Intense Near-Infrared Light-Emitting NaYF4:Nd,Yb-Based Nanophosphors for Luminescent Solar Concentrators

In this study, we synthesized NaYF4-based downshifting nanophosphors (DSNPs), and fabricated DSNP-polydimethylsiloxane (PDMS) composites. Nd3+ ions were doped into the core and shell to increase absorbance at 800 nm. Yb3+ ions were co-doped into the core to achieve intense near-infrared (NIR) luminescence. To further enhance the NIR luminescence, NaYF4:Nd,Yb/NaYF4:Nd/NaYF4 core/shell/shell (C/S/S) DSNPs were synthesized. The C/S/S DSNPs showed a 3.0-fold enhanced NIR emission at 978 nm compared with core DSNPs under 800 nm NIR light. The synthesized C/S/S DSNPs showed high thermal stability and photostability against the irradiation with ultraviolet light and NIR light. Moreover, for application as luminescent solar concentrators (LSCs), C/S/S DSNPs were incorporated into the PDMS polymer, and the DSNP-PDMS composite containing 0.25 wt% of C/S/S DSNP was fabricated. The DSNP-PDMS composite showed high transparency (average transmittance = 79.4% for the visible spectral range of 380–750 nm). This result demonstrates the applicability of the DSNP-PDMS composite in transparent photovoltaic modules.


Introduction
The development of advanced building-integrated photovoltaics (BIPVs) can address space constraint issues in high-rise buildings and meet their energy needs [1]. In the BIPVs, the utilization of luminescent solar concentrators (LSCs) enables the realization of semi-transparent photovoltaic (PV) modules for converting the facades of the urban building into energy generators [2]. LSCs are typically fabricated as waveguides consisting of transparent surfaces either coated with or matrices containing luminescent materials, such as organic dyes, quantum dots (QDs), or inorganic phosphors [3,4]. Although organic dyes have been used in the LSCs, they are hampered by poor stability and self-absorption loss due to their small Stokes shift [3]. Recently, QDs have been explored as luminescent materials in LSCs owing to their higher stability and wider absorption band width than organic dyes [5]. Nonetheless, QDs have small Stokes shifts and suffer from self-absorption loss [3]. To overcome this issue, large Stokes shift-emitting QDs have been developed where host QD nanocrystals were doped with transition metal ions, such as Mn 2+ , to emit long-wavelength visible light under short-wavelength ultraviolet (UV) light excitation [6,7].
On the other hand, lanthanide-doped inorganic phosphors are promising candidates for use in LSCs [8,9]. Compared to organic dyes and QDs, lanthanide-doped inorganic phosphors are more advantageous because they generally exhibit a large Stokes shift [9]. For example, de Boer et al. applied SrB 4 O 7 :Sm,Eu phosphors to an LSC film, where the SrB 4 O 7 :Sm,Eu phosphor showed an absorption band below 600 nm and an emission peak at 685 nm [9]. Liu et al. reported an LSC film consisting of a poly(methyl methacrylate) waveguide containing CaAlSiN 3 :Eu 2+ phosphors with a Stokes shift of 112 nm [10]. In addition, Weber and Lambe reported that Nd 3+ -doped glass could be a luminescent medium because Nd 3+ -ions have strong absorption bands in the 500-900 nm range and an emission peak at 1060 nm, which is a wavelength well suited for use in silicon solar cells [11]. Thus, Nd 3+ -doped phosphors appear desirable for LSCs due to the amelioration of the above drawbacks that plague organic dyes and QDs [11]. However, micrometer-sized phosphors exhibit a scattering issue that increases non-emissive absorption and escape cone losses when these phosphors are utilized in LSCs [8]. This scattering issue can be addressed using nanophosphors. According to Do's group, light scattering decreases as the phosphor size decreases, and nanophosphors smaller than 50 nm can lead to transparent nanophosphorbased matrix [12]. Thus, Nd 3+ -doped nanophosphors, smaller than 50 nm, can be applied to a transparent LSCs coupled with silicon solar cells.
To synthesize the C/S DSNPs, YCl 3 ·6H 2 O (0.6 − x mmol), NdCl 3 ·6H 2 O (x mmol, x = 0, 0.06, 0.12, 0.18, 0.24), 6 mL of OA, and 15 mL of ODE were loaded into a 3-neck flask and heat-treated at 150 • C. After the reaction solution was cooled to room temperature, core DSNPs and MeOH solution containing NaOH and NH 4 F were added to it. After removing MeOH, C/S DSNPs were synthesized by heat treatment at 320 • C for 1 h. The synthesized C/S DSNPs were washed in the same manner as the core DSNPs and dispersed in 10 mL of chloroform.
To synthesize C/S/S DSNPs, YCl 3 ·6H 2 O (1 mmol), OA (6 mL), and ODE (15 mL) were mixed and heat-treated at 150 • C for 40 min. After the heat treatment, the reaction mixture was cooled to room temperature, followed by the addition of C/S DSNPs solution and MeOH solution containing NaOH (2.5 mmol) and NH 4 F (4 mmol) to it. The subsequent synthetic process was the same as that used to synthesize C/S DSNPs.
To prepare the PDMS-based LSC film, the C/S/S DSNP solution (0.5 mL) was mixed with SYLGARD 184 silicone elastomer (10 mL, Dow Chemical Company, Midland, MI, Materials 2023, 16, 3187 3 of 12 USA). The curing agent (1 mL) was then added to the mixture of the C/S/S DSNPs and silicone elastomer. Subsequently, the mixture was poured into a mold and baked at 80 • C for 1 h.
To investigate the feasibility of LSC applications of the NaYF 4 :Nd (30%), Yb (10%)/NaYF 4 :Nd (10%)/NaYF 4 C/S/S DSNPs, we fabricated NaYF 4 :Nd (30%),Yb (10%)/NaYF 4 :Nd (10%)/NaYF 4 C/S/S DSNP-PDMS composites. Figure 6a shows a schematic illustration of bare PDMS-and DSNP-PDMS composite-coupled solar cells. In a bare PDMS-coupled solar cell, incident sunlight directly passes through the PDMS polymer so that most sunlight cannot reach the solar cell. In contrast, in the DSNP-PDMS composite-coupled solar cell, NIR photons of incident sunlight are absorbed by the DSNPs, and the DSNPs emit NIR light in all directions. The emitted light is directed to the silicon solar cell attached to the edge of the DSNP-PDMS composite, which can result in electricity generation from the silicon solar cell. Figure 6b shows the transmittance spectra of the bare PDMS and the C/S/S DSNP-PDMS composite. The average transmittance values of the bare PDMS and the C/S/S DSNP-PDMS composite in the visible region (λ = 380-750 nm) were 92.1 and 79.4%, respectively. Although the transmittance of the C/S/S DSNP-PDMS composite was lower than that of the bare PDMS polymer, it was still highly transparent, as shown in the inset of Figure 6b. Therefore, we believe that the C/S/S DSNP-PDMS composite can be applied to transparent PV modules. Additionally, in the transmittance spectrum of the C/S/S DSNP-PDMS composite, peaks were observed at 574, 740, and 794 nm, which were attributed to the light absorption via 4 I 9/2 → 4 G 5/2 , 4 I 9/2 → 4 F 7/2 , and 4 I 9/2 → 4 F 5/2 transitions of Nd 3+ ions in the NaYF 4 :Nd (30%),Yb (10%)/NaYF 4 :Nd (10%)/NaYF 4 C/S/S DSNPs, indicating the presence of NaYF 4 :Nd (30%),Yb (10%)/NaYF 4 :Nd (10%)/NaYF 4 C/S/S DSNPs in the PDMS composite [25].   Figure S8a shows the current density versus voltage curves of the bare PDMS-and C/S/S DSNP-PDMS composite-coupled silicon solar cells. The C/S/S DSNP-PDMS composite-coupled silicon solar cell showed increased short-circuit current density compared with the bare PDMS-coupled silicon solar cell. The efficiencies of the bare PDMS-and the C/S/S DSNP-PDMS composite-coupled silicon solar cells were measured to be 0.92 and 1.96%, respectively, under Air Mass (AM) 1.5G illumination using a solar simulator. Since the transmittance of the C/S/S DSNP-PDMS composite is lower than the bare PDMS, the light scattering effect may contribute to the increase in solar cell efficiency. Since the absorption of the C/S/S DSNP-PDMS composite was low (Figure 6b), the contribution of light scattering to the increase in solar cell efficiency seems to be larger than that of the  Since the transmittance of the C/S/S DSNP-PDMS composite is lower than the bare PDMS, the light scattering effect may contribute to the increase in solar cell efficiency. Since the absorption of the C/S/S DSNP-PDMS composite was low (Figure 6b), the contribution of light scattering to the increase in solar cell efficiency seems to be larger than that of the luminescence from the C/S/S DSNPs in the DSNP-PDMS composite to the increase in solar cell efficiency. However, when the C/S/S DSNP-PDMS composite was excited with weak 800 nm NIR light (1 mW), it showed clear emission band in the NIR spectral region ( Figure S8b). Thus, it seems that both light scattering and luminescence due to the C/S/S DSNPs in the DSNP-PDMS composite contribute to the increase in solar cell efficiency, although the contribution of the luminescence from the C/S/S DSNPs may be low. Since the Nd 3+ ions exhibit strong absorption peaks in NIR spectral region (Figure 3a), the absorption of the C/S/S DSNP-PDMS composite in the NIR spectral region can be enhanced after further optimization of the C/S/S DSNP-PDMS composite preparation. The transmittance of the C/S/S DSNP-PDMS composite can be further increased via further optimization of the DSNP-PDMS composite preparation. The contribution of the luminescence from the C/S/S DSNPs in the DSNP-PDMS composite will then increase, and the light scattering effect will decrease for the increase in solar cell efficiency. The optimized DSNP-PDMS composites can be suitable as LSCs for transparent PV modules.

Conclusions
We successfully synthesized NaYF 4 :Nd,Yb/NaYF 4 :Nd/NaYF 4 C/S/S DSNPs that absorb 800 nm NIR light and emit broad-band NIR light peaking at 978 nm. The synthesized NaYF 4 :Nd (30%),Yb (10%)/NaYF 4 :Nd (10%)/NaYF 4 C/S/S DSNPs exhibited a uniform rod shape and a single hexagonal phase. The EDS analysis confirmed the formation of the C/S/S structure. The Nd 3+ ions doped in the core and shell can effectively absorb 800 nm NIR light, and the absorbed energy is transferred from the Nd 3+ ions to the Yb 3+ ions, resulting in a strong and broad NIR emission from the Yb 3+ ions in the core. As a result, the luminescence intensity of the C/S/S DSNPs was significantly higher than that of the core DSNPs. The C/S/S DSNPs also exhibited high thermal stability and photostability. A transparent composite film was fabricated by incorporating C/S/S DSNPs into the PDMS polymer. This study shows that NaYF 4 :Nd,Yb-based C/S/S DSNPs can be used in LSCs and have potential applications to transparent PV modules.