To make silicon thin film solar cells attractive to the market, their efficiencies should reach the efficiencies of the dominant crystalline silicon solar cell. Therefore the search for ways to improve the efficiency of silicon thin film solar cells continues. TCO front contact layers and back reflector layers play a significant role in the increase of thin film solar cell efficiencies. For the optical characterization of these TCO layers, commonly used methods are found to differ significantly in results regarding the extinction coefficient which makes their accuracy questionable.

This research consists of two main parts. For the first part, the commonly used optical characterization methods spectroscopic ellipsometry and spectrophotometry + data analysis in SCOUT are analyzed in accuracy and compared to newly introduced methods using spectrophotometry + data analysis in GenPro4 and photothermal deflection spectroscopy + data analysis in GenPro4 to build a guide on optical characterization of TCO materials. For the second part, the optical response determined using the software GenPro4 of a double junction silicon thin film solar cell for a novel bi-layer front contact design consisting of IOH and i-ZnO will be compared to the optical response for standard used AZO, and ITO single layers to find the best front contact design. Furthermore, the optical response of a double junction silicon thin film solar cell for a back reflector containing an i-ZnO layer on top of the silver back contact will be compared to the optical response for a back reflector containing an AZO layer on top of the silver back contact to find the best back reflector design.

From the results of the optical characterization methods, it is concluded that photothermal deflection spectroscopy + data analysis in GenPro4 is the most accurate method for determining the extinction coefficient of a TCO material. The results of the optical simulations for front contact TCO and back reflector TCO designs showed that the bi-layer can enhance the optical response of a double junction silicon thin film solar cell significantly compared to the AZO and ITO single layers. The i-ZnO TCO back reflector layer was found to induce less parasitic absorption and therefore a better optical response of the double junction silicon thin film solar cell.

The Bullet Cluster is a galaxy cluster, which is often used as evidence for dark matter [10]. In this thesis, Modified Newtonian Dynamics (MOND) is studied for various interpolation functions on a model of the Bullet
Cluster. MOND is a theory proposed by Milgrom which modifies the Newtonian gravity law, such that it explains the flat rotation curves observed in all the galaxies and galaxy clusters [13]. It is an alternative theory
to the dark matter model and in this thesis, the results of MOND are compared to results from the paper of
Paraficz et al which have studied the Bullet Cluster with the dark matter model [15]. In MOND, the Newtonian
gravity law is changed at low accelerations, for a around and below the value of 1.2 · 10^-10  m/s²
, which is introduced as a constant a₀ by Milgrom. Accelerations much smaller than a₀ are in the deep MOND regime and
should satisfy certain conditions. Combining the low acceleration regime (a ≪ a₀) and the Newton regime
(a ≫ a₀), an interpolation function is needed. The following interpolation functions are studied: the standard
interpolation function, the Verlinde interpolation function and the Angus interpolation function.
First the Newtonian gravitational potential Φ_N is introduced which can be calculated for a certain mass
distribution ρ using the Poisson equation. Also the acceleration field can be obtained from ΦN . It could
be seen that in the model used in this thesis for the Bullet Cluster, the strength of the acceleration field a
is mostly below a₀, but not much. Similarly the MOND potential Φ_M can also be calculated for ρ with the
MOND equations, which are different equations for each interpolation function. The MOND equations are
non-linear and can not be solved analytically in most cases. Thus a numerical iterative method is introduced
to solve the MOND equations and to obtain Φ_M . After obtaining Φ_M for each interpolation function, the acceleration field f can also be calculated. We found that Φ_M is steeper than Φ_N for all interpolation functions.
Also the acceleration field f is larger than a, and f is mostly above a₀ in the model of the Bullet Cluster used
in this thesis.
By substituting Φ_M in the Poisson equation, another mass distribution could be obtained: apparent matter. This would be the matter distribution needed to give the acceleration field f using the Newtonian gravity
law. From this matter distribution, the apparent dark matter could be obtained. We can compare this apparent dark matter distribution with the dark matter model found in the paper in Paraficz et al. Also the apparent
matter distribution can be compared to the image of the Bullet Cluster. For both apparent dark matter and
apparent matter, the MOND model is not in agreement with the dark matter model and the observation respectively. In general, the dark matter did not spatially coincide with the galaxies. By increasing the number
of galaxies or increasing the mass of the galaxies, dark matter distributions that spatially coincide with the
galaxies can be obtained

In this thesis the behaviour of a Bose Einstein condensate is explored that consists of bosons that annihilate. In order to do this a system where bose einstein condensation occurs is modeled as a Zero Range process which is a special case of a Markov process.
First we made a single slow site Zero range process with uniform rates and see how this would be influenced by annihilating particles. For this system it is shown that, in the large system limit, the number of particles in the slow site is given by a differential equation. A series of realization of different sizes of this system are done to support that the fraction of the particles in the ground state converges
to the differential equation.
In order to relate this mathematical approach to physical Bose Einstein condensates it is established that the equilibrium distributions of multiple bosons in one system is the same as the detailed balance of a zero range process where the transition rates depend on the occupation of a site, with an indication function, that is equal to 0 when there are no particles at that site and some constant 𝑐 if there is one
or more particles at that site.
In statistical physics we need to take the energy of the state of the system into account. If the particles do not have interaction with each other this is the sum over all particles of the energy of the particle site. As a zero range model only has rates with one particle hop we can see that the difference in energy of the system is equal to the difference in energy of the sites of hop. In order to stay in detailed
balance the rate for the specific hop must be exp(BΔ𝐸) times the opposite rate, where B = 1 / 𝑘_b 𝑇.
Now we can go from any system with a set of spin orbitals and energies of those spin orbitals and design a zero range model where the spin orbitals correspond to sites of the zero range model with rates between those sites, such that the systems detailed balance distribution is the same distribution as the equilibrium of the physical system. In this zero range model an annihilation term is introduced.
This can be any function of occupation for the site, but in this report a simple relation that decreases when Δ𝐸 increases is used.
In order to see the effect on such systems the system is numerically approached for a 100 particles system in a 3 dimensional harmonic potential. This system is too complicated to get an differential equation that can be easily solved. Therefore it is approximated. This numerical approximation appears to have similar behaviour to the numerical approximation done in [3]. However the results are not identical.
The advantage of the model in this thesis is that an annihilation therm can be implemented. This approximation of a 3 dimensional harmonic oscillator with annihilating particles is done. If annihilation is slow enough, the exited sites form a different equilibrium compared to a Bose Einstein condensate of non annihilating particles. This happens as long as there is a condensate in the ground state.

In this thesis Modified Newtonian Dynamics (MOND) is explored in galaxy clusters similar to the Virgo cluster. MOND is a theory proposed to explain the flat rotation curves of galaxies and the velocities of galaxies within galaxy clusters, as an alternative to the Dark Matter (DM) model. MOND states that Newton’s law of gravitation is incorrect at accelerations of the order of and smaller than Milgrom’s constant a_0 = 1.2 · 10^−10m/s^2 [1]. 
The MOND potential φ_M created by a certain mass distribution ρ satisfies the MOND equation, a non-linear partial differential equation. For accelerations much smaller than a0 this equation gives a quadratic relation between the gradient of the potential (∇φ_M) and the mass distribution ρ, this is called deep MOND. This is much different from the Poisson equation, that infers a linear relation between ∇φ_M and the mass sources, and which still holds for accelerations much larger than a_0 [1], referred to as Newtonian Dynamics (ND). For accelerations around a0 an interpolation of deep MOND and ND is used. It appears that the potential and acceleration in Virgo-like clusters is according to ND at the center, and approaches deep MOND at the edge. Therefore an interpolation function µ is necessary to model such clusters accurately.
When the MOND potential φ_M is substituted into the Poisson equation, a new mass distribution is found, the apparent mass distribution ρ_AM, which would need to match the actual mass distribution in DM models, which use the Poisson equation. This apparent mass distribution ρ_AM is the sum of the actual mass distribution ρ extracted from optical observations and the apparent dark mass distribution ρ_ADM, a distribution that is interpreted as a theoretical DM halo. This allows us to compare MOND and DM. With our method, realistic mass configurations of galaxy clusters that are Virgo-like, generate apparent mass distributions ρ_AM with regions containing negative mass. The existence, shapes and locations of these regions are in agreement with what Milgrom found [2]. The total mass of the actual mass distribution is M = 10^15M_sun, while the sum of the negative mass is M_negative ≈ −0.09 · 10^15M_sun = −0.09M is approximately 9% of the total mass. Since negative mass is not acceptable, this gives us the opportunity to create conditions to falsify either the MOND model or the DM model. 

Perovskite solar cells (PSCs) are an emerging and promising photovoltaic technology that have demonstrated impressive power conversion efficiencies exceeding 25% after only fourteen years of development. This rapid progress can mostly be attributed to the development and optimisation of PSCs fabricated by solution-based methods, because of their easy processing and inexpensive equipment. However, to commercialise PSCs, alternative methods are required. Among these methods, the vacuum-based technique thermal evaporation is a suitable candidate. In thermal evaporation, solid precursors are evaporated and deposited onto a substrate in a high vacuum chamber. Thermal evaporation is a mature technique in the semiconductor industry to fabricate pinhole-free and highly uniform thin films at a large scale on different types of substrates. To date, the efficiency of PSCs by thermal evaporation is lagging behind their solution-based counterpart, partly due to limited research on vacuum deposited PVK films and several challenges unique to thermal evaporation.
In this MSc thesis project, organic-inorganic metal halide perovskite absorbers based on thermally evaporated FAxCs1-xPb(IyBr1-y)3 and spin-coated MAPbI3 have been developed together with the additional supporting layers for device fabrication. Several PSCs with different p-i-n architectures have been fabricated and characterised. The device comprises of: ITO as front electrode, spiro-TTB, spiro-OMeTAD and/or MoOx as hole transport layer, PVK absorber layer, C60 as electron transport layer, BCP as buffer layer, and silver and aluminium as metallic back electrode. The goal of this work is to develop and optimise the structural and opto-electronic properties of different device layers and demonstrate a working perovskite solar cell.
PVK engineering is carried out on both thermally evaporated and spin-coated PVK thin films with the aim at obtaining perovskite that possesses desirable opto-electronic properties. For FAxCs1-xPb(IyBr1-y )3 fabricated by sequential layer thermal evaporation, the uniformity and tooling factors were optimised to grow layers with a correct precursor ratio showing a high crystallinity and absorptance. No clear effect of post annealing could be detected for the temperatures and times tested in this work. Achieving the target thickness and obtaining a satisfactory reproducibility were not fully reached. For spin-coated MAPbI3, it was found that increasing the precursor solution concentration helped to achieve a desirable thickness for PSCs. Moreover, an adequate bandgap, diffractogram and absorptance were measured. However, based on the small crystal size determined with SEM, and poor device performance, further improvement is needed.
Transport, contact, and buffer layers were successfully fabricated by developing new deposition recipes for silver, BCP, and spiro-TTB. Furthermore, spectrophotometry measurements show that thermally evaporated HTLs (MoOx and spiro-TTB) exhibit much lower parasitical absorption losses compared to spin-coated spiro-OMeTAD. An optical model to fit spectroscopic ellipsometry data measured on spiro-TTB thin films is created to determine its thickness and optical properties. Moreover, time resolved microwave conductivity results indicate that C60 effectively extracts electrons from MAPbI3. In contrast, spiro-TTB films did not appear to possess any hole extracting abilities. Strong quenching of the steady-state PL signal is observed for bi-layers of MAPbI3 with either C60, spiro-OMeTAD and MoOx . This quenching is possibly caused by extraction of either electrons or holes and/or enhanced non-radiative recombination at the interface.
The optimised films were combined in complete PSCs with most layers fabricated by thermal evaporation through metal masks structuring cells with active areas of 0.16 and 0.36 cm2. Solar cells were characterised in the dark and under illumination using a solar simulator integrated with a glove box to prevent degradation. A wide range of J-V characteristics are identified and classified: (1) ohmic responses, possibly caused by poor quality PVK absorber layers with pinholes filled with thermally evaporated metal, (2) S-shaped curves under illumination, possibly caused by a mismatch in energy band alignment or increased non-radiative recombination at the MoOx/MAPbI3 interface, (3) diode behaviour in the dark, and (4) a high series resistance and low shunt resistance, possibly caused by low mobility, non-optimal thicknesses, internal currents, and a mismatches in band alignment. The most promising PSC featuring the ITO (200 nm)/Spiro-OMeTAD/MoOx (5 nm)/MAPbI3/C60 (20 nm)/BCP (4 nm)/Ag (150 nm) architecture, had a short circuit current of 10.0 mA/cm2 and an open circuit voltage of 0.67 V.
The experiments carried out in this MSc thesis project supported the development of several steps of PSC fabrication process. This work contributes towards understanding thermal evaporation processes of perovskite films and fabricating fully evaporated devices with high efficiency and a large area.

The need for more efficient, cheaper, easily producible solar cells is growing as this combination of attributes can decrease the levelized cost of energy (LCOE). A two­terminal (2T) Perovskite and galliuminidium­gallium­selenide (Pvk/CIGS) tandem cell is an excellent candidate, as it can have a power conversion efficiency (PCE) of +30%, can be flexible (making it suitable for roll­-to­-roll production), and
the deposition of a Pvk solar cell (PSC) on top of established techniques like CIGS will only add one extra step to the production process, thereby improving the PCE significantly

This thesis is part of LAFLEX­-2T project which aims to design and engineer a highly efficient flexible 2T thin­-film solar device based on the Pvk/CIGS tandem configuration. The aim of this thesis is to develop an opto­-electrical model of the tandem configuration which replicates the inner physics of a reference solar cell. This can be used to give insights on the current losses occurring in the cell and on the limitations in the charge carrier transport towards the electrodes. Based on this analysis, a route of improvements is proposed with could result in more efficient solar cells.

The simulation template uses optical and electrical simulations based on GENPRO4 and TCAD Sentaurus, respectively. An extensive model for Pvk/CIGS tandem cells is presented and validated using experimentally obtained J­-V curve measurements. It was found that charge transport in the tunnel recombination junction (TRJ) depends on direct energy and in­direct energy transfer, in terms of two tunneling mechanisms: band to band tunneling (B2BT) and trap assisted tunneling (TAT). For TAT, a non­-local model facilitated by trap states is successfully implemented.
Simulation results reveal that the transport in the TRJ of the reference solar cell is based on TAT. The loss analysis points out that reflectance losses are responsible for a loss of 10.92 mA/cm². Similarly, losses due to parasitic absorption are equal to 5.32 mA/cm². The CIGS bottom was identified as the current limiting layer. The dominant recombination mechanisms in the Pvk and CIGS absorber layers are Shockley-­Read­-Hall (SRH) and surface recombination calculated as 4.71 mA/cm² and 1.59 mA/cm² for top Pvk and bottom CIGS, respectively. An increase in losses in the absorber layers in maximum power point (MPP) compared to short circuit (SC) conditions exposed charge transport issues in the collecting path of charge carriers.
After assessing the loss mechanisms, a road­map for efficiency improvements is proposed. After implementation, an increase in the PCE of the reference tandem cell was observed from 10.63% to 26.69%.

Solar cells can play a key role in the transition towards a sustainable future. This transition is one of the major challenges our society faces during the coming decades. Development of high-efficiency photovoltaic solutions at reasonable costs will help accelerating the transformation of our energy system. In this respect, perovskite solar cells are very promising due to their outstanding opto-electronical properties and low-cost fabrication. Obtaining a complete understanding of the device physics and charge transfer mechanisms inside perovskites is crucial for further device improvements. This thesis focuses on the notorious hysteresis in the current-voltage characteristics of perovskite solar cells. So far, this remarkable phenomenon has usually been explained using ion migration, despite the lack of clear experimental evidence. We implement a simulation platform of perovskite solar cells to analyse the charge transfer mechanisms among energy states including those with energy within the forbidden bandgap. We evaluate transient behaviour and identify the limiting physical mechanisms. To explain anomalous hysteresis in perovskite solar cells we use a novel approach in which charge accumulates near the material interfaces due to defects with relatively low capture cross-sections. Defects in lead halide perovskites create shallow sub-gap energy states, that act as charge carrier traps. Near the interfaces, this leads to accumulation of trapped charge carriers, effectively screening the electric field inside the perovskite layer. This reduces the device performance. A slow release of trapped charge due to low capture cross-sections results in hysteresis in the current-voltage curve at commonly used scan rates. TCAD Sentaurus is used as a platform to simulate J-V scans of a planar non-inverted architecture based on the archetypal perovskite MAPbI3, with TiO2 as electron transport layer and spiro-OMeTAD as hole transport layer. This thesis presents a systematic study of different trap distributions, both in the spatial and energetic domain. The capture cross-sections, densities, energy levels and locations of traps are varied and also the effect of scan rate is analysed. This work analyses both tail state defects and deep defects, based on reported values in literature. It is found that defects near the ETL/perovskite interface potentially cause anomalous hysteresis in the current-voltage curve. These defects have their transition energy around 0.25 eV and are possibly attributed to iodine interstitials.