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. ... Read more

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. ... Read more

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. ... Read more

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.
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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. ... Read more

A potential approach to minimizing cabling losses in Photoelectrochemical (PEC) water splitting devices is adapting a wireless stand-alone configuration. With all components integrated into a single device, this configuration also helps in reducing system cost, size, and complexity. The issue with this structure, however, is that the proton transport distance between the electrodes is quite large, as ions need to travel around the cell to reach the opposite electrode. This leads to a pH gradient between the electrodes, resulting in high ohmic losses and risking cross-over between the product gases, which is a safety hazard. This problem can be eliminated by integrating pores into the device that serve as ionic shortcuts between the electrodes, resulting in a Porous Monolithic Photoelectrochemical (PMP) cell. In this thesis, the effect of pore size and distribution on the performance of PMP cells was analyzed in pursuit of finding a range of optimal pore patterns for this application. A theoretical method involving 2D COMSOL simulations of PMP cells is devised to evaluate losses associated with proton transport (Electrochemical) and the active area available for light absorption (Photovoltaic). It was found that optimal pore size and distribution for proton transport trends towards smaller pore dimensions (diameter and pitch). It was also found that for pore diameters between 20 – 80 µm, the PMP cell can retain up to 70% of the ideal (lossless) photocurrent, if the pH gradient can be suppressed to < 0.36 pH units. Moreover, two pore-processing techniques were compared, namely Deep Reactive Ion Etching (DRIE) and Pulsed Laser Drilling (PLD), to determine their suitability for this application. DRIE processed holes can be near perfectly cylindrical compared to PLD processed pores, which have rougher sidewalls and exhibit significantly more tapering, in comparison. However, DRIE requires lithographic patterning, which is a more expensive and tedious process that adds several steps to the fabrication process. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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.
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