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Since quite some time similarities have been noted between the photo-generation of charge carriers (electrons and holes) in a molecular semiconductor and the photosynthetic reaction centre of bacteria. For example, the mechanism for charge carrier generation by light in phthalocyanine films is largely dependent on the presence of ionised impurities. These impurities have been proposed to play a role similar to that of the special pair in bacterial reaction centra. In addition, the redox chain cascade in the reaction centra has a comparable function for charge transport as the internal field of a Schottky barrier. Although the present work applies to entirely different molecular system, this analogy is confirmed by the results of our experiments.Chapter 1 contains a general introduction and the theoretical framework to understand the basics of charge separation processes in organic donorlacceptor combinations and in p/n heterojunctions. In this study the expertise at the Department of Molecular Physics with water-soluble porphyrin heterodimers has been used as a starting point to examine the photophysical and charge-transfer properties of molecular assemblies upon organising them as two thin layers on top of each other. A complication arises from the requirement to produce pinhole-free films for (photo) electrical measurements.Experimental procedures to perform photo-electric measurements, optical characterisation and other techniques are described in Chapter 2.In Chapter 3 we describe the adsorption of a fourfold positively charged tetrakis(4 - methyl pyridinium) porphyrin and its metal derivatives on glass surfaces from basic aqueous solutions. The thickness and the depth profile of the chemical composition of these films has been determined using optical and ion beam techniques. Upon exposure to a buffered porphyrin solution, the glass surface is initially almost completely coated with a monomolecular porphyrin layer (5*10 13molecules*cm -2). Subsequently, the area of the glass surface available for deposition increases due to leaching of the glass by the buffer solution, resulting in an almost linear increase of the amount of adsorbed organic material with the period of exposure.In Chapter 4 a different technique to produce thin films on transparent, electrically conducting substrates (ITO) is introduced, i.e. the electro-polymerisation of hydroxy-phenyl substituted metallo-porphyrins. During electro-polymerisation an ether bond is formed between the phenyl rings of two porphyrin units, leaving the porphyrin macrocycles undisturbed. Using this deposition technique the thickness as well as the redoxpotentials of the layer can be accurately controlled. Using spectro-electrochemical techniques we can conclude that the oxidised film effectively consists of dimeric porphyrin mono-cations as the building blocks. To characterise charge carrier conduction in the polymerised films, diffusion coefficients were determined and found to be (1-2).10 -10cm 2s -1for films of porphyrins with varying number of hydroxyphenyl substituents, indicating that no correlation exists between the value of the diffusion coefficient and the number of cross links per porphyrin unit. This suggests that charge transport is mainly from ring to ring.Chapter 5 demonstrates a photovoltaic effect in organic heterojunctions, made by the consecutive deposition of two different types of porphyrins onto an ITO substrate equipped with a mercury top contact, using a home-built measuring cell. From photoelectric measurements of the separate and composite electron donor- and acceptor porphyrin layers with varying order of the layers it can be concluded that the photoactive region is located at or near the interface of both porphyrin layers. Changing the redoxpotentials for the porphyrin donor/ acceptor combinations results in different values of the observed short circuit current, demonstrating the dependence of the photo-induced electron transfer on the exothermicity of the electron transfer. Depending on the layer thickness and type of porphyrins, the open circuit voltage is typically≤0.4 Volt and the short circuit current≤130μA.cm -2using illumination of 13 mWatt.cm -2at 440 nm.To clarify the photovoltaic mechanism of these porphyrin heterojunctions cells in greater detail Chapter 6 reports the results of impedance spectroscopy providing evidence for the formation of a depletion layer upon contacting the two porphyrin films. This depletion layer can be described in the same way as in a conventional p/n heterojunction of inorganic semiconductors. From Mott-Schottky plots the doping concentration is found to be≈10 17cm -3for electro-polymerised zinc tetra-(hydroxy phenyl) porphyrin films and two orders of magnitude higher for spin-coated films of free base tetra (methyl pyridinium) porphyrin. Excitation of these cells with a nanosecond laser pulse reveals a lower limit for the photocurrent response time of < 10.From the results of Chapters 5 and 6 we conclude that charge separation and thus the photo- active part of the cell is confined to the interface of both layers. For charge collection, consecutive dissociation of the electron/hole pair competes with charge recombination, in particular in organic heterojunctions with presumably low values of the dielectric constant. The internal field over the heterojunction space charge layer causes the photo-induced charge carriers which are generated at the interface to drift through the bulk layer to the electrodes. This explanation for the photovoltaic effect in organic heterojunctions implies that for a device to be efficient, the rate for electron transfer at the interface must be optimised, next to the presence of an internal field. This also implies that for a sandwich-type cell, such as described in this work (but also many other cells reported in literature) the efficiency (η≤2%) is restricted by the absorbance of only a few dye monolayers. For application of this kind of charge generation layers in organic solar cells an antenna layer must be added to the cell.

For centuries we have relied on fossil fuels to produce energy for our needs, causing significant damage to the environment and our own health. To make an energy transition possible, technology has to step up, providing solutions for cleaner and cheaper energy production. In the field of solar energy, perovskite-based devices can offer a feasible alternative to conventional technologies, involving less energy intensive and cheaper manufacturing processes. Despite the great technological advancements of the past years, open circuit voltage losses and especially poor long-term stability are two of the main bottlenecks that still have to be overcome in order to bring the technology to market. In this thesis we have addressed such issues by investigating the origin and impact of electronic trap states on charge carrier dynamics in perovskite thin films of different composition...

The uncommon photochromism, photo-conductivity, and H-mobility of RE oxyhydride materials make them promising candidates for application in optics, opto-electronics and electrochemical devices alike. Additionally, their extreme compositional flexibility and the connected variety of possible (meta)stable phases make them an excellent case study to advance our understanding of the link between composition, structure, and properties in mixed-anion materials. Further, the possibility of producing RE oxyhydride not only under thermodynamic control (e.g., high temperature/pressure solid state reaction) but also under kinetic control (e.g., topochemical anion-exchange, or post-oxidation of reactively sputtered polycrystalline/epitaxial REHx thin films) largely expands the possibility of tuning their properties, influencing other aspects such as concentration of defect, material morphology, film texture, filmstress, etc.

Control over the charge carrier density of semiconductor materials is essential for various electronic devices. Unfortunately, common electronic doping methods have not always been successful for new generations of semiconductors, such as organic semiconductors and colloidal quantum dots. Therefore, a new doping method that offers a great control over the charge carrier density is needed. Electrochemistry is a powerful way of doping porous semiconductor films, where the charge carrier density can be controlled by a button on a potentiostat. Unfortunately, when the semiconductor film is disconnected from the potentiostat, injected charges leave the film. The work performed in this thesis is aimed to understand electrochemical doping and the instability with the final goal of producing stable electrochemically doped semiconductor films at room temperature for the use in devices.

Conjugated organic materials are interesting for application in opto-electronic devices where they can act as a light absorbing layer, for instance in solar cells or as a light emitting layer in light-emitting diodes. In addition, they can also be used as the semiconducting materials in for instance field-effect transistors. Conjugated organic materials have certain desirable properties that are generally found in organic materials such as light weight, flexibility and cheap processing from solution. A particularly attractive aspects of such materials is that their solid-state properties can be tuned by making changes in the molecular structure by organic synthesis techniques. This also makes it possible to modify the materials so that they exhibit more uncommon processes that may be beneficial for solar cells. Two of such processes, singlet exciton fission (SF) and triplettriplet annihilation upconversion (TTA-UC) are the main subjects of this thesis. The first (SF) is a process in which a singlet excited state, formed by absorption of light, is transformed into a combination of two triplet excited states each with half of the energy. This can, in principle, double the number of electrons that are injected in a solar cell device, and hence double the current from the device. The second (TTA-UC) is the reverse process of SF in which two triplet states with low energy can be combined into a single higher-energy singlet excited state from which an electron can be injected in a solar cells. Exploiting these two processes can in principle lead to considerable improvements in the efficiency of solar cells based on these devices. SF can be exploited to use the excess energy in photons with more than twice the bandgap energy to excite an additional electron. In this way, the excess energy that is otherwise lost as heat, is used to increase the current and therefore the overall energy efficiency of the device. TTA-UC addresses another energy-loss in solar cells, that of photons in the solar spectrum that have a lower energy than the bandgap of the active material of the solar cell. TTA-UC can be used to combined the energy of two of these photons, that are normally not absorbed by the solar cells, to generate a single higher-energy excited state that has sufficient energy to charge separate at an interface. Together, these two processes can address two of the factors that cause major energy losses in solar cells. In order to efficiently exploit these processes, a detailed fundamental understanding of these processes is required, with particular emphasis on the effect of molecular packing in the solid-state as this is the state where they are to be used in devices.

In this thesis we have aimed to tune and control the optoelectronic properties of organic-inorganic metal halide perovskites by systematically changing components in the structure and studying the charge carrier dynamic mechanisms....

Metal halide perovskites (MHPs) are interesting candidates for application in photovoltaic devices. During the past decade the efficiency of perovskite solar cells has improved enormously exceeding now values over 24%. In an MHP absorber layer, charge carriers are generated on optical excitation, and decay by band-to-band recombination or by other undesired loss mechanisms. In this thesis, the relationship between material properties, including constituents and morphology of the MHP layer and these processes is studied. The charge carrier dynamics describing these light-induced processes are studied by the time resolved microwave photoconductivity (TRMC) technique, in combination with standard optical techniques such as time-resolved photoluminescence (TRPL)…