Solar energy conversion technologies are considered among the most promising strategies of exploiting renewable energy resources for sustainable development of our society. These technologies have a common ultimate goal: converting solar energy into electricity (in photovoltaics) or storing solar energy into chemicals (solar fuels) at high efficiency/cost ratios. In order to achieve high efficiency/cost ratios, many different solar cell and solar fuel cell approaches have been developed and implemented. Specifically, employing inorganic-organic hybrid nanostructured materials as active or passive building blocks show great potential for fulfilling this goal. Among them, large area-to-volume ratio architectures like those employed in dye-sensitized solar cells as well as photocatalytic water splitting cells are among the most interesting concepts in each field thanks to their low cost of fabrication, linked with the low energy input of processing and the employment of abundant materials on their development. However, the efficiencies for these technologies are yet not fully optimized. The photoconversion efficiencies for these technologies are closely determined by the photo-induced charge carrier dynamics in the bulk as well as at the interface of components comprising the devices. Therefore it is of a huge practical significance to precisely measure, characterize, model and ultimately tune the charge carrier dynamics (charge carrier transport and interfacial transfer) via a careful selection and design of key components. Time-resolved terahertz spectroscopy (TRTS) is a femtosecond-laser pump- probe based technique where the material is first excited by a pump pulse and then probed by a THz pulse with sub-picosecond resolution. This technique enables us to study the non-equilibrium carrier dynamics of materials. Furthermore, we can retrieve the complex optical properties at any pump-probe delay, such as the pump induced frequency-resolved complex conductivity of a given sample. In Chapter 3 and 4 we studied photo-induced electron transfer (ET) processes in nanographene-sensitized metal oxide films. Nanographenes are carbon-based and non-toxic. Their size/edge/shape morphology and hence their optoelectronic properties can be precisely defined. The size-dependent energy gap of nanographenes holds the potential for using them in order to absorb the entire solar spectrum, a unique advantage compared with other conventional molecular sensitizers. The high surface area provided by the mesoporous metal oxide films, 139 SUMMARY together with the strong extinction coefficient of nanographenes make sensitized solar cells and solar fuels made from these constituents to promise a superior performance when compared with conventional approaches. However, the efficiencies of current prototypes based on these components are yet much lower than those based on e.g. ruthenium dyes. This has been linked from several authors to, among other factors, the poor control of the bonding geometry at the interfaces defined between donor and acceptor. Using TRTS, we provided the first experimental evidence on how physisorption or chemisorption of nanographenes onto mesoporous metal oxides affects related interfacial carrier dynamics (and hence photoconversion efficiency). In Chapter 3 we presented these results, where we resolved an almost 2-order-of-magnitude speed-up of the photo- induced electron transfer at the interfaces of the chemically absorbed (via carboxylate groups) C 42 nanographenes-sensitized metal oxides compared to that where sensitizers are physically absorbed. Our observables can be rationalized by an enhanced donor-acceptor coupling strength (i.e. donor- acceptor wavefunction overlap) in chemisorbed case. These findings indicate that functionalizing the nanographenes with anchoring head groups opens an avenue for the enhancement of photocurrent and eventually overall efficiency in carbon- based sensitizer/oxide electrodes employed in solar energy conversion schemes. In Chapter 4 we analyzed how the size of nanographenes (defining their absorption onset) affects interfacial electron transfer rates in sensitized systems. The energy gap of nanographenes exhibited a power-law dependence on the size (i.e. the number of fused rings), in good agreement with theoretical predictions. Applying TRTS, we resolved that ET in these systems was characterized by a biphasic mechanism, likely linked with hot and cold ET channels respectively. We also found a sensitizer size dependence towards both ET rate components, that can be interpreted and modeled by the many-body Marcus theory. Within this theoretical framework, electrons are transferred from the nanographene towards the oxide faster as nanographenes decrease their size (i.e. enlarging the energy gap). In order to achieve the maximum charge collection efficiency at the metal electrode, ET from the sensitizer should favorably kinetically compete with radiative relaxation within the nanographene, while having an absorption onset (defined by its size) enabling best matching to the solar spectrum (around 1.4eV following the Shockley-Queisser formalism). In Chapter 5 we analyzed novel two-dimensional (2D) electrically conductive metal-organic frameworks (MOFs). Due to their large and tunable porosity and 140 SUMMARY thus large surface-to-volume ratios, MOFs have been considered as ideal candidates for photocatalysis. However, this aspect is strongly limited by the fact that most of the MOFs developed to date are insulating, and therefore do not absorb visible light. In this work, we investigated the optoelectronic properties as well as the nature of charge transport in a novel semiconducting, π-d conjugated, porous Fe3(THT)2(NH4)3 2D MOF. TRTS was employed to model the frequency- resolved photoconductivity of the material, from which a Drude response was obtained, indicating that band-like charge carrier transport (rather than hopping) was operative in the systems. From the Drude fit, a record charge mobility of ~220 cm2/Vs at room temperature was demonstrated. Moreover, the temperature dependence for the complex photoconductivity revealed that scattering rates and thus mobilities are primarily limited by impurity scattering. Therefore, the mobilities inferred from TRTS refer to the lower limit. By comparing the optical and electrical conductivities of this material, we found a quantitative agreement, indicating that the long-range charge transport in our 2D MOFs was not affected by grain boundary scattering, an aspect that was rationalized by the strong p- doping in analyzed samples. As an outlook, we anticipate that the conductivity, mobility, and bandgap of all graphene-like MOF analogs can be controlled through appropriate chemical design. Specifically, in-plane engineering can be pursued via metal substitution, modification of functional groups and organic ligands, and even by selecting appropriate guest molecules within the pores. Out-of-plane engineering is also possible by the design of 2D van der Waals heterostructures by, e.g., controlling the spacing between layers and/or the relative alignment between planes. Furthermore, controlled stacking of 2D MOF layers of dissimilar materials might allow the development of heterostructures with unique physical and chemical properties. The large degree of chemical and structural tunability for this planar (and porous) class of 2D MOF materials provides plenty of room for development in this exciting field of research. Important challenges include identifying reliable correlations between chemical composition, electronic structure, conductivity and charge carrier mobility. From a synthetic point of view, developing single crystals and delaminating them into single layers will enable not only fundamental studies of structure-properties relations, but also the development of MOF-based functional devices where long-range free carrier motion is required.weiterlesen