Single junction solar cells have a theoretical efficiency limit of 33.1% due to spectral mismatch. To overcome this, multi-junction devices are generally fabricated with two or more junctions, so as to achieve better energy conversion efficiency by optimum spectral utilization. The c-Si bottom cell of a thin-film Si triple-/quadruple-junction device does not utilize the low energy photons (below 1.1 eV). The photons in the range of 0.7-1.1 eV, have an available current density of 15.9 mA/cm^2. A fraction of this current density would be large enough to not limit the output current of these thin-film Si-based multi-junction devices. Ge, belonging to the same group IV as Si and being heavier than it, forms weaker covalent bonds. Hence, it has a lower bandgap energy, making it the preferred choice of material. In this work, a low bandgap material such as a Ge-based absorber layer is fabricated that can be used in the bottom cell of a thin-film Si-based quadruple-junction device. This thesis will focus on the influence of a set of deposition parameters on the various properties of the Ge:H films. This will result in a set of Ge:H films from which a specific few are used as absorber layers to analyze the performance of a single-junction cell. The fabrication of the Ge:H films is carried out on a CASCADE setup which is based on the RF-PECVD technique. A processing range is identified to be in the range of 1-5 mbar pressure with RF powers ranging between 5-25 W for a fixed electrode distance of 20mm. nc-Ge:H are processed in the range of 20-25 W for pressures of 2 mbar and higher at a high dilution of 400. A strong correlation is found between the refractive index of the films and the presence of GeOx. The films with low refractive index possibly indicate a porous network with high void density show substantial oxygen contamination and vice-versa. Water vapour in the ambient is responsible for the oxidation. The oxygen contamination significantly impacts the properties of the films. The E04 optical bandgap increases with oxygen contamination which hinders the development of a low-bandgap absorber layer, while the Eact decreases to values as low as around 50 eV . Generally, the pre-exponential factor (sigma_o) decreases significantly by 1-5 orders of magnitude which outweighs the decrease in Eact, resulting in the decrease in dark conductivity by 1-3 orders of magnitude. Consequently, highest photo/dark conductivity ratios of 5-6 are obtained for these films. Amongst the films without oxygen contamination, the lowest E04 optical bandgap reported is 1.2 eV, with a Etauc bandgap of 0.93 and a photo/dark conductivity ratio around 3.4. Most of the single-junction cells processed at absorber layer thicknesses of 100nm and above show resistor-like behaviour. The substantially high values of Rs losses associated to the Rsh degrade the cell parameters significantly. Although, for low absorber layer thickness of around 50nm, the closest resemblance to a cell-behaviour is observed. Therefore, there is a scope for improvement with regards to processing of these single-junction cells.

The maximal conversion efficiency on a single pn-junction solar cell is calculated to be 33.7% at a band gap energy of 1.34 eV. The main reasons for this maximum are the non-absorption of low energy photons and the thermalization of high energy photons. To counter this, multijunction devices can be constructed to make optimal use of the spectral range of the sunlight reaching the earth’s surface. While the introduction of thin film multi-junctions based on silicon has managed to overcome the problem of spectral mismatching, photons with an energy below 1.12 eV are still not able to be collected. Where usually a high current density is traded off against a high open circuit voltage in multi-junction devices, the addition of a low band gap germanium-based bottom cell (Eg = 0.67 eV) could improve open circuit voltage without limiting the current density in the device. The photon flux in the range of 0.67-1.12 eV is high enough such that a current density of 15.9 mA/cm2 can be produced when they can be effectively collected. This current density is so high that it will never limit the output current of a multi-junction device, making germanium a very attractive material for studying and integration in structures where the low energy photons are not yet utilised. This thesis is focused around the growth and characterization of hydrogenated germanium films for future use as active absorbers in multi-junction deivces. The influence of a set of deposition parameters on the morphological, optical and electrical properties of the films is studied with the aim of finding a deposition regime where device quality germanium films can be produced within the CASCADE reactor in the EKL clean room. The thin films are all be deposited using the RF-PECVD technique. Uniformly deposited Ge:H films under a stable plasma can be deposited in CASCADE in the range of 1-5 mbar pressure and 5-30 W RF power at a fixed electrode distance of 20 mm. The films start to crystallize and form nc-Ge:H at a pressure of around 3-4 mbar and an RF power of 15-25 W when the germane precursor gas is diluted with hydrogen in a ratio of 1:400. A strong correlation between the refractive index and the presence of post deposition oxidation is investigated. Films with a low refractive index are characterized as having a low material denisty, making it easy for ambient water molecules to diffuse into the lattice and react with germanium dangling bonds present there. By studying the effect and extent of the post-deposition oxidation on other film properties it was found that the activation energy of the films decreases to values as low as 50 meV. Despite this, due to a significant decrease of the σ0 by 1-5 orders of magnitude, the dark conductivity is found to decrease by 1-3 orders of magnitude. With a high photo/dark conductivity of 5-6 as a result. These results have led to the belief that the formation of Ge-O bonds in the films decreases the amount of defective states in the film, but that the defect states are moved up to an energy level closer to the conduction band. The presence of Ge-O bonds also inhibits the development of low band gap absorbers as seen by the low E04 optical band gap. In the denser films without oxygen contamination, the lowest E04 that has been reported is 1.2 eV along with a Tauc gap of 0.93 eV and a photo/dark conductivity ratio of 3.4.

Multi-junction solar cells with Si alloys have true potential for high conversion efficiency, because of the spectrum splitting capability. For optimal spectral utilization, a multi-junction silicon based device needs a silicon alloy with a bandgap between that of nc-Si (1.1 eV) and a-Si (1.8 eV). a-SiGe:H has a tunable bandgap between 1.4-1.6 eV by varying the GeH4 flow rate in the layer. An investigation of deposition parameters on PECVD processed a-SiGe:H films showed that variation of the GeH4/SiH4 flow rate is the most influential deposition parameter to achieve bandgap tuning. It influences the optical-, electrical- and material properties due to the fact that it directly changes the GeH4 flow rate in the material. Subsequently, a n-i-p substrate single junction a-SiGe:H solar cell is fabricated. This is the reversed p-i-n superstrate a-SiGe:H solar cell, fabricated by the PVMD group. It has a Voc of 508 mV, a FF of 0.39, a Jsc of 9.9 mA/cm2 and a final efficiency of 2.9%. Due to the low performance, manipulation techniques were introduced. The different proposed shapes consist of bandgap grading in the absorber through GeH4 flow rate profiling. A study on buffer type and thickness, different profiling schemes, grading widths and total absorber thickness strongly improved the device performance. The overall champion single junction a-SiGe:H solar cell has a 3.2 nm n/i i-a-Si buffer combined with a 5 nm n/i i-a-SiGe:H buffer, a V-shape GeH4 peak flow rate of 2.4 sccm, a 134 nm n/i grading width, a 36 nm i/p grading width and a total absorber thickness of 170 nm. This cell generates a Voc of 735 mV, a FF of 0.64, a Jsc of 13.23 mA/cm2 and a final efficiency of 6.18%. This is a relative increase of 113.8% in efficiency. The single junction solar cell fabrication is followed by demonstration of a two-junction device. The overall champion tandem solar cell consists of a 200 nm a-Si:H top cell and a 170 nm a-SiGe:H bottom cell with a V-shape GeH4 peak flow rate of 5.3 sccm. This champion tandem solar cell generates a Voc of 1395 mV, a FF of 0.69, a current limiting Jsc of 8.34 mA/cm2 and a final efficiency of 7.99%. Finally, research proved that the change in top and bottom cell thickness, the maximum GeH4 flow rate and the U- and V-shape profiles are interesting manipulation techniques in a multi-junction device due to their flexibility to increase the current limiting Jsc and improve current matching.

Single junction solar cells have a theoretical efficiency limit of 33.1% due to spectral mismatch. To overcome this limitation, two or more single junction cells with different bandgaps can be coupled together to achieve higher energy conversion efficiency by optimum spectral utilization. In this work, thin film silicon-based alloys are used to obtain a high potential and a current matched 2-junction solar cell is fabricated. The top cell is a-Si which has high bandgap of 1.8eV and the bottom cell is a-SiGe:H which has a tuneable bandgap between 1.4-1.6eV. This thesis will focus in optimizing the window layer of the top a-Si solar cell and the effects of Hydrogen Plasma Treatment (HPT) at the i/p and p/TCO interface. This top cell is fabricated in a PECVD cluster tool in n-i-p substrate configuration on ASAHI glass substrate. The use of PECVD allows for a better control of the layer properties by changing the gas flow rates and the deposition environment. Initially a reference cell is fabricated which had an open circuit voltage (Voc) of 804 mV, a fill factor (FF) of 0.63, a short circuit current density (Jsc) of 12.8 mAcm-2 and an efficiency of 6.56 %. To improve the performance of the solar cell the the effects of HPT on the material properties of the i-layer and the p-layer are studied at varying power, pressure and duration of exposure. The next step is to study the effects of the precursor gas flow rates for the deposition of p-nc-SiOx layer and optimize the thickness of the individual components of the window layer including the TCO. The optimization process involved trading off certain properties in favour of a high Voc and high FF. The final cell had an open circuit voltage (Voc) of 863V, a fill factor (FF) of 0.655, a short circuit current density (Jsc) of 12.5 mAcm-2 and an efficiency of 7.06 %. Thus, by only optimizing the non-active window layer an open circuit voltage gain of 60mV is achieved with an improved FF. This single junction cell is then fabricated on top of a-SiGe:H solar cell. With some iteration in the absorber layer properties of both the top and bottom cells, the final tandem cell had a Voc of 1395 mV, a FF of 0.69, the current limiting Jsc of the top cell at 8.34mAcm-2 and an overall efficiency of 7.99%. To further improve this tandem cell, the tunnel recombination junction can be further optimised.

A next step in the energy transition is to produce green fuels from sunlight. One way to achieve this is via high voltage photovoltaic (PV) devices which can directly drive electrolysis reactions to produce hydrogen or other chemical fuels. Group IV based multijunction (MJ) PV devices are a candidate to function as these high voltage devices. This work is focused on improving the efficiency of crystalline silicon/nanocrystalline silicon/amorphous silicon (c-Si/nc-Si/a-Si) triple junction (3J) solar cells through better current matching. On top of that, the low bandgap material germanium tin (GeSn:H) is characterized as it shows potential for use as bottom junction in multijunction photovoltaic devices. Several methods were tried to improve current matching in the 3J PV devices. Varying the nc-Si middle absorber thickness was successful at improving the shortcircuit current density (J 𝑠𝑐 ) in the middle junction, which was the current limiting junction. A range between 35μm ncSi absorber thickness is effective at keeping the middle junction J𝑠𝑐 high while keeping opencircuit voltage (V𝑜𝑐 ) and fill factor (FF) losses at an acceptable level. Varying the n-type nanocrystalline silicon oxide (n-nc-SiO𝑥) intermediate reflective layer (IRL) thickness was also effective at improving the J𝑠𝑐 of the middle junction. Introducing a silver IRL between the n-layer of the middle junction and the player of the bottom junction was ineffective, as was introducing various transparent conductive oxides (TCOs). The J𝑠𝑐 of the middle junction did not increase with the introduction of these layers. In the case of the TCOs, the shunt resistance decreased drastically resulting in decreased V𝑜𝑐 s and FFs. The best performing device in this thesis was manufactured using a 400nm a-Si:H absorber, a 4.5μm i-nc-Si:H absorber and a 60nm n-nc-SiO𝑥 intermediate reflective layer. This device had a V𝑜𝑐 of 1.947V, FF of 0.789, a J𝑠𝑐 of 9.51mA/cm2 and an efficiency of 14.6%, which, to the best of our knowledge, is the highest conversion efficiency achieved to date with this device architecture. Introducing tetramethyltin (TMT) to process GeSn:H films resulted in carbonization of the films. Carbon passivates Ge dangling bonds better than hydrogen. This reduces the defect density, leading to higher activation energy (E𝑎𝑐𝑡) with high TMT injection rates. A high germane flow rate resulted in dense, relatively defectfree films with a high photoconductivity, low bandgap and near intrinsic semiconductor properties. A high germane flow of around 2 sccm is therefore recommended. Varying the pressure in the deposition chamber offers some control over postprocessing oxidation, where higher pressure results in less oxidation. Changing the radiofrequency power offers a tradeoff between film growth rate and film quality.

The world is in need for an energy transition from the usage of fossil fuels to renewable energy source to reduce the harmful effects climate and health. Focusing on accelerating the research and development of photovoltaic technologies is an important step towards energy transition. The thin-film silicon solar modules has shown a great potential and a high market prospect. It is flexible, light-weight and also has a low manufacturing and installation cost. It is used for solar pumps and building integration. However, the highest initial conversion efficiency for a micromorph is 14.8%. Thus, we focus on further optimizing the multi-junction solar cells to reach higher efficiency by focusing on the improving the p-layer, texturing and silicon oxide intermediate reflector(SOIR). Further, standardized and error-free external quantum efficiency (EQE) measurement was done to obtain reliable Jsc values. This work introduces deposition of single, micromorph and triple-junction thin-film solar cells on glass and wafers. The experiments were designed to study the effects of varying the material, thickness of the p-layer and n-SiOx reflective layer and textures of the solar cells. The Jsc from the EQE, Voc, FF and efficiency were the main parameters used to analyze the results. The Jsc from the EQE measurement was not reliable as the EQE had an artifact while measuring micromorph devices. It was solved by increasing the bias light intensity and decreasing probe light intensity. The forward voltage bias was also found to be important for a cell with low shunt resistance and to measure the target cell in short circuit condition. The standard bias lights to be used are 1-3 to bias the top cell and 7-8 to bias the bottom cell combined with 50% reduction in the probe light intensity. After correcting the EQE, the optimization of the top cell p-layer was performed on a-Si single junction solar cells on Asahi substrates. It was found that both the contact layer and the window layer of the p-layer have a high influence on the performance of the solar cell. A maximum conversion efficiency of 10% and Voc of 910mV were achieved for the a-Si single junction solar cells. Further, it was also found that there is more than one way to reach high efficiency with an optimal p layer by changing the material composition and thickness of both the contact and window layer. Additionally, p-SiOx contact layer resulted in good performing solar cells for larger range of F_B2H6 of the window layer than p-nc-Si contact layer and the insertion of the buffer layer in the i/p interface showed no significant improvement in the performance of the solar cell. Furthermore, comparison of the performance of textures of different feature sizes was done on solar cells in p-i-n and n-i-p architecture. It was found that the best optoelectronic performance was obtained from asahi and smooth pyramidal textured solar cell in p-i-n and n-i-p configuration respectively in both double and triple junction solar cell. Further, the honeycomb textured substrate with R_rms around 270nm is not suitable to deposit on double and triple junction solar cell as it resulted in shunted cells. Finally, the SOIR can be used in a-Si/nc-Si/nc-Si triple junction solar cell to divide current within middle and bottom junctions. The reflection of light in the infrared region increases with the thickness of the SOIR layer. A thick 90nm n-SiOx layer with F_B2H6=3.2sccm gives the best electrical performance and current distribution property for a micromorph.

With the rise of various renewable energy sources, comes the possibility for combining the different type of sources together to balance their shortcomings. The goal is to find a renewable energy system that can be reliable year-round and be accessible for everyone. This research tries to model such a system. A model of a grid-tied PV-battery-electrolyser-fuel cell power system, which is based on a continuation of a series of master thesis projects, was expanded to include a neighbourhood with a fully electrical load or a combination of electrical and hydrogen loads. This model was developed to answer the following question. What is the techno-economic feasibility of a grid-tied PV-battery-electrolyser-fuel cell power system for a household area in the Netherlands which is either fully electrical or hydrogen integrated? This hybrid system is simulated by using the graphical interface program TRNSYS. The system size of the PV, batteries, electrolyser, fuel cell and hydrogen gas storage tank are optimised by the GenOpt, an add-on for TRNSYS. The optimisation algorithm will try to find the lowest levelised cost of energy(LCOE) while keeping the system self-sufficiency ratio(SSR) around 1 [%]. This will mean that only 1 [%] of the load is allowed to be extracted from the grid. The simulation is based on a neighbourhood that consists of 630 houses located in Pijnacker Netherlands. All houses will be equipped with a roof mounted solar PV system with centralised batteries, electrolyser, fuel cell and a hydrogen storage tank. If needed the model can be extended to include a small solar park next to the neighbourhood. The model will simulate two scenarios for a simulation time of one year, the first being that the neighbourhood is fully electrical and the second for a neighbourhood with integrated hydrogen gas in its consumption. The first one is the base, with only the electrical load demand of houses. Then the load profile will be extended by adding vehicle to the neighbourhood, including the heat demand of the house. These additional load profiles will either be electrical energy based for the fully electrical scenario or hydrogen gas based for the integrated hydrogen scenario. To estimate the economic development of this hybrid system, a price projection of PV, battery, electrolyser, fuel cell, hydrogen heating, heat pumps and inverters components were determined for the years 2020, 2030, 2040 and 2050. a, the cases will all be simulated for these years. The economic analysis will be over the systems lifetime, which is 25 years. Before the cases were simulated the model undertook a sensitivity analysis. From this resulted that the simulation start time can be moved from the 1st of January to the 2nd of March to relief the storage tank of getting depleted at the start of the simulation. A battery discharge constraint was lifted and this led the batteries to provide more energy. A forecasting method was applied to the system that effectively reduced the electrolyser on/off cycles by 60 [%], which increased the lifetime of the electrolyser component. From a technical feasibility analysis of the cases, it resulted that the integrated hydrogen scenario was not technical feasible with the PV system (roof mounted with the PV park) of this model. All the integrated hydrogen scenario cases resulted in a depleted hydrogen storage tank, which forced the system to buy the hydrogen demand externally. The system will rely on an external source more than the allowed 1 [%] (hydrogen gas SSR >> 1 [%]) of the load demand. From the fully electrical scenario the 2020 C-E-(V+H) case resulted not be technical feasible with a SSR value of 2.1 [%]. All the other cases were technical feasible. From an economic and cost perspective, the cases resulted that the LCOE reduced with the years. The lowest LCOE value found was for the C-E-(Base) case, which reduced from 0.44 [€/KWh] in 2020 to 0.21[€/KWh] in 2050. The cost breakdown of the cases resulted in the PV system and the storage tank to be the most expensive components of this system. Due to the fact that the C-H2-(H) case had to buy a significant amount of hydrogen from an external source, this became a significant expensive cost of the system. Comparing the two scenarios resulted that the integrated hydrogen scenario system sizes were smaller, but this is an effect of the system being more eager to buy hydrogen gas then to expand the hydrogen production components. As both scenarios had different SSR values of their respected energy demands, a conclusion of which scenario is more beneficial will be inadequate.

The availability of clean drinking water and various energy sources has always been taken for granted and considered to be infinite throughout human history. However, nowadays, we have reached a historical moment in which we are over-exploiting the Earth's resources. The reduction of the high level of pollution in the air and in the water is one of the main challenges that the next generations will have to face. Photovoltaic technologies offer a clean and cheap solution for both water purification and hydrogen generation for energy storage. The main goal of this thesis is to upscale lab-scale thin film solar cells, by a factor of 600, optimized for water treatment devices. To this end, two alternative solutions have been explored. On the one hand, a metallic front contact grid has been designed to minimize the power losses in large area pin superstrate solar cells. On the other hand, a thin film mini-module has been manufactured by isolating small area nip substrate solar cells and connecting them in parallel. The implementation of the front metallic grid has enabled the upscaling of pin superstrate amorphous silicon solar cells by a factor of around 10. Additionally, the laser ablation process needed for dividing the sample into multiple cells has been studied and optimized for nip substrate amorphous silicon devices. Finally, some of the major problems that cause the formation of shunts have been identified. However, the relatively high density of shunts through the amorphous silicon bulk prevents the device from achieving higher efficiencies at larger dimensions.

Multijunction devices based on thin film silicon are useful for a range of applications due to their low costs, better performance at higher temperatures, potential for flexibility, and light weight. One particularly interesting application of such devices is in solar-to-fuel conversion. In light of this, the goal of this thesis was to suggest ways to better design multijunction devices based on thin film silicon. Three main research objectives were identified. The first objective was the development of a high VOC top junction. Cells based on a-Si and a-SiOX were optimized by varying gas flows during the deposition of the intrinsic absorber layer. A maximum VOC of around 890 mV was obtained from cells based on both absorbers. Based on these results, it was possible to identify unique advantages of using either a-Si or a-SiOX as the top junction. For the second research objective, solar cells based on a-SiGe were studied. The impact of varying the bandgap profile in a-SiGe absorbers on the device performance was investigated with a set of experiments. Subsequently, a set of semi-empirical equations were motivated and fitted to the experimental data. These relations could approximately show how bandgap profiling affects device performance in a more general way. The presented approach may be used to expedite the design of a-SiGe based cells for use in different types of multijunction devices. Finally, a systematic optimization of tunnel recombination junctions (TRJ) was conducted. Experiments were done to study the impact of using different TRJs in two types of tandem devices: a-Si/a-SiGe and a-SiGe/nc-Si. Different features of the TRJ were varied separately: p-doped layer(s) used, thicknesses of the p-doped layer(s), and the n-doped layer(s) used. The best performing p-doped layers was identified among the studied combinations. For the n-layer(s), the experimental data suggested the different roles played by n-a-Si, n-nc-SiOX, and n-nc-Si in the TRJ. This observation could be used to design an optimized combination of n-layers for the TRJ. Collectively, the work done as part of each research objective aims to contribute to the better design of multijunction devices based on thin film silicon alloys.

In a future energy system, chemical energy carriers that can easily be stored, like hydrogen, are of vital importance. One possible way of producing carbonneutral hydrogen is by direct solar to hydrogen conversion in a photoelectrochemical cell (PEC). Silicon based multijunction solar cells are a possible candidate for the photovoltaic stack of these PEC devices. To make high efficiency PEC devices, the photovoltaic stack has to be optimised, especially with respect to reaching high current densities in the middle (ncSi) cell. In this work it was attempted to increase the current density of a triple junction device based on cSi/ncSi/aSi absorber layers by varying the ncSi absorber thicknesses and implementing various intermediate reflecting layers (IRLs) between the middle and bottom cell. These layers were based on silicon oxide, transparent conducting oxides (TCOs) and thin silver films. For each method it was attempted to give a quantitative comparison of how much the electrical performance is affected per unit of current gained in the middle cell. Both increasing the ncSi absorber thickness and the silicon oxide reflector thickness were found to be feasible methods of increasing the middle cell current density. The ncSi absorber thickness leads to an initial rapid current gain, of about 2.6mA/cm2 between 2.5 and 3.75 μm, reducing to just 0.8mA/cm2 between 3.75 and 5 μm. The electrical performance cost between 2.5 and 3.75 μm was calculated as a 2% reduction in 푉 oc*퐹퐹 product per mA/cm2 current gain. This cost increased to about 5% per mA/cm2 between 3.75 and 5 μm. Increasing the absorber thickness beyond 5 μm is not considered feasible due to reducing current gains and mounting electrical performance losses. Increasing the silicon oxide thickness can result in a current gain of about 1mA/cm2. This comes at an electrical loss of 3% 푉 oc*퐹퐹 product per mA/cm2. TCO based IRLs were found to quickly result in shunting, and in the case of indium doped tin oxide (ITO) based IRLs also damage to the surrounding cell structure resulting in poorly performing cells. Aluminium doped zinc oxide (AZO) based reflectors with proper electrical isolation from the edges gave only slightly reduced electrical performance, but failed to lead to a current gain in the middle cell. Thin silver films were found to quickly rearrange into nanoparticles, effectively forming a plasmonic IRL. This IRL however suffered from high parasitic absorption, and as a result also did not lead to a current gain. A very thin silver film was however shown to slightly improve the electrical performance, at the cost of bottom cell current density. Finally, a device was fabricated with both a thick ncSi absorber as well as a silicon oxide IRL. The resulting device achieved a current density of 9.5mA/cm2, a 푉 oc of 1.947V and a FF of 0.789, giving an efficiency of 14.6%. To the knowledge of the author this is the highest efficiency reported for this device configuration to date. Furthermore, attempts were made to incorporate these stacks into PEC, by fabricating an microstructured anode, cathode and ionconducting pores, to form a porous membrane PEC (PMP). The cathode and anode were successfully demonstrated, but the pore etching is yet to be optimised.

While the renewable energy sources are rising more and more throughout the years, and the effects of climate change starting to become more visible in our everyday life, some actions need to be taken. Combining different renewable energy sources gives the opportunity to build systems that can help limit these effects and provide energy in different types. The main goal of this project is to model a system that can offer electricity and hydrogen to a steel industry, in this case Tata Steel, and provide an insight for their next steps towards sustainability not only to cover their electricity needs but also investigate alternative steel making routes for their primary processes which are the iron making and further the steel making with hydrogen injection. The models developed for this project aim to answer the following research question: ”What is the feasibility to cover with a solar - wind - hydrogen system, the loads of the different steel making routes of Tata Steel ?” Different models were build through the software TRNSYS for the scenarios investigated that are linked with different combinations of components and loads so more sustainable steel making routes can be followed in the future. The steel making routes refer to the iron making process and the electrification of the industry with green electricity. For the year of 2030 a fuel mix of 70% natural gas and 30% of hydrogen and for the year of 2050, 100% of hydrogen will be used for the iron making process while for both of them 100% of the electricity loads were investigated to be covered by renewable energy sources. The scenarios were divided in a combining structure of local - non local generation and the loads corresponding to the different steel making routes. The local scenario refers to generation in the Netherlands while the non local scenario refers to the generation of hydrogen in the Arabic Peninsula and the generation of the electricity loads of the processes in the Netherlands. The components that were used were wind turbines, solar panels, batteries and electrolyzers. Each system was optimized with the help of GenOpt, an add - on of TRNSYS, to which was set to minimize the levelized cost of electricity, considering also the load coverage both for the hydrogen and electricity loads not to deviate more than 1% from the full coverage. Further than the optimization, a sensitivity analysis with the Sobol method through the programming environment of python and more specifically the SALib library and the parametric analysis of TRNSYS was conducted for all the different scenarios investigated to give an insight how the amount of the different components affects the levelized cost of electricity. Combining different metrics that were calculated as the Levelised cost of electricity (LCOE), the Self Sufficiency Ratio (SSR), the SSR of hydrogen, the total cost, the avoided emissions per MWh, the area ratio and the avoided emissions per area, a final comparison was done through the different scenarios. The most attractive choices both in a feasibility and economical perspective were the scenarios for local generation of hydrogen and electricity for the projected steel making routes of the years of 2030 and 2050. For the local generation scenario of 2030 an LCOE of 0.383 AC/kWh with an electricity coverage of 99.007 % and a hydrogen load coverage of 99.135% were resulted. On the other hand, for the scenario of the local generation of 2050, an LCOE of 0.421 AC/kWh, with an electricity load coverage of 99.875% and a hydrogen load coverage of 99.207% were calculated. The scenario that was the most attractive throughout the different metrics was the one for the local generation of 2050 while the next to come was the one for the local generation of 2030. Given the aforementioned study and its respectful results, it is a first step to evaluate the impact that such a system can have not only in the emissions reduction of such an intensively emitting industry but also to bring into perspective all the different aspects needed to be considered to realize such a project and evaluate them in more depth in the future.

In this thesis, two new types of crystalline silicon texturization were developed, eventually aimed to be used in a triple junction solar cell from monocrystalline silicon and two thin film silicon alloy layers. The bottom of the two thin film silicon layers is made of hydrogenated nanocrystalline silicon (nc-Si:H). The current problem with this technology is that the adhesion of the nc-Si:H to a flat surface of crystalline silicon is not always ideal. This can be improved by texturing the monocrystalline silicon. This also improves the optical efficiency. The standard pyramidal texture method of crystalline silicon has proven to be too sharp which leads to cracks in the layer of nc-Si:H during deposition. The two investigated texturing methods are a periodic hexagonal texture with semispherical walls created using photolithography and a random texture of craters using a sacrificial layer of polycrystalline silicon. For the hexagonal texture, a photolithography process was designed and tested on the ability to grow crack-free nc-Si:H. It was found that a texture period of 3 µm was most suited for the desired nc-Si:H layer thickness of 3-3.5 µm. For the sacrificial layer process, the effect of the different parameters on the crater size was investigated. These tests showed that an amorphous silicon (a-Si) layer thickness of 1-2 µm gives the largest craters. The ideal implantation energy is dependent on both the implanted ion and the a-Si layer thickness. It was also found that the crater size increases with increasing dopant concentration. An anneal temperature of 950 °C for 60 minutes was determined to result in the largest craters. Furthermore, argon implantation using an energy of 250 keV in 1 or 2 µm of a-Si showed the biggest promise in terms of crater size, with an average hole diameter of around 320 nm. In terms of optical performance, the hexagonal texture results in a slightly lower mean reflectance over a wavelength range of 300 to 1200 nm. The angular distribution of the reflectance showed that the reflectance of the hexagonal texture was very dependent on the measured angle and this angular dependence varies for different wavelengths. This indicates good light scattering of this texture. The results for the large craters show light scattering at approximately the same level as the photolithography samples, but more gradual over different angles. The best hexagonal and sacrificial layer textures were used to create SHJ's. Unfortunately, the minority charge carrier lifetime measurements of these SHJ's showed that the deposited passivation layers were likely to be too thin, which resulted in very low lifetimes. Consequently, the efficiency of the sacrificial layer SHJ's was only 0.1%. The efficiency of the hexagonal texture SHJ's were much higher, at 10.0%. Still, this efficiency was lower than anticipated. The reflectance measurements of these solar cells showed that the average reflectance of the SHJ's using the photolithography texture was 13.4% with TCO. This is lower than the reflectance of the sacrificial layer SHJ, which was at 14.4% with TCO.

For the far majority of current photovoltaic devices, photons with an energy below 1.1eV are not utilized. Adding a Plasma Enhanced Chemical Vapor Deposition (PECVD) processed germanium bottom cell to a multi-junction device has the potential to provide a low-cost boost in conversion efficiency by utilizing a part of this spectral range below 1.1eV. For this thesis, 89 amorphous/nano-crystalline hydrogenated germanium (a-/nc-Ge:H) films were PECVD processed with the objective of creating a device quality material that is stable, intrinsic and has a high photoresponse. These characteristics are required to enable p-i-n bottom cell integration in a multi-junction device and prevent post-deposition oxidation and carbonation. To this end, the following three approaches were applied: I. Boron doping to increase the activation energy; II. Hydrogen plasma treatment (HPT) to decrease the defect density of the material with hydrogen passivation; III. A decreased electrode gap to improve the photoresponse. These approaches were integrated in the sample preparation steps. Subsequent film characterization was carried out by using vibrational analysis, elemental analysis, and opto-electrical analysis. The influence of boron doping was considered insignificant and the influence of hydrogen plasma treatment was considered inconclusive. Furthermore, a decreased electrode gap did not result in a higher photoresponse. Reported photo-/dark conductivity ratios were in the range of 1-7. However, the decreased electrode gap resulted in lower deposition rates and higher refractive indices of 4.9-5.0 (compared to 4.1-4.9 for a 20mm electrode gap), indicating denser films. A substantial fraction of the films was processed at an increased temperature of 275°C. It was found that increasing the processing temperature from 200°C to 275°C resulted in considerably denser films with refractive indices of up to 5.2, leading to higher activation energy (up to 314meV) and very low post-deposition oxidation and carbonation. Therefore, by increasing the processing temperature, it was accomplished to produce denser, more stable films, which have a lower bandgap energy and are more intrinsic.

The development in solar cells began with wafer based cells, also called the first generation photovoltaic technology. Most of these wafer based cells were made of crystalline silicon. As a crystalline silicon cell must be relatively thick to absorb most of the incoming energy, the second generation (thin film solar cells) was introduced. In the third generation the focus is more on achieving high efficiencies at low production costs. A way to achieve higher efficiencies is by the use of multi-junction devices. In such devices, different sub cells are stacked onto each other and in this way a larger part of the solar spectrum can be utilized. Moreover, the fabrication costs of multi-junction devices can significantly be reduced by using a cheap processing technique, like plasma enhanced chemical vapor deposition (PECVD). When germanium and germanium-tin are passivated by hydrogen atoms, Ge:H has its theoretical bandgap in the 0.9-1.1eV range and GeSn:H its bandgap in the 0.6-1.0eV range. The use of such low bandgap materials facilitates absorption of photons in the infrared spectrum, what reduces the non-absorption losses. Their low bandgaps make them perfect candidates to act as the absorber material in a bottom cell in a multi-junction device. Both materials can also be processed by PECVD. In this thesis about 100 Ge(Sn):H films were PECVD processed in the CASCADE reactor located in the Else Kooi Lab. The objective was optimizing the plasma conditions to obtain device quality thin films. A device quality bottom cell material must fulfil some requirements, like having a low bandgap, being intrinsic and having a high photo response. The influence of various deposition parameters was investigated to characterize their effect on the material properties. It was found that a densification of Ge(Sn):H generally lead to lower bandgap energies. Densification of these materials can be caused by increasing the substrate temperature (in the 250-300°C range). Next to this, a decrease in hydrogen dilution (in the 100-400 range) also leads to lower bandgap energies for the amorphous Ge(Sn):H films. By combining a substrate temperature of 290°C with a hydrogen dilution of 100, promising a-Ge:H films were processed containing refractive indexes above 5.3, optical bandgap energies below 1.1eV, activation energies above 330meV and dark conductivities below 5∙10-4 Ω-1cm-1. The material properties of the processed a-GeSn:H films were even closer to a device quality bottom cell material. Nevertheless, processing device quality GeSn:H layers remains challenging. Adding relatively large amounts of tetramethyltin (TMT) into the plasma chamber led to clusters of tin and significant oxygen and carbon concentrations throughout the layer. Managing the atomic carbon, oxygen, germanium and tin fractions could be crucial in obtaining device quality bottom cell absorber layers based on GeSn:H in the future.