Clemens Tummeltshammer

A bioinspired solution for spectrally selective thermochromic VO 2 coated intelligent glazing

We present a novel approach towards achieving high visible transmittance for vanadium dioxide (VO(2)) coated surfaces whilst maintaining the solar energy transmittance modulation required for smart-window applications. Our method deviates from conventional approaches and utilizes subwavelength surface structures, based upon those present on the eyeballs of moths, that are engineered to exhibit broadband, polarization insensitive and wide-angle antireflection properties. The moth-eye functionalised surface is expected to benefit from simultaneous super-hydrophobic properties that enable the window to self-clean. We develop a set of design rules for the moth-eye surface nanostructures and, following this, numerically optimize their dimensions using parameter search algorithms implemented through a series of Finite Difference Time Domain (FDTD) simulations. We select six high-performing cases for presentation, all of which have a periodicity of 130 nm and aspect ratios between 1.9 and 8.8. Based upon our calculations the selected cases modulate the solar energy transmittance by as much as 23.1% whilst maintaining high visible transmittance of up to 70.3%. The performance metrics of the windows presented in this paper are the highest calculated for VO(2) based smart-windows.

Homeotropic alignment and Förster resonance energy transfer: The way to a brighter luminescent solar concentrator

We investigate homeotropically aligned fluorophores and Forster resonance energy transfer (FRET) for luminescent solar concentrators using Monte-Carlo ray tracing. The homeotropic alignment strongly improves the trapping efficiency, while FRET circumvents the low absorption at homeotropic alignment by separating the absorption and emission processes. We predict that this design doped with two organic dye molecules can yield a 82.9% optical efficiency improvement compared to a single, arbitrarily oriented dye molecule. We also show that quantum dots are prime candidates for absorption/donor fluorophores due to their wide absorption band. The potentially strong re-absorption and low quantum yield of quantum dots is not a hindrance for this design.

Efficiency and loss mechanisms of plasmonic Luminescent Solar Concentrators

Using a hybrid nanoscale/macroscale model, we simulate the efficiency of a luminescent solar concentrator (LSC) which employs silver nanoparticles to enhance the dye absorption and scatter the incoming light. We show that the normalized optical efficiency can be increased from 10.4% for a single dye LSC to 32.6% for a plasmonic LSC with silver spheres immersed inside a thin dye layer. Most of the efficiency enhancement is due to scattering of the particles and not due to dye absorption/re-emission.

Flexible and fluorophore-doped luminescent solar concentrators based on polydimethylsiloxane.

We demonstrate a simple and inexpensive method to fabricate flexible and fluorophore-doped luminescent solar concentrators (LSCs). Polydimethylsiloxane (PDMS) serves as a host material which additionally offers the potential to cast LSCs in arbitrary shapes. The laser dye Pyrromethene 567 is used as a prototype fluorophore, and it is shown that it has a high quantum yield of 93% over the concentration range investigated. The optical efficiency and loss channels of the flexible LSCs are investigated; it is also demonstrated that the efficiency remains high while bending the LSC which is essential for flexible LSCs to make an impact on solar energy.

Fundamental limits of concentration in luminescent solar concentrators revised: the effect of reabsorption and nonunity quantum yield

Luminescent solar concentrators (LSCs) are devices theoretically able to condense both direct and diffuse solar radiation into thin dielectric layers with extremely high efficiencies. A theory based on thermodynamic principles was developed in the past to estimate the concentration limits that can be achieved with an LSC and facilitate researchers’ efforts to predict the potential of their designs to convert optical to electrical power. However, while concentration efficiencies of thousands or even tens of thousands of suns are supported by this model, values of only a fraction of those have ever been recorded experimentally. This is because in the calculation of the thermodynamic limits the quantum yield of the luminophores is assumed to be equal to unity and any processes that quench the intensity of the trapped field are completely ignored. In an attempt to better match theory with reality and provide more accurate performance estimates, we have revised the limits of concentration based on a statistical optics framework. The new model gives insight into the main mechanisms inhibiting the concentration of LSCs and can be used to extract design rules for efficient LSCs. Comparisons between the method presented in this paper and results obtained with Monte Carlo ray-tracing simulations demonstrate excellent agreement between the two. Finally, we discuss the conditions for validity of the thermodynamic limits, and we show that in some circumstances these can actually be surpassed.

Impact of curvature on the optimal configuration of flexible luminescent solar concentrators

Flexible luminescent solar concentrators (LSCs) could deliver integrated photovoltaics in all aspects of our lives, from architecture to wearable electronics. We present and experimentally verify a model for the optimization of the external optical efficiency of LSCs under varying degrees of curvature. We demonstrate differences between the optimization of flat and bent LSCs, showing that optimal fluorophore concentrations can differ by a factor of two.

Influence of Depth of Interaction upon the Performance of Scintillator Detectors

The uncertainty in time of particle detection within a scintillator detector, characterised by the coinci- dence time resolution (CTR), is explored with respect to the interaction position within the scintillator crystal itself. Electronic collimation between two scintillator detectors is utilised to determine the CTR with depth of interaction (DOI) for different materials, geometries and wrappings. Significantly, no rela- tionship between the CTR and DOI is observed within experimental error. Confinement of the interaction position is seen to degrade the CTR in long scintillator crystals by 10%.