Building integrated thin film luminescent solar concentrators: Detailed efficiency characterization and light transport modelling
Highlights
► We have quantified all the different loss processes in a thin film LSC based on Eu3+. ► We have measured significant scattering losses in a thin film LSC with no self-absorption. ► Total LSC light transport efficiency was calculated under homogeneous illumination. ► BI-LSC power efficiencies were related to window color, size and attenuation length.
Introduction
One of the big aims in photovoltaic (PV) research is to make photovoltaic energy competitive in prize with conventional energy resources. Besides making the PV-cells more efficient, there is a second option, namely concentration of the sunlight onto the PV-cells.
Luminescent solar concentrators (LSCs), dating back from the 1970s [1], [2], are a very attractive concept for a concentrating PV-module for numerous reasons: production can take place at low costs, the LSC can operate in diffuse light, therefore not needing expensive solar tracking devices, and, due to the luminescence, only ‘cold’ light will be collected by the PV-cells, resulting in higher PV efficiencies. In recent years there has been a renewed interest in LSCs due to the urgency of development of sustainable technology combined with new ideas on light trapping, and new photo stable luminescent (nano) materials and dye molecules with broader absorption and less self-absorption [3], [4], [5], [6], [7].
In general LSCs consist of a slab of dielectric material containing a luminescent material (organic dyes, quantum dots, or rare-earth complexes), with PV-cells connected to the perimeter of the slab. The luminescence centers absorb the sunlight incident on the face of the LSC, and isotropically emit light at a slightly lower energy. The major part of the emitted light is trapped inside the slab, and is guided to the solar cells at the perimeter. Two different types of LSCs can be identified. The first is the single plate concentrator, in which the luminescence centers are homogeneously spread throughout the plate. The second type, which is considered in this paper, is the thin film LSC. Such LSCs consist of an optically passive glass- or polymer-based substrate coated with one or more luminescent layers.
Though the concept of LSCs is very promising, problems like photo-degradation [8], [9], [10], limited spectral absorption [11], or self-absorption losses [12], have so far prevented large scale use of LSCs.
All problems have been addressed individually, like Wu et al. [13], who reported rare earth complexes showing absorption in the vis-spectrum and IR emission with zero self-absorption loss. Earp et al. [14] reported in 2004 an LSC with a light transport half-length of 1.2 m (corresponding to a waveguide attenuation length of about 1.7 m). Kennedy et al. [15] presented an LSC based on quantum dots, overcoming the problem of photo-stability at the cost of lower luminescent quantum yield. Such limitations of the total LSC PV-module cause the application of LSCs to still remain a castle in the air.
An interesting application would be building integrated (BI) LSCs [16], in which the windows of e.g. office buildings are coated with an absorbing luminescent thin film, and can therefore act as (additional) power supplies for such buildings. Either broad band UV–vis absorption resulting in greyish shaded glass, or absorption of just part of the UV–vis spectrum resulting in colored windows are considered. Both approaches result in better usage of the available sunlight and increasing feasibility of the BI LSC.
The performance of an LSC is directly linked to its optical efficiency (ηopt), as defined in [17]The optical efficiency is given by the product of the light harvesting efficiency (ηLHE) [12], the luminescent quantum efficiency (ηLQE) [18], Stokes' efficiency (ηStokes), the light trapping efficiency (ηtrap) [19], the self-absorption (ηSA) [20], the waveguide efficiency (ηWG) [14], and (1−R), in which R represents the Fresnel reflection coefficient of the LSC surface. In short it is given by the product of the quantum efficiency of the LSC (ηQE) and Stokes' efficiency.
The light harvesting efficiency can be calculated for a given source spectrum and absorption spectrum [12]. In this research however, the source spectrum consists of only one wavelength, resulting in a light harvesting efficiency equalling the absorption at that wavelength.
Optical efficiencies of up to 12% have been reported for dye–polymer-based thin film LSCs by Dienel et al. [12]. A similar optical efficiency of up to 13.2% is predicted by Kennedy et al. [15] using a QD doped single plate concentrator. In both papers though, the waveguide efficiency ηWG is assumed to be unity, which, as is shown by Earp et al. [14], can deviate from unity significantly.
In the first part of this paper we will investigate the efficiencies of the various processes that are involved in the concentration of sunlight in the Lu2O3:Eu3+ thin film luminescent solar concentrator. This means that the specular reflection, the absorption efficiency, the luminescent quantum efficiency, Stokes efficiency, the calculated light trapping efficiency and the calculated waveguide efficiency, are all quantified separately.
In the second part we will present a light transport model that considers the linear attenuation of light transport and the dimension of the waveguide. It can be used to calculate the fraction of the trapped luminescence light reaching the perimeter of the LSC under homogeneous illumination of the entire LSC. The model allows for the calculation of the light transport efficiency ηt=ηSA ηWG with an easy to perform measurement of the linear attenuation. Finally, we discuss the boundary conditions for light transport efficiency and window size for realizing a worthwhile building integrated thin film LSC.
Section snippets
Experimental procedures
The used LSC sample has been made by Boston Applied Technologies, Inc. A 3 μm thin film of Lu2O3:8% Eu3+ was deposited by a dip coating procedure onto a 250 μm Al2O3 (c-cut) sapphire substrate.
The transmission measurements have been performed using an AvaLight-DHc light source, emitting in the UV–NIR spectral region, and an AvaSpec-3648 Czerny–Turner spectrometer.
The photo-luminescent excitation spectrum was taken using a 450 W Xenon lamp, connected to a double monochromator, and photon counting
Results
In Fig. 2 the transmission spectrum of our LSC is plotted in the UV–NIR spectral region. The spectrum immediately shows that our LSC is not intended to work as an efficient solar concentrating device as it is not capable of absorbing any sunlight. At wavelengths shorter than about 350 nm strong absorption is observed that reaches a maximum at about 215 nm.
A reflection of 10% at 250 nm is measured, which is caused by the relatively high index of refraction of Lu2O3 of 1.93. At this wavelength we
Light transport modelling of thin film BI LSCs
Formula (4.1) gives ηt,rectangular for a rectangular LSC with dimensions (a formula for a circular LSC with diameter D is given by Formula (6.1) in Appendix A).
The triple integral contains basically two parts. The first part, the integral over theta, sums over the radiation reaching one edge of the LSC emitted by an infinitesimal area, dy dx, of the LSC. The 1/
Discussion
From Table 1 it becomes clear that the optical efficiency of the presented LSC is not 100%, mostly due to a low luminescent quantum efficiency of the luminescent material (44%) and the low light transport efficiency (45%). As the light transport in our thin film LSC is only controlled by the linear attenuation length that is not affected by self-absorption, the waveguide loss must be due to scattering at the various interfaces.
One way to improve the waveguide efficiency is to use a thicker
Conclusion
We have quantified the different loss processes in a thin film luminescent solar concentrator based on Eu3+ doped Lu2O3. The attenuation length for light transport has been determined, which in out self-absorption free LSC is entirely due to scatter losses. It is shown how to calculate total light transport efficiency of an LSC (for homogeneous illumination) based on an easy process to perform light transport attenuation measurement. Light transport efficiency has been fitted with an easy
References (34)
New developments in luminescence for solar energy utilization
Optical Materials
(2010)- et al.
Fluorescent planar concentrators
Solar Energy Materials
(1984) - et al.
Photostable solar concentrators based on fluorescent glass-films
Solar Energy Materials and Solar Cells
(1994) - et al.
Spectral-based analysis of thin film luminescent solar concentrators
Solar Energy
(2010) - et al.
Hybrid solar concentrator with zero self-absorption loss
Solar Energy
(2010) - et al.
Maximising the light output of a luminescent solar concentrator
Solar Energy
(2004) - et al.
Improving the optical efficiency and concentration of a single-plate quantum dot solar concentrator using near infra-red emitting quantum dots
Solar Energy
(2009) - et al.
Enhancing the performance of building integrated photovoltaics
Solar Energy
(2011) - et al.
Thermoluminescence excitation spectroscopy: a versatile technique to study persistent luminescence phosphors
Journal of Luminescence
(2011) - et al.
Preparation and spectroscopic characterization of Lu2O3:Eu3+ nanopowders and ceramics
Optical Materials
(2008)
Surface treatments and properties of CuGaSe2 thin films for solar cell applications
Thin Solid Films
Luminescent greenhouse collector for solar-radiation
Applied Optics
Solar energy conversion with fluorescent collectors
Applied Physics A: Materials Science and Processing
Luminescent solar concentrators—a review of recent results
Optics Express
High-efficiency organic solar concentrators for photovoltaics
Science
Solar concentrators: thirty years of luminescent solar concentrator research: solar energy for the built environment
Advanced Energy Materials
Cited by (57)
Universal measure of photon collection efficiency of dye luminescent solar concentrators
2023, Solar Energy Materials and Solar CellsCitation Excerpt :In this way, it is possible to manufacture LSC of relatively neutral colour [10]. Our approach helps to select specific dyes and concentrations (absorbencies) needed to achieve desired visual needs, e.g., optical density, colour, which are essential for use of large scale LSCs in BIPV [39–43]. B. Dzurnak: Visualization, Validation, Software, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization, Writing – original draft, Writing – review & editing.
A comprehensive review on optics and optical materials for planar waveguide-based compact concentrated solar photovoltaics
2022, Results in EngineeringCitation Excerpt :Rather than using an optics design system, a luminescent material-based waveguide LSC CPV system is a direct approach to the problem. Simple LSC-based CPV is now becoming a popular BIPV glazing solution, with more aesthetic diversity compared to existing systems and an additional prospectus of visible light selectivity and transmissivity [28–30]. The technological advancement of LSC was majorly focused on developing the key luminophore as the optical materials, which range from material distinctions of organic and inorganic materials to their hybrids [31–33].
Recent advances in photoluminescent polymer optical fibers
2021, Current Opinion in Solid State and Materials ScienceThe potential of transparent sputtered NaI:Tm<sup>2+</sup>, CaBr<inf>2</inf>:Tm<sup>2+</sup>, and CaI<inf>2</inf>:Tm<sup>2+</sup> thin films as luminescent solar concentrators
2021, Solar Energy Materials and Solar CellsPlasmonic luminescent solar concentrator
2021, Solar Energy