Building integrated thin film luminescent solar concentrators: Detailed efficiency characterization and light transport modelling

https://doi.org/10.1016/j.solmat.2012.04.016Get rights and content

Abstract

An inorganic thin film luminescent solar concentrator (LSC) is characterized experimentally in detail in terms of all the separate light transport steps, which result in the concentration of sunlight. The results are discussed in the context of application as windows in buildings, called building integrated (BI) LSCs. A 3 μm thin film of Eu3+ doped Lu2O3 was chosen because of its large Stokes shift, which excludes all waveguide losses due to self-absorption and allowed to study losses caused by scattering at interfaces of the LSC. A model is presented that can be used to calculate the LSC light transport efficiency as a function of window size, which only needs the easily measurable linear attenuation as input. Measurements show that the quantum efficiency (ηQE) of our LSC is 13%, which is less than ideal, mainly due to a 44% luminescence quantum efficiency and a 45% waveguide efficiency. Our modelling relates BI-LSC efficiency to window color, and reveals that the linear attenuation coefficient of an LSC should be higher than 1000 mm. This is in contrast to values in the order of tens of millimeters which have been measured for the LSC in this paper.

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]ηopt=ηLHEηLQEηStokesηtrapηSAηWG(1R)=ηQEηStokesThe 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 H×W (a formula for a circular LSC with diameter D is given by Formula (6.1) in Appendix A).ηt,rectangular=1πHW(0H0Warctan[HyWx]arctan[xHy]+π2exp{Hyμsinθ}dθdxdy+0W0Harctan[WxHy]arctan[yWx]+π2exp{Wxμsinθ}dθdydx)

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

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