Multiligand Europium Complexes Incorporated Polyvinylpyrrolidone for Enhanced Solar Cell

Intense reddish-orange fluorescence of Eu is observed in multiligand europium complexes Eu(BA)(TTA)2 and Eu(BA)(TTA)2Phen doped polyvinylpyrrolidone (PVP). Under 367 nm UVA-LED pumping with 708.5 μW, the net emission powers are as high as 204.8 and 243.2 μW, and the total emission photon numbers are derived to be 635.9×10 and 754.9×10 cps for Eu(BA)(TTA)2 and Eu(BA)(TTA)2Phen doped PVP thin films, respectively. It is incredible that the quantum yield (QY) is up to 57.87% in the former; furthermore, as the introduction of Phen, the QY of Eu is increased to 62.03%, verifying the effectiveness of multiligands in photon conversion. Conclusive photon quantification and efficient fluorescence emission reveal the potential of europium complexes/PVP as UV-visible conversion layers for enhanced solar cells.


Introduction
Energy harvesting by means of the photovoltaic system has attracted extensive attention, while the inadequate utilization of ultraviolet (UV) radiation restricts application and development for solar cells [1][2][3].In order to solve this problem, the photoluminescence materials are introduced into solar cells as photonic conversion layers [4][5][6][7][8][9][10].As is well known, the fluorescent materials are able to absorb the short-wavelength photons and convert them into longwavelength photons with better spectral responsiveness for solar cells [11][12][13].Proceeding from this angle, rare-earth europium complexes can be introduced into solar cells with a significant impact on the photoelectric conversion efficiency [14][15][16][17].Consequently, the optical conversion materials are capable of absorbing UV radiation effectively, which are worthy of further research and development.
Compared to other photoluminescence materials, rareearth complexes have high fluorescence quantum efficiency and extremely narrow emission bands [18][19][20].Moreover, the fluorescence of rare-earth complexes can be enhanced by synthesizing a stable complex with a suitable organic ligand.In the rare-earth organic complexes, the composite of β-diketone and carboxyl possesses better stability and higher fluorescence efficiency than those of single-ligand complexes [21][22][23].Furthermore, the combination of europium complexes and organic polymers has good thermal properties, which is favorable in solar cells [24][25][26][27].Herein, as a typical amphiprotic organic polymer, polyvinylpyrrolidone (PVP) is introduced into rare-earth complexes with low toxicity, excellent chemical stability, and good compatibility [28][29][30][31].Hence, europium complexes/PVP as the emitters in organic photoluminescence devices is a potential candidate as an UV-visible conversion layer for enhanced solar cells, and the schematic diagram is proposed and depicted in Figure 1.
In this work, the europium complexes/PVP luminescence films with high absorption efficiency and intense fluorescence emission have been synthesized.e brilliant reddish-orange fluorescence indicates the validity of energy transfer from multiligands to Eu 3+ through intramolecular energy transfer.Under UVA-LED radiation, the absorption and emission photon numbers are obtained, and the quantum yields are estimated as high as 57.87% and 62.03% for Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP, respectively.e multiligand europium complexes doped PVP thin film protects the solar cells components on the one hand and improves the photon conversion e ciency on the other hand, which is prospective as UV-visible conversion layers for enhanced solar cells.

Experimental
Ternary complex Eu(BA)(TTA) 2 and quaternary complex Eu(BA)(TTA) 2 Phen were prepared by the chemical synthesis using benzoic acid (BA), thenoyltri uoroacetone (TTA), and 1,10-phenanthroline (Phen).For example, according to Eu(BA)(TTA) 2 Phen chemical formula, initially, EuCl 3 , BA, TTA, and Phen with the molar ratio of 1 : 1 : 2 : 1 were dissolved in 30 ml 99.7% ethanol and placed in a water bath at 50 °C, and then, sodium hydroxide (NaOH) solution was dripped to the pH value of 6-7.Instantaneously, the white precipitate of Eu(BA)(TTA) 2 Phen was formed, which was ltered, washed, and dried at room temperature.In the fabrication process of europium complexes doped PVP, 1 g PVP powder was dissolved in 0.95 g N, N-dimethylformamide (DMF) solution and 10 ml ethyl alcohol solution followed by the addition of 0.01 g Eu(BA)(TTA) 2 and 0.01 g Eu(BA)(T-TA) 2 Phen, respectively.e well-mixed solutions were heated at 50 °C for 1 hour using a thermostat water bath, and the samples were obtained by evaporating the solvent slowly.Finally, the resultant solution was deposited on the quartz sheet and dried to obtain transparent lms.
Fourier-transform infrared (FT-IR) spectra were measured at room temperature by a PerkinElmer FT-IR/NIR spectrometer in the range of 4000-400 cm −1 .e elemental analysis was carried out by energy dispersive X-ray spectroscopy (EDS) at an accelerating voltage of 5 kV.e emission and excitation spectra were taken on a Hitachi F-7000 uorescence spectrophotometer equipped with a photomultiplier tube (PMT) as a detector and a commercial CW Xe-lamp as a pump source.Transmittance spectra were recorded by a UV-1600 spectrophotometer.e di erential scanning calorimetry (DSC) and the thermogravimetric analysis (TGA) were carried out by the American TA company SDT 600 at a heating rate of 20 °C/min under a dynamic nitrogen atmosphere.By the Metricon 2010 prism coupler, the refractive indices of the Eu(BA)(TTA) 2 and Eu(BA)(TTA) 2 Phen doped PVP were measured to be 1.5215 and 1.5256 at 635.96 nm, and 1.5091 and 1.5115 at 1546.9 nm, respectively.e refractive indices of other wavelengths can be reckoned by Cauchy equation n (A + B)/λ 2 with A 1.5066 and B 6035.2 nm 2 for Eu(BA)(TTA) 2 doped PVP, and A′ 1.5086 and B′ 6862.6 nm 2 for Eu(BA)(TTA) 2 Phen doped PVP.All of the spectral power distributions of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2- Phen/PVP were carried out in an integrating sphere of 25 cm inner diameter (Labsphere) which was connected to a QE65000 and a USB4000 standard CCD detector (Ocean Optics) with a 600 μm-core optical ber.e currents of the exciting 308 nm UVB and 367 nm UVA light emitting diode were xed at 20 mA with reference voltages 5.780 and 3.607 V, respectively.e samples covered the upper part of LED completely, and the measurement system is calibrated with a standard halogen lamp (Labsphere, SCL-050).

Structural Analysis of Multiligand Europium Complexes.
e structures of Eu(BA)(TTA) 2 , Eu(BA)(TTA) 2 Phen, Eu(BA)(TTA) 2 /PVP, Eu(BA)(TTA) 2 Phen/PVP, and PVP are measured in the 4000-400 cm −1 region by FT-IR spectroscopy, as shown in Figure 2.Such as the case of Eu(BA)(TTA) 2 Phen (curve 2), after the coordination formation between organic ligand and Eu 3+ , the carbonyl group (-C O) stretching vibration absorption peak of TTA has a redshift from 1680 to 1605 cm −1 [32,33], which is caused by the reduction of bond length and bond energy of the covalent bond because the isolated electrons transfer from part of the oxygen to the outer layer of a hollow orbit in Eu 3+ .In addition, carbonyl is conjugated with Eu 3+ after the deprotonation, subsequently forming a stable chelating ring, and then the conjugate e ect of the system makes the density of π-electron cloud more uniform; therefore, absorption frequency is decreased, and polarity of the covalent bonds is weakened [34,35].
e peaks located in the range of 1304-1357 and 1538-1569 cm −1 belong to symmetric and antisymmetric stretching vibrations of carboxyl, respectively, indicating the carboxylic acid of BA removes the proton and coordinates with the Eu 3+ ions.Compared with Eu(BA)(TTA) 2 (curve 1), the absorption peak shifts from 1545 to 1538 cm −1 due to the introduction of the third ligand suggesting the energy required for vibration is higher, and the group is more stable.And the noncharacteristic absorption C N stretching vibration peak of Phen appears a slight redshift from 1422 to 1415 cm −1 , demonstrating that the lone pair of electrons on the two N atoms of the Phen coordinates with the Eu 3+ to form the chelate ring [33].Moreover, the spectra of PVP-loaded composite lms have changed signi cantly in contrast to powders, and the corresponding absorption peaks of the complex powders are weakened due to the overlapping of PVP vibration peaks. 2 Advances in Materials Science and Engineering PVP (1663 cm −1 ) occurs remarkably in contrast to purephased PVP (1648 cm −1 ), which further con rms the e ects of dopants on the FT-IR spectra of PVP.
For the sake of examining the elemental composition in the lms, the EDS spectra of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are obtained, as shown in Figures 3(a) and 3(b), respectively.Both of them show that the C, O, N, Eu, S, and F elements existed in europium complex originate from BA, TTA, and Phen ligands, which provides a basis for the synthesis of the europium complexes doped PVP composites.Meanwhile, the schematic diagram based on the molecular formula of Eu(BA)(TTA) 2 and Eu(BA)(TTA) 2 Phen is illustrated in Figure 3.In Eu(BA)(TTA) 2 Phen, the center Eu 3+ connects to three ligands (BA, TTA, and Phen), among which Phen as a synergic shielding ligand enhances the luminescence intensity of the complexes [36,37].e presence of the surrounding ligand acts as a physical bu er between the Eu 3+ and other ions.

Fluorescence Behaviors of Multiligand Europium Complexes/PVP. Figures 4(a) and 4(b)
show the emission spectra of Eu(BA)(TTA) 2 and Eu(BA)(TTA) 2 Phen powders under the excitation of 278 and 366 nm, both of them exhibit the typical emission peaks of Eu 3+ located at 578, 591, 615, 651, and 701 nm, which are ascribed to the Eu 3+ transitions from the 5 D 0 to 7 F J (J 0-4) [38,39], respectively.Among them, the 5 D 0 ⟶ 7 F 2 electric dipole transition has the strongest relative uorescence intensity, which reveals the better monochromaticity of europium complexes.e excitation spectra of multiligand europium complexes monitored at 615 nm are depicted in Figure 4(c), and a broad band almost covers the whole ultraviolet region, which mainly corresponds to π ⟶ π * electronic transition of the ligands.Meanwhile, the excitation of coligand Phen exists in the long-wavelength region, indicating that the introduction of the Phen enlarges the range of conjugated π ⟶ π * and facilitates energy transfer.
In order to facilitate the application in solar cells, the preparation of Eu(BA)(TTA) 2 and Eu(BA)(TTA) 2 Phen doped PVP lms is carried out, and the emission and excitation spectra are shown in Figure 5, respectively.e broad band from ligand at about 310 nm is mainly attributed to the transition of π ⟶ π * of the BA under 278 nm excitation.Simultaneously, the larger emission intensity of Eu(BA)(TTA) 2 Phen/PVP at 366 nm can be explained in accordance with the introduction of Phen as a synergistic screening ligand for BA and TTA.Bright red uorescence is observed, and the e ect of Phen is recon rmed upon enhanced emission, indicating the availability of conversion from UV to visible photons.Figure 5(c) reveals the excitation spectra of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2- Phen/PVP by monitoring the ligand emission at 381 nm, respectively.It can be e ectively activated within the scope of 200-360 nm, and the optimum excitation wavelength locates at 278 nm.As shown in Figure 5(d), an excited broad band of europium complexes/PVP in the UV region indicates the e ective absorption of the organic ligands.
Taking an example of quaternary complex, in order to clearly show the crosslink interaction between the europium complexes and PVP, the normalized excitation spectra of Eu(BA)(TTA) 2 Phen and Eu(BA)(TTA) 2 Phen/PVP monitored at 615 nm are depicted in Figure 6.It can be seen that the excitation peak located at 373 nm shows a slight blue shift and the width of excitation peak narrows in the Eu(BA)(TTA) 2 Phen/PVP lms.Moreover, the uorescence intensity decreases signi cantly in the short-wavelength range, which indicates the strong interaction between PVP and rare-earth complex with the occurrence of energy transfer.Meanwhile, the two broad bands further con rm that both Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/ PVP can be stimulated under the entire UV range, and the latter is more e cient.
e reason for the intensity increment is that the cooperative ligand Phen improves the conjugation and the electron delocalization of the whole system, which facilitates the absorption and the utilization of UV radiation [40][41][42].
In rare-earth complexes, energy transfer e ciency mainly depends on the energy level matching between the excited state of rare-earth ions and the triplet state of the ligand, and the better the energy level matching, the stronger the uorescence emitting [43].e possible energy transfer mechanisms of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are portrayed in Figure 7.In Eu(BA)(TTA) 2 Phen/PVP, the ligands absorb the energy from the ground state to the rst two lowest energy singlet excited states S 0 ⟶ S 1 and S 0 ⟶S 2 under 278 and 366 nm excitation.As shown in Figure 7(b), the energy ows from BA-S 1 to Phen-S 1 , then to TTA-S 2 , and internal conversion to TTA-S 1 under 278 nm excitation.Because the energy levels of Phen-S 1 and TTA-S 2 are close, the  resonance is allowed to transit from S 1 to S 2 , and vice versa.Subsequently, partial energy of TTA is released in the form of fluorescence (FL), and the other is transferred to its triplet excited state through intersystem crossing (ISC) and then released FL.In addition, under the excitation of 366 nm, TTA gets directly excited to S 1 due to changes in the circumstance, and some energy is transferred to TTA-T 1 through ISC and others to Phen-T 1 .Finally, the energy transfers from the ligands to the resonantly excited level 5 D 2 of Eu 3+ ion [11,[44][45][46][47][48]]. e 5 D 0 level is populated by the nonradiative relaxation (NRR); afterward, the radiative transition takes place accompanying the release of visible emissions.

Absolute Spectra and Radiometric Flux Quantitation.
To accurately reflect the fluorescence performance of multiligand europium complexes/PVP thin films, the absolute spectral parameters were determined which provides an external quantum yield to assess luminescence and laser materials.Spectral power distributions of Eu(BA)(TTA) 2 / PVP and Eu(BA)(TTA) 2 Phen/PVP are recorded using an integrating sphere with a 367 nm UVA-LED as the pump source when the sample on the quartz sheet is located on the top of the LED pump source.By removing the influence of the quartz sheet, the net spectral power distributions of europium complexes/PVP are obtained, as depicted in  e photon distribution provides the fundamental information in optical eld and relevant applications.Based on the net spectral power distributions, the photon distributions can be derived by where v, λ, h, c, and P(λ) represent wavenumber, wavelength, Planck constant, vacuum light velocity and spectral power distribution, respectively.e net absorption and emission photon distributions of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are calculated by equation ( 1) and depicted in Figure 9, and the integrated values are listed in Table 1.Meanwhile, the total emission photon numbers are derived to be 635.9× 10 12 and 754.9 × 10 12 cps for Eu(BA)(TTA) 2 and Eu(BA)(TTA) 2 Phen doped PVP thin lms, respectively.Quantum yield (QY) is used to re ect the utilization of luminescent materials to the absorption of photons and is de ned as the photon number ratio of emission and absorption [49].Namely,

QY
emitted photons absorbed photons All the QY values for di erent emission peaks of europium complexes/PVP under the excitation of the 367 nm UVA-LED are listed in Table 1.
e total QYs of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are derived to be 57.87% and 62.03%, respectively, which are far    higher than those of 9.34% in Eu(DBM) 3 Phen/PMMA [50], 18% in Eu(DBM) 3 PIP/PVP [51], and 36% Eu(DBM-D) 3 (Bath)/PMMA [52].e large QYs re ect the e ectiveness of the photon utilization in the multiligand europium complexes.In the table, the absolute quantum e ciency is accordingly changed with the introduction of Phen.Due to the low inversion symmetry at Eu 3+ ion sites, the mixture of uneven ligand eld components and the electric dipole transitions is no longer strictly forbidden in the ligand elds, resulting in faster electron transition radiation rates.Luminous ux refers to the amount of radiant ux that is evaluated according to the international standard of human visual characteristics [53].e total luminous ux Φ V of the luminescence can be calculated by where V(λ) is the relative eye sensitivity and K m is the maximum luminous e cacy at 555 nm (683 lm/W).e luminous e cacy is obtained by dividing total luminous ux Φ V by the electric power (W) on the LED.ereupon, the luminous ux distributions of the Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP lms can be obtained under the excitations of 308 nm UVB-LED and 367 nm UVA-LED, as presented in Figure 10, and the relevant total luminous uxes Φ V and luminous e cacy in the whole visible spectral region are listed in Table 2.
Under the excitation electric power of 115.61 mW for 308 nm UVB-LED, the total luminous uxes Φ V of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are integrated to be 13.20 and 16.60 mlm by equation ( 3) and luminous e cacy get 11.4% and 14.4% lm/W, respectively, which are higher than 2.63 mlm and 2.27% lm/W in TAH/ PMMA [54].Under the excitation electric power of 72.14 mW for 367 nm UVA-LED, the luminous uxes of residual LEDs are 0.33 × 10 −2 and 0.25 × 10 −2 mlm, and its role is negligible in Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP, respectively.e total luminous uxes of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are as high as 44.55 and 52.34 mlm, and luminous e cacy are calculated to 61.8% and 72.6% lm/W when excited by 367 nm UVA-LED, respectively.As the excitation wavelength changes, both of luminous ux values and luminous e cacy show upward tendency due to the enhancement of conversion uorescence intensity.Moreover, the 367 nm UVA-LED is con rmed as a more e ective excitation device for the sample, and the higher luminous e cacy reveals that the multiligand europium complexes have good luminescence ability under UV radiation.
Judd−Ofelt (J-O) intensity parameters Ω t (t 2, 4, 6) are used to evaluate rare-arth ion interaction with the substrate, which re ect some characteristics of the local coordination eld and estimate the population of odd parity electron transitions [55,56].Since the Eu 3+ has limited absorption transitions and the special energy level structure, radiative transition properties of Eu 3+ in Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are derived from their emission behaviors [57].e Ω t (t 2, 4, and 6) are determined based on emission photon numbers and listed in Table 3, and the  .579× 10 −20 cm 2 , respectively.In particular, Ω 2 is more sensitive to the asymmetry and the sequence of ligand elds, which strongly depends on the local environments of rareearth ions, such as high inversion asymmetrical and strong covalent environments.In addition, the Ω 2 of Eu(BA)(T-TA) 2 Phen/PVP is smaller than that of Eu(BA)(TTA) 2 /PVP, which con rms that the covalence degree of Eu 3+ in the rst coordination shell tiny decreases after the addition of Phen [58,59].e J-O intensity parameters Ω t , spontaneous transition probabilities A ij , branching ratios β ij , and calculated radiative lifetimes τ rad are calculated and listed in Table 4. e A ij of 5 D 0 ⟶ 7 F 2 transition are up to 954.7 s −1 and 942.6 s −1 in Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP, respectively, and the β ij are also found as high as 83.99% and 82.94%, respectively, which are stronger than other transitions indicating that the 5 D 0 ⟶ 7 F 2 transition is e cient and dominant in europium complexes doped PVP thin lms.ese results further con rm the suitability of multiligand europium complexes/PVP as UV-visible conversion layers for enhanced solar cells.
Stimulated emission cross section σ em is one of the most basic physical quantities to evaluate the energy extraction e ciency from rare-earth doped optical materials, not only for a fundamental understanding of the excitation and depopulation process but also for the optimization of laser materials, which is determined through the Fuchtbauer-Ladenburg equation [60]: where n, A rad , and N(λ) represent the refractive index, spontaneous transition probability, and emission photon distribution, respectively.As is clearly seen in Figure 11, there are four visible emission bands, whose peaks are located at 592, 615, 701, and 810 nm corresponding to the 5 D 0 ⟶ 7 F J (J � 1, 2, 4, 6) transitions of Eu 3+ , respectively.e maximum values σ em-max for 5 D 0 ⟶ 7 F 2 transitions of Eu 3+ in Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP peaking at 615 nm both are derived to be 7.90 × 10 −21 cm 2 , indicating both of them have approximate emission capability.As for the increment of fluorescence intensity, which can be interpreted that the addition of Phen leads to the combination of the heterogeneous ligand field and the antiparity of the 4f configuration levels, resulting in the intense absorption property and the prominent radiation behavior [61].e large emission cross section enables the europium complexes/PVP to be a promising material for photon conversion materials.

Optical Absorbability and ermal
Stability.e transmission spectra and the images of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are depicted in Figure 12(a), respectively.Both of them show up to 95% transparency at 350-800 nm and the broad band at 200-350 nm exhibits an intense absorbability of ligands, indicating the PVP is a suitable matrix for doping rare-earth complexes into photon conversion materials.
e optical absorption exhibits excellent absorption properties for multiligand europium complexes/PVP upon UV radiation, as depicted in Figure 12(b).As for the Eu(BA)(TTA) 2 /PVP, the absorption of BA at 234 and 273 nm is primarily attributed to the n ⟶ σ * transitions of E absorption band and π ⟶ π * transitions of B absorption band [62,63], respectively, and the strong absorption of TTA at 350 nm is assigned to the n ⟶ π * electron transition.With the addition of Phen, the energy transfer is more effective; meanwhile, the effect of Phen is reconfirmed; that is, Phen has no contribution in increasing the emission section while enhances the absorption of the UV. e absorption coefficient curve is nearly unchangeable after 400 nm, which implies that the ligands can effectively transfer the excited state energy to the emission state of Eu 3+ through the nonradiated transition, thus sensitizing luminescence and making up for the small absorption coefficient of Eu 3+ in the UV region [64].And the multiligand europium complexes are put into a dark room with a hole for guiding sunlight through a glass tube and attached with foil in the inner wall to avoid energy losses.Meanwhile, the bright red fluorescence of Eu(BA)(TTA) 2 / PVP and Eu(BA)(TTA) 2 Phen/PVP exhibits an effective response to the sunlight, as shown in Figure 12(b).Advances in Materials Science and Engineering ermodynamic property is essential to europium complexes/PVP for practical applications as optical materials; therefore, the DSC-TGA analyses of Eu(BA)(TTA) 2 / PVP and Eu(BA)(TTA) 2 Phen/PVP have been carried out to explore their degradation behavior, as illustrated in Figure 13.e TGA curves of Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP are similar, and both of them show 5% weightlessness at 100 °C, which is ascribed to the water detach from the samples with an obvious endothermic peak.Furthermore, in the 200-400 °C temperature ranges, an exothermic peak appears due to the sample melting.Besides, the curve reveals that the major weightlessness events occur at the temperature interval (400-500 °C), implying the thermal decomposition behavior of PVP appears with the emergence of endothermic peaks as well.e DSC curve tends to a constant at 470-600 °C, indicating the decomposition of the coordination compound is complete.e results further confirm that the composition of the Eu(BA)(TTA) 2 and Eu(BA)(TTA) 2 Phen doped PVP has excellent thermal stability for the research and development of electronic devices.

Conclusions
e films of Eu(BA)(TTA) 2 and Eu(BA)(TTA) 2 Phen doped polyvinylpyrrolidone (PVP) have been synthesized, and the bright red fluorescence is observed under ultraviolet radiation.Absorption and emission spectra confirm that the composite material with good thermodynamic stability can convert ultraviolet radiation into visible light efficiently and suggest the interaction between PVP and Eu 3+ .Under 367 nm UVA-LED pump source, the highest emission powers, emission photon numbers, and luminous efficacy are derived to be 204.8 and 243.2 μW, 635.9 × 10 12 and 754.9 × 10 12 cps, and 61.8% and 72.6% lm/W for Eu(BA)(TTA) 2 /PVP and Eu(BA)(TTA) 2 Phen/PVP thin films, which verifies the effective utilization of UVA radiation by introducing multiligands.Simultaneously, the quantum yields are up to 57.87% and 62.03% for without and with Phen, respectively.Meanwhile, the J-O intensity parameter Ω 2 of Eu(BA)(TTA) 2 /PVP is higher than Eu(BA)(TTA) 2 Phen/PVP, confirming that the local asymmetry around Eu 3+ increases after the addition of Phen.Efficient photon emission demonstrates the availability of multiligand europium complexes as an attractive UV radiation conversion material for enhanced solar cells.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.Advances in Materials Science and Engineering )(TTA) 2 Eu(BA)(TTA) 2 Phen Eu(BA)(TTA) 2