Advanced intermediate reflector layers for thin film silicon tandem solar cells

Tandem solar cells based on thin film silicon benefit from an intermediate reflector layer between the top and bottom cells since it enhances the absorption in the top cell. The top cell can thus be manufactured thinner and less prone to light induced degradation. Made from a thin layer of nanocrystalline silicon oxide, the interlayer provides a second functionality since it aids in the spatial separation of local shunts occurring in both sub-cells. Recently, the reflector morphology received attention since it can provide a third function; here, a substantial difference exists between the commonly used configurations, i.e. superstrate or substrate. In the former, the thin layer of nanocrystalline silicon oxide reproduces the morphology of the underlying top cell. Its surface may thus be too rough for the growth of the bottom cell. In the latter configuration, reflectors made from a thick layer of ZnO can yield an adequate texture for the top cell, but conductive ZnO loses the effect of shunt quenching. We present our recent progress with improved intermediate reflector layers in both cell types. For silicon oxide based interlayers, we introduce a smoothing lacquer layer with self-organized openings that allow current transport. For ZnO based interlayers, we demonstrate that a treatment in oxygen plasma is capable of tuning the in-plane resistivity

Tuning the porosity of zinc oxide electrodes: from dense to nanopillar films

Thin films with tunable porosity are of high interest in applications such as gas sensing and antireflective coatings. We report a facile and scalable method to fabricate ZnO electrodes with tuneable porosity. By adjusting the substrate temperature and ratio of precursor gasses during low-pressure chemical vapor deposition we can accurately tune the porosity of ZnO films, from 0 up to 24%. The porosity change of the films from dense layer to separated nanopillars results in an effective refractive index reduction from 1.9 to 1.65 at 550 nm, as determined by optical and x-ray spectroscopy. The low-refractive-index ZnO films are incorporated into amorphous silicon solar cells demonstrating reflection losses reduction down to 4% in the visible wavelengths range. © 2015 IOP Publishing Ltd

Light-Management Strategies for Thin-Film Silicon Multijunction Solar Cells

Light management is of crucial importance to reach high efficiencies with thin-film silicon multijunction solar cells. In this contribution, we present light-management strategies that we recently developed. This includes high quality absorber materials, low-refractive index intermediate reflectors, and highly transparent multiscale electrodes. Specifically, we show the fabrication of high-efficiency tandem devices with a certified stabilized efficiency of 12.6%, triple-junction solar cells with a stabilized efficiency of 12.8%, recently developed smoothening intermediate reflector layers based on silicon dioxide nanoparticles, and periodic-on-random multiscale textures

Selective intra-carotid blood cooling in acute ischemic stroke : a safety and feasibility study in an ovine stroke model

Selective therapeutic hypothermia (TH) showed promising preclinical results as a neuroprotective strategy in acute ischemic stroke. We aimed to assess safety and feasibility of an intracarotid cooling catheter conceived for fast and selective brain cooling during endovascular thrombectomy in an ovine stroke model. Transient middle cerebral artery occlusion (MCAO, 3 h) was performed in 20 sheep. In the hypothermia group (n = 10), selective TH was initiated 20 minutes before recanalization, and was maintained for another 3 h. In the normothermia control group (n = 10), a standard 8 French catheter was used instead. Primary endpoints were intranasal cooling performance (feasibility) plus vessel patency assessed by digital subtraction angiography and carotid artery wall integrity (histopathology, both safety). Secondary endpoints were neurological outcome and infarct volumes. Computed tomography perfusion demonstrated MCA territory hypoperfusion during MCAO in both groups. Intranasal temperature decreased by 1.1 °C/3.1 °C after 10/60 minutes in the TH group and 0.3 °C/0.4 °C in the normothermia group (p < 0.001). Carotid artery and branching vessel patency as well as carotid wall integrity was indifferent between groups. Infarct volumes (p = 0.74) and neurological outcome (p = 0.82) were similar in both groups. Selective TH was feasible and safe. However, a larger number of subjects might be required to demonstrate efficacy

Performance Improvement of Organic Solar Cells by Metallic Nanoparticles (Prestatieverbetering van organische zonnecellen door metalen nanodeeltjes)

The aim of this thesis is to investigate the utilization of plasmonic metal nanostructures for efficiency enhancement of organic solar cells. This efficiency enhancement strategy exploits the strong near-field enhancement and highly efficient light scattering that originates from localized surface plasmon resonances (LSPRs) excited in metal nanostructures and leads to an increased absorption in the solar cell active layer. In the first part of this thesis, the near-field enhancement is systematically studied by employing a model system of metal nanoparticles (NPs) covered by thin-films of organic molecules. Absorption and both steady-state and transient photoluminescence measurements are used to probe the interactions between the LSPRs excited in the NPs and the excitons in the organic thin-film. By introducing a transparent spacer layer, the range of these near-field plasmon-exciton interactions is determined. The absorption enhancement is found to be accompanied by a strong reduction in exciton lifetime, which is detrimental to the device efficiency. Since the range of both interactions is comparable, isolating the NPs with a layer thick enough to prevent exciton quenching may simultaneously eliminate any beneficial effect due to the near-field absorption enhancement. These results demonstrate that the exploitation of near-field enhancement in organic solar cells is quite challenging. Far-field light scattering, on the other hand, shows promise for producing plasmonically-enhanced organic solar cells. This is demonstrated by equipping an organic solar cell with a plasmonic nanostructured Ag rear electrode. The LSPR of this electrode is tuned to the red absorption tail of the active layer and generates enhanced absorption by light scattering, as verified by experiments and numerical simulations. Due to this absorption enhancement, the plasmonic nanostructured cell exhibits an enhanced power conversion efficiency compared to an optimized, high performance organic solar cell.Abstract v Contents xiii List of Figures xvii List of Tables xxi 1 Introduction 1 1.1 Motivation 1 1.2 Organic molecules 3 1.2.1 Molecular orbitals 3 1.2.2 Optical transitions 5 1.3 Organic solar cells 8 1.4 Optical properties of metal nanoparticles 12 1.4.1 Localized surface plasmon resonances 12 1.4.2 Light extinction by metal NPs 14 1.4.3 Metal NP-embedding medium Interactions 19 1.5 Plasmonic solar cells 21 1.5.1 Plasmonic enhancement strategies 21 1.5.2 Literature review for plasmonic organic solar cells 24 1.6 Thesis outline 26 2 Optical properties of metal nanoparticles covered by organic thin-films 29 2.1 Introduction 29 2.2 Experiments 30 2.3 Three-dimensional finite-element numerical simulations 32 2.4 Results and discussion 33 2.4.1 Morphology and optical properties of the Ag NP layer 33 2.4.2 Excitation of multiple dipole resonances 37 2.5 Conclusions 41 3 Plasmon-exciton near-field interactions: absorption 43 3.1 Introduction 43 3.2 Results and discussion 44 3.2.1 Absorption enhancement in CuPc 44 3.2.2 Spectral dependence and influence of the permittivity 51 3.2.3 Effect of a spacer layer 53 3.3 Conclusions 57 4 Plasmon-exciton near-field interactions: emission 59 4.1 Introduction 59 4.2 Experiments 60 4.3 Theory 62 4.4 Results and discussion 64 4.4.1 Analytical calculations 64 4.4.2 Experimental results 71 4.4.3 Comparison between experiment and theory 77 4.5 Conclusions 81 5 Organic solar cells with nanostructured metal rear electrode 83 5.1 Introduction 83 5.2 Experiments 84 5.3 Results and discussion 87 5.3.1 Reference device optimization 89 5.3.2 Optical properties of the nanostructured Ag layer 92 5.3.3 Organic solar cells with nanostructured Ag rear electrode 97 5.4 Conclusions 101 6 General conclusions and outlook 103 Bibliography 107 Curriculum vitae 123 List of publications 125nrpages: 156status: publishe

Perovskite/silicon tandem solar cells: marriage of convenience or true love story? - An overview

Perovskite/silicon tandem solar cells have reached efficiencies above 25% in just about three years of development, mostly driven by the rapid progress made in the perovskite solar cell research field. This review aims to give an overview of the achievements made in this timeframe toward the goal of developing high-efficiency perovskite/silicon tandem cells with sufficiently large area and long lifetime to be commercially interesting. The developments that led to the recent progress in tandem cell efficiency, as well as the fac-tors currently still limiting their performance, including parasitic absorption, reflection losses, and the nonideal perovskite absorber layer bandgap, are discussed. Based on this discussion, guidelines for future developments are given. In addition, crucial aspects to enable the commercialization of pero-vskite/silicon tandem solar cells are reviewed, such as device stability and upscaling. Finally, economic considerations show how the number of steps and/or the costs associated to these steps for realizing the perovskite cell must be kept to a minimum to keep up with progress in the field of silicon photovoltaics

In situ TEM analysis of structural changes in metal-halide perovskite solar cells under electrical bias

Organic-inorganic metal-halide perovskite solar cells are emerging as a promising photovoltaic technology to harvest solar energy, with latest efficiencies now surpassing 22%1 - an impressive increase from the first reported value of 3% in 2009.2 In addition to low manufacturing costs, the optical properties of such cells can be tailored to form efficient tandems when combined with high-efficiency silicon solar cells.3 A typical perovskite cell structure as investigated here is based on a methylammonium lead trihalide absorber (MAPbI3) that is placed between hole- (Spiro-OMeTAD) and electron-selective contacts (a fullerene-based material).While new record efficiencies are frequently reported, the commercial application of this solar cell technology remains hindered by issues related to thermal and operational stability. Different mechanisms that are still debated modify cell properties with time, temperature, illumination and general operating conditions.4 In order to correlate applied voltage (V) and resulting current (I) to changes in active layer chemistry and structure on the nanometre scale, we performed both ex situ and in situ transmission electron microscopy (TEM) experiments, involving (scanning) TEM (STEM) imaging, selected-area electron diffraction, energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Samples were prepared by focused ion beam (FIB) milling, with exposure to air during transfer to the TEM minimised to <5 minutes to reduce any degradation of MAPbI3.First, the effects of exposure to air and electron beam irradiation were assessed in relation to FIB final thinning parameters. Once adequate sample preparation and observation conditions were identified, changes in morphology during cell characterisation were assessed ex situ by comparing lamellae extracted from as-manufactured and tested cells and then in situ by contacting FIB-prepared samples to a microelectromechanical systems (MEMS) chip mounted in a TEM specimen holder5 (Fig. 1a). Cell manufacturing parameters led to iodine diffusion into the hole collector, with the width of this diffused layer remaining constant during I-V characterisation. Similarly to ex situ experiments, the MAPbI3/Spiro interface was observed to delaminate during in situ electrical measurements, resulting in the presence of a ~5 nm Pb-rich layer on the hole-transparent-layer side (Figs. 1b-c). In addition, PbI2 nanoparticles were observed to nucleate within the MAPbI3 layer at the hole-collector interface and at the positions of structural defects (Figs. 1b-d).Overall, the active MAPbI3 layer was observed to be sensitive to sample preparation, exposure to air, observation conditions and I-V stimulus, resulting in the need for great care to deconvolute each effect. Different mechanisms that may all contribute to the decrease in efficiency of the cell were identified both ex situ and in situ, including ionic migration, PbI2 formation and local delamination of interfaces

Chancen und Risiken von Methoden zur Entnahme und Speicherung von CO₂ aus der Atmosphäre: Empfehlungen aufgrund der Analyse des Wissensstandes und einer systematischen Befragung von Fachleuten in der Schweiz

Mit dem Pariser Klimaabkommen von 2015 hat sich die Staatengemeinschaft verpflichtet, die globale Erwärmung auf deutlich unter 2°C zu begrenzen. Der Weltklimarat geht inzwischen aber davon aus, dass die bisher beschlossenen Massnahmen zur Reduzierung der Treibhausgasemissionen nicht genügen, um den menschengemachten Klimawandel in den Griff zu bekommen. Technologische Lösungen könnten dabei helfen, das Netto-Null-Ziel bis 2050 trotzdem zu erreichen: Sie sollen schwer vermeidbare Restemissionen aus Landwirtschaft, Tierhaltung und Abfalldeponien durch negative Emissionen kompensieren. Die Studie analysiert fünf Negativemissionstechnologien (NET), deren Einsatz in der Schweiz erwägt wird. Sie stützt sich dabei auf die von der Universität Zürich und der Empa entwickelte und hier erstmals angewandte partizipative Online-Befragungsmethodik LOTA (Landscape of Opinions for Technology Assessment). Ziel ist es, Politik und Öffentlichkeit transparent über Chancen, Grenzen (Kosten, Machbarkeit, Dauerhaftigkeit, Klimawirksamkeit) und Risiken (Umweltaspekte, Auswirkungen auf Landwirtschaft und Bevölkerung) zu informieren, Handlungsoptionen aufzuzeigen und damit eine faktenbasierte Debatte über den Stellenwert von NET in einer nachhaltigen und gesellschaftsverträglichen Klimastrategie zu unterstützen

Vapor Transport Deposition of Methylammonium Iodide for Perovskite Solar Cells

Vapor-based processes are promising options to deposit metal halide perovskite solar cells in an industrial environment due to their ability to deposit uniform layers over large areas in a controlled environment without resorting to the use of (possibly toxic) solvents. In addition, they yield conformal layers on rough substrates, an important aspect in view of producing perovskite/ crystalline silicon tandem solar cells featuring a textured silicon wafer for light management. While the inorganic precursors of the perovskite are well suited for thermal evaporation in high vacuum, the sublimation of the organic ones is more complex to control due to their high vapor pressure. To tackle this issue, we developed a vapor transport deposition chamber for organohalide deposition that physically dissociates the organic vapor evaporation zone from the deposition chamber. Once evaporated, organic vapors, here methylammonium iodide (MAI), are transported to the deposition chamber by a carrier gas through a showerhead, ensuring a spatially homogeneous conversion of PbI2 templates to the perovskite phase. The method enables the production of homogeneous perovskite layers on a textured 6 in. wafer. Furthermore, small-scale methylammonium lead iodide solar cells are also processed to validate the quality of the absorbers produced by this hybrid thermal evaporation/vapor transport deposition process