Up-scalable plasmonic and diffractive nanostructures

Since most of the academic photonic and plasmonic nanostructures are based on slow and expensive electron beam lithography processes, there is a need for innovative alternatives suitable for industrial manufacturing. Large areas need to be patterned with a high throughput, which is for example achieved with roll-to-roll machines. Up-scalable designs of photonic and plasmonic nanostructures are therefore studied in this thesis. Typical industrial processes include embossing and evaporation, which are consequently used throughout the thesis. I propose oblique evaporation of high refractive index materials to render binary gratings highly efficient for first order transmission. Zinc sulfide coatings are employed to diffract close to 70% of unpolarized green light. Simulations further show that they can be encapsulated to protect them from environmental influences like humidity, wear or dust, while retaining their exceptional diffractive properties, which is very appealing for outdoor applications. I also show that thin metallic coatings can attain similar efficiencies for TE polarized light. The effect is asymmetric and shows a maximum at the Wood-Rayleigh anomaly, which results in orientation dependent coloration of the zero order as well as first order transmittances. A large part of the standard RGB gamut can be covered through proper adjustment of the grating parameters. Combination of zero order and first order effects allows creation of color appearances that switch when rotating or flipping the device. I finally present how floating images become apparent when a patterned light source like e.g. a mobile phone is used in conjunction with the metallized grating. Direct transfer of plasmonic technology from universities to industry is often not possible: in academia, metallic nanostructures often require a lateral resolution of a few nanometers, which is challenging to achieve in up-scalable processes. Thicknesses on the other hand can be controlled in this regime using evaporation techniques, which are hence powerful methods for high-throughput production. In this thesis, Fano-resonant, U-shaped nanowires are created with oblique metal deposition. In order to make them available for mass-production, aluminum is chosen as the plasmonic material. The surface integral equation method is used to investigate near-fields and charge distributions, which shed light onto the physics behind the present resonances. A surface plasmon polariton is found to couple to a localized plasmonic mode with a hexapolar charge distribution. It is finally shown that the Fano-resonance can be accurately tuned by adapting evaporation angle and metal thickness. These two parameters can easily be accessed and would allow for good control over the optical response even in an industrial environment. The applicability of the above insights is then demonstrated by creating a strain sensor. To that end, the process is transferred to a stretchable polymer and when elongating the structure perpendicular to the wires, the polymeric spacing between them is expanded. The sensitivity of the Fano-resonance to this change in inter-wire distance is investigated and a strong damping is observed. Through careful design, a clearly visible color switch from purple to green is achieved for elongations less than 20%. The sensor was deemed to be very durable, as no deterioration in the color or the spectral response was observed even after several strain cycles

Advanced light management concepts for perovskite photovoltaics

Um die rasante Zunahme der Treibhausgasemission zu bremsen und damit die globale Erderwärmung, ist ein schneller Umstieg von fossilen Brennstoffen auf erneuerbare Energien unabdingbar. In dieser Hinsicht spielt die Photovoltaik (PV) eine entscheidende Rolle, um eine effiziente Dekarbonisierung der globalen Stromerzeugung voranzutreiben. Dafür wird gegenwärtig sowohl an bestehender Silizium-PV, als auch an neuen PV-Technologien geforscht. Der prominenteste Kandidat unter den neuen Technologien sind die Perowskit-Solarzellen. Diese haben in den letzten 10 Jahren eine beispiellose Effizienzsteigerung durchlaufen und erzielen heute Rekordwirkungsgrade über 25%. Die rasche Entwicklung der Perowskit-basierten PV ist vor allem durch das Versprechen einer kostengünstigen, effizienten und skalierbaren Technologie motiviert. Sie gilt zum einen als Konkurrenz zur bestehenden Silizium-PV und zum anderen als Partner für die Anwendung in Perowskit/Silizium Tandem-PV. In dieser Hinsicht bietet die Perowskit-basierte Tandem-PV die Aussicht, den derzeitigen Rekordwirkungsgrad von Silizium (c-Si) Solarzellen (≈27%) und sogar die Shockley-Queisser-Grenze für Einfachsolarzellen (≈34%) zu übertreffen. Eine verbleibende Herausforderung, sowie ein aktuell stark untersuchtes Forschungsthema von Perowskit/c-Si-Tandemsolarzellen, ist ihre geringere Lichtausbeute im Vergleich zu konventionellen c-Si Solarzellen. Dies ist insbesondere auf zusätzlich erforderliche Funktionsschichten, wie die transparenten Elektroden, Ladungstransportschichten und Passivierungsschichten zurückzuführen, die gemeinsam zu Reflexionsverlusten und Verlusten durch parasitäre Absorption beitragen. Dies reduziert sowohl den Wirkungsgrad (PCE) als auch den Energieertrag (EY) der Tandem-Solarzelle. Um Reflexions- und Absorptionsverluste zu minimieren, ist ein fortschrittliches Lichtmanagement unerlässlich. Da sich die realistischen Einstrahlungsbedingungen stark von typischen Standardtestbedingungen unterscheiden (z.B. spektrale Variation und variabler Einfallswinkel des Sonnenlichts), ist es zwingend notwendig, PV-Module nicht nur für den PCE, sondern auch für den EY zu optimieren. Daher ist ein ausgeklügeltes Lichtmanagement nicht nur auf Tandem-Solarmodule beschränkt, sondern für jede Art von Solarmodul wichtig. In dieser Arbeit werden verschiedene Lichtmanagementkonzepte für die Perowskit-basierte-PV diskutiert und in Bezug auf den PCE und den jährlichen EY bewertet. In diesem Zusammenhang werden Mikrotexturen für eine verbesserte Lichteinkopplung an der Luft/Glas-Grenzfläche untersucht, was für alle PV-Technologien relevant ist. Die Mikrotexturen an der Vorderseite des Solarmoduls bieten die Möglichkeit, die Luft/Glas-Reflexion fast vollständig zu eliminieren und bei schrägen Einfallswinkeln (z.B. 80°) um ca. 80%rel zu reduzieren. Die experimentelle Realisierung zeigt die Erhöhung des PCE um 12%rel bzw. 5%rel für planare und texturierte Siliziumsolarzellen. Darüber hinaus werden Mikrotexturen auf Perowskit/c-Si-Tandem-Minimodulen realisiert, die den PCE um 10%rel verbessern. Aufgrund der ausgezeichneten Winkelstabilität der Mikrotexturen spiegelt sich die Verbesserung des PCE auch im EY wider, was durch Simulationen gezeigt wird, bei denen die Verbesserungen im EY die des PCE um 2%rel übertreffen. Zusätzlich zur ersten Grenzfläche jedes Solarmoduls werden die Reflexionsverluste an den vorderen halbtransparenten Indiumzinnoxid (ITO) Elektroden der Perowskit-Solarzellen untersucht. Mit Hilfe von nanotexturierten Glas/ITO-Grenzflächen können diese Verluste minimiert werden, was zu einem verbesserten Strom in der oberen Perowskit- und unteren c-Si-Solarzelle führt. Dies verbessert den Tandem-PCE um 2%rel. Darüber hinaus sind die nanotexturierten Elektroden winkelstabil und versprechen in den Simulationen eine Erhöhung des EY um 10%rel, was höher ist als die simulierte Verbesserung des PCE um 9%rel. Weitere nanophotonische Modifikationen der Absorberschicht der Perowskit-Solarzelle führen zu einer verbesserten Absorption in der Nähe der Bandlücke, indem das einfallende Licht in quasi-geführte Moden eingekoppelt wird. Simulationen zeigen, dass dies die Stromerzeugung in den Perowskit-Solarzellen um bis zu 6%rel verbessert. Erste experimentelle Ergebnisse demonstrieren eine Verbesserung um 2%rel. Darüber hinaus bieten die nanophotonischen Perowskit-Solarzellen eine einfache Möglichkeit, den um-weltschädlichen Bleigehalt in den Perowskit-Solarzellen bei gleichbleibendem Wirkungsgrad, um 30%rel zu verringern. Darüber hinaus verändert die nanophotonische Modifikation des Absorbers die Winkelabhängigkeit der Perowskit-Solarzellen nicht und führt zu den äquivalenten Verbesserungen des EY. Schließlich wird ein neuartiges Herstellungsverfahren für Perowskit-Solarzellen vorgestellt, dass eine einfache Laminierung der Perowskit-Solarzellen ermöglicht. Damit umgeht die Laminierung Inkompatibilitäten bei konventionellen Schichtabscheidungs-techniken und bietet somit mehr Flexibilität und Freiheit bei der Wahl der Ladungstransportmaterialien für die Perowskit-Solarzellenherstellung. Erste Prototypen zeigen eine ausgezeichnete Langzeit- und Temperaturstabilität der laminierten Perowskit-Solarzellen mit einem PCE über 14%. Das vorgestellte Laminierungskonzept bahnt damit den Weg für eine direkte Laminierung von Perowskit-Solarzellen auf die bestehende Siliziumtechnologie und hat so ein großes Potential für die aktuelle Perowskit-basierte Tandemforschung

Microstructured glazing for daylighting, glare protection, seasonal thermal control and clear view

The appropriate choice of glazing in a facade depends on many factors. They include amongst other criteria: location, orientation, climatic condition, energetic efficiency, usage of the building, required user comfort, and the architectural concept. On the south facade of high-rise buildings in particular, it is a challenge to have simultaneously large glazed area, no glare, no excessive cooling loads, a clear view and sufficient natural light flux. In Switzerland, electric lighting, heating and air conditioning account for about 74% of the total energy demand in private housing and 32% of the overall Swiss electricity usage. This energy consumption can be strongly influenced by using the most appropriate fenestration system. A software was developed during this thesis to engineer new complex fenestration system (CFS) that have a two dimensional profile. The originality of the implemented Monte Carlo ray tracing algorithm is the separation of intersection and interaction. The model also calculates an accurate bidirectional transmission distribution function that is used in combination with Radiance to obtain a rendering of the daylighting distribution in an office space or dynamic daylight metrics such as the daylight factor and daylight autonomy. Finally, to estimate the thermal performances, a simple nodal thermal model was added to simulate the temperature evolution and the thermal loads in a given office. This tool was validated. A glazing combining several functions and that can contribute to significantly reduce energy consumption in buildings was developed using this novel ray tracing approach. It was designed to obtain a strongly angular dependent transmission and a specific angular distribution of transmitted light. The engineered geometry provides elevated daylight illuminance by redirecting the incoming light towards the depth of the room. This redirection simultaneously reduces the glare risk. For an optimised usage of available solar radiation, the transmission of direct sunlight is maximised in winter and minimised in summer. Taking advantage of the changing elevation of the sun between seasons, such a seasonal variation can be created by a strongly angular dependent transmittance. A fabrication process was identified and samples of embedded micromirrors were produced to demonstrate the feasibility. The fabrication of such structures required several steps. The fabrication of a metallic mould with a high aspect ratio and mirror polished surfaces is followed by the production of an intermediate polydimethylsiloxane mould that was subsequently used to replicate the structure with a ultraviolet (UV) curable polymer. Selected facets of these samples were then coated with a thin film of reflective material. Finally, the structures were filled with the same polymer to integrated the mirrors. The blocking effect can be obtained by a combination with well placed reflective stripes, those were fabricated by lift-off lithography. The samples were characterised during the various fabrication steps using various microscopy techniques, energy-dispersive X-ray spectroscopy, profilometry and optical measurements. A setup was built for the measures of angular dependent transmittance. The final samples redirect up to 70% of the light flux and are very transparent when looking through at normal incidence

An investigation into the theoretical and practical aspects of office mechanisation

The leaders of industry are so often leaders because of their distaste for regulation and orthodox methods. They are often reluctant to be restricted in their activities by static requirements of a mechanised system. It may, however, be observed that, with the rapid strides in the development of machinery itself, the makers are very willing to make favourable trade -out conditions in order that users may take advantage of the most recent improvements. It may well be that, owing to improvements, ten years might be regarded as the period of most useful life for the more expensive office machinery, by which time the initial cost should have been fully saved to the user.The Question of installing mechanical aid in the office is largely an economic one, and must be determined by the need of such aids, and in the return which they are likely to yield. Needs, on the one hand, are essentially relative, and cannot be assessed by simple standards. On the other hand, returns are difficult to translate into £.s.d. where speed, accuracy, security, and even prestige, may be involved. It will be the purpose of the pages which follow to endeavour to afford some guidance on these issues

Investigation of Photopolymer-based Holographic Optical Elements for Solar Applications

The aim of this research was to explore the potential of photopolymer Holographic Optical Elements (HOE) for use in the collection of light from a moving source, such as the sun, and its direction into a fixed detector/convertor for application in solar concentrators. In order to increase the acceptance angle and the wavelength range of operation of the holographic device, low spatial frequency holographic recording was explored. The challenge was to record high diffraction efficiency HOEs at this spatial frequency, since it requires a material with relatively fast monomer diffusion. The acrylamide-based photopolymer developed at the Centre for Industrial and Engineering Optics has been selected, because it has previously shown such diffusion properties. In order to achieve large acceptance angle, the theoretical modelling of the angular and wavelength selectivity of the HOEs was carried out. The theoretical results confirmed that the gratings with just a few hundred lines per mili meter were of most interest in this study because the selectivity is lower

USE OF MICROSCALE HYDROPHOBIC SURFACE FEATURES FOR INTEGRATION OF 3D CELL CULTURE INTO MULTI-FUNCTIONAL MICROFLUIDIC DEVICES

3D cell culture and microfluidics both represent powerful tools for replicating critical components of the cell microenvironment; however, challenges involved in integration of the two and compatibility with standard tissue culture protocols still represent a steep barrier to widespread adoption. Here we demonstrate the use of engineered surface roughness in the form of microfluidic channels to integrate 3D cell-laden hydrogels and microfluidic fluid delivery. When a liquid hydrogel precursor solution is pipetted onto a surface containing open microfluidic channels, the solid/liquid/air interface becomes pinned at sharp edges such that the hydrogel forms the “fourth wall” of the channels upon solidification. We designed Cassie-Baxter microfluidic surfaces that leverage this phenomenon, making it possible to have barrier-free diffusion between the channels and hydrogel; in addition, sealing is robust enough to prevent leakage between the two components during fluid flow, but the sealing can also be reversed to facilitate recovery of the cell/hydrogel material after culture. This method was used to culture MDA-MB-231 cells in collagen, which remained viable and proliferated while receiving media exclusively through the microfluidic channels over the course of several days. Further modifications were made to create a multi-functional 3D cell culture platform. Gas impermeable polymer structure and deoxygenated flow were used to lower the oxygen content in the device, and the oxygen content was monitored in real-time using embedded oxygen sensors. This is particularly useful in replication of the tumor microenvironment where hypoxic conditions affect the cellular behavior and morphology. Also, by incorporating two inlets in the microfluidic device, binary concentrations of solutes were introduced into the system which created a lateral concentration gradient across the fluidic path. This allows studying of cell migration and response to various chemoattractant and drug doses. And finally, two high throughput designs to create 4-well and eight-well microfluidic devices were proposed and tested. This enables conducting more replicates of an experiment and even comparative studies on a single chip

Biomimetic nanostructured surfaces for antireflection in photovoltaics

A key consideration in the design of any solar cell is the reduction of reflectance from the top surface. Traditional thin film antireflection schemes are being challenged by new techniques that involve texturing on the subwavelength scale to form ‘moth-eye’ arrays, so called because they are inspired by Nature’s answer to unwanted reflections, the arrays of pillars found on the eyes and wings of some species of moth. In this work, a new method is presented for the optimization of thin film coatings that accounts for the angular and spectral variations in incident solar radiation from sunrise to sunset. This approach is then extended to silicon moth-eye arrays to assess how effectively these surfaces can provide antireflection for silicon solar cells over a full day. The reflectance spectra of moth-eye surfaces are found to depend on the period of the arrays and the height and shape of the pillars, and consequently these parameters can be optimized for the solar spectrum. Simulations predict that replacing an optimized double layer thin film coating with a moth-eye array could increase the full day cell performance by 2% for a laboratory cell and 3% for an encapsulated cell. Compared to a perfectly transmitting interface, this corresponds to losses in short circuit current of only 5.3% and 0.6% for a laboratory and an encapsulated cell, respectively. Furthermore, fabrication of silicon moth-eye arrays by electron beam lithography and dry etching leads to predicted percentage losses at peak irradiance, compared to an ideal antireflective surface, of only 1%. The potentially more scalable technique of nanoimprint lithography is also used to fabricate antireflective moth-eye arrays in silicon, over areas as large as 1 cm2, demonstrating great potential for stealth and antiglare applications in addition to photovoltaics

Fabrication of Large Mechanically Flexible Multi-Layered PDMS Optical Devices

Mechanically flexible large area polydimethylsiloxane (PDMS) optical devices are fabricated using soft-lithography techniques based on replica moulding. These non-rigid optical devices can be designed as sheets to act as either light concentrators (collectors) or diffusers (illuminators) based on the position and geometry of micro-optical structures (MOSs) embedded within the sheet or imprinted on its surface. The active surface area of the device can range from less than a sq. cm to several sq. m. The performance of the large area optical device is a function of the location and geometry of micro-optical structures, thickness and shape of the flexible waveguide, core and cladding material (ie. refractive indices), and the wavelength of the incident light source. A centrifugal casting technique that simultaneously de-gasses and fills a patterned, thin mould cavity is introduced as the backbone to the proposed fabrication methodology. Combined with the ability to control the refractive index of PDMS and a partial curing technique that bonds subsequent layers, a bottom-up layer-by-layer fabrication process is proposed and described in detail