Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published versio

Chapter 5 Optical Properties of Excitons in ZnSe-based Quantum Well Heterostructures

Publisher Summary This chapter reviews the most recent experimental and theoretical work about the optical properties of excitons in ZnSe-based quantum wells. It briefly summarizes the basic theoretical concepts used to model quasi-two-dimensional excitons in quantum wells. The chapter discusses the linear optical properties of excitons, including the electrostatic and thermal stability, the strength of the quantum size effect in quantum wells and thin films, the phototransport properties, and the temporal evolution of the excitonic transitions. The nonlinear optical properties of excitons are considered. The chapter discusses the interaction of excitons with a single-carrier and a two-carrier plasma and the role of excitons in the lasing processes of II-VI quantum wells. The role of excitons in the operation of optoelectronic devices operating in the blue-green region has been emphasized in the discussion of the nonlinear optical properties. The unique combination of large exciton binding energy and reduced screening and phonon coupling occurring in quantum wells permits the observation of novel phenomena connected with the coexistence of the exciton gas and the free-carrier gas at high density

Transient optical studies of photoinduced charge transfer in semiconductor quantum dot solar cells

Semiconductor quantum dots (also referred to as 'nanocrystals‘) are well suited as light-harvesting agents in solar cells because they are robust, have tuneable effective band gaps, and are easy to process. The research presented in this thesis is targeted towards the study of excitonic solar cells employing semiconductor nanocrystals as a light harvesting component. Gaining control of the interfacial charge transfer processes in operation in these devices forms a crucial part of any attempt to optimise their performance. In particular, the use of transient spectroscopic techniques reveals how efficient and long-lived charge separation can be achieved in these solar cell architectures. The primary focus of this research is to investigate the parameters influencing charge transfer in dye-sensitised solar cells (DSSCs) using colloidal quantum dots as light-absorbers. One aim is to study the impact of varying the thermodynamic driving forces provided for interfacial electron transfer on the yield of both the electron injection and hole regeneration reactions occurring within the DSSC; this can be achieved by varying the energetics of each component of the system (metal oxide, quantum dot and hole conductor) in turn. In addition, the interfacial morphology can be modulated by changing the passivating ligands present at the QD surface, and by modifying the structure of the redox mediator (or hole conductor). In doing so, we also attempt to improve our understanding of how charge carrier trapping in quantum dots impacts upon solar cell performance. Furthermore, new strategies towards solar cell design are presented, which show great potential as a result of their favourable photophysical properties. One of these approaches (presented in the final chapter) is to effect the in situ growth of CdS nanocrystals in a conducting polymer, a method which circumvents many of the processing issues associated with the use of nanocrystals in polymer blend solar cell architectures. It is hoped that the work presented in this thesis is used to develop design rules for the construction of semiconductor nanocrystal-based excitonic solar cells. By identifying which key parameters control the rates and yields of electron transfer at the nanocrystal interface, improvements in device efficiency can be realised. It is believed that these studies fill an important gap in our current understanding, and highlight some of the potential benefits and shortcomings of using semiconductor nanocrystals in cheap, solution-processed solar cells

Étude des propriétés électroniques et de la dynamique des charges dans diverses nanostructures semi-conductrices par la spectroscopie térahertz

Cette thèse présente les résultats de l’étude des propriétés électroniques de diverses nanostructures semi-conductrices. Le but principal est de déterminer l’influence du dopage, du désordre et des dimensions des nanostructures sur le transport des charges électroniques en utilisant des techniques de spectroscopie térahertz. Trois types de nanostructures sont étudiés dans cette thèse, notamment, des nanocomposites de silicium mésoporeux graphénisés, des nanofils de silicium ayant différents niveaux de dopage, et enfin des couches polycristallines d’InGaAs. L’étude des propriétés structurelles, optiques et diélectriques dans le domaine du térahertz des nanocomposites de silicium mésoporeux graphénisés est pionnière. Elle met en relief la stabilité de la morphologie et le caractère diélectrique des nanocomposites. La température du dépôt de la couche graphénisée est un paramètre important. La spectroscopie térahertz révèle que l’augmentation de la température de dépôt augmente l’indice de réfraction, diminue la porosité et induit la formation des états de surface aux interfaces Si/graphène. Par ailleurs, des mesures de photoluminescence indiquent que ces états de surface sont en partie responsables de la recombinaison non-radiative des photoporteurs. L’étude de la dynamique des photoporteurs par la spectroscopie pompe-optique sonde-térahertz montre que les états de surface aux interfaces Si/graphène se comportent comme des pièges de photoporteurs. La capture des photoporteurs s’effectue de 2 à 4 ps après l’excitation des nanocomposites graphénisés et est suivie de la recombinaison des photoporteurs dans les états des pièges pendant typiquement 25 ps. Les temps de capture et de recombinaison observés pour les nanocomposites sont considérablement plus petits que les temps de 74 ps et 730 ps dans la membrane libre de silicium mésoporeux en raison de la forte densité des pièges. Le transport de charges est affecté par une barrière de potentiel qui tend à confiner les photoporteurs dans le volume des nanocristallites de silicium. Cette barrière de potentiel est corrélée à la densité de pièges chargés. La mobilité effective dans les nanocomposites est la même que celle de la membrane libre de silicium mésoporeux malgré l’augmentation de la localisation des porteurs avec la température. Les travaux portant sur les nanofils de silicium crûs sur substrat de silicium révèlent une contribution très importante des porteurs crées dans le substrat à la dynamique générale ayant des temps vie de quelques nanosecondes pour des excitations à 400 nm et d’une dizaine de nanosecondes pour des excitations à 800 nm. Le transfert des nanofils sur une membrane d’acétate permet d’observer la dynamique des charges propres aux nanofils. Nos résultats démontrent que pour ces derniers échantillons, le temps de vie des photoporteurs diminue avec le dopage, typiquement de 100 ps dans des nanofils non-dopés, à typiquement 25 ps dans les échantillons dopés à 5 × 1019 cm−3. L’étude de l’influence de la fluence laser sur le temps de vie révèle que la chute du temps de vie avec le niveau de dopage résulte principalement d’une augmentation de la densité de pièges de surface avec le niveau du dopage. Toutefois, ces temps de vie relativement grands montrent une amélioration dans la passivation de surface des nanofils de silicium par rapport à ceux étudiés lors de travaux antérieurs. Des études précédentes montrent que l’implantation en ions Fe à hautes énergies d’une couche d’InGaAs cristalline amorphise la couche et augmente considérablement sa résistivité au-delà de 1000 Ω.cm. Dans cette thèse, la dynamique des photoporteurs est décrite selon un modèle physique de capture de photoélectrons par des pièges profonds créés durant l’implantation, suivie de leur recombinaison avec des trous en excès dans ces pièges chargés. Le temps de vie des photoporteurs augmente de 0,7 ps à 7 ps tandis que la densité de pièges diminue pour des températures de recuit dans l’intervalle de température allant de 300∘C à 700∘C. L’étude de la photoconductivité révèle l’impact de la localisation des charges par les défauts aux interfaces des grains et aussi par la taille de ces derniers. La mobilité effective déduite de l’analyse des courbes de photoconductivité est d’environ 2750 cm2/(V.s). D’un point de vue global, les résultats de la présente étude montrent que l’ingénierie des défauts d’interface dans les nanostructures semi-conductrices est d’intérêt tant pour la fabrication de matériaux photoconducteurs ultra-rapides que pour le développement de matériaux optoélectroniques

Photoelectrochemistry of nanocrystaline semiconductor metal oxides in contact to liquid electrolytes : photocatalytic and photovoltaic applications

Programa de Doctorado en Ciencia y Tecnología de Coloides e InterfasesComo consecuencia de la demanda energética a nivel mundial y los impactos medioambientales derivados del uso de combustibles fósiles, la búsqueda de nuevas fuentes de energía limpia han recibido mucha importancia. Desde entonces, la comunidad científica ha tomado consciencia de la problemática ambiental y ha centrado sus esfuerzos en aprovechar las fuentes de energías renovables como la luz solar. Es conocido desde mucho tiempo que los procesos inducidos por la luz en la interface entre un semiconductor y un electrolito pueden ser utilizados para la conversión de luz en otras formas de energía. Como consecuencia, estos sistemas han sido ampliamente estudiados por químicos, físicos y por miembros de otras ramas científicas. Actualmente, el uso de estructuras basadas en nanomateriales recibe mucha atención al ser considerados sistemas de bajo costes capaces de capturar, almacenar y convertir la energía solar en energía química y electricidad. Los electrodos mesoporosos basados en óxidos semiconductores constituyen sistemas complejos cuyas propiedades fisicoquímicas no solo dependen de la naturaleza de los nanocristales, sino también de la interacción entre las unidades cristalinas que forman la fina película. La estructura mesoporosa asegura una alta superficie interna. Por tanto, su complejidad se incrementa bajo condiciones relevantes para su aplicación, como por ejemplo, bajo la presencia de un gas o liquido circundante. La comprensión de cómo manipular sistemáticamente estas interfaces y los procesos interfaciales que ocurren entre el semiconductor y la fase circundante es un requisito previo para la optimización de las tecnologías emergentes y aplicaciones como baterías, sensores y células solares entre otros. En aplicaciones fotocatalíticas y fotovoltaicas, una especial atención ha sido prestada a la relación entre las propiedades microscópicas de la fina película semiconductora (propiedades electrónicas, estructura cristalina, tamaño y forma de la partícula) y el rendimiento macroscópico de un fotocatalizador o una celda solar, respectivamente. Por esta razón, en esta Tesis se ha desarrollado un estudio fundamental de las propiedades fotocatalíticas y fotovoltaicas de semiconductores como el TiO2 y ZnO con el objetivo de analizar el impacto de la interface semiconductor/electrolito en los procesos de transferencia de carga y la indeseada recombinación de los portadores de cargas. En las aplicaciones fotocatalíticas y fotovoltaicas la recombinación ocurre de diferentes maneras. En el primer caso, la recombinación tiene lugar entre los electrones y huecos, los cuales han sido generados en el semiconductor bajo la absorción de luz. Por otro lado, en las celdas solares basadas en la interface semiconductor/electrolito (como es el caso de las celdas Grätzel o DSSC, del inglés: Dye-Sensitized Solar Cell), la recombinación tiene lugar entre las electrones fotoinyectados en el semiconductor y la especie oxidada del par redox presente en el electrolito. Esta transferencia de carga representa la principal ruta de pérdida de eficiencia en tales dispositivos y constituyen un factor determinante para el voltaje a circuito abierto. En relación a los procesos de recombinación, dos líneas de investigación han sido desarrolladas en esta Tesis: ¿Se ha mostrado un procedimiento basado en la acumulación de electrones en la película semiconductora como una estrategia para reducir la recombinación en aplicaciones fotocatalíticas y fotovoltaicas. Este dopaje reductivo ha sido realizado in situ por medio de la polarización catódica del electrodo en una disolución ácida. El desplazamiento del nivel de Fermi hacia valores negativos ha sido llevado a cabo por una polarización externa o, alternativamente, por la acumulación de electrones fotogenerados bajo la exposición de luz UV. En ambos casos, los cambios mostrados en el rendimiento son comparables. La influencia de las propiedades estructurales y morfológicas de la película semiconductora sobre el efecto el efecto del dopaje electroquímico ha sido analizado. ¿Para aplicaciones fotovoltaicas un estudio de la recombinación electrónica basado en la influencia de la naturaleza química del electrolito ha sido desarrollado. Los efectos sobre la cinética de recombinación empleando diferentes composiciones electrolíticas y moléculas de colorantes han sido discutidos. En concreto, se ha analizado como influye propiedades como la polaridad del electrolito y la presencia de ciertos aditivos en la tasa de recombinación. A partir de las conclusiones derivadas de tal estudio, ha sido propuesta una estrategia para lograr sistemas caracterizados por una larga estabilidad y baja recombinación mediante el uso de una mezcla de solventes orgánicos y líquidos iónicos a temperatura ambiente como electrolitos. En celdas solares la recombinación tiene lugar en una escala de tiempo específica, en el orden de 0.001-1 segundos. Sin embargo, hay otros procesos que también dependen de la naturaleza del electrolito que limitan la eficiencia de los dispositivos fotovoltaicos y que ocurren en un rango de tiempo más corto. Es el caso de la inyección electrónica y la regeneración de las moléculas de colorantes. En esta Tesis, se presenta un análisis global de todos estos procesos mediante la combinación de técnicas experimentales que incluyen decaimientos de fluorescencia, espectroscopia de transitorios de absorbancia y técnicas electroquímicas como la espectroscopia de impedancia. Este análisis global ha sido utilizado para mostrar por primera vez las limitaciones fundamentales que están asociadas al ZnO cuando es utilizado como fotoánodo para la separación de carga en la interface semiconductor/electrolito.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y Naturale

Performance enhancement of quantum dot sensitised solar cells through enhanced interfacial charge transfer kinetics

There is a plethora of renewable energy sources that wait for us to be harnessed - wind, geothermal, wave, solar energies, to name a few - which are more than enough to supply our energy demand. The sun, with its enormous amount of free energy at 3 x 10^24 Joules/year, is estimated to be capable of covering 10,000 times the world's energy requirement at the beginning of the 21st century. The most common method of harvesting solar energy is through photovoltaic (PV) technology in which next-generation PV technologies are vastly becoming popular due to limitations in the mainstream solar PVs i.e. Silicon-based solar PVs. One of these next-generation PVs is the quantum dot sensitised solar cell (QDSSC), the focus in this thesis. Quantum dots (QD) which are semiconductor nanomaterials used as sensitiser in QDSSCs, are physically very small in size, usually below 10 nm. Because of this minuteness the QD's optical and electronic properties differ from those of its bulk material's properties, such that it will absorb/emit light usually from the visible to infrared wavelength in the solar spectrum. In addition, these properties can be controlled by tuning the parameters during synthesis, opening up a number of applications in biotechnology, electronics, photovoltaics, and quantum computing. This thesis focuses on the photovoltaic application of QDs specifically investigating the liquid junction QDSSC. There have been previous studies focusing on the components such as electrode, sensitiser, counter-electrode, and limited studies on electrolyte. The aim of this thesis is thus to understand how the concentration of the redox electrolyte affects kinetics and dynamics of electrons at the interfaces of the CdS QDSSC which was achieved by: • reviewing previous and current works on the enhancement of QDSSC conversion efficiency and studies on QDs. • advancing the understanding of the interfacial charge transfer kinetics in a CdS QDSSC based on aqueous Fe(CN)6^4-/Fe(CN)6^3- electrolyte. • analysing the effects of varying concentrations of the reduced (Fe(CN)6^4-) and oxidised (Fe(CN)6^3-) species in a ferrocyanide/ferricyanide electrolyte on the performance of a CdS QDSSC. • identifying the most influential factors on the output of a CdS QDSSC using the Matlab software for optimised fitting of a theoretical vs. experimental voltage-current curve based on the diode model. • disseminating the results of the investigation conducted via publication in peer-reviewed journals. The main research questions addressed in this thesis are: • What are the alternative ways of controlling and handling QDs and how these handling conditions affect QD's ageing? • How will ferrocyanide/ferricyanide redox electrolyte affect the interfacial charge transfer kinetics in a CdS QDSSC? • What are the optimal reduced and oxidised species concentrations in a ferrocyanide/ferricyanide electrolyte to maximise the performance of a CdS QDSSC? • Which parameters in the diode model of the CdS QDSSC cell have the most influence on cell performance with this redox electrolyte and which among these parameters are sensitive to tolerance changes? • To what extent does a ferrocyanide/ferricyanide electrolyte with optimised concentrations improve the overall QDSSC performance? These questions were answered by: • Synthesising and characterising quantum dots (using PbS as model) by using established and modified parameters. • Studying the interfacial charge transfer kinetics and transport of a CdS QDSSC via controlling the reduced and oxidised species of redox electrolyte. • Writing an algorithm in Matlab using a single diode equation for solar cell simulation and another algorithm to simulate the sensitivity of the fitted parameters. • Designing an optimal reduced and oxidised species concentration combination and observe its effect on the cell's conversion efficiency. Summarising the findings from this thesis: 1. PbS QD size engineering can be done by keeping the precursor ratio constant while the injection temperature variable. 2. PbS QDs can be stored in air/dark without effect on its optical properties after one bubbling in nitrogen. 3. PbS QDs remain optically stable after 60 days in air/dark environment. 4. PbS QDs can be dried when needed to be transported and re-dispersed without adverse effect on the absorption. 5. 0.2 M reduced species concentration is the optimal reduced species concentration in this study. 6. 0.01 M oxidised species concentration results in relatively slower charge recombination at the TiO2 surface hence high FF results in longer lifetime thus higher open circuit voltage (VOC). 7. At fixed oxidised species concentration (0.01 M) in the electrolyte, a sufficiently low (<0.02 M) reduced species (ferrocyanide) concentration controls the anodic limiting current. 8. A Matlab algorithm found that the ideality factor deteriorated (n>2 where the ideal value is 1) as the irradiation intensity was increased. 9.The extracted parameters that were sensitive to slight changes (± 1%) were identified as the ideality factor, n, and shunt resistance, Rh. 10. The extracted ideality factors result showed that the interfacial recombination increased once the irradiation is more than 100% i.e. 120%, 130% via solar simulation. Recommendations from this study are: 1. Size engineering studies should be extended to much larger QD sizes and temperature and molar ratios being the parameters to focus on still. Emission and excitation spectral measurements on QDs should also be conducted. 2. Further studies on the QD ageing beyond 180 days in order to establish QD's practical lifetime. 3. Further studies on this QDSSC model should be focused on other components such as the sensitiser, counter-electrode, and passivating agent since the maximum theoretical VOC has already been achieved in this study. 4. Reasons why the ideality factor increases (moving away from ideal) as the irradiation intensity is increased need to be further investigated together with ways to improve the charge recombination kinetics. 5. Since the maximum theoretical VOC is almost achieved in this thesis, it is recommended that the next study be focused on how to improve the short circuit current (JSC) and fill factor (FF). In summary, the optimised ferrocyanide/ferricyanide concentration ratio of the redox electrolyte in the QDSSC examined in this thesis has been found to be 0.2/0.01 M resulting in a VOC of 0.8 V, a FF of 0.66, and JSC of 3.8 mA/cm^2, corresponding to an IPCE of 57% at 410 nm and overall conversion efficiency of 2%

Development of Zinc Oxide Nanowires and Quantum Dot Incorporation for Photovoltaic Applications

Heterojunctions of metal oxide semiconductors with quantum dots (QD) have been deployed in a number of advanced electronic devices. Improvement in the devices’ performance requires in-depth studies on charge carrier transfer dynamics. In this work, charge carrier dynamics, at the interface on zinc oxide nanowires (ZnO NW) with cadmium selenide QDs, were investigated. ZnO NWs were synthesized and characterized through the chemical vapor deposition (CVD) and hydrothermal methods. Both methods yielded highly crystalline ZnO structures. The hydrothermally grown NWs were doped with aluminum (Al) and the spectroscopy analyses showed that Al was successfully incorporated into the ZnO crystalline structure. Colloidal cadmium selenide/zinc sulfide (CdSe/ZnS) core/shell QDs were incorporated into synthesized ZnO NW arrays. The interaction and wettability of two different QD ligands (Octadecylamine and oleic acid) on the self-assembly of QDs in the NW spacing were investigated using electron microscopy. Afterwards, the charge carrier transfer dynamics at the heterojunction of NW/QD were studied employing time resolved photoluminescence spectroscopy (TRPL). A hypothesis on charge transfer kinetics, based on the experimental measurements, was provided. It was realized that photocharging of QDs is the main reason for substantial PL quench, when holes are not effectively removed from the photoexcited QDs by a hole-transporting medium. Furthermore, the TRPL measurements showed that the hole transfer rate by a polysulfide electrolyte is slower than that of an electron; one main reason in impeding the device performance in quantum dot-sensitized solar cells (QDSSC). The NW/QD heterojunction was deployed in the structure of a QDSSC. The current-voltage behavior of the cells under various conditions was characterized in both dark and light conditions. The underlying problems hindering the device performance were identified by these characterizations. Heterojunction of ZnO NWs with a GaN thin film was also deployed in the structure of an LED. The NWs were grown on GaN film using the hydrothermal method. The fabricated device exhibited light emission under both forward and reverse bias injection currents. The electroluminescence and PL characterizations revealed that the light emission from the fabricated device depends on the point defects and interface states of the two semiconductors

Interface engineering in solid-state dye-sensitized solar cells

Dye-sensitised nanocrystalline solar cells are currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. In particular dye-sensitised cells based on spiro-MeOTAD have gained attention as promising approach towards an organic solid-state solar cell. However, the efficiency in such dye-sensitised solid-state solar devices (SSD) was so far only ca. 10 % of the values reported at AM1.5 for the classical dye-sensitised solar cell with an electrolyte hole transporting medium (DSSC). The objective of the present work is to study the limitations that emerge from the exchange of the electrolyte by the solid-state system and that oppose photovoltaic photon-to-electron conversion as high as for the DSSC. Interfacial charge recombination is an important loss mechanism in dye-sensitised solar cells. This is particularly true for SSD, as the solid hole-transporting medium is less efficient in screening of internal fields which assist recombination. A variety of strategies were tested in the SSD to minimise interfacial charge recombination. The most promising approach was the blending of the hole-transporting medium with tert.-butylpyridine (tBP) and lithium ions. Optical and electrochemical techniques, such as nanosecond laser spectroscopy, impedance spectroscopy and photovoltaic characterisation measurements, were used to study the impact of the additives on the SSD. Both lithium ions as well as tBP were found to increase the open circuit potential of the SSD. At the same time tBP was found to considerably lower the current output. The interaction of the additives was studied and their concentration in the spiro-MeOTAD medium optimised. The doping of the spiro-MeOTAD film, which was intended to support the hole transport, was found to enhance interfacial recombination significantly. The morphological properties of the TiO2, in particular layer thickness, particle size and film porosity, play a more important role in the SSD than in the DSSC. Penetration of the hole conductor into the TiO2 pores and electron diffusion length are coupled to these properties. As a result the light harvesting cannot be controlled at will via the TiO2 film thickness and the active surface area for dye adsorption. An enhanced light harvesting for thin TiO2 layers offers advantages for the charge transport and the formation of the interpenetrating network. The dye uptake in presence of silver ions was found to increase the dye loading and to significantly improve the device performance of thin TiO2 layer devices. The mechanism of this simple dye modification technique was studied by a variety of spectroscopic techniques. From spectroscopic evidence it is inferred that the silver is binding to the sensitiser via the ambidentate thiocyanate, allowing the formation of ligand-bridged dye complexes. The beneficial influence of the silver ions on the photovoltaic performance was not limited to the application of the standard N3 dye nor to the spiro-MeOTAD. SSDs were furthermore studied by frequency resolved techniques. Intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) were performed over a wide range of illumination intensities. The IMPS and IMVS responses provide information about charge transport and electron-hole recombination processes respectively. For the range of light intensities investigated, the dynamic photocurrent response appears to be limited by the transport of electrons in the nanocrystalline TiO2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO2 was found to be 4.4 μm. This value was almost independent of the light intensity as a consequence of the fact that the electron diffusion coefficient and the rate constant for electron-hole recombination both increase in the same way with light intensity but with opposite sign. The results of this work provide for a substantial improvement of the overall photovoltaic performance compared to earlier results for this type of SSD. However, this study reveals also that high conversion efficiencies as are measured for DSSC are not likely to be reachable with the spiro-MeOTAD system due to the significantly slower charge transport in the spiro-MeOTAD compared to the electrolyte redox mediator

Novel Soft Chemistry Synthesis of TiO2 for Applications in Dye–Sensitized Solar Cells and Photocatalysis

Although the high cost of solar cells prevents them being a primary candidate for energy production, great attention has been paid towards them because of the depletion of the conventional energy sources–fossil fuels–and the global warming effect, and the need to provide power to remote communities disconnected from the power grid. To reduce the cost, thin film technologies for silicon solar cells have also been investigated and commercialized, but dye sensitized solar cells (DSSC) have been considered as a promising alternative even for the silicon thin films with efficiency exceeding 10%. Compared with silicon-based photovoltaic devices, DSSCs are quite complex systems that require an intimate interaction among components. Within the last few years, conclusive smart solutions have been provided to improve the efficiency of these cells, with solar efficiency that makes them potential competitors against silicon devices. The most successful systems use titanium oxide as a core material tuned to collect and transmit the electrons generated by the photo-excitation of dye molecules. However, most of the solutions demonstrated so far require a thermal treatment of the TiO2 photoelectrodes at temperatures that preclude using any flexible organic substrate. This treatment prevents development of any roll-to-roll manufacturing process, which would be the only way to achieve cost effective large scale production. In order to overcome this major drawback, a novel synthesis of TiO2 at room temperature is described in the present document. This synthesis leads to 4-6 nm nanocrystalline anatase, the desired phase of titanium oxide for photoactive applications. An intensive study was carried out to explore the properties of these nanoparticles, via a mixture design study designed to analyze the influence of the starting composition on the final TiO2 structure. The influence of a post-synthesis thermal treatment was also explored. This 4 nm nanocrystalline TiO2 exhibits a high specific surface area and a good porosity that fulfills the requirements for an efficient photoanode; a high surface area allows high dye loading, and, hence, increases photocurrent and photo-conversion efficiency. Another important result of this study is the band gap, as it confirmed that nanocrystalline anatase has an indirect band gap and a quantum confinement for a crystal size of less than 10 nm. This result, well-known for bulk materials, had been discussed in some previous publications that claimed the effectiveness of a direct band gap. Following this synthesis and the structural and spectroscopic analyzes carried out in parallel, photocatalytic study was an important tool to further explore the semiconducting properties of this material. Additionally, our material gave very promising results in photocatalytic dye degradation, compared to the commercial products, even if it was not initially synthesized for this application. We assign these performances to the improved crystallinity resulting from thermal activation, without changing the crystal size, and to the ability to optimize the surface. This photocatalytic study gave us insights into the methods that optimize the electronic structure of the titanium oxide. Hence, we decided to thermally activate the nanoparticles before the preparation of films to be inserted into DSSCs. At this stage, as the thermal activation applies to the powder, the resulting material can still be used with flexible substrates. We have successfully integrated these nanoparticles in dye sensitized solar cells. Various organic additives were added to the TiO2 paste used to prepare photoelectrode films, to increase the porosity of the film and have a crack–free film with good attachment to the substrate. We demonstrated that the dye was chemically attached to the TiO2 surface, which led to better electron transport. Different treatment methods (UV and thermal) were applied to the film to cure it from organic additives and improve the electronic connectivity between the particles. When the UV treatment was applied as a single method, i.e. without thermal treatment, the cell performance was lower, but a combination of thermal treatment and UV enhanced this performance. We compared our nanoparticles to the reference material used in most of the studies on DSSC, that is, TiO2 Degussa, with cells prepared the same way. Our nanoparticles revealed higher overall conversion efficiency. As the dye attachment to the TiO2 surface is an important parameter that enhances the cell efficiency, so we checked via ATR-FTIR how the dye attached to the TiO2 surface. In addition, FTIR, UV-Vis, and IV measurements revealed that the amount of dye adsorbed was increased through HCl treatment of the photoelectrode. We also checked the internal resistance of the cell using impedance spectroscopy, and the analysis proved a successful integration of the nanoparticles in dye–sensitized solar cells as there was an increase in both the electron life time and the recombination resistance, and a decrease in the charge transfer resistance compared to the commercial powder.1 yea