1. Introduction

2. Materials and Methods

3. Results and discussion

3.2. Structure of the complexes

We have added the diffraction patterns of the different compounds as supplementary material (Figures S1-7). The structure of the compounds C1 and C2 consist of centrosymmetric dinuclear molecules in which each Eu atom is bound to one phenanthroline or bephenanthroline molecule and one terminal bidentate benzoate ligands (Table 1). The powder diffraction pattern and selected distances of C1 and C2 can be found in previous works [22,23]. These complexes crystallize in the triclinic system, space group P1.

The asymmetric unit of compound C4 contains two different molecules of the complex C5 which are stereoisomers. The two molecules have the same organic ligands, the same coordination number, and the same coordination environment of the central Eu(III) ion. However, the two molecules differ in the relative orientation of one of the bta- ligands. C4 and C5 complexes crystallize in the orthorhombic Pna21 and Pbca space-groups, respectively. The complex C6 crystallizes in the monoclinic P21/c space-group with its Eu(III) ion bound to one 5-methyl-phenanthroline and three β-diketonate benzoyl trifluoroacetone ligands.

Structures of C7-11 complexes are shown in Table 2. The crystal structure of C8 was solved by single-crystal X-ray diffraction and it was found to be isostructural with the corresponding C7 and C9 complexes. C8 complex crystallizes in the monoclinic P21/c space-group and has a molecular structure in which the Yb(III) ion is bound to six oxygen atoms of three β-diketonate bta- ligands and two nitrogen atoms of one 5-methyl phenanthroline ligand. C10 crystallizes in the orthorhombic Pna21 space-group and has a molecular structure with the Yb atom surrounded by three bta- β-diketonate ligands and one 2,3-pyrazine-1,10-phenanthroline ligand. C11 crystallizes in the orthorhombic Pbca space-group, and its molecular structure is formed by Er(III) ions surrounded by three tta- β-diketonate ligands and one pyz-phen ligand.

In the case of [M1M2(bz)₄(tta)₂(phen)₂] series, PXRD patterns confirmed that all compounds were isostructural. The structure of C12, solved by single-crystal X-ray diffraction, showed that these complexes crystallize in the triclinic $P\overline{1}$ space-group as neutral centrosymmetric dinuclear molecules of [M1M2(bz)₄(tta)₂(phen)₂] with four bridging benzoate (bz−), two 2-thenoyltrifluoroacetonate (tta−) and two 1,10-phenanthroline (phen) ligands.

Table 3. Molecular structures of the C12–17 complexes [M₂(bz)₄(tta)₂(phen)₂], where M₂ metal- ions can be Eu₂, Gd₂, EuTb or EuGd..

Table 3. Molecular structures of the C12–17 complexes [M₂(bz)₄(tta)₂(phen)₂], where M₂ metal- ions can be Eu₂, Gd₂, EuTb or EuGd..

Complex	Molecular structure
[Eu2(bz)4(tta)2(phen)2] (C12) (*)

(*) C12 was found to be isostructural with the corresponding C13-17 complexes.

3.3. Preparation of the luminescent down-shifting layers

Samples containing 1–7 mg of the C1–2 complexes were dissolved in 7 mL of CH₂Cl₂ in a refrigerated ultrasonic bath; (ii) then 70 mg of PMMA were added and the mixture stirred until complete solution of the polymer, obtaining samples with relative concentration of the actives species ranging from 1.4 to 10%; (iii) spin-coating method was used for film deposition (50-100𝜇m) on commercial glass substrates.

An analysis of the additional costs of a solar panel with this type of film can be seen in previous studies [22], where we estimated in euros (€) per square meter (m²) of solar panel with 15% efficiency, at approximately 0.05 €/m² for every 1% of complex product used. However, these costs were calculated in 2015, and the prices of the chemicals used have increased significantly nowadays. Thus, for complex C1, we have made an estimate of its price per square meter for the same concentration, resulting in approximately 0.48 €/m². For the other complexes, we have also estimated their price in euros per square meter for a 1% concentration, from lowest to highest price for the C1-6 series as follows: C1=0.48; C6=0.84; C5=1.24; C4=1.28; C3=1.71; C2=2.28. For the C7-11 series, C7=0.56; C8=0.58; C9=0.61; C11=0.97; C10=1.02. And for the C12-17 series, C13=0.16; C15=0.32; C16=0.36; C14=0.37; C17=0.42; C12=0.49.

To obtain the DS layers corresponding to complexes C3, C7 and series C12-17 before were encapsulated on a PV mini-module, Deltalab 1.1 mm thick EUROTUBE® Slides (25 mm × 70 mm) were cut into 20 × 20 mm squares or 25 × 20 mm pieces and immersed in a 2 M sulfuric acid solution for one hour, rinsed with a generous amount of deionised water and dried at 60 °C overnight. 10 mg of DS complex mixed with 25 mg of EVA was dissolved in 1.5 mL of CH₂Cl₂ under vigorous stirring for 20 min. 300 µL of that solution was dropped on the glasses and the solvent was evaporated at room temperature.

Figures 2a) and b) display the down-converting layers of C1 and C3 complexes, respectively, which are embedded in PMMA and deposited on bare glass substrates under UV radiation. The C3-7 and C12-17 series samples were encapsulated using a standard procedure [65] with the Lab PV Module Laminator from Fraunhofer ISE. Examples of the C3 complex can be observed in Figures 2c) and d) when exposed to UV light. Figures 2e) and f) demonstrate the down-converting layers of C12-17 complexes embedded in EVA, both without and with UV light.

Figure 2. Images of downshifter layers corresponding to complexes C1 a) and C3 b) when irradiated with ultraviolet light. In photograph c), downshifter layers of complex C7 laminated with EVA are observed on a mini module of copper indium gallium selenide (CIGS), and in photograph d) when illuminated with UV light. Photograph e) shows converter layers from the C12-17 series laminated with EVA on a multicrystalline Silicon mini module, and in photograph f), the same samples when irradiated with UV light.

Figure 2. Images of downshifter layers corresponding to complexes C1 a) and C3 b) when irradiated with ultraviolet light. In photograph c), downshifter layers of complex C7 laminated with EVA are observed on a mini module of copper indium gallium selenide (CIGS), and in photograph d) when illuminated with UV light. Photograph e) shows converter layers from the C12-17 series laminated with EVA on a multicrystalline Silicon mini module, and in photograph f), the same samples when irradiated with UV light.

3.6. EQE measurements

To quantify the effect of the DS layers on top of the PV devices, we calculated the increase in photo-generated current, ΔJ_sc, as a function of the wavelength, λ, through equation (1) [71]

$$\Delta J_{sc}=q{\int }_{}^{}\varphi \left(\lambda \right)·\Delta EQE\left(\lambda \right)d\lambda =\int E_0·\Delta SR\left(\lambda \right)d\lambda$$

(1)

where q represents the charge of an electron, 𝜙 is the incident photon flux, ΔSR is the increase in spectral response (SR), and E₀ is a standard reference of solar spectral irradiance based on ASTM G173-03 [64]. The spectral response uses the power of the light at each wavelength, with its magnitude given in Ampere/Watt. Converting EQE to SR is done using the formula SR = EQE$·\lambda$(nm)/1239.8. Equation (1) was used to determine the current density J_sc of the solar cell with DS films encapsulated. Estimations of the relative increase in conversion efficiency (η), are made while considering that the open circuit voltage (V_oc) and the fill factor (FF) remain constant when the downshifter is placed on the PV mini module. By assuming that (%) is equal to the product of J_sc, V_oc , and FF, for one sun of irradiation, the increment in relative percentage of conversion efficiency, $(\Delta \eta /\eta )\%$, can be calculated using the following expression

$$(\Delta \eta /\eta )\times 100\gtrsim (\Delta J_{sc}/J_{sc})\times 100$$

(2)

Below, we summarize the results we have obtained with the complexes used for different experimental configurations. In order to optimize our research on the use of DS films in different PV devices, we have been testing the placement of photoluminescent films on the device, the concentration of materials used in their manufacturing, the light concentration using spherical reflectors, the encapsulation of the photoluminescent films on the device, and the degradation they may undergo with prolonged exposure to ultraviolet light.

3.6.1. DS layers using different concentrations of complexes C1 and C2

Figure 12 shows the measurements obtained in EQE when using a DS film of complex C1 for different experimental configurations. Figure 12 a) shows the EQE behavior for the 280 – 350 nm spectral range when the DS layer was deposited through spin coating or tape casting on glass while Figure 12 b) shows the wavelength dependence of the EQE when the DS layer was deposited through tape casting on the rear side of the glass and covered by a UV reflector.

The analysis of the measurements obtained from EQE with the DS film of complex C1 allows us to calculate estimates of the relative increase in percentage efficiency, as shown in Table 5.

The values of (Δη/η) % are very high in the UV region where the photovoltaic device practically has no response. This increase is what can ultimately lead to a gain across the entire studied spectrum.

The C2 complex using the bphen ligand improves the absorption and photoluminescence results of the C1 complex, as shown by the results discussed earlier. The measurement of the EQE of the device with DS films continued with higher concentrations of this C2 complex, as shown in Figure 13 a) and b). Figure 13 a) displays the obtained EQE measurements for the UV region, where there is a maximum of 4.6% and 3.5% for C2 complex concentrations of 10% and 5%, respectively. The EQE measurements across the spectrum shown in Figure 13 b) demonstrate that these DS films do not worsen the efficiency in the visible region of the spectrum.

3.6.2. DS layers using different concentrations of complex C3

When the PV mini module used has virtually no response in the UV region, the EQE values increase with the concentration of complex C3 when the DS film is placed on top. This evolution can be studied based on the obtained results shown in Figure 14. Thus, for the maximum EQE value at an excitation wavelength of 340 nm, it is found that there is a maximum concentration value of complex C3 in the DS film, close to 30%, as above that value the transmittance of the DS film decreases to zero.

The maximum downshifting efficiency is determined by increasing the concentration of the active species to its solubility limit [25]. Our results are consistent with the EQE behavior, as shown in Figure 15.

The increments in the relative percentage of conversion efficiency from Figure 15 a) were 1.07% and 1.92% for 10% and 30% concentrations of complex C3, respectively. For the PV mini module based on a single p-type mc-Si solar cell with a 16% conversion efficiency, the increases were 0.17% and 0.31%, respectively.

If we use the same configuration but add a hemispherical reflector (concentrator) on top of the downshifter to collect most of the downshifted photons emitted isotropically out of the device, we obtain EQE values as shown in Figure 15 b). With a 30% concentration of complex C3, the relative percentage of conversion efficiency increased by 2.88% and the absolute conversion efficiency increased by 0.46%. This new configuration can be integrated into PV devices for specific applications related to concentrator photovoltaics

3.6.3. DS layers using complexes C4-6

Luminescent down-shifting (LDS) layers containing the complex C4, C5 and C6 embedded in polymethylmethacrylate (PMMA) have been placed on top of the mini module used to study their photovoltaic conversions compared to the PMMA when there is not complex. Figure 16 a) shows a positive increase in EQE values for the 300–400 nm spectral region and Figure 16 b) shows the EQE values for the 300–1200 nm spectral region.

The EQE measurements shown in Figure 16 b) show that the complexes C5 and C6 used as DS layers (blue and green lines) can improve the efficiency as we show in next sections.

3.6.4. DS layers encapsulated using C6 and C12-17 complexes

First, the DS films were just put on top of the PV devices without any encapsulation process, obtaining an increase of the EQE for the 310 – 370 nm spectral range. However, in the region of the solar spectrum where the mini-modules have a good response (380–1100 nm) the DS films reduce their efficiency due to poor transmittance, which indicates that the encapsulation of the films on top of the PV devices was necessary.

In the beginning, the films were made with PMMA as mentioned before, but it wasn’t possible to obtain clear and homogenous luminescent DS films after the encapsulation process. EVA is the main encapsulant polymer used in the fabrication process of PV devices, so we tried doing the films with it. Clear DS films that enhanced the EQE of the mini-modules for the 300–370 nm range were obtained using EVA (Figure 17 b)). These films also eliminated any losses between 380 nm and 1100 nm thanks to a greater transmittance. In addition, encapsulating the films prevents contact between the DS complex and atmospheric dioxygen. This results in increased efficiency of the down-shifting process by avoiding non-radiative deactivation of the excited states of the ligands due to interference from the triplet state of the dioxygen molecule. Therefore, the EQE response in the UV region of the spectrum is higher after encapsulation.

Once the conditions of the films and the encapsulation process were optimized, the EQE measurements of all complexes encapsulated on top of the mini-module were performed (Figure 18). The greatest increase in EQE was observed in the heterobimetallic compounds C14 and C17, with increases of 16.3% and 14.3%, respectively, for the 300 – 400 nm spectral range. These compounds were found to exhibit high quantum yields in the photoluminescence study. In the case of the C6 complex, there was a 10.0% increase in EQE under the same conditions. The C6 complex was subjected to a UV aging process, as explained in the next section.

Table 6 shows that all encapsulated LDS layers resulted in a significant increase in both EQE and J_sc within the 300–400 nm spectral range. The highest values were observed for samples C6, C14, and C17 when they were encapsulated in the mini module. These three complexes have positively contributed to the increase in the relative percentage of conversion efficiency within the 300–1200 nm spectral range.

In Table 6, we also wanted to show J_sc results through new experiments conducted with intensity-voltage (IV) curves measurements using a solar simulator AM1.5G, using reflectors. The increase in efficiency of the mini-module used for C12-17 complexes was obtained when a hemispherical reflector (concentrator) on top of the downshifter is placed. We have achieved, in the best case scenario for the DS layer with complex C14 an increase in conversion efficiency of 0.52% using a PV mini-module of 12.1% of efficiency [31].

3.6.5. Accelerate aging of DS layers encapsulated using complex C6

Cycles of UV radiation were applied to samples of C6 complex for varying durations: 2, 5, 10, 50, and 100 hours, totaling 2000 hours of exposure. Two samples of [Eu(bta)₃me-phen] encapsulated on top of a mini-module were used, featuring two different film configurations of complex C6 on the glass surface (face-up, FU, and face-down, FD). These samples underwent a study to assess degradation caused by UV radiation within a climatic chamber. In Figure 19, the EQE curves of the reference samples (non-degraded, without UV exposure) and the study samples after exposure to 450 kW/m2 of UV radiation (2000 hours in the climatic chamber) are presented. At the end of the study, the FU sample contributes only up to 2.5% to the EQE for the 300-400 nm spectral region. In contrast, the DS-FD sample exhibits a contribution above 20% at a wavelength of 350 nm compared to the reference sample without the down-shifting complex after 2000 hours.

Figure 20 a) shows a remarkable relative increase in percentage of efficiency, (𝛥𝜂/𝜂) (%), when the DS layer encapsulated is not subject to UV degradation for the configuration DS-FU sample around 1.2% for the 300–1200 spectral range, and 120% for the 300–400 spectral range as we show in Figure 20 b).

After 600 hours of UV radiation exposure, the DS-FU sample shows no significant relative increase in efficiency. On the other hand, the DS-FD sample demonstrates minimal UV degradation, with a relative increase in efficiency (Δη/η) exceeding 80% for the 300-400 nm spectral range and approximately 0.8% for the 300-1200 nm spectral range.

3.6.6. Application of complex C6 to a bifacial solar cell

In this section, we want to report initial studies conducted with bifacial mini-modules using one of the most studied complexes from the previous sections, C6. In future work, we are already considering other complexes for evaluation. We used two bifacial solar cells from CENER (National Renewable Energy Centre) in Navarra, Spain. In one of the bifacial mini modules (first experiment), we conducted a series of measurements when the C6 complex is directly deposited on top of the mini module by dissolving it in tetrahydrofuran (THF). 300 microliters of the compound are deposited using a micropipette via drop casting (DS film). In another bifacial mini module (second experiment), we encapsulated the compound deposited on glass with EVA (DS layer). In all cases, we performed an experimental setup where the light is incident perpendicularly on both sides.

3.6.6.1. First experiment

Figure 21 shows the measurements taken in a first experiment to obtain the EQE values of a multicrystalline silicon bifacial mini module in the spectral range of 300 to 1200 nm. The EQE dependence was obtained for the wavelength on the front side, the rear side, and for both sides. The total EQE (illumination from both sides) is not the sum of the individual EQEs (illumination from each side); it is the sum of the EQEs of each side weighted by a factor that depends on the percentage of illumination from each side, where the sum of those factors is the total incident light.

The evaluation of current density was performed using EQE measurements, and we calculated a parameter called the bifaciality factor, BF (%), which is defined as the ratio between the value of J_sc when illuminated from rear side and its value when illuminated solely from the other side [72]. Table 7 presents these calculations for all EQE measurements conducted (Figure 22 and Figure 23).

As the EQE cannot be used to assess current values with illumination from both sides of the bifacial cell (Figure 24), the main conclusion here is that, in addition to achieving an increase in the bifaciality factor when the film is placed on the rear side (+20.6%) in the UV-Vis range, there is an overall increase in the bifaciality factor across the entire spectral range when films are placed on both sides (0.76%), even though some loss occurs in the UV-Vis range. These conditions are considered non-field working conditions. Different incidences should be studied in future works.

3.6.6.2. Second experiment

In this second experiment, we have encapsulated the compound used in the first experiment with EVA on another bifacial mini module. We present preliminary EQE results in Figure 25, measuring with normal incidence from both sides simultaneously, bm-m(4), and repeating the measurement when the DS layer is encapsulated on both sides, bm-m(5).

Once the increase in EQE obtained by encapsulating the DS film onto the mini module is observed, we have decided to conduct some experiments measuring the IV curves as detailed below. As mentioned earlier, it is not possible to extract a current value from the EQE measurement with illumination from both sides, as it would be a linear combination of the currents measured on each side rather than their sum.

In Figure 26, we present IV measurements with 1 Sun illumination first on the front side and then on the rear side, considering an effective area of 4 cm².

Table 8. Calculated values of the short circuit current I_sc , bifaciality factor BF , maximum power P_max, and the efficiency 𝜂 for one Sun of illumination and 4 cm² of area.

Table 8. Calculated values of the short circuit current I_sc , bifaciality factor BF , maximum power P_max, and the efficiency 𝜂 for one Sun of illumination and 4 cm² of area.

Device	Isc (mA) Front	Isc (mA) Rear	BF(%)	Pmax (mW) Front	Pmax (mW) Rear	η(%) Front	η(%) Rear
bm-m(4)	142.96	130.58	91.34	60.22	54.37	15.05	13.59
bm-m(5)	148.18	133.80	90.29	61.39	55.66	15.35	13.91

We also calculated another parameter called the separation rate, SR(%) , which is defined to quantitatively assess the difference between the sum of J_sc when illuminated from each side of the bifacial cell and J_sc when illuminated simultaneously from both sides [73]. We have studied the separation rate in a more realistic situation where the fraction of incident light from the one side of the bifacial cell is lower than in the other side. In Figure 27, we present IV measurements with an illumination of 0.3 Sun on the rear side and 0.3 Sun on the front side.

Table 9. Calculated values of the short circuit current I_sc , the separate rate SR, and the maximum power P_max, for 0.3 and 0.7 Sun of illuminations on front and rear sides, respectively, and 4 cm² of area.

Table 9. Calculated values of the short circuit current I_sc , the separate rate SR, and the maximum power P_max, for 0.3 and 0.7 Sun of illuminations on front and rear sides, respectively, and 4 cm² of area.

Device	Isc (mA) Front	Isc (mA) Rear	Isc (mA) Both	SR (%)	Pmax (mW) Front	Pmax (mW) Rear	Pmax (mW) Both
bm-m(4)	30.04	48.09	76.85	-1.6	10.66	18.20	30.66
bm-m(5)	28.95	51.13	79.53	-0.7	10.16	19.46	31.77

Just as the bifaciality factor gives the percentage of photogenerated current between the rear and front of the mini-module, the separate rate give us the percentage of current that separates the measured current when both sides are illuminated, from the sum of the photogenerated currents by the front and rear sides of the mini-module.

4. Conclusions

References

1. Griffiths, D.J.; Schroeter, D.F. Introduction to quantum mechanics, 3rd ed.; Cambridge: Cambridge University Press, United Kingdom, 2018; pp. 279–313. [Google Scholar] [CrossRef]
2. Tanner, P.A. Lanthanide Luminescence in Solids. In Lanthanide luminescence: photophysical, analytical and biological aspects; Hänninen, P., Härmä, H., Eds.; Springer Science & Business Media: Germany, 2011; Volume 7, pp. 183–214. [Google Scholar] [CrossRef]
3. Buenzli, J.C.G. On the design of highly luminescent lanthanide complexes. Coord. Chem. Rev. 2015, 293, 19–47. [Google Scholar] [CrossRef]
4. SeethaLekshmi, S.; Ramya, A. R.; Reddy, M. L. P.; Varughese, S. Lanthanide complex-derived white-light emitting solids: A survey on design strategies. J. Photochem. Photobiol. A. 2017, 33, 109–131. [Google Scholar] [CrossRef]
5. Monteiro, J.H.S.K. Recent advances in luminescence imaging of biological systems using lanthanide (III) luminescent complexes. Molecules 2020, 25(9), 2089. [Google Scholar] [CrossRef]
6. Wu, J.; Fujii, K.; Yashima, M.; Staykov, A.; Akbay, T.; Ishihara, T.; Kilner, J. A. A systematic evaluation of the role of lanthanide elements in functional complex oxides; implications for energy conversion devices. J. Mater. Chem. A. 2018, 6(25), 11819–11829. [Google Scholar] [CrossRef]
7. Bunzli, J.C.G.; Chauvin, A.S. Lanthanides in Solar Energy Conversion. In Handbook on the Physics and Chemistry of Rare Earths; Bunzli, J.C.G., Pecharsky, V.K., Eds.; Elsevier: Amsterdam, Netherlands, 2016; Volume 44, pp. 169–281. [Google Scholar] [CrossRef]
8. Zhang, Y.; Jia, X.; Liu, S.; Zhang, B.; Lin, K.; Zhang, J.; Conibeer, G. A review on thermalization mechanisms and prospect absorber materials for the hot carrier solar cells. Sol. Energy Mater Sol. Cells. 2021, 225, 111073. [Google Scholar] [CrossRef]
9. Wan, X.; Xu, Y.; Guo, H.; Shehzad, K.; Ali, A.; Liu, Y.; Luo, J. A self-powered high-performance graphene/silicon ultraviolet photodetector with ultra-shallow junction: breaking the limit of silicon? 2D Mater. Appl. 2017, 4, 1–8. [Google Scholar] [CrossRef]
10. Richards, B. S. Enhancing the performance of silicon solar cells via the application of passive luminescence conversion layers. Sol. Energy Mater Sol. Cells. 2006, 90(15), 2329–2337. [Google Scholar] [CrossRef]
11. Grzeszkiewicz, K.; Marciniak, L.; Stręk, W.; Hreniak, D. Downconversion in Y2Si2O7: Pr3+, Yb3+ polymorphs for its possible application as luminescent concentrators in photovoltaic solar-cells. J. Lumin. 2016, 177, 172–177. [Google Scholar] [CrossRef]
12. Hovel, H. J.; Hodgson, R.T.; Woodall, J.M. . The effect of fluorescent wavelength shifting on solar cell spectral response. Sol. Energy Mater Sol. Cells. 1979, 2(1), 19–29. [Google Scholar] [CrossRef]
13. Wegh, R. T.; Donker, H.; Oskam, K. D.; Meijerink, A. Visible quantum cutting in LiGdF4: Eu3+ through downconversion. Science 1999, 283(5402), 663–666. [Google Scholar] [CrossRef]
14. Dexter, D. L. Possibility of luminescent quantum yields greater than unity. Phys. Rev. 1957, 108(3), 630–633. [Google Scholar] [CrossRef]
15. Trupke, T.; Green, M.A.; Würfel, P. Improving solar cell efficiencies by up-conversion of sub-band-gap light. J. Appl. Phys. 2002, 92, 4117–4122. [Google Scholar] [CrossRef]
16. Badescu, V.; De Vos, A. Influence of some design parameters on the efficiency of solar cells with down-conversion and down shifting of high-energy photons. J. Appl. Phys. 2007, 102(7), 073102. [Google Scholar] [CrossRef]
17. Green, A. P.; Buckley, A.R. Solid state concentration quenching of organic fluorophores in PMMA. Phys. Chem. Chem. Phys. 2015, 17(2), 1435–1440. [Google Scholar] [CrossRef]
18. Yahia-Ammar, A.; Nonat, A. M.; Boos, A.; Rehspringer, J. L.; Asfari, Z.; Charbonnière, L. J. Thin-coated water soluble CdTeS alloyed quantum dots as energy donors for highly efficient FRET. Dalton Trans. 2014, 43(41), 15583–15592. [Google Scholar] [CrossRef]
19. Binnemans, K. Lanthanide-Based Luminescent Hybrid Materials. Chem. Rev. 2009, 109(9), 4283–4374. [Google Scholar] [CrossRef]
20. Dieke, G. H.; Crosswhite, H.M.; Dunn, B. Emission spectra of the doubly and triply ionized rare earths. J. Opt. Soc. Am. 1961, 51(8), 820–827. [Google Scholar] [CrossRef]
21. Peijzel, P. S.; Meijerink, A.; Wegh, R. T.; Reid, M. F.; Burdick, G. W. A complete 4fn energy level diagram for all trivalent lanthanide ions. J. Solid State Chem. 2005, 178(2), 448–453. [Google Scholar] [CrossRef]
22. Monzón-Hierro, T.; Sanchiz, J.; González-Pérez, S.; González-Díaz, B.; Holinski, S.; Borchert, D.; Guerrero-Lemus, R. A new cost-effective polymeric film containing an Eu (III) complex acting as UV protector and down-converter for Si-based solar cells and modules. Sol. Energy Mater Sol. Cells. 2015, 136, 187–192. [Google Scholar] [CrossRef]
23. González-Pérez, S.; Sanchiz, J.; Rodríguez, V. D.; Cañadillas-Ramallo, D.; González-Platas, J.; Borchert, D.; González-Díaz, B.; Hernández-Rodríguez, C.; Guerrero-Lemus, R. Highly luminescent film as enhancer of photovoltaic devices. J. Lumin. 2018, 201, 148–155. [Google Scholar] [CrossRef]
24. Guerrero-Lemus, R.; Sanchiz, J.; Sierra, M.; Martín, I. R.; Hernández-Rodríguez, C.; Borchert, D. Alternative and fully experimental procedure for characterizing down-shifters placed on photovoltaic devices. Sol. Energy Mater Sol. Cells. 2018, 185, 312–317. [Google Scholar] [CrossRef]
25. Guerrero-Lemus, R.; Sanchiz, J.; Sierra-Ramos, M.; Martín, I. R.; Hernández-Rodríguez, C.; Borchert, D. Downshifting maximization procedure applied to [Eu(bphen)(tta)3] at different concentrations applied to a photovoltaic device and covered with a hemispherical reflector. Sens. Actuators A: Phys. 2018, 271, 60–65. [Google Scholar] [CrossRef]
26. González-Díaz, B.; Saw, M. H.; Hernández-Rodríguez, C.; Sanchiz, J.; Khoo, Y. S.; Guerrero-Lemus, R. Evolution of the external quantum efficiency of Si-based PV minimodules with encapsulated down-shifters and aged under UV radiation. Mater. Sci. Eng. B. 2020, 261, 114763. [Google Scholar] [CrossRef]
27. González-Díaz, B.; Hernández-Rodríguez, C.; Cañadillas-Ramallo, D.; Sanchiz, J.; Guerrero-Lemus, R. Ageing evaluation of different polymeric Er 3+ downshifters exposed to harsh outdoor conditions. In 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC)(A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), Waikoloa, HI, USA, 2018, pp. 2882-2885. [CrossRef]
28. Brito-Santos, G.; Gil-Hernández, B.; Hernández-Rodríguez, C.; González-Díaz, B.; Guerrero-Lemus, R.; Sanchiz, J. Degradation analysis of highly UV-resistant down-shifting layers for silicon-based PV module applications. Mater. Sci. Eng. B. 2023, 288, 116207. [Google Scholar] [CrossRef]
29. González-Díaz, B.; Sierra-Ramos, M.; Sanchiz, J.; Guerrero-Lemus, R. Durability analysis of the [Eu(bphen)(tta)₃] down-shifter on Si-based PV modules exposed to extreme outdoor conditions. Sens. Actuators A: Phys. 2018, 276, 312–319. [Google Scholar] [CrossRef]
30. Brito-Santos, G.; Gil-Hernández, B.; Martín, I. R.; Guerrero-Lemus, R.; Sanchiz, J. Visible and NIR emitting Yb (iii) and Er (iii) complexes sensitized by β-diketonates and phenanthroline derivatives. RSC Adv. 2020, 10(46), 27815–27823. [Google Scholar] [CrossRef]
31. Brito-Santos, G.; Hernández-Rodríguez, C.; Gil-Hernández, B.; González-Díaz, B.; Martín, I. R.; Guerrero-Lemus, R.; Sanchiz, J. Highly luminescent mixed-ligand bimetallic lanthanoid (III) complexes for photovoltaic applications. Dalton Trans. 2022, 51(8), 3146–3158. [Google Scholar] [CrossRef]
32. Ho, W.-J.; Liu, J.-J.; Ke, B.-X. Characterization of Luminescent Down-Shifting Spectral Conversion Effects on Silicon Solar Cells with Various Combinations of Eu-Doped Phosphors. Materials 2022, 15(2), 452. [Google Scholar] [CrossRef]
33. Correia, S. F.; Bastos, A. R.; Fu, L.; Carlos, L. D.; André, P. S.; Ferreira, R. A. Lanthanide-based downshifting layers tested in a solar car race. Opto-electron. adv. 2019, 2(6), 190006-1. [CrossRef]
34. Le Donne, A.; Acciarri, M.; Narducci, D.; Marchionna, S.; Binetti, S. Encapsulating Eu3+ complex doped layers to improve Si-based solar cell efficiency. Prog. Photovolt.: Res. Appl. 2009, 17(8), 519–525. [CrossRef]
35. Leyre, S.; Proost, K.; Cappelle, J.; Durinck, G.; Hofkens, J.; Deconinck, G.; Hanselaer, P. Experimental validation of adding-doubling modeling of solar cells including luminescent down-shifting layers. J. Renew. Sustain. Energy. 2015, 7(4), 043130. [Google Scholar] [CrossRef]
36. Klampaftis, E.; Congiu, M.; Robertson, N.; Richards, B. S. Luminescent ethylene vinyl acetate encapsulation layers for enhancing the short wavelength spectral response and efficiency of silicon photovoltaic modules. IEEE J. Photovolt. 2011, 1(1), 29–36. [Google Scholar] [CrossRef]
37. Klampaftis, E.; Richards, B.S. Improvement in multi-crystalline silicon solar cell efficiency via addition of luminescent material to EVA encapsulation layer. Prog. Photovolt.: Res. Appl. 2011, 19(3), 345–351. [CrossRef]
38. Sayed, H.; Matar, Z. S.; Al-Dossari, M.; Amin, A. F.; Abd El-Gawaad, N. S.; Aly, A. H. The design and optimization of an anti-reflection coating and an intermediate reflective layer to enhance tandem solar cell photons capture. Crystals 2022, 12(1), 57. [Google Scholar] [CrossRef]
39. Leyre, S.; Cappelle, J.; Durinck, G.; Abass, A.; Hofkens, J.; Deconinck, G.; Hanselaer, P. The use of the adding-doubling method for the optical optimization of planar luminescent down shifting layers for solar cells. Opt. Express. 2014, 22(103), A765–A778. [Google Scholar] [CrossRef] [PubMed]
40. Mohan, B. V.; Mayandi, J.; Muniasamy, K.; Veerapandy, V. Assessment of the design efficacy of Eu2+ ion embedded thick scattering layers for operational photovoltaics. Appl. Surf. Sci. Adv. 2021, 4, 100067. [Google Scholar] [CrossRef]
41. McIntosh, K. R.; Lau, G.; Cotsell, J. N.; Hanton, K.; Bätzner, D. L.; Bettiol, F.; Richards, B. S. Increase in external quantum efficiency of encapsulated silicon solar cells from a luminescent down-shifting layer. Prog. Photovolt.: Res. Appl. 2009, 17(3), 191–197. [CrossRef]
42. Huang, X.; Han, S.; Huang, W.; Liu, X. (2013). Enhancing solar cell efficiency: the search for luminescent materials as spectral converters. Chem. Soc. Rev. 2013, 42(1), 173–201. [CrossRef]
43. Farah Khaleda, M.Z.; Vengadaesvaran, B.; Rahim, N.A. Spectral response and quantum efficiency evaluation of solar cells: A review. In Energy Materials, Dhoble, S.J., Kalyani, N.T., Vengadaesvaran, B., Arof, A.K. Eds.Elsevier: Amsterdam, Netherlands, 2021, pp. 525–566. [CrossRef]
44. De la Mora, M. B.; Amelines-Sarria, O.; Monroy, B. M.; Hernández-Pérez, C. D.; Lugo, J. E. Materials for downconversion in solar cells: Perspectives and challenges. Sol. Energy Mater Sol. Cells. 2017, 165, 59–71. [Google Scholar] [CrossRef]
45. Satpute, N. S.; Mehare, C. M.; Tiwari, A.; Swart, H. C.; Dhoble, S. J. Synthesis and Luminescence Characterization of Downconversion and Downshifting Phosphor for Efficiency Enhancement of Solar Cells: Perspectives and Challenges. ACS Appl. Electron. Mater. 2022, 4(7), 3354–3391. [Google Scholar] [CrossRef]
46. Lian, H.; Hou, Z.; Shang, M.; Geng, D.; Zhang, Y.; Lin, J. Rare earth ions doped phosphors for improving efficiencies of solar cells. Energy. 2013, 57, 270–283. [Google Scholar] [CrossRef]
47. Fix, T.; Ferblantier, G.; Rinnert, H.; Slaoui, A. Evaluation of the effective quantum efficiency of photon conversion layers placed on solar cells. Sol. Energy Mater Sol. Cells. 2015, 132, 191–195. [Google Scholar] [CrossRef]
48. Kennedy, M.; Ahmed, H.; Doran, J.; Norton, B.; Bosch-Jimenez, P.; Pirriera, M. D.; Puigdollers, J. Large Stokes shift downshifting Eu (III) films as efficiency enhancing UV blocking layers for dye sensitized solar cells. Phys. stat. sol. (a). 2015, 212(1), 203–210. [Google Scholar] [CrossRef]
49. Wei, C.; Ma, L.; Wei, H.; Liu, Z.; Bian, Z.; Huang, C. Advances in luminescent lanthanide complexes and applications. Sci. China Technol. Sci. 2018, 61, 1265–1285. [Google Scholar] [CrossRef]
50. Lopez-Delgado, R.; Zhou, Y.; Zazueta-Raynaud, A.; Zhao, H.; Pelayo, J. E.; Vomiero, A.; Ayon, A. Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots. Sci. Rep. 2017, 7(1), 14104. [Google Scholar] [CrossRef]
51. Dumont, L.; Cardin, J.; Labbé, C.; Frilay, C.; Anglade, P. M.; Yu, I. S.; Gourbilleau, F. First down converter multilayers integration in an industrial Si solar cell process. Prog. Photovolt.: Res. Appl. 2019, 27(2), 152–162. [CrossRef]
52. Debnath, G. H.; Mukherjee, P.; Waldeck, D. H. Optimizing the key variables to generate host sensitized lanthanide doped semiconductor nanoparticle luminophores. J. Phys. Chem. C. 2020, 124(49), 26495–26517. [Google Scholar] [CrossRef]
53. Tawalare, P. K. Optimizing photovoltaic conversion of solar energy. AIP Adv. 2021, 11, 100701. [Google Scholar] [CrossRef]
54. Devi, R.; Rajendran, M.; Singh, K.; Pal, R.; Vaidyanathan, S. Smart luminescent molecular europium complexes and their versatile applications. J. Mater. Chem. C. 2021, 9(20), 6618–6633. [Google Scholar] [CrossRef]
55. He, Y.; Liu, J.; Sung, S. J.; Chang, C. H. Downshifting and antireflective thin films for solar module power enhancement. Mater. Des. 2021, 201, 109454. [Google Scholar] [CrossRef]
56. Li, P.; Li, H. Recent progress in the lanthanide-complexes based luminescent hybrid materials. Coord. Chem. Rev. 2021, 441, 213988. [Google Scholar] [CrossRef]
57. Down-conversion materials for organic solar cells: Progress, challenges, and perspectives. [CrossRef]
58. Yang, L.; Luo, J.; Gao, L.; Song, B.; Tang, J. Inorganic Lanthanide Compounds with f–d Transition: From Materials to Electroluminescence Devices. J. Phys. Chem. Lett. 2022, 13(19), 4365–4373. [Google Scholar] [CrossRef]
59. Sekar, R.; Ravitchandiran, A.; Angaiah, S. Recent Advances and Challenges in Light Conversion Phosphor Materials for Third-Generation Quantum-Dot-Sensitized Photovoltaics. ACS Omega. 2022, 7(40), 35351–35360. [Google Scholar] [CrossRef] [PubMed]
60. Yu, D.; Yu, T.; Lin, H.; Zhuang, S.; Zhang, D. Recent advances in luminescent downconversion: new materials, techniques, and applications in solar cells. Adv. Opt. Mater. 2022, 10(12), 2200014. [Google Scholar] [CrossRef]
61. Galico, D. A.; Calado, C.; Murugesu, M. Lanthanide Molecular Cluster-Aggregates as the Next Generation of Optical Materials. Chem. Sci. 2023. [Google Scholar] [CrossRef] [PubMed]
62. Zhang, J.; Ming, C.; Song, F. Spectrum conversion by luminescent glass for improving efficiency of solar cells. J. Non-Cryst. Solids. 2023, 606, 122217. [CrossRef]
63. GM Sheldrick, G.M. Crystal structure refinement with SHELXL. Acta Cryst. 2015, C71, 3–8. [Google Scholar] [CrossRef]
64. American Society for Testing and Materials. Standard tables for Reference solar spectral irradiances: Direct Normal and hemispherical on 37° tilted surface. Standard G173-03. 2020. [CrossRef]
65. Blieske, U.; Stollwerck, G. Glass and Other Encapsulation Materials. In Semiconductors and Semimetals; Academic Press Inc., 2013; Vol. 89, pp 199–258. [CrossRef]
66. Sun, Q.; Yan, P.; Niu, W.; Chu, W.; Yao, X.; An, G.; Li, G. NIR luminescence of a series of benzoyl trifluoroacetone erbium complexes. RSC Adv., 2015, 5, 65856–65861. [Google Scholar] [CrossRef]
67. Gavriluta, A.; Fix, T.; Nonat, A.; Slaoui, A.; Guillemoles, J. F.; Charbonnière, L. J. EuIII-Based Nanolayers as Highly Efficient Downshifters for CIGS Solar Cells. Eur. J. Inorg. Chem. 2017, 44, 5318–5326. [Google Scholar] [CrossRef]
68. González-Pérez, S.; Martín, I.R.; Lahoz, F.; HAro-González, P.; Herreros, J. Local crystallization in an oxyfluoride glass doped with Er³⁺ ions using a continuous argon laser. Appl. Phys. A. 2008, 93, 983–986. [Google Scholar] [CrossRef]
69. Alexander, O.T.; Kroon, R.E.; Brink, A.; Visser, H.G. Symmetry correlations between crystallographic and photoluminescence study of ternary β-diketone europium (III) based complexes using 1, 10-phenanthroline as the ancillary ligand. Dalton Trans. 2019, 48(42), 16074–16082. [Google Scholar] [CrossRef] [PubMed]
70. E Silva, F.G.; Menezes, J.F.S.; Rocha, G.B.; Alves, S.; Brito, H.F.D.; Longo, R.L.; Malta, O.L. Emission quantum yield of europium (III) mixed complexes with thenoyltrifluoroacetonate and some aromatic ligands. J. Alloys Compd. 2000, 303, 364–370. [Google Scholar] [CrossRef]
71. Liu, J.; Wang, K.; Zheng, W.; Huang, W.; LI, C.-H.; You, X.-Z. Improving spectral response of monocrystalline silicon photovoltaic modules using high efficient luminescent down-shifting Eu³⁺ complexes. Prog. Photovoltaics: Res. Appl. 2013, 21, 668–675. [Google Scholar] [CrossRef]
72. Liang, T. S.; Pravettoni, M.; Deline, C.; Stein, J. S.; Kopecek, R.; Singh, J. P.; Khoo, Y. S. A review of crystalline silicon bifacial photovoltaic performance characterisation and simulation. Energy Environ. Sci. 2019, 12(1), 116–148. [Google Scholar] [CrossRef]
73. Ohtsuka, H.; Sakamoto, M.; Koyama, M.; Tsutsui, K.; Uematsu, T.; Yazawa, Y. Characteristics of bifacial solar cells under bifacial illumination with various intensity levels. Prog. Photovoltaics: Res. Appl. 2001, 9(1), 1–13. [CrossRef]