and power conversion efficiency by 25.09%.

Exploring new technologies that can meet the world’s energy demands in an efficient and clean manner is critically important due to the depletion of natural resources and environmental concerns. Dye-sensitized solar cells (DSSCs) are low-cost and clean technology options that use solar energy efficiently and are being intensively studied. How to further reduce the cost of this technology while enhancing device performance is one of the demanding issues for large scale application and commercialization of DSSCs. In this research dissertation, four main contributions are made in this regard with the motivation to reduce further cost of DSSC technology. Firstly, ~10% efficiencies were achieved after developing understanding of key concepts and procedures involved in DSSCs fabrication. These efficiencies were achieved after step-by-step modifications in the DSSC design. Secondly, carbon nanotubes (CNTs) were successfully employed as an alternative to Pt in the counter electrodes of DSSCs. DSSCs fabricated with CNTs were ~86% as efficient as Pt-based cells. Non-aligned CNTs were successfully grown using four different CVD methods and finally, multi-walled vertically aligned CNTs (MW-VACNTs) were synthesized using water-assisted chemical vapor deposition (WA-CVD). Thirdly, carbon derived from pyrolysis of nanocrystalline cellulose (NCC) was successfully employed in counter electrodes of DSSCs instead of Pt. DSSCs with NCC were ~58% as efficient as Pt-based DSSCs. Fourthly, novel organic metal-free dyes were designed and employed instead of commonly used Ru-based dyes. DSSCs with these novel sensitizers were ~62% as efficient as those using the conventional Ru-based dyes. Characterization techniques including current-voltage measurements, scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), cyclic voltammetery (CV), thermogravimetric analysis (TGA), small angle x-ray scattering (SAXS), atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) were used.

La finalité de cette étude était la réalisation d’un afficheur électrochrome sur papier activable par un smartphone comme solution anti-contrefaçon. La nature cellulosique du substrat et la faible énergie délivrée par le smartphone ont représenté deux contraintes majeures dans de ce travail de thèse. Ainsi, une nouvelle méthode originale de dépôt à température ambiante de films minces WO3 a été développée en utilisant un traitement UV. Le premier dispositif électrochrome couplant le film WO3 sur papier, le Bleu de Prusse, et une membrane électrolytique plastifiée, a montré une bonne stabilité électrochimique associée à un contraste optique ΔR de 13 % pour une tension d’activation de -1 V. En vue de minimiser la tension d’activation, de nouveaux dispositifs électrochromes, à architecture simplifiée, dérivés d’une configuration classique à 5 couches ont été mis au point. Le remplacement du matériau de contre électrode et du conducteur électronique par un matériau métallique unique a conduit à des dispositifs à 4 couches. De plus, l’utilisation d’un matériau électrochrome conducteur a permis de s’affranchir de la couche conductrice électronique adjacente à l’électrode de travail menant à des dispositifs encore plus simplifiés à 3 couches. Les dispositifs simplifiés se caractérisent par des performances électrochromes très intéressantes pour une activation à seulement -0,7 V et présentent une stabilité électrochimique sur plus de 500 cycles. Le premier prototype sur emballage de parfum, à base d’afficheur électrochrome constitué de PEDOT, d’électrolyte liquide ionique polymérisé par vernis UV, et de contre électrode d’Ag, a été activé avec succès par un smartphone en moins de 5,3 secondes pour une énergie d’activation de 0,33 mJ.cm-2. / This study aims at the development of an electrochromic display on paper, activated by a smartphone as solution for counterfeit. A novel deposition method at room temperature of WO3 thin films was developed using a UV treatment. 5-layer electrochromic device (ECD), based on WO3 on paper, Prussian blue, and an electrolyte membrane, showed good electrochemical stability for more than 500 cycles associated with an optical contrast in reflectance ΔR of 13% at -1 V. In order to minimize the activation potential, ECDs with simplified architecture, derived from 5-layer conventional configuration have been developed in a second part of this work. 4-layer ECDs were constructed by replacing the counter-electrode and conductive layer by a single metal layer. In addition, the use of an electrochromic conducting material leads to 3-layer device. 4 and 3-layer ECDs exhibit high electrochromic performances at only -0.7 V with nice stability for more than 500 cycles. The first prototype integrated on perfume packaging, with 4-layer ECD based on PEDOT (poly(3,4-ethylenedioxythiophene), ionic liquid electrolyte polymerized by UV-varnish, and Ag as counter-electrode was successfully activated by a smartphone in less than 5.3 seconds for an activation energy of 0.33 mJ.cm-2.