ABSTRACT
The prospect of using Tm2+-doped halides for luminescence solar concentrators (LSCs) requires a thorough understanding of the temperature dependent Tm2+excited states dynamics that determines the internal quantum efficiency (QE) and thereby the efficiency of the LSC. In this study we investigated the dynamics in CaX2:Tm2+(X= Cl, Br, I) by temperature- and time-resolved measurements. At 20 K up to four distinct Tm2+emissions can be observed. Most of these emissions undergo quenching via multi-phonon relaxation below 100 K. At higher temperatures, only the lowest energy 5d-4f emission and the 4f-4f emission remain. Fitting a numerical rate equation model to the data shows that the subsequent quenching of the 5d-4f emission is likely to occur initially via multi-phonon relaxation, whereas at higher temperatures additional quenching via interband crossing becomes thermally activated. At room temperature only the 4f-4f emission remains and the related QE becomes close to 30%. Possible reasons for the quantum efficiency not reaching 100% are provided.
ABSTRACT
The phenomenon of self-absorption is by far the largest influential factor in the efficiency of luminescent solar concentrators (LSCs), but also the most challenging one to capture computationally. In this work we present a model using a multiple-generation light transport (MGLT) approach to quantify light transport through single-layer luminescent solar concentrators of arbitrary shape and size. We demonstrate that MGLT offers a significant speed increase over Monte Carlo (raytracing) when optimizing the luminophore concentration in large LSCs and more insight into light transport processes. Our results show that optimizing luminophore concentration in a lab-scale device does not yield an optimal optical efficiency after scaling up to realistically sized windows. Each differently sized LSC therefore has to be optimized individually to obtain maximal efficiency. We show that, for strongly self-absorbing LSCs with a high quantum yield, parasitic self-absorption can turn into a positive effect at very high absorption coefficients. This is due to a combination of increased light trapping and stronger absorption of the incoming sunlight. We conclude that, except for scattering losses, MGLT can compute all aspects in light transport through an LSC accurately and can be used as a design tool for building-integrated photovoltaic elements. This design tool is therefore used to calculate many building-integrated LSC power conversion efficiencies.
ABSTRACT
CaLaSiN3 samples doped with Eu, Yb, Sm, Ce and Pr have been prepared via solid-state reaction synthesis and the optical properties have been studied. Both Yb and Sm were only observed in the trivalent state due to the fact that their Ln(2+) ground states are located inside or very close to the conduction band of the CaLaSiN3 host lattice. Doping with Ce(3+) or Eu(2+) resulted in a very low energy Ce(3+) or Eu(2+) 4f-5d absorption band around 1.9 eV (650 nm) and 1.4 eV (885 nm), respectively. The Ce(3+) 5d-4f emission appeared to be quenched, just as the Eu(2+) 5d-4f emission, which can be explained as the result of auto-ionization.
ABSTRACT
The diffuse reflectance and photoluminescence (PL) spectra of NdI(2), SmI(2), EuI(2), DyI(2), TmI(2) and YbI(2) were measured between 225 and 12 500 nm in order to determine their 4f(n) â 4f(n-1)5d(1) optical bandgaps. The results were compared with those obtained using an empirical model of the electronic structure of LnI(2). The results can be used to explain the lanthanide valency and crystalline structure changes of other lanthanide diiodides such as PrI(2).
Subject(s)
Iodides/chemistry , Lanthanoid Series Elements/chemistry , Crystallography, X-Ray , Luminescence , Models, Molecular , Molecular Structure , Spectroscopy, Fourier Transform InfraredABSTRACT
High quality phase pure samarium monosulfide (SmS) thin films were prepared by electron beam evaporation using a samarium metal source in a H(2)S atmosphere. The optical properties (reflection, transmission, absorption) of the films in the semiconducting and metallic phase were analysed from the UV to the mid-IR and explained in terms of the electronic structure of SmS. In this paper it will be shown that metallic SmS thin films exhibit an apparently continuous thermally induced metallic to semiconducting phase transition when studied optically. Temperature dependent x-ray diffraction measurements, however, indicate that the metallic to semiconductor phase transition is in fact first order at a single grain level. The apparently continuous optical behaviour is therefore due to the polycrystalline nature of the films.
