Monte Carlo simulations of quantum dot solar concentrators: ray tracing based on fluorescence mapping

One promising application of semiconductor nanostructures in the field of photovoltaics might be quantum dot solar concentrators. Quantum dot containing nanocomposite thin films are synthesized at EPFL-LESO by a low cost sol-gel process. In order to study the potential of the novel planar photoluminescent concentrators, reliable computer simulations are needed. A computer code for ray tracing simulations of quantum dot solar concentrators has been developed at EPFL-LESO on the basis of Monte Carlo methods that are applied to polarization-dependent reflection/transmission at interfaces, photon absorption by the semiconductor nanocrystals and photoluminescent reemission. The software allows importing measured or theoretical absorption/reemission spectra describing the photoluminescent properties of the quantum dots. Hereby the properties of photoluminescent reemission are described by a set of emission spectra depending on the energy of the incoming photon, allowing to simulate the photoluminescent emission using the inverse function method. By our simulations, the importance of two main factors is revealed, an emission spectrum matched to the spectral efficiency curve of the photovoltaic cell, and a large Stokes shift, which is advantageous for the lateral energy transport. No significant energy losses are implied when the quantum dots are contained within a nanocomposite coating instead of being dispersed in the entire volume of the pane. Together with the knowledge on the optoelectronical properties of suitable photovoltaic cells, the simulations allow to predict the total efficiency of the envisaged concentrating PV systems, and to optimize photoluminescent emission frequencies, optical densities, and pane dimensions. Keywords: quantum dots, photoluminescence, fluorescent planar solar concentrators, photovoltaics, solar cells, Monte Carlo simulations, ray tracing, polarization

Material Development for Perovskite/Silicon Tandem Photovoltaics

Peter Joseph Fiala

The developed world is built on the fact that energy is readily available and functionally infinite. The electricity from the wall, the gas at the station, and the heat in our homes are reliable and low-cost. But this comfort is so far only possible through the combustion of fossilized carbon sources. This is not a sustainable system. Not only are these sources finite, but the long-term effects of this much combustion are catastrophic for our planet and species. Transitioning from current energy production to sustainable sources will require the development and deployment of a diverse group of technologies. Chief among these is the generation of electricity from sunlight, or photovoltaics (PV).Most modern PV is based on crystalline silicon. Silicon PV alone could be scaled to provide the necessary energy production, but this could be realized more quickly and efficiently if more advanced technologies are used. The defining metric is the cost per generated electricity. In the case of silicon, the current cost breakdown of materials, manufacturing, installation, and system costs leaves little room for further price reduction; and light-to-electricity efficiencies are near their practical limit. Of the proposed alternatives, the pairing of silicon with a thin film of lead-halide perovskite in a multijunction or "tandem" architecture is one of the most promising. This splits the solar spectrum in two, and raises the theoretical maximum efficiency from 33.3% to 45.1% based on each material better using its segment than any one material could use the full spectrum. The central goal of this thesis was therefore to advance the capability of this type of solar cell while remaining within the constraints of industrially-plausible production methods.Towards this goal, we first isolated perovskite layers to study what limits their performance. This led to improvements in the bulk material quality, the interfaces with adjacent materials, and the layer morphology. We reduced bulk disorder via balancing the annealing temperature, time, and environment. The interfaces were improved by selecting passivating materials which were compatible with our deposition methods. We further observed a complex interplay between the materials used and the environment of the annealing, which illuminated critical dependencies of device performance and gave us a more complete understanding of our material. Finally the morphology was improved via high-temperature annealing. This was enabled by a surface-binding additive which reduced the detrimental effects of such an anneal while preserving the benefits, resulting in a perovskite with less internal resistance. From there, we moved outside of the perovskite layer and developed a light scattering ZnO layer which increased the optical performance and absorption of our cells. Returning to the main goal of the thesis, the lessons learned from work on isolated perovskites were applied in tandems on textured silicon. We additionally increased the perovskite layer thickness and made the bottom-cell more robust to processing damages. Together, the result was a stabilized efficiency of 27.3%. This is better than any silicon cell to date, but work remains to achieve 30% and beyond, extend material stability, scale up the deposition area, and transfer these capabilities to industry. Together, the work done as a part of this thesis represents a step forward in the journey towards carbon-neutral energy production.

EPFL2022

Nanocomposite Coatings for Solar Energy Conversion: Large Opportunities for Small Structures

Andreas Schueler

Due to their fascinating optical and electronical properties, nanometer-scaled structures play an important role in solar energy conversion [1]. Nanocomposite coatings consist typically of dielectric, semiconducting or metallic nanocrystals embedded in a host matrix. In this presentation it will be described how such nanostructures can result in interesting new material properties which might be exploited in solar energy conversion. This overview tries to illustrate the large variety of existing and envisaged solar energy applications of nanostructured thin films. Examples include antireflection coatings on solar collector glazing, colored coatings with high solar transmittance for novel glazing of solar thermal facades, photoluminescent quantum dot solar concentrators for photovoltaic energy conversion, nanostructured photovoltaic cells, and optical selective absorber coatings for thermal solar collectors. Dielectric nanostructured films are used as coatings on solar collector glazing. Nanoporous silicon dioxide films exhibit a low refractive index suitable for broad band anti-reflection, and such coatings enter the market now. Novel nanocomposite quaternary Mg-F-Si-O films exhibit also a low refractive index in the desired range (see Fig.2), and might be more resistant regarding gradual pore-filling by hydrocarbons [2]. Nanocomposite Ti-Si-O films are used in novel multilayered coatings on glazing of solar thermal facade collectors [3] (see Fig.3). Such coatings combine a colored reflection with a high solar transmittance and open new possibilities for the architectural integration of solar collectors in building facades [4]. The optical performance of the multilayered interference filters is optimized by computer simulations of thin film interference. Such thin film interference calculations are also used in order to design novel sun protection coatings with a better overheating protection at a comparable level of daylighting [5]. This novel glazing, developed by the research group of Prof. P. Oelhafen at the University of Basel, offers the possibility of tremendous energy savings due to the reduction of cooling needs in hot summers. One promising application of semiconductor nanocrystal containing composite films might be planar photoluminescent concentrators for photovoltaic solar energy conversion (see Fig.4). Quantum dot containing nanocomposite thin films have been synthesized by a low cost sol-gel process and characterised by photoluminescence spectroscopy. A tool for ray tracing simulations has been developed on the basis of Monte-Carlo methods that are applied to polarization-dependent reflection/transmission at the involved interfaces, photon absorption by the semiconductor nanocrystals and photoluminescent reemission [6]. Such simulations allow estimating the efficiency of the envisaged devices. Embedded nanometer-sized metal clusters play an important role for selective absorber coatings for solar thermal collectors. Metal doped amorphous hydrogenated silicon-carbon films (a-Si1-xCx:H/Me) can be deposited by a combined physical vapor deposition/plasma enhanced chemical vapor deposition process (PVD/PECVD). Multilayered solar absorber coatings have been developed on the basis of this promising material. The PVD/PECVD process has been upscaled from the laboratory to industrial production [1]. The highly durable coatings are extremely stable at elevated temperatures in air [7], and service lifetimes above 25 years are expected. The most recent research topics include the development of a low-cost sol-gel process for the deposition of nanocomposite selective solar absorber coatings, and novel thermochromic films for overheating protection of solar thermal systems.

2011

Optimisation of Urban Form by the Evaluation of the Solar Potential

Marylène Montavon

What will the cities of tomorrow look like? This is a crucial question because in the four and a half billion years of its existence, the Earth has never been so threatened, as it is today. Since 2008 half of the world's population has lived in towns – more than 70% in rich nations. Three quarters of CO2 emissions are produced in towns, 75% of the world's energy is consumed in towns. The battle for our climate, for the continuation of our species and all other life, will be largely waged in cities. Will it be possible to create the ideal ecological town? The objective of this study is to compare actual and theoretical urban forms in order to explore the diverse effects of daylighting and solar potential on densely built-up sites. Indeed, the search for ideal solar orientations has been a persistent problem from the 19th century until the beginning of the 20th. Even today, for all our scientific knowledge and empirical experience of solar energy, making the correct choice has still not crossed the boundary from an enigmatic, tricky art into the field of an exact science. As a first step, six existing urban sites in Switzerland and Brazil, as well as two projects in the United Kingdom, a few generic models and a utopian city, were modelled and analyzed. The goal was not merely the construction of new, exemplary districts but also the transformation of existing towns. The findings have been used to assess the potential of façades and roofs located in urban areas for active and passive solar heating, photovoltaic electricity generation and daylighting. The results obtained from these different case studies revealed large variations of the potential for solar energy collection on the buildings' façades and roofs. Ideally, these investigations are expected to help architects and town planners understand how to optimise solar collection on buildings design. In addition, a study was made of the Contemporary City of Three Million Inhabitants created by Le Corbusier. According to the latter, the Contemporary City could increase the urban capacity and at the same time improve the urban environment and the efficiency of the city. Therefore the present study extracted the solar potential of this utopian city in order to assess Le Corbusier's propositions. Since 1922, Le Corbusier's studies have influenced architects and town-planners in their choice of orientations and urban forms: it was therefore crucial to discover if this reliance on Le Corbusier was justified. Solar irradiation and illuminance values obtained through numerical simulations form the core part of the method. The method adopted for the entire set of computer simulations consisted in modelling — via a computer-aided drawing piece of software, namely AutoCAD — the digital model of each building or group of buildings prior to exporting them to an urban performance analysis software (PPF/RADIANCE software package). Spatial distributions of solar irradiation and daylight fluxes over the overall building façades and roofs were calculated using ray-tracing simulation techniques to determine the appropriate placement of different solar technologies (passive and active solar, photovoltaic and daylighting). Performance indicators were used to assess the solar utilisation potential of the urban sites (e.g. statistics of sky view factors and daylight factors) in order to determine the optimal solar strategies for a given urban context. In addition, various works and sun-related recommendations that have appeared mainly since the end of the nineteenth century, together with the various scientific models and software tools pertaining to this topic, are presented in this study.

EPFL2010