Visible Quantum Cutting in NaGdF4:Eu Nanocrystals for Enhancing Solar Cell Efficiency

Nonlinear Phenomena in Complex Systems, vol. 28, no. 3 (2025), pp. 242 - 247 Visible Quantum Cutting in NaGdF4:Eu Nanocrystals for Enhancing Solar Cell Efficiency Evgeny O. Mityushkin,1, ∗Artemi G. Shmelev,1, †Ramilya M. Gataullina,1Andrey V. Leontyev,1, †Larisa A. Nurtdinova,1, †Dmitry K. Zharkov,1, †and Victor G. Nikiforov1, † 1Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Kazan, RUSSIA (Received 1 November, 2024) NaGdF4:Eu nanocrystals are known as quantum cutting systems, that can convert UV radiation into the visible range with a quantum yield close to 200%. Typically, 202 nm photons are used to excite the 6GJlevels of Gd3+ ions. However, this mechanism has drawbacks due to low absorption efficiency at 202 nm and the relatively large amount of absorbed energy, which dissipates through non-radiative transitions. Additionally, the absence of vacuum ultraviolet radiation in sunlight renders this quantum cutting method useless for designing any type of sunlight convertors. In this study, we demonstrate the ability of NaGdF4:5%Eu to convert 250 nm UV light into visible emission of Er3+ ions. We employ an alternative quantum cutting approach based on cross-relaxation between Er3+ ions, which exhibits a quantum yield of 188%. This technique offers promising prospects in the field of green photovoltaics. PACS numbers: 42.65.-k, 42.70.-a, 78.55.-m, 78.67.-n, 88.40.-j Keywords: Cross relaxation, Energy transfer, Fluorides, Green photovoltaics, Hydrothermal synthesis, Luminescence, NaGdF4:Eu, Nanocrystals, Nanophosphors, Quantum cutting effect, Quantum efficiency, Rare-earth ions, Solar cells, Ultraviolet excitation DOI: https://doi.org/10.5281/zenodo.17236960 1. Introduction The use of traditional fossil energy sources (coal, natural gas, and petroleum) in the power industry causes irreparable harm to the environment. The development and implementation of solar energy technologies is one of the most promising and competitive areas for solving the problem of energy supply to consumers by converting solar energy into electricity [1–5]. Solar energy contribution to global electricity generation is relatively small, amounting to 3.6% in 2022. By 2050, however, photovoltaic technologies are expected to provide about 25% of total electricity demand [6]. Nevertheless, despite advances in photovoltaic technology, the main drawback of solar panels is ∗E-mail: [email protected]; Also at Federal Research Center "Kazan Scientific Center Russian Academy of Sciences, Kazan, Russia †Also at Federal Research Center "Kazan Scientific Center Russian Academy of Sciences, Kazan, Russia their low efficiency [7, 8]. The spectral distribution of sunlight consists of photons with wide wavelength ranging from UV to IR radiation (280–2500 nm, 0.5–4.4 eV) [9]. However, modern photovoltaic panels use only a relatively small fraction of the solar photons, mainly in the long-wave part of the spectrum [10]. The energy of short-wave radiation is inefficiently used and released as heat during thermalization process. As a result, the maximum theoretical efficiency of silicon solar cells is 33.7% (the Shockley-Queisser limit). In effect, the efficiency of the most silicon solar panels doesn’t exceed 20% [11]. The efficiency of solar cells can be drastically increased by depositing a quantum cutting material to their surface [12]. Quantum cutting (also known as down-conversion) is the process of absorbing a high-energy photon and emitting two or more photons of lower energy [13]. The quantum yield of such a process can significantly exceed 100%. Estimates of the efficiency of silicon photovoltaic cells modified with a down242 Visible Quantum Cutting in NaGdF4:Eu Nanocrystals for Enhancing Solar Cell Efficiency 243 conversion layer on the front surface are up to 38.6% [14]. The Gd3+-Eu3+ ion system is a prime candidate for efficient spectral conversion by the process of quantum cutting due to its rich energy level structure [15]. In such systems Gd3+ ions absorb one vacuum ultraviolet (VUV) photon at 202 nm while Eu3+ ions emit two photons of visible light (Fig. 1) [16–19]. In the first step, excitation of 6GJlevel of Gd3+ ion results in energy transfer by cross-relaxation to the 5D0state of Eu3+ ion. In the second step, Gd3+ transfers the remaining excitation energy to another Eu3+ ion, which then undergoes fast cascade non-radiative relaxation to the 5DJ states. Both steps lead to the emission of a visible light due to 5DJ→7FJtransitions of Eu3+ ions. It may seem that excitation at 394 nm can be used to calculate the quantum cutting efficiency as well. In this case, however, no cross-relaxation and no quantum cutting is observed, so for every ultraviolet photon absorbed one visible photon is emitted. A serious drawback of this quantum cutting mechanism is the absence of VUV light in the solar spectrum at Air Mass 1.5 and very low absorption efficiency at 202 nm. Therefore, it is necessary to propose an alternative quantum cutting mechanism capable of converting shortFIG. 1: Quantum cutting mechanisms for Gd3+-Eu3+ system under VUV excitation. The solid and dotted arrows represent radiative, non radiative transitions and interionic transitions, respectively. wave radiation of sunlight into the visible light. In this study, we consider and analyze the quantum cutting process through cross-relaxation of two adjacent Eu3+ ions in the NaGdF4 nanocrystals synthesized by the hydrothermal method. Photoluminescence excitation and emission spectra were investigated to determine photophysical properties. For NaGdF4:5%Eu3+ nanoparticles the quantum yield of up to 188% was obtained. 2. Experimental 2.1. Materials All materials were used as received without further purification. Gadolinium chloride hexahydrate (GdCl3·6H2O, 99%), europium chloride hexahydrate (EuCl3·6H2O, 99.9%) and oleic acid (C18H34O2, 90%) were supplied by Sigma-Aldrich, USA. Sodium fluoride (NaF, 99%) and sodium hydroxide (NaOH, 99%) were purchased from JSC "Vecton" (Saint Petersburg, Russia). Cyclohexane was obtained from JSC "EKOS-1" (Moscow, Russia). 2.2. Preparation of NaGdF4:5%Eu3+ nanocrystals The NaGdF4nanocrystals doped with 5 mol% of Eu3+ were synthesized by hydrothermal method using oleic acid as a stabilizing agent [20– 22]. In a typical procedure, 1.2 g NaOH was first dissolved in 2 ml of distilled water, and then 8 ml ethanol and 20 ml oleic acid were added dropwise while stirring to form metal-oleic acid complexes. After vigorous stirring for 30 min, 0.95 mmol GdCl3·6H2O and 0.05 mmol EuCl3·6H2O (the total amount of rare earth metal salts is 1 mmol) were added to the resulting mixture. After dissolving the precursors, 8 ml of an aqueous solution of NaF with a concentration of 1.0 M were added to the solution. The resulting solution was stirred for 30 min to form a white foamy mixture, then placed in a Teflon-lined autoclave and heated at 190◦C for 24 h. Lastly, autoclave Nonlinear Phenomena in Complex Systems Vol. 28, no. 3, 2025 244 E. O. Mityushkin, A. G. Shmelev, R. M. Gataullina, A. V. Leontyev, L. A. Nurtdinova, D. K. Zharkov, V. G. Nikiforov was naturally cooled to room temperature. The precipitates of NaGdF4:5%Eu3+ at the bottom were collected by centrifugation and washed several times with ethanol and deionized water to remove oleic acid and other residues. The samples thus obtained were dried in air at ambient temperature for 12 h and then placed in cyclohexane to prevent aggregation. To obtain a well-dispersed suspension of particles in cyclohexane, the solution was subjected to ultrasonic treatment. 2.3. Characterization The X-ray-powder diffraction (XRD) measurement was carried out using a LANScientific Xrd-602 X-ray diffractometer with Cu-Kαradiation (λ= 1.5406 ˚ A). The measurement time was 5 s per 0.025◦step and the diffraction angle range was 8◦–90◦. The microstructural characteristics of the synthesized nanoparticles were examined using a Carl Zeiss EVO 50 XVP scanning electron microscope (SEM). The photoluminescence excitation and emission spectra were acquired using a Horiba Fluorolog QM-75-22-C modular spectrofluorimeter with an external 75 W xenon lamp as an excitation light source and registration using a Hamamatsu R13456 photomultiplier with a multi-alkali cathode. All the tests were performed at room temperature. 3. Results and discussion 3.1. Crystal structure and morphology The powder X-ray diffraction (XRD) patterns of the NaGdF4nanocrystals doped with 5% Eu3+ were measured and are shown in Fig. 2. It can be clearly seen, that as-prepared phosphors exhibited fairly good crystallinity and all of the diffraction peaks can be readily indexed to the pure hexagonal β-NaGdF4fluoride crystallized in a P¯ 6space group. No other impurity reflexes were detected, indicating that the obtained particles were of high purity and Eu3+ doping did not significantly change the crystal structure. The size and morphology of NaGdF4:5%Eu3+ nanocrystals were characterized by scanning electron microscope (SEM). As shown in Fig. 3, the as-prepared nanophosphors have a spherical shape with rough surfaces and characteristic diameter distributed in the range 45-484 nm. The average size of hundreds of nanoparticles is about 145 nm. 3.2. Excitation and emission properties Fig. 4A shows the excitation spectrum of NaGdF4nanocrystals doped with 5 mol% europium obtained by monitoring the 5D0→ 7F0emission of Eu3+ ions at 620 nm. There is a series of narrow absorption bands related to the characteristic f-f transitions of Gd3+ and Eu3+ ions. Bands peaked at 252, 273 and 310 nm correspond to the transitions from the ground 8S7/2state to the excited 6DJ, (∼ 40,000 cm−1)6IJ(∼37,000 cm−1) and 6PJ(∼ 32,000 cm−1) states of Gd3+ ions. The 7F0→ 5D3transition of Eu3+ ions corresponds to the excitation bands at 394 nm (∼25,000 cm−1). In addition, the band at 203 nm due to 8S7/2→ 6GJtransition of Gd3+ ions was not observed because of low efficiency of the xenon lamp at FIG. 2: Powder XRD pattern of the as-synthesized NaGdF4:%Eu3+ nanophosphors and the corresponding reference pattern of β-NaGdF4. Нелинейные явления в сложных системах Т. 28, № 3, 2025 Visible Quantum Cutting in NaGdF4:Eu Nanocrystals for Enhancing Solar Cell Efficiency 245 200 nm. ItвЂTMs worth mentioning that the 252 and 273 nm quanta used to excite the Gd3+- Eu3+ system have energies of 40,000 and 37,000 cm−1, respectively. This is two times higher than the photon energy of Eu3+ emission at 612 nm (∼17,000 cm−1) and therefore meet the requirements of the quantum cutting mechanism. The NaGdF4:5%Eu3+ emission spectra obtained at excitation wavelengths of 252, 273, and 394 nm are shown in Fig. 4B. The spectra are normalized to the 5D1→7FJemission intensity. The intense europium luminescence upon excitation at 252 and 273 nm implies energy transfer from Gd3+ ions to Eu3+ ions. Irradiation at 394 nm results in direct single-quantum excitation of Eu3+ ions via the 7F0→5D3transition. All observed emission spectra upon excitation at 252 nm (blue line), 273 nm (green line) and 394 nm (red line) correspond to the transitions from the 5DJ(J = 0, 1, 2) excited levels to the 7FJ(J = 0, 1, 2, 3, 4) ground state of Eu3+ ions. As shown in Fig. 4B, NaGdF4:5%Eu3+ emission spectra vary significantly with excitation wavelength. Relative intensities of red luminescence from the upper 5D0level of Eu3+ are twice as strong under excitation at 252 nm as compared to direct excitation of Eu3+ ions at 394 nm. This fact demonstrates the higher yield of the quantum cutting effect as compared to simple downshifting process. Interestingly, the normalized emission spectra upon excitation at 273 and 394 nm are practically identical. Thus, the 273 nm FIG. 3: SEM image of the as-prepared NaGdF4:5%Eu3+ nanocrystals. excitation of NaGdF4:5%Eu3+ nanocrystals does not lead to quantum cutting. 3.3. Alternative mechanism for quantum cutting in Gd3+ - Eu3+ system under 252 nm excitation In contrast to the conventional quantum cutting process in the Gd3+ Eu3+ system which requires excitation by VUV radiation, we propose FIG. 4: (A) The excitation spectrum of NaGdF4:5%Eu3+ monitored at 620 nm, the 5D0 →7F2transition of Eu3+. (B) Emission spectra of NaGdF4:5%Eu3+ upon excitation of the 6DJ and 6IJstates of Gd3+ at 252 nm (blue line), 273 nm (green line), respectively and upon excitation of the 5D3state of Eu3+ at 394 nm (red line). The emission spectra are normalized to the 5D1→7FJemission bands. Nonlinear Phenomena in Complex Systems Vol. 28, no. 3, 2025 246 E. O. Mityushkin, A. G. Shmelev, R. M. Gataullina, A. V. Leontyev, L. A. Nurtdinova, D. K. Zharkov, V. G. Nikiforov a different mechanism with excitation at longer wavelength. This mechanism is schematically shown in Fig. 5. After excitation at 252 nm, direct energy transfer occurs from the 6DJlevel of Gd3+ ion to the resonant level of a nearby Eu3+ ion. In the next step, fast cross-relaxation takes place between the two neighboring Eu3+ ions. Then, cascade non-radiative multiphoton transitions populate the emissive 5D0−3levels of the first Eu3+ ion. According to the proposed energy transfer scheme in Fig. 5, both Eu3+ ions emit visible photons due to the 5D0→ 7FJtransitions, which explains increased emission efficiency from 5D0upon excitation at 252 nm. Experimentally, the quantum cutting efficiency can be estimated by comparing the emission due to quantum cutting with the emission due to any other process following direct excitation. For instance, excitation at 394 nm, as shown in Fig. 5 (right), results in luminescence corresponding to the 5D0−3 →7FJtransition (see Fig. 4B). It should be noted that the energy of the 394 nm excitation (∼25,000 cm−1) is not sufficient for the quantum cutting process to occur, and a much higher energy (∼2×17,000 cm−1) is required to emit FIG. 5: Quantum cutting mechanisms for Gd3+-Eu3+ system under UV radiation excitation (left). Direct excitation (DE) of Eu3+ ions by 394 nm radiation (right). The dashed and solid arrows represent cross-relaxation (CR), interionic energy transfer (ET) and radiative transitions, respectively. two photons of red light. The quantum cutting efficiency QCE of the process presented in Fig. 5 can be defined as: QCE =nCR nET (1) where nCR is the number of photons emitted from the 5D0level, which is excited by crossrelaxation process between Eu3+ ions; and nET is the number of photons emitted due to the Gd3+ →Eu3+ direct energy transfer and the subsequent cascade non-radiative transitions. Since the emission is proportional to the number of photons, the expression (1) can be presented in form of QCE =ICR IET (2) where ICR and IET are integral emissions following the cross relaxation and the direct energy transfer processes, respectively. The value of IET one can calculate by using the luminescence spectrum measured upon 394 nm excitation and normalized to the intensities of the emissions from the 5D1,2,3levels. This normalization procedure formally provides equal powers for the direct excitation process and the direct energy transfer process, which populate the 5D0level. Therefore, we can set IET =I394nm in further calculation, where I394nm is the normalized integral emission upon 394 nm excitation. Under these conditions, the ICR intensity can by expressed as the difference between the normalized integral emissions upon excitations at 252 nm and 394 nm ICR =I252nm − I394nm. So, the final expression for QCE is as follows: QCE =I252nm −I394nm I394nm (3) The calculated value of QCE is about 0.88 for NaGdF4:5%Eu3+. This indicates the probability of emitting a second red photon via the crossrelaxation mechanism between Eu3+ ions. Since the non-radiative relaxation of the 5D0state in NaGdF4:5%Eu3+ nanocrystals is negligible compared to radiative transitions, the quantum Нелинейные явления в сложных системах Т. 28, № 3, 2025 Visible Quantum Cutting in NaGdF4:Eu Nanocrystals for Enhancing Solar Cell Efficiency 247 yield of the Eu3+ emission due to the Gd3+ → Eu3+ direct energy transfer is very close to 100%. Overall quantum yield consists of two contributions due to the quantum cutting and the direct energy transfer. Therefore, in NaGdF4:5%Eu3+ nanocrystals overall quantum yield reaches 188% at the 252 nm excitation at room temperature. 4. Conclusions The hexagonal phase of NaGdF4 nanocrystals, doped with 5 mol% Eu3+ ions, was synthesized using a hydrothermal method. As-synthesized phosphors exhibited bright red luminescence under excitation at 252 nm. A comparative analysis revealed the alternative quantum cutting mechanism based on crossrelaxation between neighboring Er3+ ions, which led to a quantum yield of 188%. Thus, the realized conversion of 252 nm radiation into visible light is an important step towards increasing the efficiency of solar cells. Acknowledgements The synthesis of nanoparticles, SEM and XRD studies were carried out with the financial support of Agreement with the Ministry of Science and Higher Education of the Russian Federation No. 075-15-2024-624 dated 12/07/2024. References [1] R. Dallaev, T. Pisarenko, N. Papez, and V. Holcman. Materials 16, 5839 (2023). [2] F. Rehman, I. H. Syed, S. Khanam, S. Ijaz, H. Mehmood, M. Zubair, Y. Massoud, and M. Q. Mehmood. Energy Adv. 2, 1239 (2023). [3] A. S. 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