Due to their size-dependent properties, high photoluminescence quantum yield and relatively cheap solution-based processing, colloidal quantum dots (QDs) are of great interest for application in optoelectronic devices. However, the efficiency of these devices is often limited by the presence of trap states: localized electronic states that lead to energy levels in the bandgap. Although much research has been geared to passivating (i.e., removing) these trap states, our understanding of the atomic configurations that lead to traps remains limited. Therefore, the work presented in this thesis is aimed at investigating trap states and the QD surface on an atomistic scale. We use a combination of experimental and computational techniques to show that reduced metal sites can lead to trap-formation, and that these trap states can be dynamic in nature. In addition, we find suggestions that the QD surface is more complex than often assumed and that surface reconstructions may play a pivotal role in the delocalization of the wavefunction. Lastly, we study the formation of deep traps in CsPbBr3 perovskite nanocrystals. We find that the traditional picture of defect tolerance in these materials is incomplete and should also include the local electrostatic potential in order to explain deep traps.@en

Among the emerging photovoltaic (PV) technologies under development, perovskite solar cells (PSCs) stood out in the last decade as one the fastest advancing solar technology to date, reaching a power conversion efficiency of 25.5% in 2020. Perovskite (PVK) absorbers show great versatility thanks to compositional and bandgap engineering. In this MSc Thesis Project, multications mixed-halides PVK thin-films of composition CsxFA1-xPbI3-xBrx are developed. Multiple-source layer-by-layer thermal evaporation and sequential thermal evaporation/spin coating hybrid synthesis techniques are investigated because of their compatibility with non-flat substrates. As they would enable the conformal growth of PVK on the microsized pyramidal textured surface of a c-Si- based solar cell, the long-term goal is the application in a monolithic (2-T) PVK/Si tandem. PVK engineering is carried out through tuning the main processing parameters with the aim of obtaining high-quality absorber layers. The optimisation of the PVK thin-films is based on criteria such as composition, phase, bandgap, crystallinity, homogeneity, uniformity and charge carrier transport properties. After optimising the annealing thermal treatment, the homogeneous and uniform thin-films resulting from the thermal evaporation of CsBr, PbI2 and FAI show a highly crystalline photoactive α phase of PVK, as well as a sharp-edge absorption onset corresponding to a bandgap (Eg) of Eg=1.60 eV and high absorption coefficient (α) in the order of α=10^4-10^5 cm^-1. Then, the two-step spin coating technique is explored as a preliminary study to develop the hybrid method, with the aim of analysing the surface wettability and reactivity of an inorganic layer of PVK inorganic precursors (CsBr and PbI2) when an organic solution (FAI in IPA) is spun on it. In spite of the optimisation of the FAI solution spun volume and concentration and the annealing parameters, the spin-coated thin-films present a mixture of photoactive and non-photoactive phases, i.e. PbI2-xBrx and α-, γ- and δ-PVK phases. The ineffective interdiffusion reaction leading to inhomogeneous and non-uniform PVK thin-films shows that the processing parameters need further fine-tuning. However, as spin coating does not allow to deposit PVK on top of non-flat substrates, the thermal evaporation/spin coating hybrid method is developed. The solution of FAI in IPA has been spun on a thermally evaporated inorganic bilayer of CsBr/PbI2. Albeit incomplete conversion to PVK is under suspicion, the resulting α-PVK thin-film shows good crystallinity and homogeneity and acceptable uniformity. It also presents high absorption (A=70%) for λ<500 nm (blue light) and low absorption for λ>500 nm (from green to red light), although the bandgap of Eg=1.53 eV needs to be further optimised by adjusting the content of Cs+ and most importantly Br–. On top of that, for all the synthesis techniques under research some unsuccessfully deposited PVK thin-films show signs of thermal and atmospheric degradation, leading to the decomposition of the α phase of PVK and formation of PbI2 and other non-photoactive phases, along with poor optical properties. Such degradation phenomena, which might be ascribed to the ineffective incorporation of Cs+ leading to structural instabilities, highlights the difficulties and limitations of PVK engineering.

The future of our energy supply cannot continue to depend on the use of exhaustible fossil fuels. The successful transition to a society powered by renewable energy sources is one of the major challenges our current generation faces. Renewable systems will only fully establish themselves if high conversion efficiencies can be obtained at a reasonable cost, in order to compete with conventional, carbon-based sources. With this in mind, photovoltaic solar energy can be a promising solution.
This study focuses on the investigation of the potential efficiency of a thin-film tandem solar cell, combining amorphous silicon and microcrystalline silicon for a high performance device. With the use of flexible, light substrates, inexpensive modules can be manufactured in a roll-to-roll configuration to produce high efficiency products for a large scale implementation in society. The semiconductor modelling software ASA was used to optimise the structure of the tandem, identifying the absorber layer thicknesses that maximise the efficiency. Moreover, the impact of possible optical variations in the design of the device on the performance were analysed.
To start, model parameters of single-junction amorphous silicon and microcrystalline silicon cells were calibrated to recreate the performance of such devices based on experimental data. By conducting a sensitivity analysis of the model parameters, this was effectively achieved. The single-junction models were combined in a tandem structure to forecast the operation of the multi-junction device.
It was found that the performance of the tandem was highly sensitive to the choice of refractive indices of the layers at the junction between the two subcells. A high mismatch
between the refractive indices of these layers and the absorber layers of the subcells results in an increased reflection of light to the top cell. This is beneficial for the current production of this subcell, but is too detrimental for the operation of the bottom cell. By bringing the refractive indices closer together, an efficiency of 13.0% was predicted. The inclusion of an encapsulation at the front of the cell boosts this efficiency to 13.7%, at a top/bottom absorber layer thickness combination of 160 nm/0.6 µm.
The possibilities of enhancing the efficiency with an intermediate reflector were examined. The optical conditions for the ideal light distribution with such a reflector were
computed to provide estimations of the potential increase in performance. For the best performing device, an increase of the initial efficiency of 13.0% to 13.2% was foreseen, at slightly thinner top cells and similar bottom cell thicknesses.
All in all, the predicted efficiencies of the amorphous silicon/microcrystalline silicon tandem solar cell evidences the potential of these devices for the production of cheap,
highly efficient modules. The developed analysis of the ideal intermediate reflector can be generalised to other multi-junction solar cells to establish the potential efficiency increase such a layer brings along. This can help in the decision of whether the increase in performance justifies the added manufacturing complexity and costs.

In connection with attempts to increase the Solar Redox Flow Battery (SRFB) viability, an investigation was done into the charge carrier transport mechanisms entailed at the photoelectrode interlayer interfaces. On the basis of a preliminary study, the investigation focused on the development of a suitable conducting layer at the electrolyte/electrode interface as well as the development and control of a hole extraction layer, based on the device electronic structure and energy levels matching. Research was conducted into the properties of various prospective materials that could improve charge carrier transport at the electrolyte/photoelectrode interface. Once the optimal configuration is determined, the photoelectrode and redox couples are prepared and characterized and the photocharging performance of the device is optimized. Furthermore, the control of the formation conditions of a hole extraction layer took place, in order to yield improved internal charge carrier separation and transport. Various hole transport layers were assessed in SRFB system in relation to the photo-charging performance. The described process resulted in the development of a conducting layer through the growth of well distributed islands, which led to a high solar-to-chemical conversion of 9.4%. Through the control of the formation conditions of the hole extraction layer, better spatial separation of the photogenerated charge carriers was achieved, leading to improved device performance. Despite the enhanced viability of each device in terms of efficiency, further research into the long-term stability is essential for its practical implementation. This mainly relates to electrolyte degradation to form solid precipitates and photocorrosion of the absorber semiconductor materials in acidic conditions.

Quantum dots are semiconductor nanocrystals that exhibit size specific photophysical properties. Therefore, they can be applied as phosphors in LED lighting and television screens. The main issue regarding currently used quantum dots is that most of them are cadmium based (e.g. cadmium selenide). Because cadmium is highly toxic and carcinogenic, its use is restricted from most commercial applications.
As solution to this problem is an indium based quantum dot, which is far less toxic. The main problem with indium based quantum dots, is a lower photoluminescence quantum yield, a lower stability and a lower color purity. For this reason, in this thesis we aimed to improve the photoluminescence quantum yield by a better quantum dot surface passivation, to reduce the number of non-emitting recombination sites (dangling bonds). To passivate the surface, zinc magnesium selenide was chosen as a shell material that has an improved lattice match compared to currently used materials (such as zinc sulfide and zinc selenide). Furthermore, this zinc magnesium selenide shell should protect the quantum dot from the outside, therefore increasing its stability to air and moisture. Also, the bandgap energy of this material is higher compared to currently used shell materials, so the electron and hole are more confined in the quantum dot core. This automatically should improve the color purity.

The research performed showed that the synthesis of zinc magnesium selenide is quite difficult and cannot be approached the same way as a zinc selenide shell is usually synthesized. Therefore, a method using organometallic precursors was developed to synthesize this material. Results showed that zinc magnesium selenide as a shell material has an improved effect on the photoluminescence quantum yield, stability and color purity of indium phosphide quantum dots, compared to a zinc selenide shell. The synthetic method needs to be optimized before the indium phosphide quantum dots with a zinc magnesium selenide shell are able to compete with cadmium selenide quantum dots.