Lithium-ion batteries are the most widely used energy storage devices, and currently dominate the portable electronic device market and automotive industry, owing to their superior energy density and long cycle life. Currently, the market of lithium-ion batteries is dominated by graphite as a material for anode. This material has proven to be very stable and reliable. However, the capacity delivered by graphite is quite low (372 mAh.g−1). As a result, there is a need to explore alternative anode materials which can deliver a superior capacity, and thereby a higher energy density. A promising alternative to overcome this limited capacity issue is the use of silicon anodes, which can theoretically deliver a specific capacity of 3579 mAh.g−1. However, silicon is prone to volumetric change (almost 200-300% of its original volume) upon cycling. This leads to loss of active material due to pulverization and delamination, severely affecting its capacity retention and hence the battery cycle life. Of the several ways in which the capacity retention of silicon anodes can be improved, we have combined two approaches: alloying silicon with nitrogen and making this material porous. This work aims at investigating the influence of the chemical composition, porosity and mass loading of the SiNx anodes developed, on the electrochemical performance when used as an anode for lithiumion battery. The monolithic SiNx anodes used in this work were fabricated using Plasma Enhanced Chemical Vapor Deposition (PECVD) and were deposited directly on copper foil, which is the most widely used current collector material for batteries. By varying the relative flow rate of silane and ammonia (precursor gases), and by varying the deposition power, the composition of the anode, its porosity and mass loading can be varied. The resultant anodes were employed to assemble coin cell against lithium metal as the counter electrode. Compositional analysis revealed that the anodes deposited at different power and at a particular flow-rate ratio (flow of ammonia to the total flow of gas) lead to the same stoichiometry of SiNx. When tested as anode for lithium-ion battery, the material with composition SiN0.32 could deliver a specific capacity of 1567 mAh.g−1 at a current density of 75 mA.g−1, which compares to a C-rate of C/20. With an increased nitrogen content in SiNx a decrease in the specific capacity and better capacity retention was observed. Furthermore, this work directs that a good balance of porosity and areal mass loading of SiNx is crucial to achieve high performance lithium-ion battery anode.

Global warming represents the most significant threat to humankind, making the need for renewable energy more crucial than ever. However, in densely populated areas near the coast, electricity production faces competition from various sectors such as agriculture, housing, and tourism. To address this challenge, one viable solution is to explore offshore electricity production.

Building upon this context, this research delves into investigating the wave-induced effect on power mismatch losses along a PV string in offshore floating photovoltaic (OFPV) systems. OFPV offers a promising solution for generating electricity in unused marine areas, complementing offshore wind energy. Although OFPV holds great potential, our understanding of its complexities remains limited, particularly regarding the impact of wave-induced power mismatch losses. To bridge this knowledge gap, a comprehensive approach is taken. A floating structure is modeled using the Bernoulli-Euler beam theory, while the fluid domain is analyzed using potential flow/linear wave theory. Structural behavior is examined in the frequency domain through the application of a FEM with the package Gridap in Julia. The wave amplitude spectra are determined using the Jonswap sea spectrum, with consideration given to four distinct sea states based on the Douglas sea scale: slight, moderate, rough and very rough. The optoelectrical modeling is conducted in pvlib in Python.

The results reveal that monthly energy losses due to power mismatch are negligible during summer months for all sea states studied. However, in winter months, monthly energy losses exceed 1%, with daily losses reaching up to 6%. Additionally, the orientation of the PV string is identified as a crucial parameter for minimizing losses. Finally, the findings indicate that using either a thick structure with a stiff and dense or a thin structure with a flexible and lightweight material can help reduce energy losses caused by power mismatch.

An increasing penetration of renewable energy sources in the global energy market necessitates an increasing amount of energy storage. Batteries are an excellent source of short term storage, used in vehicles, home storage and mobile applications. However modern technologies such as Li-ion batteries face resource scarcity and hence introduce geopolitical dependence. To this end Si-air batteries were recently, in 2009, explored in an attempt to devise a battery with high energy density and abundant materials.
The Si-air battery makes use of a silicon anode and an porous carbon cathode, to allow the circulation of air into the battery. Two electrolytes have been looked at in the past, KOH and the Room Temperature Ionic Liquid (RTIL) EMIm(HF)2.3F. The Si-air battery has excellent energy density in theory, with volumetric density being theoretically as high as 1 · 104 Wh/L. However, this theoretical energy density is as of yet far from reached, current research uses electrolytes which have a parasitic corrosion reaction with the silicon anode. This work aims to explore the usage of a new RTIL, BMPyr[NTf2], as the electrolyte in this battery. This RTIL has seen usage in Zn-air batteries, however it is yet untested for Si-air. The objective is to determine the discharge characteristics for a Si-air battery using this RTIL through comparison with KOH, with focus on the conductivity.
To do so first the relationship between conductivity and discharge potential for KOH was determined experimentally. The result found is that the discharge potential using KOH decreases with decreasing conductivity, however, this decrease is much larger than what can be solely attributed to the conductivity. Next the chosen RTIL BMPyr[NTf2], having a conductivity of 2.2 mS/cm at room temperature in a pristine state, was used in the battery discharges, where it was established to have an OCP of 0.7-0.8 V. From measurements no detectable consumption of Si was found after 1 hour of OCP. Further during the discharges it was found that the potential drops rapidly when discharge current is equal or greater to 2.5 μA. In an attempt to decrease the resistance in the battery cell a new design was created where the distance between the two electrodes is
decreased from 2 cm to 0.8 cm, using this second design a maximum OCP of 1.1 V was measured.
Finally by mixing in 1wt% and 3wt% water into the RTIL two mixtures are obtained with conductivity of 2.6 mS/Cm and 3.0 mS/cm at room temperature. Discharging these two mixtures at 20 nA, 100 nA, 500 nA and 2.5 μA it was found that for the 1 wt% the highest potential was found for 20 nA at 0.8 V. Meanwhile for the 3 wt% mixture the 20 nA discharge exhibited significantly lower potential at 0.4 V. For the 100 nA, 500 nA and 2.5 μA increasing conductivity led to increased potential, however similar to the KOH experiment, this difference in potential is larger than what is to be expected from purely conductivity changes. Finally, reproducibility of these experiments is low as a series of discharges with the same materials and current showed different potentials. These results combined lead to the conclusion of this work, thatthe relationship between conductivity and potential for the RTIL BMPyr[NTf2] is inconclusive, there are unknown factors influencing the discharge potential.

The mass production of lithium-ion batteries used in, for instance, electric vehicles led to significant cost reductions. However, concerns are emerging about the availability of resources and the environmental impact of the production. Metal-air batteries use more resource-efficient materials and potentially have higher energy densities than lithium-ion batteries. A little over a decade ago, a promising new type of metal-air battery was discovered, the silicon-air (Si-air) battery.
The Si-air battery uses a c-Si anode and a porous air cathode, combined with a liquid electrolyte. Si-air batteries have a theoretical cell potential up to 2.2 V and a very high theoretical energy density of 8,470 Wh/kg. However, Si-air batteries that use alkaline electrolytes, such as potassium hydroxide (KOH), are not rechargeable and the Si anode suffers from passivation and corrosion. Currently, corrosion is considered as the dominant factor that limits the conversion efficiency of alkaline Si-air batteries. In this work, the role of the electrons participating in the corrosion reaction is further investigated. This is based on a model where electron transfer is considered as a rate-limiting step. The research objective of this work is the mitigation of the corrosion reaction in alkaline Si-air batteries through nonuniform doping of the Si anode.

The corrosion in the Si-air battery setup was first quantified for uniformly doped c-Si. These results function as reference to nonuniformly doped Si. The corrosion rates for uniformly doped n-type and p-type Si using 6.6M KOH were found to be constant at 1.25 µm/h and lower than the 2.2 µm/h measured in previous research. This lower corrosion rate increased the conversion efficiency from 2.0% to 3.4%, corresponding to a specific capacity of 130 mAh/gSi. Based on these results, corrosion in Si-air batteries is still significant.
Nonuniformly doped wafers were made by depositing thin impurity layers on c-Si through PECVD or epitaxial growth. Subsequently, drive-in of the impurity atoms into the c-Si was performed at a temperature of 1250°C for 36 hours. The shape and junction depth of the dopant profiles were verified by the four-point probe and spreading resistance profiling. These results show the presence of doping profiles with junction depths up to 36 µm.
In the samples made with PECVD, corrosion slightly increased in nonuniformly doped p-type and n-type Si with respect to uniformly doped Si. However, the corrosion is not clearly affected by the implemented doping profiles. For nonuniformly doped p-type Si samples made with epitaxial growth, corrosion clearly decreased to 0.82 µm/h compared to 1.25 µm/h with uniformly doped Si. Corrosion decreases by having a high boron concentration above 2·10^19 atoms/cm^3 in the Si and not through the implementation of a dopant profile.
It is concluded that nonuniform doping does not affect the corrosion in alkaline Si-air batteries and, therefore, does not mitigate the corrosion reaction.

To extend Moore’s law, silicon carbide devices attend the researcher’s attention due to their irreplaceable advantages such as high critical breakdown electrical field, wide bandgap and excellent thermal conductivity without sacrificing too much charge carrier mobility. However, the defects on the SiC-oxide interface degrades the performance of the device and even get worse at high temperature, such as the threshold voltage shifting problems, limiting the design of the integrated circuits. The thesis models, characterise, analyses, measures and extracts the traps of SiC MOSCAP, to understand the trapped charge transportation and unreliability mechanism under high temperature.
Effectively using SiC materials necessitates the need to understand the physical properties itself. Due to the large bandgap, the inversion layer cannot be observable in low frequency C-V measurement. The electrical field, space charge region, surface potential and C-V curve in SiC devices all differ from Si devices. More importantly, the trapped charges fluctuate the surface potential of the SiC devices. To give insight into the mechanism governing the trapped charges, the mathematical solution of the trapped charges in the whole bandgap is solved.
To be specific, the trap behaviour at a single energy level is then followed. A nonzero transient current is generated due only to the capture and release of the trapped charges when the equilibrium condition is broken. The trapped charges with high energy are thermalized and an equivalent admittance is obtained which further be split into a capacitance and a conductance.
Rising temperature activates the dopant atoms that are not ionised, and the Fermi level shifts towards the midband. The variation of equivalent circuit components and physical parameters responds to the temperature. High temperature expands the SiC crystal and the trapped charges are easy to receive energy from phonons so that the interaction between traps and the conduction band is enhanced and more empty states are waiting for the recombination of the charge carriers.

Energy storage is a critical component in decreasing the unpredictability of
renewable energy. Silicon-air battery is a type of storage technology that potentially has a higher energy density than lithium-ion battery. The silicon-air
battery, on the other hand, must overcome corrosion and passivation reactions in order to discharge continuously. One method of reducing the passivation reaction is by increasing the dissolution rate of the discharged product higher than its generation rate. This is possible by increasing the surface area of silicon anode by making it porous. The objective of this research is to evaluate the effect of porosity on the discharge of a silicon-air battery. In this work amorphous silicon (a-Si:H) based anodes made by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique are used. The deposition power and deposition pressure are altered to obtain a-Si:H anodes of different porosities. The influence of these deposition conditions on porosity and conductivity is studied. In order to find the porosity, the refractive index of the deposited a-Si:H layer was obtained from optical characterization. Bruggeman’s Effective Medium approach is followed to calculate porosity. The deposition power has the greatest impact on the refractive index, porosity, and conductivity of the a-Si:H layer. Increasing the deposition power raises the porosity and the conductivity. This thesis also looks into the effect of varying the fraction of dopant gas in the gas mixture on porosity and
conductivity. The experiments show that both porosity and conductivity increase with higher fraction of dopant gas. The deposited a-Si:H layer and c-Si wafer were used as anode in the discharge of silicon-air battery.

Renewable energy sources such as solar and wind energy rely on climate­ and weather conditions, like sun irradiation in the case of solar energy, and wind speed in the case of wind energy. These change throughout the day and with the seasons. There are periods of little wind, and during the night there is no sunlight. During periods of no sunlight and little to no wind, there is still a demand for energy. This leads to a shortage of energy. On the other hand, there are periods when the amount of available wind­ and solar energy will surpass the demand for energy, leading to an energy excess. To mitigate this mismatch between energy production and energy demand the excess energy can be stored to be used during periods of shortage. Many different solutions for this have been investigated in recent years. One of the storage technologies that is currently quite dominant is battery storage. Lithium-ion batteries are used quite widely, among others in battery electric vehicles. However, the use of batteries as a storage device to overcome energy mismatch is not yet implemented on a large scale, as most battery technologies are still quite novel, making them
uneconomical for this use compared to traditional hydrocarbon fired power plants. Furthermore, many battery technologies depend on scarce and expensive minerals. Recently, a battery utilizing silicon as its anode and oxygen from the air at the cathode has been proposed. This so­called silicon­-air battery utilizes mainly silicon and oxygen, which are the two most common elements on earth. Furthermore, the theoretical energy density of this battery type was shown to be significantly higher than the energy density of lithium­-ion batteries. Because of this, the silicon­-air battery has been a growing area of research in the last years.

Battery models help to simulate batteries based on empirical data and electrochemical systems. These models are a powerful tool in the evaluation of the performance of batteries. Parameters of the battery can be altered quickly and specifically. This can provide a powerful analysis tool to determine weaknesses in a batteries. They can also help in further developing an understanding of the operating principles of the battery technology. A specific type of model is the finite element model. In this type of model the object that is modeled is divided into small pieces and for each piece a set of (partial) differential equations is evaluated. Different electrochemical, chemical, physical and mathematical models can be modelled and combined in this tool. For this thesis a finite element model of an alkaline silicon­-air battery is developed in COMSOL. The model is based on an earlier model that was developed in 2020.

Besides the discharge mechanism, alkaline silicon-­air batteries are subject to two secondary reactions that hinder the performance of the battery: corrosion and passivation. Corrosion consumes a large part of the silicon without contributing to the discharge. Passivation creates an oxide layer on the surface of the silicon electrode, stopping the discharge reaction. Both these reactions have been implemented in the model. Besides that, a metal contact on the silicon anode is implemented in the model. The parameters used in this model are supported by empirical values for these parameters. Finally, the model was compared to experimental results.

The simulation of the discharge of the alkaline silicon­air battery was improved in several ways compared to the pre-­existing model. The corrosion was shown in the simulations, although the mechanism is somewhat simplified because of the 1D nature of the model. The passivation reaction was shown in the simulations as well, and was improved on compared to the previous model by breaking it up into two steps. Using this model, experimentally observed trends could be simulated reasonably well. The simulated discharge potential was a close representation of the experimental data, although the open circuit potential was somewhat higher, and for higher current densities the potential was somewhat lower. For different electrolyte concentrations the model showed results similar to what was found in experiments.

Lithium-ion batteries are currently most commonly used in most electronic devices. These batteries are used because of their superiority in gravimetric energy and cyclability compared to other battery technologies. The most common anode used in lithium-ion batteries is currently graphite. Graphite has proven to be a very stable cycling material. However, with a specific capacity of 372 mAh/g, there are alternatives with higher specific capacities.
One of those alternatives is silicon, which has a theoretic capacity of 3000 mAh/g. However, a volume change of 200-300% occurs when a pure silicon anode is cycled. Thereby cracking the material and losing the majority of this theoretic capacity. The capacity retention of a silicon-based anode can be increased by various techniques. Of those techniques, two are used in this
work, alloying the silicon with carbon and creating a porous material.
This research aims to evaluate the effect of carbon concentration and porosity in the hydrogenated amorphous silicon carbide (a-SiCx:H) on the cycling performance of Li-ion batteries when this material is used for the anode. The a-SiCx:H used in this research is deposited on carbon fiber paper (CFP) using Plasma Enhanced Chemical Vapour Deposition (PECVD). By varying the precursor gas flows used during the PECVD the structure of the a-SiCx:H is changed and
samples with varying porosity and carbon concentration are obtained. These anodes are then tested using coin-cell batteries in a half-cell configuration. The results of a stability test indicated that the sample with an estimated carbon concentration of 8% and a porosity of 29% had the best capacity retention, retaining 61% of the initial 1800 mAh/g during 60 cycles at 0.3C.
This result is achieved with the sample having the highest carbon concentration and porosity that was researched in this work, suggesting that both a high carbon concentration and a high porosity value increase the capacity retention of the a-SiCx:H anode. Individual contributions of the carbon concentration and the porosity to the capacity retention have not been researched.
Therefore, conclusions to these individual contributions cannot be drawn.

In recent years, the world has witnessed a dramatic advancement in sustainable energy development. Due to the inconsistent supply of such energy, a more efficient energy storage method is in need. Among many options, lithium-ion battery stands out due to its lightweight, high energy density, and high discharge potential. Currently, the most commonly adapted anode materials in lithium-ion batteries are carbon-based, most often graphite. It shows a layered structure that can be used to store Li+ ions based on the intercalation and de-intercalation mechanism. Although this material is stable and successfully commercialized, due to its low specific capacity efforts have been put into searching other potential anode materials. Potential materials are aluminum, tin, and silicon. Among them, silicon shows an ultra-high (theoretical) specific capacity that is 12 times higher than that of carbon. However, the volume taken up by the material increases by about 300% upon lithiation and de-lithiation. Hence, silicon anodes show a poor capacity retention ability comparing to its graphite counterpart. In this work, by using a silicon alloy, we aim to alleviate the effects of volume expansion of Si by introducing alloying species and by providing a porous structure. In this work it is demonstrated that this material structure is able to absorb the expansion, while still rendering a high specific capacity. Silicon alloy samples over a wide range of alloy concentration and porosity were synthesized using PECVD. Samples were assembled into pouch-cells and coin-cells and tests were performed to compare the battery performance of each sample. A FEM model was built, enabling more investigation opportunities. Together with the experiments, they revealed how alloy concentration and porosity influence the specific capacity and cycling ability of the anode.

The objective of this research is to evaluate the effect the air electrode has on the discharge performance of an alkaline silicon-air battery. Experiments are conducted to show that alkaline pre-treatment of the air electrode of up to eight hours leads to an increase in both discharge time and discharge potential of the battery. Furthermore, it is shown that alkaline pre-treatments of sixteen and 24 hours also increase the discharge time and discharge potential of the battery with respect to no pre-treatment. However, the increase in discharge time and discharge potential for these pre-treatments is much smaller than for pre-treatments between three and eight hours. A pre-treatment of 110.75 hours results in a discharge time and discharge potential similar to that of no pre-treatment. In the experiments it is also shown that pre-treating the air electrode with water only, instead of an alkaline solution, has no effect. Finally, the experiments show that discharging the battery at a current higher than 150 μA is not supported. A computer model is then used to evaluate the impact of some qualities of the air electrode on the discharge performance of the silicon-air battery. It is found that the electrical conductivity of the air electrode has very little impact, with a very large increase in electrical conductivity resulting in only a very small increase in discharge potential. The micro-pore surface area in the air electrode has slightly more influence on the discharge performance. Still, a relatively large increase in this parameter only results in an increase of approximately 0.2 V in discharge potential. Both of these parameters are found to have no effect on the discharge time of the battery. To explain the significant increase in both discharge potential and discharge time after alkaline pre-treatment found in the experiments two possible reasons are suggested. Firstly, the adsorption of OH- ions contributing to the oxygen reduction reaction activity of the air electrode material. Secondly, other atmospheric gases besides oxygen might be suffocating the micro-pores of the air electrode. Increased micro-pore area as a result of alkaline pre-treatment could explain the extended time before the air electrode is fully suffocated.

Renewable energy sources (RES) such as Solar and Wind energy rely on the availability of natural resources like sunlight in the case of Solar and wind speed in the case of Wind energy generation which is variable in nature. There are periods where there is excess energy production than needed and periods of energy shortage where not enough energy is produced to meet the demand. To mitigate this mismatch, a short term solution is to use batteries in order to store energy at times where energy production is more than the energy demand. This stored energy would be later used at times where energy production is low and meet the energy demand. However, the current battery technology is still novel for this application making it uneconomical when compared to current energy infrastructure of using power plants. The current battery market is held by Li-ion batteries which uses lithium as a raw material which is a rare earth material. In 2009, a battery cell utilizing Si as its anode and air as its cathode was discovered. As this system relies on two of the most abundant elements in the earth's crust which is silicon and oxygen and has much higher theoretical energy density than Li-ion batteries, it has become a growing area of research and development. Battery models are created to simulate battery operations based on empirical formulas and electrochemical reactions taking place in the battery. Development of these models are very critical as they provide results and optimum condition evaluations much faster than physical testing with minimal resources. A battery model for the alkaline Si-air battery which utilizes KOH as the cell electrolyte is developed in Simscape (MATLAB) as part of this thesis. The modelling parameters are also subjected various physical conditions such as varying electrolyte concentration and change in electrode materials and the variation is investigated for model validation to study whether changing physical conditions of the Si-air cell has an effect on the modelling cell parameters. It is supported with experimental results obtained from discharging a fabricated Si-air cell. It was concluded that there are cell parameters which are dependent only on the state of charge (SOC) of the cell and one cell parameter that is a function of both the SOC as well as the discharge profile of the cell. The fabricated Si-air cell gives higher open-circuit potential (OCP) values than what was reported constant 1.4 V in literature which is speculated to be due to the usage of a 99% Aluminum and 1% Silicon (Al:Si) back contact layer. Average OCPs ranging from 1.5 V to 1.45 V which varies due to change in electrolyte (KOH) concentration is achieved. The MATLAB battery block is calibrated to be integrated with energy system models as a Si-air battery.

The development of solar photovoltaic systems has seen a rapid increase in the recent years propelled by mass manufacturing and constantly declining costs of PV modules. In some regions, however, the levelized cost of energy is still rather high, posing some challenges for this technology to take a leading roll in the energy mix. In order to overcome this, two approaches are being taken by the industry. On the one hand, improvements on the PV technology itself are nabling new PV systems to achieve higher yields of energy. One of this novel echnologies is bifacial PV modules. On the other hand, some research is being done in the deployment of PV systems in alternative configurations such as floating on water. This allows countries with land limitations, such as the Netherlands, to consider PV energy as a feasible and cost effective way to supply its energy demand.
A pilot system called InnoZoWa is being developed by the water authority Rivierenland in the Netherlands. This system will evaluate the feasibility of floating bifacial PV systems in the country. At the current phase of the project a monitoring system is developed. This system serves as means of data acquisition for research purposes from a PV system that can potentially be deployed in larger areas of inland shallow water. Delft University of Technology is one of the partners involved in the development of InnoZoWa. The PVMD group from the university is in charge of the electrical specifications and data analysis. The same group has developed in recent years a toolbox for the design and simulation of PV systems. This tool must go through different validation stages. This project is an opportunity to test out the toolbox and establish its accuracy and reliability. The toolbox presents also an opportunity to simulate the outcome from the InnoZoWa study system . Therefore, the toolbox is used to reproduce the different scenarios and configurations proposed in the pilot. As a result of this simulation, it is concluded that bifacial floating systems with reflectors are the technology with the highest potential in the setup. Other configurations including tracking functionalities show an even larger potential but their self energy consumption is a concern.

Single junction solar cells suffer from two primary loss mechanisms due to spectral mismatch: thermalisation and non-absorption losses. A photon with energy greater than the bandgap of the solar cell can be absorbed to excite an electron-hole pair and the excess energy received is released as heat constituting thermalisation losses. On the other hand, a photon with energy lower than the bandgap of the solar cell is not absorbed and leads to non-absorption losses. William B. Shockley and Hans-Joachim Queisser worked out the theoretical efficiency limit of the single junction solar cell based on the spectral mismatch and assuming band-to-band recombination, arrived at a conclusion that the maximum possible theoretical efficiency of a single junction solar cell is 33.1%.

To go beyond the theoretical efficiency limit, a high bandgap solar cell (top cell) can be optically coupled to a low bandgap solar cell (bottom cell) such that the high energetic photons are absorbed in the top cell and the non-absorbed photons from the top cell in the bottom cell. This explains the principle of tandem solar cells. In this work, a tandem solar cell structure known as four terminal device (4TD) is utilised. In a four terminal device tandem cell, the top and bottom cells are mechanically stacked together to have the same optical path, but are electrically isolated to ensure independent performance. This reduces the fabrication complexity involved in a two terminal tandem device structure.

To access the efficiency potential of various 4TD configurations before the actual fabrication of such devices, a theoretical model was derived in this work. The model predicted an absolute efficiency gain of 2% in the bottom cell for a combination of a-SiOx:H-CIGS (top cell-bottom cell) and a 3.5% efficiency gain for a a-SiOx:H-CIS devices. Based on the theoretical model, different combinations of 4TD’s were fabricated. All such devices showed a gain in efficiency in the bottom cell and the combination of a lower bandgap a-SiOx:H-CIS devices showed a maximum efficiency gain of 4.40%.

The performance analysis of different combinations of 4TD’s revealed that the extent of efficiency gain in the bottom cell is higher for a low performing bottom cell and the potential of the current 4TD configuration is limited by the transmission of the top cell. Using GenPro4 simulations, the optical system of the 4TD were analysed and possible solutions were explored. The infrared (IR) transmission of the top cell showed a drastic improvement from 55% to 80% by replacing the Asahi substrate with textured IO:H substrate. The new optimised top cell possess a combination of excellent spectral response in shorter wavelength and a good transparency in the IR region, thus making it more suitable for high efficiency 4TD configuration.

In this thesis, graded bandgap energy a-SiO_x:H solar cells are developed to attain a high JSC without compromising their high V_OC × FF product. First, a grading method which takes into account the interdependence of bandgap energy, deposition rate and the desired grading profile for a-SiO_x:H materials is developed. Then, graded bandgap energy a-SiO_x:H solar cells are fabricated to investigate the optimum grading width. The bandgap energy is linearly graded from 2.1 eV to 1.96 eV. It was observed that, when fabricating graded bandgap energy a-SiO_x:H solar cells, the grading width at both the ends of the absorber layer should be as small as possible. A 10-nm thick grading at the front end and 20-nm thick grading in the rear of the absorber layer showed the best result among all the graded bandgap cells developed during the optimization, with an efficiency of 8.2 % (V_OC : 0.96 V, J_SC : 13.36 mA/cm2, FF: 0.64).

The effects of grading the bandgap energy on the performance of a-SiO_x:H solar cells were also investigated. For instance the bandgap variation within the graded region was investigated. We found that increasing the range over which the bandgap energy is varied within a fixed graded region was detrimental to the J_SC and FF. When increasing the thickness of the graded bandgap energy absorber layer for a-SiOx:H, J_SC as high as 14.4 mA/cm2 was achieved. Also, the increase in the thickness of these graded bandgap energy absorber layers did not result in a drastic drop in efficiency as seen for a-SiOx:H solar cells without bandgap energy grading.

Finally, optical simulations of graded bandgap energy a-SiO_x:H solar cells were attempted. Graded bandgap energy i-a-SiO_x:H layers were simulated. The matching of the simulated absorbance to the measured absorbance of the graded bandgap absorber layers were found to be satisfactory. However, the simulations of the graded bandgap energy a-SiO_x:H solar cells were not satisfactory and requires more attention.

Further research into improving the FF of graded bandgap energy solar cells can help increase the VOC × FF product and efficiency of such devices. Moreover, the increase in J_SC obtained by grading the bandgap energy makes the device a suitable candidate for a top cell in four-terminal (4T) applications.

A tremendous amount of research in improving the efficiency of the single junction crystalline silicon (c-Si) based solar cells has brought its efficiency (26.6%) close to the theoretical maximum achievable conversion
efficiency (29.43%) [1–3]. Thus to further improve the efficiency, new avenues of reducing losses need to be opened up. One of the major loss, spectral mismatch loss, can be reduced by utilising tandem structures. Still, there are shortcomings associated with the conventional tandem device structure.
This thesis describes how higher efficiencies can be achieved by utilising four terminal mechanically stacked structures. This structure utilises two solar cells, which are electrically isolated but optically connected. Isolating the cells electrically negates the problem of current matching which is present
in conventional tandem cells. This structure also removes constraints such as lattice matching for the two cells. Hence many different types of four terminal cells can be developed.
In this work, high bandgap hydrogenated amorphous silicon oxide (a-SiO፱:H) thin film top cell was fabricated using plasma-enhanced chemical vapor deposition (PECVD). Three different types of c-Si cells namely, poly-Si, interdigitated back contact (IBC) and silicon heterojunction (SHJ (Hybrid)) were
utilised as the bottom cells. This is the first instance where an a-SiO፱:H based cell has been used with c-Si cells in a four terminal application.
Before actual fabrication, theoretical calculations using two parameters called as Response 4T and Spectral Response 4T were made to determine the optimal configuration as well as the efficiency enhancement for the four-terminal device. From this theoretical calculation, an efficiency gain of almost 4% can be obtained when an a-SiOx:H top cell is used with a poly-Si cell in four terminal
configuration. Gain between 1% and 2% can be realised by utilising four terminal topology for IBC and SHJ (Hybrid) bottom cells. These gains in efficiency are in comparison to the efficiency of the bottom cell alone.
A first of its kind- bifacial a-SiO፱:H cell with the efficiency of 6.60% (V_oc:0.97 V, J_sc:10.31 mA/cm² and fill factor:0.66) was developed. Using this cell as the top cell four terminal devices were fabricated. A gain in efficiency of 0.46% was obtained for the four terminal device based on the poly-Si bottom cell.
Further analysis of the four terminal devices using GenPro4 simulation tool was also performed. The analysis pointed to the reflection due to the top cell substrate glass as the limiting factor in improving the efficiency of the four terminal cell further. Parasitic absorption in the carrier selective layers of the
thin film cell was another contributor to the losses. Through this work, the potential of four terminal device concept was analysed and actual development of the device was carried out. From the work done, future steps in improving the efficiency of the four terminal device were also deduced.
Present work positively brought hope for further improvement in efficiency of crystalline silicon solar cells.

Solar cells are showing significant promise to become the solution for the growing energy needs of our world. However for this to happen, new disruptive technologies with high efficiency and low cost are needed in the market. One possibility comes from multijunction thin film solar cells based on a-Si alloys and nc-Si. For this purpose a–SiO_x:H is an interesting material since it can have V_OC values above 1 V and FF above 0.7. However when paired in a tandem structure with a material of an advantageous bandgap, like nc-Si:H. The performance of the tandem solar is limited in output current by the a–SiO_x:H layer.



Research to increase J_SC by increasing thickness of the absorber layer show it is not practical to increase the thickness of i-a–SiO_x:H above 250 nm. Spillover knowledge from other thin film solar cells (GaAs, a-SiGe_x and CIGS), showed bandgap grading was able to increase performance of the electrical parameters. Grading in a solar cell means that in one of the layers a material property is varied continuously in concentration in order to achieve a different performance.

Our aim was to experiment with bandgap grading in the absorber layer of a–SiO_x:H solar cells, to try to achieve a higher J_SC while still retaining the high V_OC x FF product. Test layers were deposited at different CO₂/SiH₄ ratios to determine the dependence of the bandgap (E₀₄) and the deposition rate on the CO₂/SiH₄ ratio. With the experimental data and fitted polynomial equations a method was devised to vary continuously the bandgap in a step wise manner. Using this grading method, experiments were designed where the intrinsic a–SiO_x:H layer of a total length of 200 nm was subdivided in 3 graded bandgap regions. The first graded region named p-i started from the end of the p-layer with a high bandgap (2.1 eV). Decreasing the bandgap over a certain length until reaching a region with no added oxygen with a low bandgap (1.96 eV.) From here the central i region started, maintaining a constant bandgap for a certain length until the bandgap starts increasing again. This marks the 3rd region called i-n, where the bandgap continues to increase over a certain width until reaching 2.1 eV at the beginning of the n-layer.



Graded experimental cells results showed that it was beneficial to have a small graded region width in the p-i and i-n region (10-30 nm). Since with this grading length J_SC could be increased significantly (8% increase) from reference cell values without grading performed. However a small compromise in a drop of V_OC and FF values around (1-2%) was observed at the same time from reference cell values. The gains in J_SC are bigger than the loss of V_OC x FF product and result in a relative efficiency gain of (4-5%) from reference cell. This technique paired with other cutting edge techniques to increase photocurrent can lead to a new record for an a–SiO_x:H thin film solar cell. Or it can lead to better current matching in tandem or triple junction solar cells.