Irene
Motta
a,
Gregorio
Bottaro
*bc,
Maria
Rando
a,
Marzio
Rancan
*bc,
Roberta
Seraglia
d and
Lidia
Armelao
ace
aDepartment of Chemical Sciences (DiSC), University of Padova, via F. Marzolo 1, 35131, Padova, Italy
bInstitute of Condensed Matter Chemistry and Technologies for Energy (ICMATE), National Research Council (CNR), c/o Department of Chemical Sciences (DiSC), University of Padova, via F. Marzolo 1, 35131, Padova, Italy
cNational Interuniversity Consortium of Materials Science and Technology (INSTM), Florence, Italy
dInstitute of Condensed Matter Chemistry and Technologies for Energy (ICMATE), National Research Council (CNR), Corso Stati Uniti, 4, 35127, Padova PD, Italy
eDepartment of Chemical Sciences and Materials Technologies (DSCTM), National Research Council (CNR), Piazzale A. Moro 7, 00185, Roma, Italy
First published on 3rd June 2024
The integration of photovoltaic (PV) devices into the built environment has become a key factor for the development of energy efficient buildings. Luminescent solar concentrators (LSCs) are well suited for this application, as they could be installed over architectural elements inaccessible to conventional PVs. In the present work, we report the synthesis of super-bright [Eu2L4]2− cages bearing bis-β-diketonate ligands and their subsequent embedding in polymethylmethacrylate (PMMA) by casting of 5 × 5 × 0.27 cm3 planar LSCs. With a measured average visible transmittance (AVT) of 92%, the proposed LSCs possess excellent transparency and aesthetic quality fit for glass destined for neutral colour applications. Moreover, the obtained materials absorb from 60% to 90% of solar UV radiation, exposure to which is harmful for human health and responsible for fading of building interiors. The performance of devices attained by edge-coupling LSC slabs with monocrystalline silicon solar cells matches that of classic Eu3+-based concentrators. Nonetheless, the employment of super-bright [Eu2L4]2− luminophores allows the reduction of the Eu3+ ion content of our devices down to 0.01% wt. This denotes a most efficient use of a valuable material like europium in the developed LSC-PVs, as Eu3+ weight contents reported in relevant literature were one to two orders of magnitude higher.
Complexes of rare earth ions with β-diketonates have become a widespread choice due to the versatility in molecular design conferred by this class of ligands, and the relative ease in their synthesis and functionalization.15,16 The literature on lanthanide-based LSCs is mainly centred on β-diketonates,17–19 and more specifically Eu3+ and Tb3+ since they display the highest photoluminescence quantum yields (PLQY) among the rare earth series.20
Herein, we report the synthesis and characterization of highly transparent LSCs based on a series of supramolecular, binuclear lanthanide bis-β-diketonates with the general formula [Ln2L4]2− (L = LA, LB, LM, LF) and structure depicted in Fig. 1c. Eu(tta)3phen (tta = thenoyltrifluoroacetone and phen = 1,10-phenanthroline, Fig. 1d) is also included in the study, due to its documented use in the literature regarding lanthanide-based LSCs.17,18 PMMA has been adopted as the host medium, due to its high transmittance in the UV and in the visible region. Emitters have been dissolved in a methyl methacrylate (MMA)/PMMA syrup that has been then poured into a casting mould and successively polymerized to prepare the LSCs. Spectroscopic characterisation of the luminophores has been conducted before and after embedment in the polymeric matrix. Average visible transmission (AVT), colour rendering index (CRI) and CIELAB coordinates have been derived from the transmittance spectra of the fabricated LSCs. The prepared LSCs were used to study the behaviour of the devices obtained by edge-coupling the luminescent planar waveguides with monocrystalline Si solar cells. Device performances have been evaluated according to the latest published protocols21,22 and terminology23 used in LSC research. A perspective on the literature regarding Eu3+-based concentrators is also offered, with the aim to assess the stand of the present study over the past-decade results.
Photoluminescence spectra were collected on a Horiba JobinYvon Fluorolog-3 spectrofluorimeter equipped with double-grating monochromators on the excitation side and iHR320 spectrograph on the emission side. A R928P Hamamatsu photomultiplier or Horiba Sincerity CCD detectors were employed. A 450 W Xe arc lamp was used as the excitation source. PLQY measurements were performed on the same instrument equipped with a poly(tetrafluoroethylene)-coated integration sphere, using a 90° excitation-collection geometry.
The synthesis of Eu(tta)3phen comprised a first deprotonation of tta using NaOH and consequent addition of the Eu3+ salt to the mixture of phen and deprotonated tta. The desired product was obtained by employing a metal:tta:NaOH:phen molar ratio of 1:3:3:1. ESI-MS (Fig. S4†) confirmed the nature of the synthesised product.
UV/vis absorption and photoluminescence (PL) spectra (Fig. 3a) have confirmed effective sensitization through the antenna effect: absorption of light is carried out entirely by the β-diketonate ligands in the 300–380 nm region, resulting in a ligand-centred excitation, while the sole PL profile observed is that of the Eu3+ ion. The Eu3+ PL spectrum is composed of five sharp peaks, corresponding to the transitions originating from the 5D0 excited state towards the 7FJ (J = 0, 1, 2, 3, and 4) ground state. The 5D0 → 7F2 hypersensitive transition at 612 nm dominates the emission spectrum of Eu3+ β-diketone complexes and is responsible for the red luminescence.15,28 Overlap between absorption and emission is null, with a pseudo-Stokes shift of ≈200 nm.
When selecting the most promising luminescent complexes/molecules for the development of LSCs, we must consider both the values of the photoluminescence quantum yield (PLQY) and the absorption coefficient (ε). Often, seeking the best emitters, there is a tendency to consider only the PLQY. However, this is a partial evaluation since it only takes into account the fraction of emitted photons but does not provide information on the number of absorbed photons (indicated by ε). The parameter that should be maximized is the number of photons emitted per unit area and per unit time. For molecules and complexes in solution, this parameter is well represented by the brightness (B = ε × PLQY),29,30i.e., the product of the absorption coefficient and the emission quantum yield. The luminophores here discussed have similar absorption and emission spectra and hence it is sufficient to compare the brightness values relative to the maximum absorption (Fig. 3b and Table 1). In MMA solutions, [Eu2LB4]2− is the brightest complex of the family with B = 96880 M−1 cm−1, followed by [Eu2LA4]2− and [Eu2LF4]2−, having a B of around 70000 M−1 cm−1, and then [Eu2LM4]2− with B close to 45000 M−1 cm−1. The difference in brightness between Eu(tta)3phen (13500 M−1 cm−1) and the binuclear lanthanide bis-β-diketonates [Eu2L4]2− (L = LA, LB, LM) is noteworthy, reaching about one order of magnitude. Considering the brightness values in MMA solutions, the most promising candidate for the development of LSCs seems to be [Eu2LB4]2−. Prior to material synthesis and characterization, we investigated the compatibility of the complexes with the PVC gasket used as lateral walls of the LSC casting mould employed for MMA polymerization. Gasket strips (approximately 1 × 3 × 0.3 cm3) were immersed in MMA solutions of the investigated luminophores, at the same concentrations used for LSC castings. After 20 h (approximately the polymerization time) the PVC pieces were removed from the solutions, dried at room temperature and observed under a UV lamp at 365 nm to detect eventual traces of the cages. The PVC strip immersed in the [Eu2LB4]2− solution displayed a markedly brighter luminescence with respect to the others, suggesting a strong affinity between [Eu2LB4]2− and PVC and eventually possible issues during material synthesis. Moreover, we observed that solubility of Eu(tta)3phen in MMA is about two orders of magnitude larger than those of [Eu2LA4]2−, [Eu2LB4]2− and [Eu2LM4]2− (Table 1). To address this aspect, we synthesized ligand LF by slightly modifying LA in order to obtain higher cage solubility in MMA, achieving in the end a twenty-fold increase in it. Once again, the compatibility of the corresponding bright europium complex, [Eu2LF4]2−, with the PVC gasket was investigated, detecting no type of interaction.
ε max × 105/M−1 cm−1 | PLQY | B/M−1 cm−1 | Solubility/mM | |
---|---|---|---|---|
a PLQY and εmax were not measured for these samples in the polymeric matrices because [Eu2LB4]2− and [Eu2LM4]2− give rise to issues during material synthesis and were not employed to develop LSC devices (for details see LSC preparation and characterization section). | ||||
[Eu2LA4]2− | 1.61 (1.65) | 0.41 (0.39) | 66010 | 0.0184 |
[Eu2LF4]2− | 1.42 (1.43) | 0.50 (0.53) | 71000 | 0.321 |
[Eu2LB4]2− | 1.73 (—a) | 0.56 (—a) | 96880 | 0.0195 |
[Eu2LM4]2− | 1.01 (—a) | 0.48 (—a) | 48480 | 0.0216 |
Eu(tta)3phen | 0.50 (0.53) | 0.27 (0.25) | 13500 | 5.02 |
(1) |
An active area of 25 cm2 has been chosen because it has been proposed as a reasonable standard size for easily comparing results between independent LSC studies on a laboratory scale.22,31 The amount of available luminophores for the castings depended on their solubility in MMA, which was found to be largely different in the investigated luminophores. Maximum solubilities achieved for Eu(tta)3phen and [Eu2LF4]2− in MMA were 0.5 wt% and 0.1 wt%, respectively, while those of the other cages [Eu2LA4]2−, [Eu2LB4]2− and [Eu2LM4]2− were two orders of magnitude lower. Five samples have hence been prepared using each system at 0.005 wt%, and an additional LSC has been prepared for both Eu(tta)3phen and [Eu2LF4]2− at their solubility limit. An undoped PMMA tile has also been fabricated. Labelling of the prepared LSCs is reported in Table S4.†
Absorption spectra of the luminophores in PMMA (Fig. 4a) match the absorbance profiles collected for the MMA solutions. Upon UV excitation at λ = 365 nm, all LSCs displayed bright red luminescence and concentration of the emitted light on the tiles edges (Fig. 4b–d). The same light guiding properties could already be appreciated by the naked eye for all doped LSCs under daylight (Fig. 4e and f).
Since the Eu3+ emission spectrum is sensitive to the ion's coordination site28 and displays different shapes accordingly, such a feature has been adopted as a qualitative tool to detect possible alterations in the luminophores' structure caused by embedment into the PMMA matrix. The PL profiles recorded for LSCs and solutions well agree with one another for all the investigated Eu3+ complexes, meaning that neither the PMMA matrix nor material processing affects the structural integrity of such luminophores. Accordingly, absorption coefficients and PLQY are fully consistent between solutions and PMMA tiles (Table 1). The resultant LSC-A, LSC-F and LSC-T were homogeneous and defect-free. Instead, LSC-B showed luminophore segregation towards its edges, as observed under 365 nm excitation (Fig. 4c). Such spatial distribution was ascribed to a preferential absorption of the luminophore into the PVC gasket used to seal the casting mould, thus determining loss of control over the loading of [Eu2LB4]2− into PMMA. This behaviour was already presumed by the reported preliminary PVC swelling tests. When illuminated with UV light, LSC-M (Fig. 4d) exhibits several bright spots dispersed randomly throughout the polymeric matrix. These bright spots are due to the precipitation of [Eu2LM4]2−, likely occurring during polymerization, within the matrix. Such behaviours make these complexes not suitable for device development. We emphasize that material processing may affect single luminophores differently, despite small differences in the molecular structure of the ligands as in the present case. The solubility increase obtained by introducing a –CF2– moiety in the terminal chain of the LA ligand (Fig. 1c) is noteworthy. Hence, the suitability of an emitter for the LSC application should be evaluated based on its characterization after embedment into the matrix of choice.
Considering the application field of the studied materials, their aesthetic appearance is of paramount importance, especially if they are intended as replacements for residential or shop windows. The parameters used to quantitatively assess this aspect are (i) the Average Visible Transmittance (AVT), which measures the amount of visible light transmitted by the LSC, (ii) the Colour Rendering Index (CRI), which quantifies how accurately the colours of objects are reproduced when illuminated with a particular light source, and (iii) (a*, b*) CIELAB coordinates, which define the colour of the material in a perceptually uniform space (a* gives the colour variation along the green-red axis and b* along the yellow-blue one). For the determination of these parameters, AM1.5G and AM1.5G × T(λ) (T(λ) being the transmittance spectrum of the material) are used as source spectra.32
Fig. 5a shows the UV/vis transmittance spectra of the [Eu2LA4]2−, [Eu2LF4]2− and Eu(tta)3phen doped LSCs, as well as that of undoped PMMA. The fabricated LSCs show very high transparency, as they transmit all visible light with AVT ≈ 92%. They also determine minimal visual impact on the transmitted light, as calculated CRI values are above 98 for all samples (Table S5†).
A recent survey on a wide inventory of commercially available and mass-market glass products has provided reference key levels for the aesthetic quality of transparent window products in the glazing industry.10 A CRI ≥ 85, and CIELAB coordinates of −7 < a* < 0 and −3 < b* < 7 have been found as acceptable standards for products destined for neutral colour applications. Besides presenting (a*, b*) coordinates widely within the specified intervals, our samples possess excellent colour and imaging fidelity, where the attribute “excellent” refers to CRI values falling within the 95–100 interval.10 Furthermore, the whole series of doped LSCs shows a CRI and CIELAB coordinates comparable to those of plain PMMA, indicating that the presence of the luminophores does not modify the visible appearance and the light transmitting properties of the pristine material.
The optimal aesthetic quality of the LSCs containing [Eu2LF4]2− and Eu(tta)3phen remains unchanged even at higher luminophore loadings. LSC-Fc and LSC-Tc both retain high transparency and absence of heavy coloration while a higher luminophore content of 0.1 wt% and 0.5 wt%, respectively, allows them to harvest more light on a wider portion of the UV region and is responsible for a larger total absorptance (eqn (2) and (3)) over the less concentrated samples (Table S5†). Absorption onsets, in fact, shift from 380 nm up to 395 nm and 410 nm, respectively. To better evidence the light absorption capability of the materials it is convenient to refer to the total absorptance ηs,abs, defined as follows:23,33
(2) |
(3) |
LSC-Fc and LSC-Tc have ηs,abs values of 1.11% and 1.81% (eqn (2) and (3)), respectively, enabling them to absorb up to 90% of the available UV photons. Notably, UV photons in the 280–400 nm range constitute only about 2% of the AM1.5G spectrum when expressed in terms of photon flux.
Given the large gap between absorbing and emitting regions displayed by the investigated LSCs (Fig. 4 and 5), the possibility of self-absorption losses, arising from the sensible luminophore loading increase in LSC-Fc and LSC-Tc, can be safely excluded. Consequently, these two samples are expected to produce significantly higher electrical outputs when coupled to Si cells in the LSC-PV configuration with respect to their less concentrated analogues. Additionally, UV-blocking capabilities are highly desirable for residential and shop windows to prevent fading of furniture, artwork, and flooring. This is typically achieved by applying passive window treatments that function solely by absorbing UV light. Luminescent solar concentrators based on europium antenna complexes offer an innovative approach, transforming passive elements into active ones by converting blocked UV radiation into visible light while also generating electricity.
The current density–voltage (J–V) characteristics of the LSC-PVs were investigated by illuminating the LSC active area and optically coupling one of its edges to two monocrystalline, single module Si cells with an active area of 2.2 × 0.7 cm2 each. An apposite cell holder was designed to lodge the 0.27 cm thick LSC edge for the coupling, while covering the PV cell excess area to avoid any direct illumination from the light source. The uncoupled edges were masked with black tape and a matte black background was used.
The J–V curves are shown in Fig. 6a, while the short-circuit current density (JSC), the open-circuit voltage (VOC), the fill factor (FF), the maximum power (PMAX) and the Power Conversion Efficiency (PCE, eqn (4)) are listed in Table 2.w
(4) |
J SC/mA cm−2 | V OC/mV | FF | P MAX/mW | PCE/% 1 edge (4 edges)b | η int /% | ||
---|---|---|---|---|---|---|---|
Experimental | Calculated from EQEa | ||||||
a Eqn (6). b Calculated by multiplying PCE by 4. c Defined in eqn (7) and (8). | |||||||
LSC-FcPV | 0.097 | 0.097 | 490 | 0.65 | 0.780 | 0.031 (0.124) | 55 |
LSC-TcPV | 0.090 | 0.091 | 488 | 0.62 | 0.680 | 0.027 (0.108) | 39 |
PMPV | 0.007 | — | 292 | 0.50 | 0.024 | 0.001 (0.004) | — |
The highest JSC, VOC and Pmax values were observed for LSC-FcPV and LSC-TcPV indicating that higher luminophore loadings positively affect the electrical outputs. In particular, for LSC-FcPV we observed a maximum output power of 0.780 mW. PCE values for LSC-FcPV and LSC-TcPV stand one order of magnitude higher than those of LSC-APV, LSC-FPV and LSC-TPV.
We focus the following discussion on LSC-FcPV and LSC-TcPV, i.e., on the best performing devices among the series. The data relative to the devices based on lower concentrated LSC tiles can be found in Table S7.†
The two doped samples produce comparable electrical outputs, with slightly better performance exhibited by LSC-FcPV. About 4% of the calculated PCEs arises from simple waveguiding of the incident white light by the PMMA matrix, as deduced from the study of the undoped LSC. Since J–V measurements were performed following the suggested guidelines, only one LSC edge has been coupled with solar cells. A multiplicative correction factor of 4 can be applied to estimate the energy that would be collected if all four edges were PV-coupled.21 The recalculated PCE values would then equal to 0.124% and 0.108% for LSC-FcPV and LSC-TcPV, respectively. It is noteworthy that the former produces higher electrical outputs, even if by a small margin, than the latter although containing one fifth of its wt% concentration of luminophores.
Position dependent External Quantum Efficiency (EQE) spectra (Fig. 6b) have also been collected within the luminophores absorption range by means of a custom-built measurement station. The EQE of a photovoltaic device (eqn (5)) evaluates its capability to convert incident photons into charge carriers, and is proportional to the ratio between the produced electrical current (I(λ)) and the incident electrical power (P(λ)) at each given wavelength, also known as the spectral response (SR(λ)).
(5) |
(6) |
When both EQE and J–V curves are available, a consistency check for LSC-PV devices can be performed by comparing the experimental JSC value with that calculated from the EQE spectrum by means of eqn (6). This comparison is important because it reduces the possibility of photocurrent overestimation due, for example, to a non-perfect masking of the PV cell from direct illumination. Since such artifacts may be difficult to detect, this consistency check should be used to validate the measured data. Integrated JSC values for LSC-FcPV and LSC-TcPV are reported in Fig. 6b and in Table 2. We found a very good agreement between experimental and integrated JSC values, thus validating the experimental data and consequently the whole measurement setup. Furthermore, the good agreement between the measured and calculated JSC values confirms the absence of scattering centres or particulates inside the waveguides, already assessed qualitatively by the naked eye and quantitatively through AVT determination.
The EQE spectra present similar shapes to the absorptance spectra and have the same absorption onsets (Fig. S6†). Any examined LSC-PV device should satisfy the photon balance A(λ) + T(λ) + R(λ) = 1 at all wavelengths, with A(λ), T(λ) and R(λ) being the absorptance, transmittance and reflectance spectrum, respectively. Since the EQE(λ) quantifies how many of the incident photons on the LSC-PV generate a charge carrier pair, its value can at best match that of A(λ), at each wavelength. Given the relation EQE(λ) ≤ A(λ), independent measurements of EQE, transmittance and reflectance should always satisfy the condition EQE(λ) + T(λ) + R(λ) ≤ 1.21,32 This provides a useful consistency check for the photon balance of the device, which is fulfilled for both LSC-FcPV and LSC-TcPV (Fig. 6c and d).
After correction for the four sides, maximum EQE values become ≈32% at 360 nm and ≈23% at 320 nm for LSC-FcPV and LSC-TcPV, respectively. LSC-FcPV displays higher EQE than LSC-TcPV, which yields the larger integrated JSC between the two LSC-PVs, and consequently the higher electrical output (Fig. 6a). Conversely, the latter has a lower EQE but a wider absorption range which allows it to capture an additional portion of the AM1.5G spectrum, wherein the photon population rapidly increases. This produces a J–V curve and a PCE value close to those of LSC-FcPV, despite the considerable gap between EQEs. Widening of absorption bands due to increased luminophore loadings is then a favourable factor for extracting more electrical power, provided that the LSC aesthetic quality is preserved. Further functionalization of [Eu2L4]2− cages oriented towards increased solubility in MMA could facilitate full utilization of the UV region by the proposed LSC-PVs.
Internal photon efficiency (ηint), defined in eqn (7) where nout is the number of photons emitted at the edges and nabs is the number of photons absorbed from the active area, was derived from collected electrical parameters according to a verified method (eqn (8)).23,33
(7) |
(8) |
Calculated internal photon efficiencies are 55% and 39% for LSC-FcPV and LSC-TcPV, respectively. Since ηint accounts for the optical losses of the sole LSC (i.e., excluding the coupling to the photovoltaic component), scattering, PLQY and re-absorption are the three main factors contributing to this efficiency. Scattering and re-absorption losses have been found to be absent from our LSC-PVs, so the better ηint of LSC-FcPV is ascribed to the higher PLQY of [Eu2LF4]2− with respect to Eu(tta)3phen.
Furthermore, we performed some preliminary outdoor stability tests on LSC-Fc and LSC-Tc by exposing them, south-facing, to atmospheric ageing in Padova (Italy) from September to November 2023. During this period, the average temperature during the day went from 20–25 °C (Sep, Oct) to 15 °C (Nov). The average (2006–2022) Global Horizontal Irradiation (GHI) in Padova for September, October and November are 4.343, 2.737, and 1.542 kW h m−2, respectively.34 At sample retrieval, a drop in the measured ISC of 20% and 30% was observed for LSC-Fc and LSC-Tc, respectively.
We finally compared the performance of our devices with that of other published Eu3+-based concentrators (Table 3). Being recommended as the primary parameter to describe a LSC-PV, we chose the PCE as a metric for comparison.23 To include precedent studies to the publication of the reporting guidelines,21,23,31 where absent, PCE values have been calculated by us using experimental data provided in the cited references. To accurately compare PCE data, the device structure needs careful consideration: (i) the number of PV-coupled edges should be clearly specified, (ii) an efficient shielding from direct illumination on the edge-coupled photovoltaic cells and a matte black backdrop are required, and (iii) for measurements relative to one edge, the other three should be blackened. This configuration minimizes the contribution of stray light (light not generated by the LSC luminescence) to the photocurrent measurement.
Luminophore | Matrix | G | Device structure | PCE/% | Eu3+/wt% | |
---|---|---|---|---|---|---|
a Value not reported in the publication, calculated based on provided experimental data. | ||||||
This work | [Eu2LF4](NEt4)2 | PMMA | 4.63 | 5.0 × 5.0 × 0.27 cm3 bulk tile | 0.031 | 0.01 |
This work | Eu(tta)3phen | PMMA | 4.63 | 5.0 × 5.0 × 0.27 cm3 bulk tile | 0.027 | 0.08 |
Dev-A (ref. 17) | Eu(tta)3phen | Poly vinylbutyral (PVB) | 6.50 | Film (thickness not specified) deposited onto 7.8 × 7.8 × 0.3 cm3 glass | 0.044 | 0.24 |
Eu(tta)3Dpbt | 0.050 | 0.20 | ||||
Dev-B (ref. 19) | [B(TMSP)Im] | PMMA | 7.85 | 5.1 μm thick film deposited onto 7.5 × 2.0 × 0.1 cm3 glass | 0.002 | 2.28 |
[Eu(tta)4] | ||||||
Dev-C (ref. 35) | Eu(2mCND)3 | Siloxane-polyether (di-ureasil) | 2.89 | 320 μm thick film deposited onto 5.0 × 5.0 × 0.4 cm3 acrylic | 0.138a | 1.05 |
Concerning the literature examples reported in Table 3, all the data are relative to one edge measurements, but device structures are quite different. While Dev-A has blackened edges and the backdrop is not specified, in Dev-B both edges and background are reflective. The edges of Dev-C are not taped and a white paper foil is used as reflective backdrop, allowing a 92% enhancement of the output power, as declared by the authors of the study.36 To date, finding literature data on devices having analogous structures is quite demanding. The comparison between the results of our study and the literature shows that LSC-FcPV and LSC-TcPV produce electrical outputs of the same order of magnitude of most published Eu3+-based LSC-PVs. This is a significant result, considering that the europium weight content of our devices (0.01 wt% for LSC-FcPV and 0.08 wt% for LSC-TcPV) is inferior to that found in the explored literature down to a one hundred factor.
The presented literature survey compares our LSC-PVs to other analogous Eu3+-based systems with aesthetic properties (high transparency and lack of colour) comparable to ours. Interestingly, Jin et al.37 recently reported LSCs with very similar aesthetic quality, based on a different type of emitter such as Ag,Mn:ZnInS2/ZnS quantum dots (QDs). Moreover, this study followed the suggested reporting guidelines as well, allowing for a direct comparison of device metrics. The QD-based LSCs possess an AVT of 90% and CRI of 95.8, and has a PCE (4 edges) of 0.09%. Such values closely resemble our results, indicating that the devices herein developed rank competitively among the best UV-selective LSC technologies. Owing to the significantly higher intensity of solar radiation in the visible region compared to the ultraviolet region, LSCs based on UV absorbers inherently exhibit lower power conversion efficiencies than LSCs prepared employing visible absorbing luminophores.6,23,38–42 However, this gain in PCE inevitably comes at the expense of a decreased AVT and CRI, and therefore their applicability is strongly dependent on the context of use.
The obtained LSCs appear highly transparent and colourless, with excellent visible transmission and colour rendering properties, comparable to those of widely industrialized glazing products. The increased [Eu2LF4]2− loading makes LSC-FcPV the best performing sample in terms of PCE and EQE, with the latter exceeding 30% in the UV region. The increase in concentration does not affect transparency or colour neutrality of the material. The proposed materials are therefore promising for a seamless integration into buildings as photovoltaic windows, blocking UV light and producing energy while preserving functionality and aesthetics. Finally, the PCEs displayed by LSC-FcPV and LSC-TcPV are in line with those found in the literature regarding Eu3+-based concentrators, but in our case the same performances are obtained with the use of a sensibly lower quantity of the lanthanide ion, down to two orders of magnitude less in terms of Eu3+ weight concentration. Particularly for LSC-FcPV, these results open the way for deployment of LSCs based on europium as highly transparent BIPVs while minimizing its usage.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2349361 and 2349362. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ta03244f |
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