Characterisation and selective modification of semiconductor nanowires for electrical applications

Among the large variety of bottom-up grown semiconductor nanostructures, semiconductor nanowires are of outstanding interest, since they are promising candidates for integration into future electronic and photonic devices. For the use in devices like transistors, sensors, LEDs, and solar cells, it is essential to know and understand the nature of the nanowires. Furthermore, the selective control of the nanowire properties, by modifying either the wire composition or the surface conditions, allows the development of novel, functional elements for next generation semiconductor devices. This thesis reports on both, the characterisation and the selective modification of semiconductor nanowires. Scanning electron microscopy based methods are shown for the determination of the carrier diffusion lengths in ZnO and GaAs nanowires. The change of the electrical properties of ZnO nanowires during ion beam implantation is investigated as an example for the targeted modification of the nanowire properties. Here, the ion-nanowire interaction enables the detection of single ion impacts. Biofunctionalisation of ZnO nanowires is also shown, demonstrating the suitability of surface-modified nanowires for the use in biosensing devices. The thesis is divided into three parts, addressing the diffusion length measurements, the ion implantation experiments, and the surface biofunctionalisation

Large-scale spatiotemporal calculation of photovoltaic capacity factors using ray tracing: A case study in urban environments

Photovoltaics (PVs) on building facades, either building-integrated or building-attached, offer a large energy yield potential especially in densely populated urban areas. Targeting this potential requires the availability of planning tools such as insolation forecasts. However, calculating the PV potential of facade surfaces in an urban environment is challenging. Complex time-dependent shadowing and light reflections must be considered. In this contribution, we present fast ray tracing calculations for insolation forecasts in large urban environments using clustering of Sun positions into typical days. We use our approach to determine time resolved PV capacity factors for rooftops and facades in a wide variety of environments, which is particularly useful for energy system analyses. The advantage of our approach is that the determined capacity factors for one geographic location can be easily extended to larger geographic regions. In this contribution, we perform calculations in three exemplary environments and extend the results globally. Especially for facade surfaces, we find that there is a pronounced intra-day and also seasonal distribution of PV potentials that strongly depends on the degree of latitude. The consideration of light reflections in our ray tracing approach causes an increase in calculated full load hours for facade surfaces between 10% and 25% for most geographical locations

Kerfless exfoliated thin crystalline Si wafers with Al metallization layers for solar cells

We report on a kerfless exfoliation approach to further reduce the costs of crystalline silicon photovoltaics making use of evaporated Al as a double functional layer. The Al serves as the stress inducing element to drive the exfoliation process and can be maintained as a rear contacting layer in the solar cell after exfoliation. The 50-70 μm thick exfoliated Si layers show effective minority carrier lifetimes around 180 μs with diffusion lengths of 10 times the layer thickness. We analyze the thermo-mechanical properties of the Al layer by x-ray diffraction analysis and investigate its influence on the exfoliation process. We evaluate the approach for the implementation into solar cell production by determining processing limits and estimating cost advantages of a possible solar cell design route. The Al-Si bilayers are mechanically stable under processing conditions and exhibit a moderate cost savings potential of 3-36% compared to other c-Si cell concepts. © Materials Research Society 2015

Thin crystalline macroporous silicon solar cells with ion implanted emitter

We separate a (34 ± 2) μm-thick macroporous Si layer from an n-type Si wafer by means of electrochemical etching. The porosity is p = (26.2 ± 2.4)%. We use ion implantation to selectively dope the outer surfaces of the macroporous Si layer. No masking of the surface is required. The pores are open during the implantation process. We fabricate a macroporous Si solar cell with an implanted boron emitter at the front side and an implanted phosphorus region at the rear side. The short-circuit current density is 34.8 mA cm-2 and the open-circuit voltage is 562 mV. With a fill factor of 69.1% the cell achieves an energy-conversion efficiency of 13.5%.Federal Ministry for Environment, Nature Conservation, and Nuclear Safety/FKZ 032514

Thermomechanical Spalling of Epitaxially Grown Silicon from Porosified Substrates

We combine two kerfless approaches to unite advantages of both processes: the epitaxial layer transfer based on porous silicon (PSI process) and the lift-off of a thin silicon layer from a substrate via controlled spalling by a stress-inducing layer. For this, we deposit an Al stressor layer on top of an epitaxially grown silicon layer. A porous double layer underneath the epitaxial layer serves as determined breaking point. We directionally heat this sample stack and cool it afterwards for controlled spalling of the epitaxial layer from the substrate. We achieve a lift-off rate of 34 out of 36 detached samples. The porous silicon layer enables a smooth surface of the detached epitaxial layer and the remaining substrate. Compared to our standard spalling process the thickness variation of the detached layers is significantly reduced from ≤ 25 μm to less than 2 μm. Furthermore we show that the lifetime of the detached epitaxial layers does not suffer from the Al deposition and the lift-off process.Federal Ministry for Environment, Nature Conservation, and Nuclear Safety/FKZ 0325461State of Lower Saxon

The Influence of Falling Costs for Electrolyzers on the Location Factors for Green Hydrogen Production

A fast and extensive build-up of green hydrogen production is a crucial element for the global energy transition. The availability of low-cost renewable energy at high operating hours of the electrolyzer is a central criterion in today's choice of location for green hydrogen production. It is analyzed how decreasing electrolyzer costs that are expected by many may influence this choice. The energy system optimization framework ESTRAM is used to find the optimum configuration of wind turbine, photovoltaic (PV), and electrolyzer capacity for covering a given hydrogen demand by locally produced green hydrogen in different European locations. It is found that PV is part of the cost-optimal solution in 96% of 1372 statistical regions in Europe. Decreasing electrolyzer costs are favoring the utilization of PV in wind–solar hybrid plants. At low electrolyzer costs, pure solar hydrogen outperforms the hybrid variant in many places if hydrogen storage is available, even with few full operating hours per year. At the same time, production costs are converging significantly. The article adds a new perspective to the discussion, as it is systematically shown how further technology development may lead to a shift in locational advantages for green hydrogen production, what should be considered to avoid stranded assets when building infrastructure

Effiziente Wärmesysteme für Wohngebäude

Die Entwicklung effizienter Wärmesysteme für Wohngebäude ist eine zentrale Aufgabe der Energieforschung. Der Wohngebäudesektor hat seit Jahren einen – witterungsbedingt leicht schwankenden – Anteil von 25 – 30 % am Endenergieverbrauch in Deutschland. Dabei werden über 80 % der im Sektor Haushalte verbrauchten Energie zur Bereitstellung von Wärme verwendet, im Jahr 2013 z. B. 585 TWh oder mehr als ein Fünftel des gesamtdeutschen Endenergieverbrauchs. Da die Wärmeversorgung hauptsächlich noch über die fossilen Energieträger Erdgas und Heizöl betrieben wird, bieten Wärmesysteme für Wohngebäude ein erhebliches Potenzial für CO2-Einsparungen. Die Energiewende im Heizungskeller beruht dabei – genau wie in anderen Sektoren auch – auf den beiden Säulen erneuerbare Energieversorgung und Effizienz. Durch verschärfte Regelungen für Neubauten und energetische Sanierungen von Bestandsimmobilien wurde in den vergangenen Jahren ein leichter Rückgang des durchschnittlichen flächenspezifischen Heizenergiebedarfs in Wohngebäuden erreicht. Diesem Trend stand jedoch ein steigender Wohnflächenverbrauch pro Person entgegen, so dass der gesamte Heizenergieverbrauch langsamer sinkt als der spezifische Heizenergiebedarf in den Gebäuden. Die jährliche Totalsanierungsquote im Wohnbereich liegt zudem weit unterhalb der als für die Erreichung der Effizienzziele 2050 nötigen postulierten 2,7 %. Ein verstärkter Ausbau des Einsatzes erneuerbarer Energien im Heizungskeller bietet einen zusätzlichen Freiheitsgrad, der zur Erreichung der angestrebten CO2-Minderung beitragen kann

Flexible und robuste Wege zur Energiewende : Neue Transformationspfade zu einem nachhaltigen Energiesystem

[no abstract available

Digitalisierung und Energiesystemtransformation : Chancen und Herausforderungen

Welche Rolle spielt die Digitalisierung mit der Vielzahl ihrer Methoden und Anwendungen für die Energiewende - also für die Transformation unseres Energiesystems im Sinne der vereinbarten Klimaschutzziele? Ist sie notwendige Voraussetzung für den Systemumbau und ermöglicht beispielsweise erst den Übergang auf ein nahezu vollständig erneuerbares Energiesystem (Enabler) oder ist sie lediglich ein nützliches, den Umbau beschleunigendes Hilfsmittel (Facilitator)? Welche Veränderungen sind durch die Ziele der Energiewende getrieben und welche durch die Verbreitung von Techniken der Digitalisierung? All dies waren Fragen, die im Rahmen der Jahrestagung 2018 des Forschungsverbunds Erneuerbare Energien unter dem Titel "Die Energiewende - smart und digital" behandelt wurden. Dieser einführende Beitrag versucht einige Anhaltspunkte zur Beantwortung dieser Fragen zu liefern und in das Thema einzuführen