Στην παρούσα διατριβή µελετάται η ανάπτυξη υποστηριγµένων καταλυτών Pt και καταλυτών (σύνθετων και απλών) µεταλλικών οξειδίων υποστηριγµένων σε γ-Al2O3 για την αντίδραση της οξείδωσης του τολουολίου σε χαµηλές θερµοκρασίες καθώς και η κινητική της εν λόγω αντίδρασης. Τα πειράµατα πραγµατοποιήθηκαν στην θερµοκρασιακή περιοχή 100-500oC µε τροφοδοσία αποτελούµενη από µίγµα 0.1% C7H8 σε αέρα. Η καταλυτική ενεργότητα των καταλυτών Pt/MxOy εξαρτάται από την φύση του φορέα (CeO2, TiO2, SiO2, Al2O3, La2O3, κ.λ.) µε το Pt/CeO2 να παρουσιάζει την µεγαλύτερη ενεργότητα σε χαµηλές θερµοκρασίες. H αύξηση της ποσότητας του Pt από 0.5% έως 5.0% οδηγεί σε σηµαντική µετατόπιση της καµπύλης µετατροπής του τολουολίου προς χαµηλότερες θερµοκρασίες, ενώ η συχνότητας αναστροφής του τολουολίου (TOF) δεν εξαρτάται από τη φόρτιση σε µέταλλο, τουλάχιστον για καταλύτες Pt. Τα αναγώγιµα µεταλλικά οξείδια, όπως η δηµήτρια, είναι ενεργά για την οξείδωση του τολουολίου και η καταλυτική τους ενεργότητα αυξάνεται µε αύξηση της ειδικής επιφάνειας. Ωστόσο, ο εγγενής ρυθµός ανά m2 επιφάνειας καταλύτη παραµένει ο ίδιος για όλα τα δείγµατα που δοκιµάστηκαν. Μελετήθηκε η καταλυτική συµπεριφορά διάφορων µεταλλικών οξειδίων υποστηριγµένων σε γ-Al2O3 (MxOy/Al2O3). Τα αποτελέσµατα δείχνουν ότι η διασπορά των MxOy σε αδρανή φορέα υψηλής επιφάνειας, όπως γ-Al2O3, οδηγεί σε καταλύτες που χαρακτηρίζονται από σχετικά µεγάλη καταλυτική δραστικότητα, η οποία είναι σηµαντικά υψηλότερη για τα αναγώγιµα από ότι για τα µη-αναγώγιµα. Η καταλυτική συµπεριφορά µπορεί να βελτιωθεί µε την κατάλληλη επιλογή της ποσότητας φόρτισης σε MxOy. Καλύτερη απόδοση σε αυτή την σειρά καταλυτών παρουσιάζουν οι καταλύτες 60%MnOx, 90%CeO2 και 5%CuO υποστηριγµένοι σε Al2O3, οι οποίοι, κάτω από τις παρούσες συνθήκες αντίδρασης, είναι ικανοί να επιτυγχάνουν ολική µετατροπή τολουολίου σε θερµοκρασίες χαµηλότερες από 350oC. Η προσθήκη του Pt σε MxOy/Al2O3 βελτιώνει σηµαντικά την καταλυτική συµπεριφορά των µη-αναγώγιµων MxOy, αλλά δεν µεταβάλλει, ουσιαστικά, την ενεργότητα των αναγώγιµων MxOy. Προκειµένου να βελτιωθεί περαιτέρω η καταλυτική συµπεριφορά µελετήθηκε η καταλυτική ενεργότητα σύνθετων οξειδίων µετάλλων (MxOy = CuO, CeO2, MnOx)διεσπαρµένων σε γ-Al2O3 για την αντίδραση της οξείδωση του τολουολίου. Τα αποτελέσµατα των πειραµάτων που πραγµατοποιήθηκαν στη θερµοκρασιακή περιοχή 150-450οC έδειξαν ότι η ενεργότητα των σύνθετων καταλυτών εξαρτάται σηµαντικά από τη φύση, τη φόρτιση και την αναγωγιµότητα των επιµέρους οξειδίων. Βέλτιστη καταλυτική συµπεριφορά παρατηρήθηκε για µικτά οξείδια 10%CuO-60%MnOx, 15%CuO-75%CeO2 και 30%MnOx-50%CeO2 σε γ-Al2O3, η ενεργότητα των οποίων είναι συγκρίσιµη µε τους καταλύτες διασπαρµένων ευγενών µετάλλων. Η συµπεριφορά των βέλτιστων σύνθετων καταλυτών σε σύγκριση µε τα επιµέρους απλά µεταλλικά οξείδια υποστηριγµένα στον ίδιο φορέα (γ-Al2O3) µελετήθηκε περαιτέρω. Μετρήσεις του εγγενούς ρυθµού που πραγµατοποιήθηκαν σε διαφορικές συνθήκες αντίδρασης έδειξαν ότι η ενεργότητα αυτών των υλικών είναι περισσότερο από µία τάξη µεγέθους υψηλότερη από αυτή των αντίστοιχων απλών οξειδίων, υποδεικνύοντας την ύπαρξη φαινοµένων συνέργειας. Τα αποτελέσµατα πειραµάτων XRD που ελήφθησαν από τους σύνθετους καταλύτες δεν έδειξαν το σχηµατισµό κάποιας καινούργιας φάσης σε σύγκριση µε τα αντίστοιχα απλά οξείδια. Οι αλληλεπιδράσεις µεταξύ των ενεργών φάσεων και του φορέα εξετάστηκαν µέσω τεχνικών TPD και TPO. Στα πειράµατα TPD παρατηρήθηκε ότι οι πιο ενεργοί καταλύτες είναι αυτοί που εκροφούν µεγαλύτερες ποσότητες τολουολίου και παραγάγουν περισσότερο CO2 ή/και σε χαµηλότερη θερµοκρασία. Τα πειράµατα TPO έδειξαν ότι οι βέλτιστοι καταλύτες παράγουν µικρότερες ποσότητες CO2. Η επίδραση της παρουσίας δεύτερου VOC (προπανίου) ή υδρατµών στην τροφοδοσία µελετήθηκε στους βέλτιστους σύνθετους καταλύτες. Τα αποτελέσµατα δείχνουν ότι η παρουσία του νερού ή του προπανίου επιδρά παρεµποδιστικά στην καύση του τολουολίου, η παρουσία του οποίου, γενικά, δεν επηρεάζει σηµαντικά την οξείδωση του προπανίου στους τρεις βέλτιστους καταλύτες. Η επίδραση της µερικής πίεσης του τολουολίου στον εγγενή ρυθµό της αντίδρασης µελετήθηκε µε χρήση των τριών βέλτιστων σύνθετων καταλυτών στην θερµοκρασιακή περιοχή 270-320oC. Η χρησιµοποιούµενη τροφοδοσία αποτελείται από 0.036– 0.341 (% κ.ο.) C7H8 και σταθερή συγκέντρωση οξυγόνου (20.9 κ.ο % O2). Τα αποτελέσµατα δείχνουν ότι ο ρυθµός της αντίδρασης αυξάνεται, άλλα όχι πολύ σηµαντικά, µε αύξηση της µερικής πίεσης του τολουολίου. Τα κινητικά αποτελέσµατα προσαρµόστηκαν σε εµπειρική εκθετική εξίσωση (Power Law), από την οποία προέκυψαν οι τάξεις των αντιδρώντων, η φαινόµενη ενέργεια ενεργοποίησης της αντίδρασης καθώς και η αντίστοιχη εξίσωση ρυθµού. Συµπεραίνεται ότι κατάλληλος συνδυασµός οξειδίων µετάλλων διεσπαρµένων σε γ-Al2O3 µπορεί να οδηγήσει στην ανάπτυξη καταλυτών µε ενεργότητα συγκρίσιµη µε αυτή των υποστηριγµένων καταλυτών ευγενών µετάλλων. / Volatile organic compounds (VOCs) present at low concentrations in industrial waste streams are considered as significant air pollutants due to their toxic and malodorous nature, as well as their contribution to the formation of photochemical smog. Catalytic combustion over supported noble metal catalysts provides an effective method for the elimination of VOCs in exhaust gases and this technology seems to be able to satisfy strict emission standards. Efforts in this field are currently directed toward the development of cheaper, noble metal-free catalytic materials characterized by high activity at low temperatures and long-term stability under reaction conditions. In the present thesis, oxidation of toluene has been investigated over supported platinum catalysts as well as over single and mixed metal oxide (MxOy) catalysts dispersed on high surface γ-Al2O3. Catalysts were characterized with respect to their specific surface area (BET), metal dispersion (selective chemisorption of CO), phase composition and MxOy crystallite size (XRD) and reducibility (H2-TPR, CO-TPR). Catalytic performance for the title reaction was investigated in the temperature range of 100-500oC, using a feed composition consisting of 0.1% toluene in air. For Pt/MxOy catalysts, it has been found that catalytic performance depends on the nature of the support, with Pt/CeO2 being the most active catalyst at low temperatures. The intrinsic reaction rate per surface platinum atom does not depend on Pt loading (0.5-5 wt.%), at least for Pt/Al2O3 catalyst, but the global reaction rate increases with increase of exposed metallic surface area. Reducible metal oxides, such as ceria, are active for the title reaction and catalytic performance is improved significantly with increase of specific surface area (SSA). However, the intrinsic reaction rate per unit surface area is the same regardless of SSA. Dispersion of MxOy on high surface inert supports, such as Al2O3, results in materials with relatively high catalytic activity, which is considerably higher for reducible, compared to irreducible metal oxides. Catalytic performance of MxOy/Al2O3 catalysts can be optimized by proper selection of MxOy loading. Best performing catalysts of this series include 60%MnOx, 90%CeO2 and 5%CuO on Al2O3 which, under the present experimental conditions, are able to completely convert toluene toward CO2 at temperatures lower than 350oC. Dispersion of Pt on MxOy/Al2O3 catalysts improves significantly the catalytic performance of irreducible MxOy but does not alter appreciably activity of reducible MxOy/Al2O3 catalysts. The catalytic oxidation of toluene has been investigated also over single and composite metal oxide catalysts supported on γ-Al2O3. Catalysts were synthesized with the impregnation method and were characterized with respect to their specific surface area (BET method), crystalline mode and mean crystallite size (XRD technique), as well as with respect to their reducibility (temperature programmed reduction with H2 or CO). The effects of the nature, loading and composition of catalytic materials on their performance for VOC combustion has been investigated. Optimal results were obtained over Al2O3-supported CuO, CeO2, MnO2 catalysts and their mixtures. For certain metal oxide combinations, e.g., 10%CuO-60%MnOx, 15%CuO-75%CeO2 and 30%MnOx-50%CeO2, activity was found to be comparable to that of supported noble metal catalysts. Measurements of reaction rates under differential reaction conditions showed that specific activity of these materials was up to one order of magnitude higher, compared to that of the corresponding single metal oxides, implying that synergistic effects are operable. Results of XRD experiments did not show formation of new phases, but mixed oxide catalysts were found to exhibit a higher reducibility compared to catalysts consisting of the corresponding single metal oxides. The synergic effect of metal oxides interaction on the oxidation reaction was studied employing TPD and TPO techniques. The more active catalyst, the higher the amount of desorbed toluene and the higher the amount of CO2 production in the in the TPD experiments. The TPO experiments indicate that the optimized composite catalysts produce lower amounts of CO2 at lower temperature, compared to the corresponding single metal oxides. The influence of the presence of a second VOC (propane) or of water on the oxidation of toluene was also investigated. Results showed that the presence of water or propane in the feed results in a decrease of catalytic activity, while the presence of toluene doesn’t have any influence in the catalytic oxidation of propane over the optimized composite catalysts. The effect of partial pressure of toluene on the kinetic reaction rate has been investigated over the optimum composite catalysts in the 270–320oC range using a feed stream consisting of 0.036– 0.341 vol%C7H8 and a constant concentration of oxygen (20.9 vol% O2). Result showed that increasing the partial pressure of toluene leads to an increase of the reaction rate. The orders of the reaction with respect to reactants for the optimized catalysts were determined by fitting the experimental data to an empirical power-law rate expression according to which the reaction rate is given by the following relationships: 2.abtolOrkPp= (1) .0.aERTkke= (2) Results of the present study show that the catalytic performance of certain Al2O3-supported composite metal oxide catalysts is comparable to that of conventional supported noble metal catalysts. These materials could provide the basis for the development of cost-effective catalysts for combustion of VOCs present in waste gas streams.

In the latter half of the 20th century the first active environmentalist movements such as Greenpeace and the International Energy Agency were born and initiated a gradual rethinking of environmental awareness. Against all expectations the sole agency under international law for climate protection policy, called the United Nations Framework Convention on Climate Change, was formed 20 years later. Today the awareness of sustained, regenerative and environmental policies permeates throughout all areas of life, science and industry. But energy provision is the most decisive topic, especially since the discussions concerning the phase out of nuclear power where the voices calling for alternative energy sources have become much more vociferous. In addition the depletion of fossil fuels is expected to occur in the not too distant future. All new energy generation methods are required to meet the present and future energy demands, need to be ecological and need to exhibit the same or significantly lower cost expenditure than current energy sources. Unfortunately mankind is confronted with the problem that current commercial alternative energies are more expensive and not yet remotely as efficient as the present energy sources. Although energy provision based on water, wind, sun and geothermal sources have a huge potential because of their continuous presence, unfortunately, they are plagued by inefficient energy conversion caused by the state of technology i.e. the conversion of sun light into electricity loses energy through heat emission, reflection of the sun light, the inability of the material to absorb the entire sun spectrum and the ohmic losses in the transmission of electric current. The sun power is the most exhaustless resource and moreover through photovoltaic action, one of the most direct and cleanest source for use in energy conversion. Presently incoming sun light is not transformed in its entirely, as much degradation occurs during photon absorption and electron transfer processes. A number of other innovative possibilities have also been researched. With respect to cost and efficiency one of the most promising devices is injection solar cells (ISC). By dint of the dye sensitised solar cell (DSSC) Grätzels findings provided the foundations for much research into this type of solar cell where the light absorbing molecule employed in is a dye.[1] The current is obtained through charge separation in the dye, which is initiated through the connection between the dye and a metal oxide on the one hand and a matched redox couple on the other. In a variant of the DSSC the charge separation processes can also occur between a nanoporous metal oxide and nanoparticles giving rise to a quantum dot sensitised solar cell (QDSSC).[2] The use of nanoparticle (NP) properties can be utilized for the harvesting of solar energy, as demonstrated by Kamat and coworkers[3] who were able to exploit these findings subsequently and prepare a number of nanoparticle based solar cells. Nanoparticle research has comprised a wide field of science and nanotechnology for a number of years. As the size of a material approaches dimensions on the nm scale the surface properties contribute proportionally more to the sum of the properties than the volume due to the increase in the surface to volume ratio. These dimensions also constitute a threshold in which quantum physical effects need to be taken into account. Hence the properties of devices or materials in this size regime are inevitably size dependent. The basic principles can be described by two different theories, one of which is based on molecular orbital theory in which the particle is treated as a molecule. For this reason n atomic orbitals with the same symmetry and energy can build up n molecular orbitals through their linear combination based on the LCAO method (Linear Combination of Atomic Orbitals).[4] In the case of solids the orbitals build up energy bands, where the unoccupied states form the quasi continous conduction band (CB) and the occuppied states form the quasi continous valence band (VB). The energy \"forbidden\" area in between these two bands is called the band gap. The band gap is a fixed material property for bulk solids but depends on size in the case of the nanoparticles. In contrast to the LCAO method, simplified solid state theory will be used throughout the present work, the theoretical background of which is provided by the effective mass approximation.[5] When an absorption of a photon occurs, an exciton (electron-hole pair) can be generated. By promoting an electron (e-) from the valence band into the conduction band a hole (h+) may be said to remain in the valence band. By comparison to bulk solids, in a small particle the free charges can sense the potential barrier i.e. the edges of the nanoparticle. Analogous to the particle in a box model this potential barrier interaction results in an increase in the band gap as the particle size decreases. In a solar cell NPs with a particle size which possess a band gap energy in the near infrared (NIR) may be utilised and therefore the NPs will be able to absorb in this spectral region. However NPs also have the ability to absorb higher energy photons due to the continuum present in their band structure, so that almost the entire sun spectral range from the NIR up to UV wavelengths may be absorbed just by using the appropriate NP material and size. Suitable NPs are metal chalcogenides e.g. MX (where M = cadmium, zinc or lead and X = sulfur, selenium or tellurium) because of their bandgap size[6–10] and their relative band positions compared to those of the semiconductor oxide states. Both the TiO2/CdSe[11–14] and TiO2/CdTe[15–18] systems have already been successfully fabricated and many of the anomalies reported.[3] Much interest in the lead chalcogenides has been generated by reports that they may feature the possibility to exhibit multiple exciton generation (MEG) where the absorption of one high energy photon can result in more than one electron-hole pairs.[19–25] Currently electrochemical impedance spectroscopy (EIS) is being used more and more to clarify processes at polarisable surfaces and materials such as nanoparticles. Likewise this method has been rediscovered in photovoltaic research and its use in the characterisation of DSSCs has been discussed in the literature.[26–31] In a number of publications the evaluation of nanoporous and porous structures has been quite extensively explored.[28,29,32–34] Since the mid-20th century Jaffé’s[35] theoretical work concerning the steady- state ac response of solid and liquid systems lead to the formation of the basics of EIS. Further developments in the measurement technology have lead to a broader range of analysis becoming possible. Nevertheless the most challenging part still remains the interpretation of the results and especially to merge the measured data with the theoretical model. EIS quantifies the changes in a small ac current response at electrode electrolyte interfaces i.e. the rate at which the polarized domain will respond, when an ac potential is applied. In this way dielectric properties of materials or composites, such as charge transfers, polarization effects, charge recombination and limitations can be measured as a function of frequency and mechanistic information may be unveiled. Hence EIS allows one to draw a conclusion concerning chemical reactions, surface properties as well as interactions between the electrodes and the electrolyte. Other very useful tools that may be employed for quantifying electron transfer processes and their time domains are intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS). IMPS permits the generation of time-resolved plots of particular photo-processes in the system, each of which may be specifically addressed through varying the excitation wavelength. For the IMPS technique a sinusoidal wave with a small amplitude is applied, analogous to that of electrochemical impedance spectroscopy, but in this case the modulation is applied to a light source and not to the electrochemical cell as in EIS.[35] The current response is associated with the photogenerated charge carriers which flow through the system and finally discharge into the circuit. The amount of generated and discharged charge carriers is often different due to the presence of recombination and capture processes in surface or trap states. Ultimately the phase shift and magnitude of these currents reveal the kinetics of such processes. The only processes that will be addressed will be those that occur in the same frequency domain or on the same time scale as that of the modulated frequency of the illuminated light. In the literature some explanation of the kinetics of simple systems can be found and basic theories and introductive disquisitions may be found elsewhere.[36–38] Furthermore in solar cell research a multiplicity of studies are available which give an account of IMPS measurements on TiO2 nanoporous structures. Such studies permitted proof for the electron trapping and detrapping mechanism in TiO2 surface states.[39,40] An analysis of TiO2 electrodes combined with a dye sensitization step was established in the work of Peter and Ponomarev.[41–43] Hickey et.al.[44,45] have previously published kinetic studies on CdS nanoparticle (NP) modified electrodes. A theory was presented which allows for the IMPS data to be the interpreted in the case of CdS NP based electrodes. The back transfer, recombination and surface states have been demonstrated to be important as was determined from their inclusion in the theory. Similar attempts to explain the kinetics of CdS quantum dots are described by Bakkers et.al.[46]. In the present work the most important questions concern the behaviour of the photovoltaic assembly. Such assemblies can be equated with an electrode in contact with an electrolyte. Preliminary remarks about such electrodes as components of an electrochemical cell will be introduced in the first part of chapter 2. Thereafter the properties of electrodes in contact with the electrolyte and under illuminated conditions are illustrated. This is followed by a description of the important electrochemical and opto-electrochemical methods which have been employed in these studies. In particular, two separate subsections are dedicated to the methods of EIS and IMPS and the experimental section which are then linked to the theoretical section. The synthesis of all substances used and the preparation of the solar cell substrates are also dealt with in this section as will the equipment used and the instrument settings employed. The optical response of the working photoactive electrode is not only dependent on the substances used but also on their arrangement and linkage. The substrate which was employed in chapter 3 consists of a nanoporous ZnO gel layer upon which an organic linker has been placed in order to connect the oxide layer with the light absorbing component, the PbS NPs. Chapter 3 deals with the linker dependence on the ZnO layer and reports the typical optical characteristics and assembly arrangements of six different linkers on the ZnO layer which is an important intermediate stage in the fabrication of an ISC. The questions concerning how the type of linking affects the photo response and other electrochemical interactions of the complete solar cell substrate will be outlined in chapter 4. Further an examination of the electrochemical and opto-electrochemical behaviours of the samples will be presented similar to that presented in chapter 3. The most interesting substrate resulting from the investigations as described in chapter 3 and 4 will be used for a more in-depth characterisation by EIS in chapter 5. A suitable model and the results of the calculation of the ISC and the intermediate stages will be presented. The potential dependence, the dependence on the illuminated wavelength and also the size dependence of the PbS nanoparticles will be discussed. It will be revealed that ZnO is chemically unstable in contact with some of the linkers. For that reason the same linker study has been repeated with the more stable TiO2 employed as the wide band metal oxide. Comparisons between the different semiconductor metal oxides are made in chapter 6. In addition a number of open questions which previously had remained unanswered due to the instability of the ZnO can now be answered. In chapter 7 another highly porous structure different from that of the ZnO gel structure has been studied to determine its suitability as an ISC substrate. The structure arises from the electrodeposition of a ZnO reactant in the presence of eosin Y dye molecules. In the end the desorption of the dye provides a substrate with a high degree of porosity. Compared to the ZnO gel which was prepared and used for measurements in chapter 3 and 4, the electrodeposited ZnO is of a higher crystallinity and possesses a more preferential orientation. This results in a lower amount of grain boundaries which in turn results in fewer trap processes and subsequently yields a higher effective diffusion of the electron through the layer.[47,48] Optical and (opto-)electrochemical methods have been used for the basic characterisation of the untreated ZnO/Eosin Y and all other materials used in the fabrication of the ISC and a comparison with the ZnO gel used in chapter 3 and 4 will be made. Finally in chapter 8 an alternative metal oxide structure will be discussed. The background to this last chapter is to examine the influence of the ISC where the oxidic layer is present as a highly periodic arrangement, known as a photonic crystal. The TiO2 metal oxide which was also used in chapter 6 has been structured to form an inverse opal. First preparative findings and the first illustration of the (opto-)electrochemical results are presented. Consequently suggestions for improvements will be made. It is envisaged that the information gathered and presented here will help to achieve a deeper understanding of solar cells and help to improve the device efficiency and the interplay of the materials. Elementary understanding paves the way for further developments which can also contribute to providing devices for more efficient energy conversion.

In the latter half of the 20th century the first active environmentalist movements such as Greenpeace and the International Energy Agency were born and initiated a gradual rethinking of environmental awareness. Against all expectations the sole agency under international law for climate protection policy, called the United Nations Framework Convention on Climate Change, was formed 20 years later. Today the awareness of sustained, regenerative and environmental policies permeates throughout all areas of life, science and industry. But energy provision is the most decisive topic, especially since the discussions concerning the phase out of nuclear power where the voices calling for alternative energy sources have become much more vociferous. In addition the depletion of fossil fuels is expected to occur in the not too distant future. All new energy generation methods are required to meet the present and future energy demands, need to be ecological and need to exhibit the same or significantly lower cost expenditure than current energy sources. Unfortunately mankind is confronted with the problem that current commercial alternative energies are more expensive and not yet remotely as efficient as the present energy sources. Although energy provision based on water, wind, sun and geothermal sources have a huge potential because of their continuous presence, unfortunately, they are plagued by inefficient energy conversion caused by the state of technology i.e. the conversion of sun light into electricity loses energy through heat emission, reflection of the sun light, the inability of the material to absorb the entire sun spectrum and the ohmic losses in the transmission of electric current. The sun power is the most exhaustless resource and moreover through photovoltaic action, one of the most direct and cleanest source for use in energy conversion. Presently incoming sun light is not transformed in its entirely, as much degradation occurs during photon absorption and electron transfer processes. A number of other innovative possibilities have also been researched. With respect to cost and efficiency one of the most promising devices is injection solar cells (ISC). By dint of the dye sensitised solar cell (DSSC) Grätzels findings provided the foundations for much research into this type of solar cell where the light absorbing molecule employed in is a dye.[1] The current is obtained through charge separation in the dye, which is initiated through the connection between the dye and a metal oxide on the one hand and a matched redox couple on the other. In a variant of the DSSC the charge separation processes can also occur between a nanoporous metal oxide and nanoparticles giving rise to a quantum dot sensitised solar cell (QDSSC).[2] The use of nanoparticle (NP) properties can be utilized for the harvesting of solar energy, as demonstrated by Kamat and coworkers[3] who were able to exploit these findings subsequently and prepare a number of nanoparticle based solar cells. Nanoparticle research has comprised a wide field of science and nanotechnology for a number of years. As the size of a material approaches dimensions on the nm scale the surface properties contribute proportionally more to the sum of the properties than the volume due to the increase in the surface to volume ratio. These dimensions also constitute a threshold in which quantum physical effects need to be taken into account. Hence the properties of devices or materials in this size regime are inevitably size dependent. The basic principles can be described by two different theories, one of which is based on molecular orbital theory in which the particle is treated as a molecule. For this reason n atomic orbitals with the same symmetry and energy can build up n molecular orbitals through their linear combination based on the LCAO method (Linear Combination of Atomic Orbitals).[4] In the case of solids the orbitals build up energy bands, where the unoccupied states form the quasi continous conduction band (CB) and the occuppied states form the quasi continous valence band (VB). The energy \"forbidden\" area in between these two bands is called the band gap. The band gap is a fixed material property for bulk solids but depends on size in the case of the nanoparticles. In contrast to the LCAO method, simplified solid state theory will be used throughout the present work, the theoretical background of which is provided by the effective mass approximation.[5] When an absorption of a photon occurs, an exciton (electron-hole pair) can be generated. By promoting an electron (e-) from the valence band into the conduction band a hole (h+) may be said to remain in the valence band. By comparison to bulk solids, in a small particle the free charges can sense the potential barrier i.e. the edges of the nanoparticle. Analogous to the particle in a box model this potential barrier interaction results in an increase in the band gap as the particle size decreases. In a solar cell NPs with a particle size which possess a band gap energy in the near infrared (NIR) may be utilised and therefore the NPs will be able to absorb in this spectral region. However NPs also have the ability to absorb higher energy photons due to the continuum present in their band structure, so that almost the entire sun spectral range from the NIR up to UV wavelengths may be absorbed just by using the appropriate NP material and size. Suitable NPs are metal chalcogenides e.g. MX (where M = cadmium, zinc or lead and X = sulfur, selenium or tellurium) because of their bandgap size[6–10] and their relative band positions compared to those of the semiconductor oxide states. Both the TiO2/CdSe[11–14] and TiO2/CdTe[15–18] systems have already been successfully fabricated and many of the anomalies reported.[3] Much interest in the lead chalcogenides has been generated by reports that they may feature the possibility to exhibit multiple exciton generation (MEG) where the absorption of one high energy photon can result in more than one electron-hole pairs.[19–25] Currently electrochemical impedance spectroscopy (EIS) is being used more and more to clarify processes at polarisable surfaces and materials such as nanoparticles. Likewise this method has been rediscovered in photovoltaic research and its use in the characterisation of DSSCs has been discussed in the literature.[26–31] In a number of publications the evaluation of nanoporous and porous structures has been quite extensively explored.[28,29,32–34] Since the mid-20th century Jaffé’s[35] theoretical work concerning the steady- state ac response of solid and liquid systems lead to the formation of the basics of EIS. Further developments in the measurement technology have lead to a broader range of analysis becoming possible. Nevertheless the most challenging part still remains the interpretation of the results and especially to merge the measured data with the theoretical model. EIS quantifies the changes in a small ac current response at electrode electrolyte interfaces i.e. the rate at which the polarized domain will respond, when an ac potential is applied. In this way dielectric properties of materials or composites, such as charge transfers, polarization effects, charge recombination and limitations can be measured as a function of frequency and mechanistic information may be unveiled. Hence EIS allows one to draw a conclusion concerning chemical reactions, surface properties as well as interactions between the electrodes and the electrolyte. Other very useful tools that may be employed for quantifying electron transfer processes and their time domains are intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS). IMPS permits the generation of time-resolved plots of particular photo-processes in the system, each of which may be specifically addressed through varying the excitation wavelength. For the IMPS technique a sinusoidal wave with a small amplitude is applied, analogous to that of electrochemical impedance spectroscopy, but in this case the modulation is applied to a light source and not to the electrochemical cell as in EIS.[35] The current response is associated with the photogenerated charge carriers which flow through the system and finally discharge into the circuit. The amount of generated and discharged charge carriers is often different due to the presence of recombination and capture processes in surface or trap states. Ultimately the phase shift and magnitude of these currents reveal the kinetics of such processes. The only processes that will be addressed will be those that occur in the same frequency domain or on the same time scale as that of the modulated frequency of the illuminated light. In the literature some explanation of the kinetics of simple systems can be found and basic theories and introductive disquisitions may be found elsewhere.[36–38] Furthermore in solar cell research a multiplicity of studies are available which give an account of IMPS measurements on TiO2 nanoporous structures. Such studies permitted proof for the electron trapping and detrapping mechanism in TiO2 surface states.[39,40] An analysis of TiO2 electrodes combined with a dye sensitization step was established in the work of Peter and Ponomarev.[41–43] Hickey et.al.[44,45] have previously published kinetic studies on CdS nanoparticle (NP) modified electrodes. A theory was presented which allows for the IMPS data to be the interpreted in the case of CdS NP based electrodes. The back transfer, recombination and surface states have been demonstrated to be important as was determined from their inclusion in the theory. Similar attempts to explain the kinetics of CdS quantum dots are described by Bakkers et.al.[46]. In the present work the most important questions concern the behaviour of the photovoltaic assembly. Such assemblies can be equated with an electrode in contact with an electrolyte. Preliminary remarks about such electrodes as components of an electrochemical cell will be introduced in the first part of chapter 2. Thereafter the properties of electrodes in contact with the electrolyte and under illuminated conditions are illustrated. This is followed by a description of the important electrochemical and opto-electrochemical methods which have been employed in these studies. In particular, two separate subsections are dedicated to the methods of EIS and IMPS and the experimental section which are then linked to the theoretical section. The synthesis of all substances used and the preparation of the solar cell substrates are also dealt with in this section as will the equipment used and the instrument settings employed. The optical response of the working photoactive electrode is not only dependent on the substances used but also on their arrangement and linkage. The substrate which was employed in chapter 3 consists of a nanoporous ZnO gel layer upon which an organic linker has been placed in order to connect the oxide layer with the light absorbing component, the PbS NPs. Chapter 3 deals with the linker dependence on the ZnO layer and reports the typical optical characteristics and assembly arrangements of six different linkers on the ZnO layer which is an important intermediate stage in the fabrication of an ISC. The questions concerning how the type of linking affects the photo response and other electrochemical interactions of the complete solar cell substrate will be outlined in chapter 4. Further an examination of the electrochemical and opto-electrochemical behaviours of the samples will be presented similar to that presented in chapter 3. The most interesting substrate resulting from the investigations as described in chapter 3 and 4 will be used for a more in-depth characterisation by EIS in chapter 5. A suitable model and the results of the calculation of the ISC and the intermediate stages will be presented. The potential dependence, the dependence on the illuminated wavelength and also the size dependence of the PbS nanoparticles will be discussed. It will be revealed that ZnO is chemically unstable in contact with some of the linkers. For that reason the same linker study has been repeated with the more stable TiO2 employed as the wide band metal oxide. Comparisons between the different semiconductor metal oxides are made in chapter 6. In addition a number of open questions which previously had remained unanswered due to the instability of the ZnO can now be answered. In chapter 7 another highly porous structure different from that of the ZnO gel structure has been studied to determine its suitability as an ISC substrate. The structure arises from the electrodeposition of a ZnO reactant in the presence of eosin Y dye molecules. In the end the desorption of the dye provides a substrate with a high degree of porosity. Compared to the ZnO gel which was prepared and used for measurements in chapter 3 and 4, the electrodeposited ZnO is of a higher crystallinity and possesses a more preferential orientation. This results in a lower amount of grain boundaries which in turn results in fewer trap processes and subsequently yields a higher effective diffusion of the electron through the layer.[47,48] Optical and (opto-)electrochemical methods have been used for the basic characterisation of the untreated ZnO/Eosin Y and all other materials used in the fabrication of the ISC and a comparison with the ZnO gel used in chapter 3 and 4 will be made. Finally in chapter 8 an alternative metal oxide structure will be discussed. The background to this last chapter is to examine the influence of the ISC where the oxidic layer is present as a highly periodic arrangement, known as a photonic crystal. The TiO2 metal oxide which was also used in chapter 6 has been structured to form an inverse opal. First preparative findings and the first illustration of the (opto-)electrochemical results are presented. Consequently suggestions for improvements will be made. It is envisaged that the information gathered and presented here will help to achieve a deeper understanding of solar cells and help to improve the device efficiency and the interplay of the materials. Elementary understanding paves the way for further developments which can also contribute to providing devices for more efficient energy conversion.:Contents List of Abbreviations vii Legend of Symbols ix 1 Introduction and Motivation 1 2 Theoretical and Experimental Introduction 7 2.1 Basics of the (Opto-)Electrochemistry . . . . . . . . . . . . . . . . 7 2.1.1 Electrode-Electrolyte Interface Non-Illuminated . . . . . . 8 2.1.2 Electrode-Electrolyte Interface Under Illumination . . . . . 10 2.1.3 The Processes in the Injection Solar Cell (ISC) . . . . . . . 12 2.1.4 Cyclic Voltammetry (CV) . . . . . . . . . . . . . . . . . . 15 2.1.5 Chronoamperometry (CA) . . . . . . . . . . . . . . . . . . 16 2.1.6 Incident Photon to Current Conversion Efficiency (IPCE) . 16 2.1.7 Electrochemical Impedance Spectroscopy (EIS) . . . . . . 17 2.1.8 Intensity Modulated Photocurrent Spectroscopy (IMPS) . 21 2.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.2.1 Synthesis of ZnO Sol-Gel . . . . . . . . . . . . . . . . . . . 23 2.2.2 Synthesis of TiO2 Sol-Gel . . . . . . . . . . . . . . . . . . 24 2.2.3 Preparation of the ZnO/Eosin Y Substrate . . . . . . . . . 24 2.2.4 Syntheses and Preparation of the Inverse Opal . . . . . . . 25 2.2.5 The Syntheses for PbS Nanoparticle . . . . . . . . . . . . . 26 2.2.6 Preparation of the PbS Coated Substrates . . . . . . . . . 30 2.2.7 Preparation of the ISC . . . . . . . . . . . . . . . . . . . . 31 2.2.8 Material Characterisations and Instrument Settings . . . . 33 3 The Linker Attachment on a ITO/ZnO Substrate 37 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.2 The ITO/ZnO Film . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.2.1 The ZnO Layer and the ITO/ZnO Substrate Preparation . 40 3.2.2 The ZnO Structure as a Function of the Sintering Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.3 The Linker on the ITO/ZnO Film . . . . . . . . . . . . . . . . . . 48 3.3.1 The Linker Orientation on the ZnO layer . . . . . . . . . . 48 3.3.2 The Linker Interaction with the ZnO Gel . . . . . . . . . . 52 3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4 The PbS Sensitized ITO/ZnO/linker Substrate 59 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.2 The ITO/ZnO/Linker/PbS Substrate . . . . . . . . . . . . . . . . 61 4.2.1 Spectroscopic Evidence for PbS on the ITO/ZnO/Linker Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.2.2 The Cyclic Voltammetry Study on the Substrates . . . . . 63 4.2.3 The Opto-Electrochemistry on the Substrates . . . . . . . 70 4.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 5 The EIS Study of the ITO/ZnO/MPA/PbS Substrate 75 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.2 The Substrate Assembly . . . . . . . . . . . . . . . . . . . . . . . 77 5.3 The Substrate Characteristics . . . . . . . . . . . . . . . . . . . . 78 5.4 The Model for the EIS Analysis . . . . . . . . . . . . . . . . . . . 83 5.5 The Results of EIS Data Fitting . . . . . . . . . . . . . . . . . . . 86 5.5.1 The EIS Results of the FTO/ZnO Substrate . . . . . . . . 86 5.5.2 The EIS Results of the FTO/ZnO/MPA Substrate . . . . 89 5.5.3 The EIS Results of the FTO/ZnO/MPA/PbS Substrate . . 92 5.5.4 The EIS Results for Shorter Illumination Wavelength . . . 96 5.5.5 The Resistance of the Linker . . . . . . . . . . . . . . . . . 111 5.6 General Remarks on the Modelling . . . . . . . . . . . . . . . . . 112 5.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 6 TiO2 based Injection solar Cell 119 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 6.2 The ITO/TiO2 Film . . . . . . . . . . . . . . . . . . . . . . . . . 121 6.3 The Linker and PbS Attachment on the ITO/TiO2 Substrate . . . 123 6.4 The Cyclic Voltammetry Study on the Substrates . . . . . . . . . 125 6.4.1 The Linker Sensitized ITO/TiO2 Film . . . . . . . . . . . 125 6.4.2 The ITO/TiO2/Linker/PbS Substrate . . . . . . . . . . . 126 6.5 The Opto-Electrochemistry on the Substrates . . . . . . . . . . . 127 6.6 Comparison Between ZnO and TiO2 Based ISCs . . . . . . . . . . 129 6.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 7 ZnO-Eosin Y based Injection Solar Cell 135 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 7.2 The FTO/ZnO-Ey Film . . . . . . . . . . . . . . . . . . . . . . . 137 7.3 The PbS Attachment to the FTO/ZnO-Ey Film . . . . . . . . . . 137 7.4 The Cyclic Voltammetry Study on the Substrates . . . . . . . . . 140 7.5 The Opto-Electrochemistry on the Substrates . . . . . . . . . . . 142 7.5.1 The Linear Sweep Voltammetry (LSV) Study on the Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 7.5.2 The IPCE Measurements on the Substrates . . . . . . . . 144 7.5.3 The Photo Transient Measurements on the Substrates . . . 145 7.6 Comparison between ZnO and ZnO-Ey based ISC . . . . . . . . . 146 7.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 8 Injection Solar Cell meets Photonic Crystal 151 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 8.2 The Opal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 8.3 The Inverse Opal . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 8.4 The Inverse Opal based ISC . . . . . . . . . . . . . . . . . . . . . 159 8.4.1 The Substrate Characteristics . . . . . . . . . . . . . . . . 159 8.4.2 The Cyclic Voltammetry . . . . . . . . . . . . . . . . . . . 160 8.4.3 The Opto-Electrochemistry . . . . . . . . . . . . . . . . . 161 8.4.4 The EIS Measurements . . . . . . . . . . . . . . . . . . . . 163 8.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 9 Overall Conclusion 167 10 Outlook 173 Bibliography I A Acknowledgement XXV B Erklärung XXVII