1,721,034 research outputs found
Novel Concepts for Silicon Based Photovoltaics and Photoelectrochemistry
No full textLong term concerns about climate change and fossil fuel depletion will require a transition towards energy systems powered by solar radiation or other renewable sources. Novel concepts based on silicon materials and devices are investigated for applications in the next generation photovoltaic (PV) devices and photoelectrochemical (PEC) water splitting for solar energy conversion and storage. Expanding thermal plasma chemical vapor deposition (ETP-CVD), a remote plasma synthesis method, is verified as an efficient process for the fabrication of free-standing silicon nanocrystals (Si NCs) on an industrial scale. The unique physical, mechanical and electrical properties of Si NCs might open routes to new PV concepts to breach the so-called Shockley-Queisser limit using mechanisms like multiple exciton generation and up- or down-conversion of the incident spectrum. Under intensive laser illumination conditions, the thermal heating effects of Si NCs become the dominant mechanism for the transverse optical (TO) mode red-shifts of the first order Si-Si peak in reference to the bulk c-Si in the Raman spectrum. The free-standing Si NCs can be heated to their melting points by a well-focused laser, and the temperature can be determined by the measured ratio of Anti-Stokes-to-Stokes TO mode intensities. In contrast, Si NCs in various matrices can hardly be heated using the same amount of laser power due to good thermal conductivity. If the free-standing Si NCs are further heated, the intensity of the blackbody radiation in Raman spectrum starts to compete with that of the TO mode. Various PEC/PV configurations for solar water splitting structures are discussed in this thesis to directly store the solar energy in the form of hydrogen fuels. In Chapter 4, the a-Si:H/a-Si:H double-junction solar cell is demonstrated as the simplest and easiest option to meet the requirements for the integration with gradient-doped W:BiVO4 photoanode, by considering the stability in aqueous solutions, simple fabrication process, matching spectral response, voltage and current density. The optimization steps of the a-Si:H/a-Si:H solar cells are carried out in both experiments and simulations, by varying the top i-layer thickness in reference to the AM 1.5 spectrum and the spectrum transmitted through the BiVO4 photoanode respectively. The stability of the a-Si:H/a-Si:H solar cell shows less sensitive light-induced degradation kinetics under the spectrum transmitted through the BiVO4 photoanode from that under the standard AM 1.5 spectrum. In Chapter 5, the performance of the front BiVO4 photoanode is further improved, comparing with the studies in Chapter 4, by improving photon absorption and carrier collection. Photon absorption is enhanced by the application of light trapping techniques on the BiVO4 photoanode using textured TCO glass substrates. The carrier collection is optimized based on our new findings on diffusion length of the photogenerated charge carriers in an undoped BiVO4. By ingenious design of the gradient W-dopant profile, the thickness of the film is extended without deteriorating the carrier separation efficiency. The catalytic limitation is overcome by electrodepositing a thin film of cobalt phosphate as water oxidation catalysts on the surface of BiVO4. The optimized front photoanode is combined with three types of solar cells to form a hybrid PEC/PV solar water-splitting. The collaboration of a BiVO4/a-Si:H/nc-Si:H photoanode demonstrates the best performance concerning the better solar spectrum utilization of nc-Si:H up to 1100 nm near-infra-red region. A 5.2% solar-to-hydrogen conversion efficiency, which is the highest ratio of metal-oxide based photoanodes ever reported, has been achieved by this PEC/PV configuration. Besides the photoanode device for oxygen evolution reaction, a photocathode based on thin-film silicon technology is designed and optimized as well, to form an unbiased photoanode /photocathode aiming to revolutionize solar water splitting. Photon absorption is enhanced by state-of-the-art implementation of light trapping techniques on the a-SiC:H photoanode using a glass substrate with integrated micro-textured photonic structures. The light traveling length is prolonged in the high quality grown nc-Si:H benefiting from the scattering morphology on the glass substrate. The carrier collection is boosted by our unprecedented design of the gradient boron dopant profile from the a-SiC:H p-layer to the i-layer. Novel spectral utilization techniques are applied in the device for the integrated PV junctions, supported by a theoretical optical model. A benchmark photocurrent density of -5.1 mA cm-2 at 0 V vs. RHE is achieved in the a-SiC:H/a-Si:H/nc-Si:H configuration. It is note-worthy to address that this photocathode does not contain a passivation layer nor any catalyst. The efficient operation of a photocathode also requires metal catalysts to facilitate charge-transfer reactions at the interface between the semiconducting light absorbers and the electrolyte. Atomic layer deposition (ALD) is employed to fabricate the Pt nanoparticles and thin films as the hydrogen-evolution catalysts under varied conditions. Using MeCpPtMe3 and ozone as the precursors and substrate temperatures as low as 200 °C, a growth rate as fast as 1.1 Å/cycle is achieved. The electro-catalytic activity of ALD-grown Pt thin films on glassy carbon electrodes shows comparable performance for the hydrogen-evolution reaction as that of Pt films deposited using electron-beam evaporation.Electrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
Improving the photocurrent of a-Si:H/CIGS hybrid tandem cells by texturing ZnO:Al
No full textTo reduce the usage of Indium (rare material) in CIGS, ultra thin, high efficiency CIGS based solar devices have been developed in our group. One approach is to fabricate hybrid tandem cells between hydrogenated amorphous silicon and CIGS. Because the a-Si:H top sub-cell limits the current of such a device, it is desirable to texture the top transparent conductive oxide of the tandem cells for better light in-coupling inside the active layers. This project focused on textured ZnO:Al as the top TCO for the hybrid tandem cells to improve the device’s current. In this project, substrate temperature and sputtering pressure were varied. By varying sputtering conditions, different texture types of ZnO:Al could be found by wet-etching in HCl 0.5%. The different morphologies of textured ZnO:Al could be explained by the compactness of the material. The textured ZnO:Al had high transparency (around 80%) and low resistivity (as low as 5.5e-4 Ohm.¬cm). Then, further examination on the effect of light trapping of different textured ZnO:Al, textured a-Si:H single junctions were made. It was found that the textured top TCO resulted in maximum 9% increase in short circuit current in wavelength range from 300nm to 600nm of the textured single junction cell. This assisted the textured ZnO:Al choices for hybrid tandem cell. In textured hybrid tandem cells, the current of the top sub-cells increased by 0.77mA/cm2 or 8% increment in device’s current. Aside the experimental work on the textured devices, a new empirical model "AID model" was developed to integrate in the GenPro4 model. This empirical model took advantage of spectrophotometer measurements to generate the vectors inside the scattering matrices to describe light’s behaviour at the interface. Incorporating the AID model with other models, GenPro4 can be used with a wider spectrum of textured surfaces.Electrical Engineering, Mathematics and Computer ScienceElectrical Sustainable EnergySustainable Energy Technolog
A Multi Junction Solar Cell for Water Splitting: Development of an a-Si:H/nc-Si:H/SHJ triple junction solar cell
No full textDespite the fact that renewable energy sources are on the march, our energy system is still dominated by fossil fuels. In this energy transition storage is one of the main bottlenecks and attracts a lot of research. In order to compensate for the intermittent character of renewable sources like solar energy, the most developed options in the field are batteries for electrical storage and hydrogen as an energy carrier for long-term energy storage. In this project, the latter has been proposed and explored as a vital solution. A sustainable way to produce hydrogen is by means of solar energy. A coupled photovoltaic-electrolysis system uses a water electrolyzer powered by a solar cell with sufficient voltage to drive the water splitting reaction. A triple junction solar cell based on hydrogenated amorphous silicon, hydrogenated nanocrystalline silicon and a silicon heterojunction solar cell has been studied. A n-type crystalline silicon wafer was used as a substrate and all thin film layers were deposited by plasma-enhanced chemical vapour deposition. The best results for a triple junction device were achieved with an intermediate layer of 6 nm n-type nc¡SiOx between top and middle cell, a 200 nm intrinsic a-Si:H layer in the top cell and a 2800 nm thick intrinsic nc-Si:H absorber layer in the middle cell. With such a structure, a STH efficiency of 2.1 % is feasible with an iridium oxide counter electrode. To further improve the device, it was shown that reducing the intrinsic a-Si:H as well as the n-a-Si:H layer in the top cell increases current density matching in the multi junction device as more light passes through to the middle cell. Furthermore, a highly doped n-a-Si:H layer in the middle cell also improved the overall performance of the triple junction.Electrical Engineering, Mathematics and Computer ScienceElectrical Sustainable EnergyPhotovoltaic Materials and Device
Development of a polished top cell interface and an intermediate Bragg reflector for a-Si:H/CIGS tandem cells
No full textElectrical Engineering, Mathematics and Computer ScienceElectrical Sustainable Energ
Solar Charging Station for Light Electric Vehicles: A Design and Feasibility Study
No full textThe growth in technology and increased public interest has brought about a rapid evolution in the realm of e-mobiilty. While generally viewed as non-polluting and environmentally friendly, the Electrical Vehicles (EVs) could still contribute significantly towards indirect emissions, depending on the source of their energy. The only way they can be made truly emission free is if they are charged from renewable energy sources. This thesis aims at designing such a charging station powered by solar energy. The charging station would cater to the most popular type of Light Electric Vehicles (LEVs) - the e-bikes, and would be located at Delft in the Netherlands. Given that the Netherlands is estimated to have close to a million e-bikes already in use, the design of such a charging station would undoubtedly heighten the positive impact of LEVs. Firstly, the amount of solar energy that can be harnessed in the Netherlands is accurately found out. A photovoltaic (PV) model is developed to predict the PV module yields based on minimal weather parameters. The PV model gives a Module Ideality Factor that is indicative of the drop in PV yield due to temperature and irradiance effects. Two main system topologies are analyzed - an autonomous charging station and a grid-connected charging station - under two different load profiles. While the system reliability is of primary concern in an autonomous charging station, the electrical autarky and the Effective Autarky Ratio are optimized for sizing the grid-connected system. Lifetime estimation models are developed to predict the PV module and battery lifetimes. A basic economical analysis is performed to determine the financial viability of such a project. At large scales (several kW), the system is shown to have a Levelized Cost of Electricity (LCOE) that is grid competitive. Finally, the environmental impact of the system is studied. It is concluded that the implementation of such a charging station is not only technically feasible, but is also environmentally friendly and economically viable, especially at large scales.Sustainable Energy TechnologyPhotovoltaic Materials and DevicesApplied Science
VHESPA 2.0: The Design of a Very High Endurance Solar Powered Aircraft
No full textWith the upcoming trend in electric aviation a good model that describes how the solar spectrum depends on altitude and solar zenith angle can ease the design process significantly. Knowing what solar spectrum to expect makes it possible to design the solar system of an aircraft more efficiently as the optimal incoming energy is included in the weight estimation. However, not only solar powered aircraft bene t from this model. Solar systems in rural mountain areas can use this model to predict more precisely the energy yield of solar systems and their return on investment. In this thesis a model for the prediction of the solar spectrum for various altitudes and solar angles is proposed. This model, that uses the physics of Rayleigh scattering and light absorption to determine the solar spectrum, is found to be accurate for clear skies without clouds and low turbidity. This is because when clear skies are assumed the majority of light interaction can be described by Rayleigh scattering and light absorption. To obtain reliable spectra at cloudy days one needs to include Mie and non-selective scattering to the model. To prove that this model can ease the design process of solar powered aircraft a case study has been performed that focuses on the design of a two-manned solar powered aircraft capable of crossing the globe. This aircraft, also known as a Very High Endurance Solar Powered Aircraft (VHESPA), has been subject to a new weight prediction model. This new weight prediction model has been based on the model proposed by Noth that uses reference solar powered aircraft together with battery and propulsion technology research to come up with a first order weight estimation of solar powered aircraft. To con rm the correctness of this model the designed aircraft has undergone a simulation that reproduces the incoming solar power as well as the power required for continuous flight and the power choices that need to be made throughout the day. It has been found that an aircraft designed using this new weight estimation model gives a reliable first order solar powered aircraft design.Electrical Engineering, Mathematics and Computer ScienceElectrical Sustainable Energy / Photovoltaic Materials and DevicesSustainable Energy Technolog
Thermal Simulation of Low Concentration PV/Thermal System using a Computational Fluid Dynamics Software
No full textCogenra company has created a low concentration PV/Thermal system that produces both thermal energy and electricity simultaneously, mainly for commercial and industrial applications. The realization of a system that combines photovoltaic modules, solar thermal collectors and concentrating mirrors makes it a complicated system to study. So far, only simple studies have been made on Cogenra’s LCPVT system including a 2-dimensional model. In this project, the possibility of using a Computational Fluid Dynamics software for analysing the low concentration PV/Thermal system of Cogenra has been studied. The CFD software Ansys Fluent has been used, in which a model was created in accordance to the Cogenra LCPVT system. After validating the results, the model has been used for analysing the system’s performance under various conditions in order to realize the system’s losses. Furthermore, due to the numerous components of the system, the analysis of the LCPVT system becomes a multi-variable problem. For this reason, three main parameters (mass flow rate, optical concentration, PV type) that affect the system’s performance has been chosen and studied in order to improve the system’s overall efficiency. Since the system has both electrical and thermal outputs, an equivalent efficiency was determined to express the two different efficiency terms. For the purpose of comparing the performance of the LCPVT system with the traditional photovoltaic modules, one other simple model was created in Ansys Fluent. This model has also been simulated under the same conditions as the Cogenra system in order to observe the difference in output between the LCPVT system and the photovoltaic modules. The low concentration PV/Thermal system has also been compared with other solar thermal systems such as a PV/Thermal system, a Concentrated Thermal system and a simple Solar Thermal System.Applied SciencesElectrical Sustainable EnergySustainable Energy Technologie
Hydrogenated amorphous silicon: Nanostructure and defects
No full textSince the first report on the fabrication of hydrogenated amorphous silicon (a-Si:H) in 1965, this material has found many applications, for instance in the fabrication of solar cells, sensors and transistors. An especially notable example in the context of this thesis is thin-film silicon solar cells, which have attracted a lot of attention. This is largely due to the fact that this type of photovoltaic technology can be implemented on flexible substrates and is a potentially cheap and lightweight product. However, a-Si:H-based solar cells suffer from light-induced degradation (LID) which can only partially be recovered by annealing. This issue, which has become known as the Staebler-Wronski effect (SWE), is obviously an undesirable material property in the context of photovoltaic applications. Although the solar cell conversion efficiency of thin-film silicon solar cells has seen significant gains over the years, research efforts are still ongoing to achieve further improvements. A significant portion of these improvements has come from the development of new materials or improvements in existing materials. This type of fundamental research is inherently linked to the understanding of the nanostructure and the defects therein to enable further material quality improvements. In this thesis it is aimed to fundamentally improve the understanding of the a-Si:H nanostructure and the defects in this material to finally enable a reduction or even elimination of the SWE. Unfortunately, the SWE has proven to be a notoriously difficult problem due to the complexity of the a-Si:H nanostructure, which is the cause of the lacking consensus on the nature of the defects in this material. Because of this complication, there is a two-stage research approach in this thesis and it is not directly aimed to fabricate more stable a-Si:H. The first objective is to improve the fundamental understanding of the nanostructure and the defects in a-Si:H. Only secondly and using this newly gained knowledge, the SWE and the nature of metastable defects are studied to pave the way towards a reduction of the SWEElectrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
The nanostructure of hydrogenated amorphous silicon, examined by means of thermal annealing and light soaking
No full textPhotovoltaic energy is one of the key components of a sustainable energy future. While the market is currently dominated by crystalline silicon solar modules, thin-film silicon technology holds the promise of a cheap, resource-efficient and versatile alternative. The major drawback of thin-film silicon PV is its relatively low conversion efficiency, in part caused by the metastable defect phenomenon known as the Staebler-Wronski effect. To fully optimize the potential of thin-film silicon solar cells, a thorough understanding of hydrogenated amorphous silicon (a-Si:H) is required. To this end, an experiment is designed in which the effects of thermal annealing and light soaking on various material properties of a-Si:H are determined. Sets of p-i-n solar cells are deposited on Asahi VU substrates and sets of intrinsic a-Si:H films are deposited on Corning Eagle XG glass and on n-type c-Si wafers. The intrinsic layer of the solar cells and the films are deposited using hydrogen-to-silane flow rate ratios of 0, 2.5, 5, 7.5 and 10. The solar cells and films are annealed in vacuum for one hour at a time at temperatures increasing from 25?C to 500?C. In between annealing steps, measurementsare performed using the following techniques and methods: Fourier transform photocurrent spectroscopy (FTPS), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, reflectance/transmittance (RT), external quantum efficiency and solar cell external parameter determination. A separate set of solar cells and films is light soaked in a degradation chamber at 25?C with a light intensity of 1 kW/m^2 and an AM1.5 spectrum. At 0, 1, 10, 100 and 1000 hours of cumulative light soaking time, the same measurements as during the annealing experiment are performed (excluding FTIR and Raman spectroscopy). Additionally, the solar cells are light soaked in-situ in the FTPS setup using blue and white light with an intensity of 1 kW/m^2. Light soaking times increase exponentially and range from 0.5 s to approximately 200 h. FTPS measurements are performed after each step. Analysis of the FTIR results suggests that divacancies agglomerate into larger vacancies and nanosized voids during annealing, which is supported by the evolution of the bandgap obtained from RT measurements. Silicon-hydrogen bonds start breaking at around 300?C, at which temperature FTPS data show that the subgap absorption starts to increase, suggesting that defects are created in the form of un- or underpassivated vacancies and nanosized voids. Analysis of the FTPS data also reveals that the subgap absorption coefficient spectrum consists of at least 4 distributions, implying that the isolated dangling bond (which results in two defect distributions) cannot be the sole type of defect in a-Si:H. Analysis of the light soaking FTPS data reveals metastable defect creation with two distinct regimes: a fast regime (defect creation ? t^? with ? = 0.4 - 0.9) at timescales ranging from several hours to several days, and a slow regime (? = 0.1 - 0.2) at timescales ranging from days to weeks. This finding contradicts the commonly assumed single value of ? = 1/3, associated with the isolated dangling bond as sole type of metastable defect. The combined results strongly support a view of the nanostructure of a-Si:H, in which unpassivated divacancies and nanosized voids contribute significantly to metastable defect creation. Performance changes due to light soaking using different wavelengths of a-Si:H solar cells with absorber layers deposited using different hydrogen-to-silane gas flow rate ratios have been observed using FTPS. These observations are correlated to the external parameters of the devices. It is suggested that the fitted subgap distributions are linked spatially or energetically to particular defect entities. This highlights the potential of FTPS for monitoring metastability effects in solar cells.Sustainable Energy TechnologyElectrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
DIrect Sunlight into CO conversion
No full textIn this abstract an overview is presented of research performed in the DISCO project, on the development of a silicon-based high voltage multijunction device for autonomous solar to fuel applications.' Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Photovoltaic Materials and Device
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