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## Creator
[Taisei Hangai](https://orcid.org/0009-0002-8165-0568), [Takuya Hasegawa](https://orcid.org/0000-0002-6170-5632), [Jian Xu](https://orcid.org/0000-0002-1040-5090), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), [Takashi Takeda](https://orcid.org/0000-0003-2510-4562), [Tomoyo Goto](https://orcid.org/0000-0003-1362-6750), [Yasushi Sato](https://orcid.org/0000-0001-5132-0301), Ayahisa Okawa, [Shu Yin](https://orcid.org/0000-0002-5449-4937)
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[Luminescence Tuning of NIR Luminescence Nanophosphor Bi3+/Yb3+-Doped RE2MoO6 (RE = Gd, Y, and Lu) and Gd2Mo1–xW xO6](https://mdr.nims.go.jp/datasets/a20c67ae-dc2e-4169-a6bd-3037e55a84fb)
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Luminescence Tuning of NIR Luminescence Nanophosphor Bi3+/Yb3+-Doped RE2MoO6 (RE = Gd, Y, and Lu) and Gd2Mo1–xWxO6Luminescence Tuning of NIR Luminescence Nanophosphor Bi3+/Yb3+-Doped RE2MoO6 (RE = Gd, Y, and Lu) and Gd2Mo1−xWxO6Taisei Hangai, Takuya Hasegawa,* Jian Xu, Takayuki Nakanishi, Takashi Takeda, Tomoyo Goto,Yasushi Sato, Ayahisa Okawa, and Shu YinCite This: J. Phys. Chem. C 2024, 128, 20360−20368 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Near-infrared nanophosphors have attracted atten-tion due to their wide application fields, including componentanalysis, bioimaging, and spectral converters of sunlight forcrystalline silicon solar cells (c-Si). Yb3+ ions exhibit near-infrared(NIR) luminescence at around 1000 nm, which is consistent withthe first biological window and the maximum responsivity range ofc-Si. Therefore, we focused on and successfully synthesized Bi3+/Y b 3 + - d o p e d N IR l um i n e s c e n c e n a n o p h o s p h o r s ,R E 2MoO 6 : B i 3 + , Y b 3 + ( RE=Gd , Y , a n d L u ) a n dGd2Mo1−xWxO6:Bi3+,Yb3+ (x = 0−0.5), utilizing a solvothermalreaction process. All samples exhibit NIR luminescence of Yb3+ions under ultra-violet (UV) light excitation and broadbandexcitation due to the charge transfer transition between the O 2p/Bi 6s and Mo 4d or W 5d orbitals, indicated by their optical properties of photoluminescence (PL), PL excitation (PLE), andreflectance spectra. Furthermore, to evaluate the contribution of the Gd2MoO6:Bi,Yb (GMO:Bi,Yb) nanophosphor to theconversion efficiency of c-Si, a phosphor-converted film was made using dimethylpolysiloxane (PDMS) and the GMO:Bi,Ybnanophosphor. The results showed that the conversion efficiency of c-Si with the PDMS/GMO:Bi,Yb film is higher than that of c-Siwith the PDMS-only film. Based on these results, the utilization of down-shifting nanophosphors is able to enhance the conversionefficiency of c-Si, which could be beneficial in addressing future energy challenges.1. INTRODUCTIONNear-infrared (NIR) light, defined as the wavelength rangefrom 700 to 1400 nm,1 plays a crucial role in many applicationfields due to its outstanding properties. For instance, NIR lightis absorbed by specific molecules, enabling its use incomponent analysis and bioimaging.2−4 These characteristicsmake NIR light indispensable in various scientific fields.Therefore, the development of near-infrared (NIR) phosphorsis crucial. As NIR luminescence centers, many researchers havechosen lanthanide ions such as Eu2+ or transition metal ionslike Cr3+ and Ni2+.5−7 However, the transition types of theseions (Eu2+: 4f-5d transitions; and Cr3+, Ni2+: 3d-3d transitions)affect their coordination environment. Consequently, anappropriate host matrix must be selected for these ions toexhibit NIR luminescence, which greatly limits the range of thehost matrix selection. This represents a significant challenge incurrent NIR phosphor research. Lanthanide ions such as Yb3+,Nd3+, and Er3+ possess an attractive property of showing NIRluminescence regardless of the host matrix due to their 4f-4fintratransitions.8−10 Specifically, Yb3+ ions exhibit a singleenergy level transition of 2F5/2 → 2F7/2, resulting in anemission around 1000 nm.11,12 Their emission wavelengthaligns perfectly with the first biological window, which is highlybeneficial for bioimaging applications.13 Furthermore, theemission wavelength of Yb3+ also matches the responsivitypeak of silicon-based solar cells.14 This alignment is expectedto enhance the overall efficiency of solar cells by converting abroader spectrum of sunlight into usable energy. Given thesecharacteristics, Yb3+ stands out as a highly promising ion forNIR luminescence applications.However, despite these advantages, Yb3+ has a relatively lowabsorption coefficient due to its forbidden transitions. Toimprove the low absorption efficiency, it is desirable to applyan energy transfer process from parity/spin-allowed transitionswith higher absorption efficiency than forbidden transition.15Recently, Ce3+ ions have often been utilized for the energydonor of Yb3+ because Ce3+ ions exhibit 4f-5d transition withallowed transition under ultra-violet (UV) light.16,17 However,the total optical absorption coefficient is still low as theReceived: July 18, 2024Revised: October 15, 2024Accepted: October 16, 2024Published: November 15, 2024Articlepubs.acs.org/JPCC© 2024 The Authors. Published byAmerican Chemical Society20360https://doi.org/10.1021/acs.jpcc.4c04814J. Phys. Chem. C 2024, 128, 20360−20368This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on December 10, 2024 at 02:06:49 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Taisei+Hangai"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takuya+Hasegawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jian+Xu"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takayuki+Nakanishi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Takeda"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomoyo+Goto"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yasushi+Sato"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yasushi+Sato"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ayahisa+Okawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shu+Yin"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.jpcc.4c04814&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/jpccck/128/47?ref=pdfhttps://pubs.acs.org/toc/jpccck/128/47?ref=pdfhttps://pubs.acs.org/toc/jpccck/128/47?ref=pdfhttps://pubs.acs.org/toc/jpccck/128/47?ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c04814?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/JPCC?ref=pdfhttps://pubs.acs.org/JPCC?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/optimum doping concentrations of Ce3+ toward optimumemission intensity are only a few mol % in the matrix due tothe concentration quenching effect, resulting in limitedabsorption. For instance, CaSc2O4:Ce3+ exhibits only a 3%reduction in reflectance due to the 4f-5d transition of Ce3+, andin Ce3+-doped Ca2YZr2Al3O12 garnet, the reflectance decreasedby 10%, suggesting high possibility for enhancing theabsorption efficiency.18,19 Employing a ligand-to-metal chargetransfer (LMCT) transition of the host matrix can overcomethe issues of limited absorption efficiency from dopantsbecause the transition utilizes the absorption of the hostmatrix rather than the dopant.20 Therefore, the NIR phosphorsshowing LMCT transitions of the host matrix are expected toabsorb sunlight more efficiently compared to the conventionalCe3+-doped phosphors.Our research has found that Yb3+-doped Gd2MoO6(GMO:Yb) nanophosphors are ideal for satisfying theseconditions.21 GMO:Yb nanophosphors exhibit a broadexcitation band from 200 to 400 nm due to the LMCTtransition between Mo and O atoms and demonstrate NIRluminescence of Yb3+ ions around 1000 nm under 363 nmexcitation (Figure 1a), suggesting that they have a suitablespectral modification.22 As shown in Figure 1b, they can besynthesized in a nanosize with around 50 nm of particle sizeusing the solvothermal method; thus, the nanosized nature ofthese phosphors suppresses the scattering of visible light fromsunlight, making them highly effective for solar energyapplications.23 Additionally, their small size is advantageousfor bioimaging applications because nanoparticles can easilypenetrate biological tissues and cells, providing enhancedimaging resolution and deeper tissue penetration compared tolarger particles.24Chemical composition adjustment plays a crucial role toinvestigate the properties of such an attractive material withexcellent optical characteristics for the GMO:Bi,Yb nano-phosphor, as the strategy effectively evaluates and enhancesoptical properties by modifying the coordination environmentand electronic structure.25−28 In the Gd2MoO6 matrix, theoptical property can be modulated by substituting Gd and Mosites with other rare earths (such as Y and Lu) and W ions.The Y2MoO6 and Lu2MoO6 matrices are isostructural with themonoclinic phase of Gd2MoO6,29,30 enabling a systematiccomparison and understanding of the substitution effect onoptical properties. On the other hand, Gd2WO6 exhibitsLMCT transitions between the O 2p and W 5d orbital,31which can significantly tune optical properties becauseGMO:Bi,Yb nanophosphors also utilize the LMCT transitions.In this study, we synthesized and characterized the opticalpropert ies of the GMO:Bi ,Yb family , inc ludingRE2MoO6:Bi,Yb (RE= Y, Lu) and Gd2Mo1−xWxO6:Bi,Yb (x= 0−1) nanophosphors. Additionally, GMO:Bi,Yb nano-phosphor films were prepared in anticipation of their excellentoptical properties, as shown in Figure 1a, and their effect onthe conversion efficiency of solar cells was also evaluated.Focusing on the excellent permeability, flexibility, and stabilityof dimethylpolysiloxane (PDMS),32 we prepared prototypefilms of phosphors encapsulated in PDMS. This study providesinsights into the development of novel near-infrared phosphorsand contributes to the application of solar cells. Therefore, theresults of this research offer guidance for the design of a newNIR luminescence phosphor.2. METHODS2.1. Materials. Raw materials for the synthesis ofRE2MoO6:Bi,Yb and Gd2Mo1−xWxO6:Bi,Yb nanophosphorsof Gd(NO3)3·6H2O (>99.95%), (NH4)6Mo7O24·4H2O(>99.0%), 5(NH4)2O·12WO3·5H2O (88.0−90.0%), andHNO3 (69.0−71.0%) were purchased from Kanto chemicalCo. Inc. Bi(NO3)3·5H2O (>99.5%), Yb(NO3)3·nH2O (78−88%), and isopropanol (99.7+%) were purchased from FujifilmWako Pure Chem. Co. Y(NO3)3·6H2O (99.8%) was purchasedfrom Sigma-Aldrich. Lu2O3 (99.9%) was purchased fromNippon Denko Co. Ltd. Lanthanide nitrate hydrates ofLu(NO3)3·nH2O and Yb(NO3)3·nH2O were obtained bydissolving Lu2O3 and Yb2O3 in 1 M HNO3 (aq) and dryingit at 80 °C for several days. The hydration amounts ofLu(NO3)3·nH2O and Yb(NO3)3·nH2O were found to be 3.7and 5.9, respectively, through thermogravimetry analysis. Tofabricate the PDMS film, chloroform (>99.0%) was purchasedfrom Kanto chemical Co. Inc., and PDMS (Sylgard-184) andthe curing agent were purchased from Dow Corning.2.2. Synthesis of RE2MoO6:Bi,Yb (RE = Gd, Y, and Lu),RE2MoO6, and Gd2Mo1−xWxO6:Bi,Yb (x = 0, 0.2, 0.5, 0.8,and 1) Nanophosphors. The RE2MoO6:Bi,Yb (REMO:-Bi,Yb) nanophosphors were synthesized by using a solvother-mal reaction method with isopropanol as the solvent. In atypical synthesis, a solution containing Gd(NO3)3·6H2O,Y(NO3)3·6H2O, or Lu(NO3)3·nH2O, Bi(NO3)3·5H2O, andYb(NO3)3·nH2O in isopropanol was prepared. The solutionwas stirred vigorously for approximately 10 min at roomtemperature (RT) before being transferred to a Teflon-linedautoclave vessel with a volume of 100 mL. The vessel was thenheated at 220 °C for 5 h in an oven. After being cooled toroom temperature, the powder was washed multiple times withwater and ethanol and then collected through filtration. Theresulting pale-gray powder was calcined at 850 °C for 5 h.RE2MoO6 (REMO: GMO, YMO, and LuMO) was synthe-sized by a similar process without Bi(NO3)3·5H2O andYb(NO3)3·nH2O. The synthesis of Gd2Mo1−xWxO6:Bi,Ybfollowed a similar procedure, with the exception thatstoichiometric amounts of 5(NH4)2O·12WO3·5H2O and(NH4)6Mo7O24·4H2O were used.2.3. Fabrication of the PDMS and GMO:Bi,Yb Films.An appropriate amount of the GMO:Bi,Yb nanophosphor wasdispersed in 3 mL of chloroform to obtain solutions of 0.5, 1,and 1.5 mg/mL. The resulting solution was then added to amixture of 3 g of PDMS and 0.3 g of the curing agent andstirred manually until the mixture became homogeneous. Theobtained pastes were deformed under vacuum conditions at 25°C for 1 h. Subsequently, the deformed pastes were depositedFigure 1. (a) PL (λex= 363 nm)·PLE (λem= 975 nm) spectra for theGMO:Bi,Yb nanophosphor (blue), responsivity for c-Si (brown), andsolar spectrum (orange). (b) TEM images for GMO:Bi,Yb nano-phosphors.The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c04814J. Phys. Chem. C 2024, 128, 20360−2036820361https://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig1&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c04814?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ason a clean board and flattened to 50 μm using the doctor blademethod. Finally, the board with the pastes was dried overnightat 60 °C.2.4. Characterization. The thermal stability was measuredby a thermogravimetry-differential thermal analyzer (TG-DTA8122, Rigaku). The chemical composition was analyzedby X-ray photoelectron spectroscopy (XPS, PHI5600,ULVAC-PHI Inc.) The crystal phase was determined bypowder X-ray diffraction (XRD) using an X-ray diffractometer(SmartLab 3G, Rigaku) with Cu Kα radiation (λ = 1.5406 Å).The detailed structural information was refined by the Rietveldanalysis technique using the RIETAN-FP program package.33To obtain the chemical states of each metal component, the X-ray absorption fine structure (XAFS) analysis for the W L3-edge and Mo K-edge was utilized by BL12C (W L3-edges andMo K-edge) in the Photon Factory (PF) at the High EnergyAccelerator Research Organization, Japan (KEK). The samplewas diluted by boron nitride powder and pelleted for themeasurement. The measurement mode was the transmissionmode. The obtained data from XAFS were analyzed by Athenaand Artemis programs.34 The diffuse reflectance spectra (DRS)were measured by a UV−vis-NIR spectrometer (V-670,JASCO Corp.) conducted with an integrated sphere attach-ment. The morphology was observed by transmission electronmicroscopy (TEM, EM-002B, Topcon Corp.). The photo-luminescence (PL) and PL excitation (PLE) spectra andquantum efficiencies were measured by a spectrofluorometer(FP-8700, JASCO Corp.) equipped with a 150 W Xe lamp asthe excitation source. The temperature-dependent PL and PLEspectra and PL lifetimes of NIR luminescence due to Yb3+ ionswere recorded by a photoluminescence spectrometer(FLS1000-SD-stm, Edinburgh Inst.) equipped with 450 WXe lamp, and the measurement temperature was controlled bythermal stage (THMS600, Linkam Scientific Instruments).The photovoltaic performance of c-Si solar cells (LR0GC02,SHARP) with and without films was measured by a sourcemeter (IVP-0605, Asahi spectra) under 1 sun (100 mW/cm2,AM 1.5) of solar light simulator illumination at 25 °C.3. RESULTS AND DISCUSSION3.1. RE2MoO6:BiYb (REMO:Bi,Yb; RE = Gd, Y, and Lu)Nanophosphors. The chemical compositions of REMO:-Bi,Yb (RE=Gd, Y, and Lu) were analyzed using X-rayPhotoelectron Spectroscopy (XPS) measurements, as shownin Figure 2a and Figure S1. Peaks corresponding to RE (Gd, Y,and Lu), Mo, Yb, and Bi elements were observed in eachREMO:Bi,Yb. These peaks were integrated to estimate themolar ratios of RE, Bi, and Yb ions for quantitative analysis(Table S1). The maximum doping level of Bi3+ inREMO:Bi,Yb occurred with RE = Gd, which correlates withthe ionic radius of RE and Bi3+. Bi3+ has an ionic radius of 1.17Å in an eight-coordination environment, and Gd3+ has theclosest radius of 1.053 Å compared to that of Y3+ (1.019 Å)and Lu3+ (0.977 Å).35 Therefore, Gd3+ is more easilysubstituted by Bi3+ in solids. Similarly, the largest dopingamount of Yb3+ is observed with RE = Y, which can beattributed to the larger ionic radius of Yb3+ (0.985 Å) and thecloser ionic radius of Gd3+.35 The evaluation of oxidation stateswas carried out based on the narrow scan analysis (FigureS1a−f) as their oxidations states were crucial factors toconsider the optical properties. The peaks observed at 149 and141 eV for Gd 4d3/2 and 4d5/2, 159 and 157 eV for Y 3d3/2 and3d5/2, and 207 and 196 eV for Lu 4d3/2 and 4d5/2 (Figure S1a−c) were attributed to Gd3+, Y3+, and Lu3+, respectively.36−38The peaks around the Y 3d spectra were deconvoluted intothree peaks due to the proximity of the Bi 4f7/2 spectra to the Y3d spectra. Two peaks at 236 and 232 eV, corresponding toMo 3d3/2 and 3d5/2 (Figure S1d), respectively, indicated theoxidation states of Mo6+.39 The Bi 4f5/2 and 4f7/2 spectra(Figure S1e) were observed at 164 and 160 eV, respectively,implying the presence of Bi3+ ions.40 The Yb 4d spectra(Figure S1f) confirmed the presence of a peak at 185 eV due tothe Yb 4d5/2 of Yb3+ ions.41 In addition to the XPS spectra, weevaluated the oxidation states of molybdenum using X-rayabsorption near edge structure (XANES) analysis becauseMo6+ ions are known to be easily reduced. The XANES spectrafor Mo K-edge in REMO:Bi,Yb (Figure S2) showed theabsorption edge and pre-edge around 20.009 and 19.992 keV,respectively, consistent with those of MoO3 as a reference.Another reference sample of MoO2 showed only an absorptionedge around 20.006 keV, which was located at a lower energydue to the presence of Mo4+. Furthermore, the peak positionsin the derivative XANES spectra for REMO:Bi,Yb (Figure S3)were consistent with those of MoO3. This indicated thepresence of Mo6+ ions in all samples, confirming the absence ofreductive Mo6+ ions in REMO:Bi,Yb.The crystal structure was determined using the powder X-ray diffraction (XRD) method, and the observed XRD patternswere refined by Rietveld analysis as shown in Figure 2b andFigure S4. In GMO:Bi,Yb, most peaks were well fitted with themonoclinic phase of Gd2MoO6, while an unknown phase wasobserved at 26°. The peaks in YMO:Bi,Yb and LuMO:Bi,Ybwere mainly attributed to the target phases of YMO andLuMO, and the peaks of RE2Mo4O15 (RE= Y and Lu) and anFigure 2. (a) Wide scan analysis of XPS spectra, (b) Rietveld analysis for GMO:Bi,Yb (inset: enlarged one). (c) TEM images for REMO:Bi,Yb(RE= Y (upper) and Lu (bottom)).The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c04814J. Phys. Chem. C 2024, 128, 20360−2036820362https://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig2&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c04814?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asunknown phase were also observed around 18° (Figure S4).The impurity phase is almost negligible as the peaks of theimpurity phase are much smaller than those of the main phase.The refined parameters are summarized in Table S2, and thereliable parameters of (Rwp, S) for RE = Gd, Y, and Lu were(2.98%, 3.38), (5.22%, 4.85), and (7.62%, 7.43), respectively.The S values were large due to the unknown phase and theinability to refine the oxygen positions and atomic displace-ment parameters, although the Rwp values are sufficiently lowto be reliable. The reasons for the failures to refine them wereas follows: (i) the samples were nanoparticles, and (ii) both ofthe target phases were monoclinic. Nanoparticles exhibit peakbroadening in XRD patterns due to their small size, whichpresents a difficulty in accurately fitting each peak. Therefinement of crystalline systems with low symmetry is acomplex process due to the increased number of diffractionpeaks and greater freedom in refinement parameters. As aconsequence of the aforementioned factors, the refinementprocess proved to be onerous, and the S values did not declineto an adequate extent. However, the primary aim of theRietveld refinement was to estimate the unit cell volumes (V).Accordingly, if only the lattice parameters are considered, it isacceptable not to refine all structural parameters even if thisresults in larger S values. The refined V values for RE = Gd, Y,and Lu were 945.4(3), 911.5(2), and 883.2(2) Å3, respectively.They gradually reduced in the order of RE = Gd, Y, and Lucorresponding to the relationship of ionic radius of the Gd, Y,and Lu (Gd: 1.053 Å, Y: 1.019 Å, Lu: 0.977 Å). The cellparameters of RE2MoO6 were also refined for comparison withREMO:Bi,Yb, and the refinement and refined parameters arelisted in Figure S5 and Table S3. The V values for RE2MoO6were found to be 948.6(1) Å3 for RE = Gd, 913.5(2) Å3 for RE= Y, and 884.3(5) Å3 for RE = Lu. The V values for RE = Gdand Y decreased due to Bi3+ and Yb3+ doping, indicating thatthe substitution of Yb3+ with a smaller ionic radius than Gd3+and Y3+ primarily contributed to the decrease. On the otherhand, there were almost no changes in V values of RE = Lu dueto Bi3+ and Yb3+ doping despite their larger ionic radiuscompared to Lu3+. This might be due to the small dopingamounts of Bi3+ and Yb3+ as summarized in Table S1. TEMimages of the REMO:Bi,Yb nanophosphor (Figure 2c) revealthe presence of spherical nanoparticles with a diameter ofapproximately 50 nm. No differences were observed betweenREMO:Bi,Yb nanophosphors because they belong to the samecrystal structure and were synthesized under the sameconditions. Based on these observations, it suggested thatREMO:Bi,Yb nanophosphors have the potential to be utilizedwith c-Si solar cells, as they are likely to minimize thescattering of visible light in sunlight.Figure 3 displays the diffuse reflectance spectra (DRS) forREMO:Bi,Yb. The amount of impurities in RE2Mo4O15 (RE =Gd, Y, and Lu) was found to be minimal and was determinedto have no significant impact on the optical properties. Allsamples exhibit efficient absorptions from 200 to 400 nm dueto the charge transfer (CT) transition between Mo 4d orbitalsand O 2p/Bi 6s hybrid orbitals. Additionally, an opticalabsorption was observed at around 1000 nm, which isattributed to the 2F7/2 → 2F5/2 transition of the Yb3+ ions.The band gap energy (Eg) was estimated using the Kubelka−Munk transformation and Tauc plot, calculated with eqs 1 and2.42h A h E( ) ( )1/2g= (1)RR(1 )22=(2)where hν, α, A, and R represent the photon energy, Kubelka−Munk function, absorption constant, and reflectance, respec-tively. The estimated Eg values for REMO:Bi,Yb with RE = Gd,Y, and Lu were 2.88, 2.82, and 2.64 eV, respectively. Those ofthe host matrix (RE2MoO6) with RE = Gd, Y, and Lu werealso estimated to be 2.96, 2.88, and 2.72 eV, respectively, asshown in Figure S6, which are reasonable Eg values because theEg values have been reported to be approximately 2.7 to 2.9eV.43−45 Compared to the Eg values of the host matrix, those ofREMO:Bi,Yb decreased with Bi3+ and Yb3+ doping, which wasattributed to the upward shift of the valence band maximum bythe formation of the hybrid orbitals between the Bi 6s/O 2porbital as observed in other Bi3+-doped molybdates.46 The Egvalues decreased following the order of ionic radius for RE ions(Gd3+ > Y3+ > Lu3+). In REMO:Bi,Yb, the Eg values wereprimally determined by the contribution of Bi3+ ions and thedistance between Mo and O atoms due to the crystal fieldeffect. The Mo 4d orbitals, forming the conduction bandminimum, are affected by the crystal field splitting effect.45From the extended X-ray absorption fine structure (EXAFS)analysis as shown in Figure S7, the peaks at 1.38 Å for RE = Yand Lu and 1.40 Å for RE = Gd were attributed to the firstcoordination sphere of the O atom around the Mo atom. Theshorter peak positions for RE = Y and Lu compared to RE =Gd suggest that the crystal fields around the Mo atom in RE =Y and Lu were stronger than those in RE = Gd, resulting in areduction of the band gap due to a lower shift in theconduction band minimum. Thus, it was reasonable to observea lower band gap of RE = Y and Lu compared to that of RE =Gd. Despite similar bond length in RE = Y and Lu, the Egvalues decreased with Lu substitution. The smaller dopingamount of Bi3+ in RE = Lu compared to RE = Y could alsocontribute to the decrease of Eg values.Figure 4a displays the photoluminescence (PL) and PLexcitation (PLE) spectra for REMO:Bi,Yb in the NIR region.The PL spectra under 363 nm excitation showed NIRluminescence from 950 to 1100 nm, with the strongest peakat 975 nm, attributed to the 2F5/2 → 2F7/2 transition of Yb3+ions. The spectra exhibited five peaks at 950, 975, 1034, 1055,and 1074 nm resulting from the Stark splitting of the 2F5/2 and2F7/2 levels.47 The normalized PL spectra for REMO:Bi,Yb(RE=Gd, Y, and Lu) (Figure S8) reveal consistent NIRemission at around 1000 nm from Yb3+ ions. The similarity inPL spectra across REMO:Bi,Yb suggests that the crystal fieldFigure 3. UV−vis-NIR diffuse reflectance spectra and Tauc plot(inset) of REMO:Bi,Yb (RE = Gd (blue), Y (dark-green), and Lu(red)).The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c04814J. Phys. Chem. C 2024, 128, 20360−2036820363https://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig3&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c04814?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aseffects on the Yb3+ ions are similar despite the variation in thehost materials. These findings imply that Yb3+-doped nano-phosphors can effectively enhance the efficiency of c-Si solarcells by providing NIR emission around 1000 nm regardless ofsolid hosts. Under 363 nm excitation, the GMO:Bi,Ybnanophosphor exhibited the highest intensity, and the internalquantum efficiencies of REMO:Bi,Yb with RE = Gd, Y, and Luwere calculated to be 3.23, 1.84, and 0.53%, respectively. ThePLE spectra monitored at 975 nm showed a broad excitationband between 300 and 500 nm for all REMO:Bi,Yb, attributedto the CT transition between Mo 4d and Bi6s/O 2p orbitals.The peak position of LuMO:Bi,Yb at 410 nm shifted towardlonger wavelengths compared to those of GMO:Bi,Yb andYMO:Bi,Yb (around 363 nm). This shift was consistent withthe reflectance property shown in Figure 3. To determine theorigin of the order of luminescence intensity, temperature-dependent PL spectra were evaluated, as shown in Figure S9and Figure 4b. The PL intensity for all REMO:Bi,Yb graduallydecreased with increasing temperature, while peak shift orspectral shape change was not observed. Figure 4b presentsplots of integrated PL intensity in the NIR regions forREMO:Bi,Yb. The temperatures at which PL intensity reducedby 50% compared to that at 80 K for REMO:Bi,Yb for RE =Gd, Y, and Lu were estimated to be 200, 221, and 170 K,respectively. The activation energy for REMO:Bi,Yb in themeasured region was evaluated using the Arrhenius equationsexpressed by the following equation (eq 3):I TI E kT( ) 11 ( / )exp( / )0 0=+ (3)where I(T) and I0 are the PL intensities at temperature T andT = 80 K, respectively, Γυ is the radiative rate, Γ0 is the attemptrate for the nonradiative process, E is the activation energy, andk is the Boltzmann constant (8.617 × 10−5 eV K−1). Theestimated E values were 34.8, 28.3, and 29.6 meV for RE = Gd,Y, and Lu, respectively. The higher E values mean that thephosphor shows less quenching with increasing temperature.However, this was not consistent with our results, suggestingthat the temperature dependency did not contribute to theorder of PL intensity. Consequently, the lifetime of Yb3+ inREMO:Bi,Yb (λex= 363 nm, λem= 975 nm) was evaluated atroom temperature as shown in Figure 4c. The decay curveswere well-fitted with the equation (eq 4).I t AtAt( ) exp exp1122ikjjjjjy{zzzzzikjjjjjy{zzzzz= +(4)where τ1,2, A1,2, and I(t) are the lifetime, decay components,and intensity at time t, respectively. In addition, the averagelifetimes (τave) were estimated by the following equation (eq5):A AA Aave1 122 221 1 2 2= ++ (5)The fitted parameters are given in Table S4. The τave valueswere 207 μs for GMO:Bi,Yb, 113 μs for YMO:Bi,Yb, and 64.4μs for LuMO:Bi,Yb, and the values were similar to other Yb3+-doped NIR phosphors, such as CaLaNb3O10:Yb (211 μs) andCdMoO4:Yb (112 μs).48,49 The estimated values decreased asthe ionic radius of the RE site decreased. It is considered thatthe shortened lifetime is caused by concentration quenching.Hence, the order of luminescence intensities in the NIRspectrum for REMO:Bi,Yb nanophosphors could be attributedto concentration quenching, which tends to occur morefrequently as the ionic radius decreases.3.2. Gd2Mo1−xWxO6:Bi,Yb Nanophosphor. BecauseGd2MoO6:Bi,Yb exhibited the highest PL intensity inRE2MoO6:Bi,Yb (RE = Gd, Y, and Lu), the Mo site wasFigure 4. (a) PL·PLE spectra (λex= 363 nm, λem= 975 nm), (b) plot of integrated PL intensity in the NIR region and Arrhenius plot (inset), and(c) decay curve (λex= 360 nm, λem= 975 nm) at room temperature of REMO:Bi,Yb (RE = Gd, Y, and Lu).Figure 5. (a) XRD patterns (*: impurity phase of Gd2(WO4)3) and (b, c) XANES spectra of Mo K- and W L3-edge with reference samples ofMoO3, MoO2, WO3, and WO2 for Gd2Mo1−xWxO6:Bi,Yb. (c) TEM image of Gd2Mo0.5W0.5O6:Bi,Yb.The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c04814J. Phys. Chem. C 2024, 128, 20360−2036820364https://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig5&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c04814?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-assubstituted into the W atom focusing on Gd2MoO6:Bi,Yb.Figure 5a displays the XRD patterns for Gd2Mo1−xWxO6:Bi,Yb.The synthesis of Gd2Mo1−xWxO6:Bi,Yb was achieved with apure phase at x = 0, 0.2, and 0.5. However, an impurity phaseof Gd2(WO4)3 was observed at x = 0.8 and 1 (marked as *),indicating that the solid solution limits of Mo and W in GMOexist between x = 0.5 and 0.8 in this solvothermal reactionmethod. Therefore, only Gd2Mo1−xWxO6:Bi,Yb with x = 0, 0.2,and 0.5 were subjected to further evaluation for comparison.Based on the XANES spectra of the Mo K-edge and the W L3-edge (Figure 5b,c), the oxidation states of Mo and W atoms inGd2Mo1−xWxO6:Bi,Yb were determined to be hexavalent. Thiswas evidenced by the absorption edge at 20.009 keV for theMo K-edge and 10.204 keV for the W L3-edge, consistent withthe reference samples of MoO3 and WO3. Additionally, fromthe derivative XANES spectra of the Mo K-edge and W L3-edge (Figure S10), the peak positions agreed well with MoO3or WO3, not with MoO2 or WO2. Therefore, the formation ofthe solid solution with Mo and W did not alter their oxidationstates, and there was no need to consider the existence ofreductive states of Mo6+ and W6+. The TEM image in Figure5d shows that Gd2Mo0.5W0.5O6:Bi,Yb has a sphericalmorphology of 50 nm, consistent with that of GMO:Bi,Yb.Thus, the morphology is not affected by W substitution, andGd2Mo1−xWxO6:Bi,Yb is expected to suppress the visible lightscattering as well as the REMO:Bi,Yb nanophosphor.Figure 6 displays the DRS spectra for Gd2Mo1−xWxO6:Bi,Yb(x = 0, 0.2, and 0.5), which exhibit absorptions from 200 to300 nm due to CT transitions between Bi 6s/O 2p orbitals andMo 4d or W 5d orbitals. Additionally, absorption due to theYb3+ ions was observed in the NIR region around 1000 nm.Furthermore, a shift of the absorption edge toward shorterwavelengths was observed as x increased. The estimated valuesof Eg, obtained from eqs 1 and 2, were 2.88, 2.97, and 3.05 eVfor x values of 0, 0.2, and 0.5, respectively. The increase of Egvalues could be attributed to the enhanced contribution of theW 5d orbital, which is located at a higher energy level than theMo 4d orbitals.43,50 These results indicated that the absorptionproperty can be tuned by the ratio of the W and Mo amount.F i g u r e 7 a shows t h e PLE spe c t r a f o r t h eGd2Mo1−xWxO6:Bi,Yb (x = 0, 0.2, and 0.5) monitored at anNIR luminescence of 975 nm. The discrepancy between thePLE spectra of the GMO:Bi,Yb spectrum and Figure 4a is dueto the presence or absence of the detector correction. In thePLE spectra for Gd2Mo1−xWxO6:Bi,Yb, no correction allowedfor the deconvolution of the excitation band. The PLE spectrafor Gd2MoO6:Bi,Yb were deconvoluted into two bands,attributed to the 6s → 6p transition of Bi3+ ions and CTtransition between Bi 6s/O 2p hybrid orbitals and Mo 4dorbitals, respectively, peaking at 303 and 380 nm.21 Incomparison, the PLE spectra for Gd2Mo1−xWxO6:Bi,Yb (x =0.2 and 0.5) were deconvoluted into three bands peaking atapproximately 283, 342, and 377 nm. Based on the attributionof PLE spectra in GMO:Bi,Yb, the bands at 283 and 377 nmshould be attributed to 6s → 6p of Bi3+ and the CT transition,respectively, as these band positions would not show drasticchanges with W substitution. Therefore, the band at 342 nmwas due to the CT transitions between the Bi 6s/O 2p hybridorbitals and the W 5d orbitals. From the PL spectra for allGd2Mo1−xWxO6:Bi,Yb under 363 nm excitation (Figure 7b),NIR luminescence was observed from 900 to 1150 nm due tothe 4f-4f transitions of Yb3+ ions. In Gd2Mo1−xWxO6:Bi,Yb,Gd2Mo0.5W0.5O6:Bi,Yb exhibited the highest PL intensity, withan increase of up to 1.7 times compared to the GMO:Bi,Yb,The internal quantum efficiencies for Gd2Mo1−xWxO6:Bi,Yb (x= 0, 0.2, and 0.5) under 363 nm excitation were found to be3.23, 2.55, and 3.81%, respectively. The results indicate thatthe concentration of the W6+ ion has an effect on theluminescence intensity. From the normalized PL spectra forGd2Mo1−xWxO6:Bi,Yb (x = 0, 0.2, and 0.5) as displayed inFigure S11, there was no shift or spectral shape difference,indicating the similarity of the crystal field effects on the Yb3+ions . F igure 7c presents the decay curve forGd2Mo1−xWxO6:Bi,Yb (λex= 363 nm, λem= 975 nm) at roomtemperature. The decay curve follows eq 5, and the fittedvalues are summarized in Table S5. The τave values remainedconsistent across different W concentrations (x = 0:207,0.2:208, and 0.5:213 μs), which mean that there were nochanges in radiative transition rate of Yb3+. Therefore, thevariation in PL intensity observed in Gd2Mo1−xWxO6:Bi,Ybmight be attributed to the energy transfer processes from CTtransitions in Gd2Mo1−xWxO6:Bi to Yb3+.3.3. Current−Voltage (I−V) Characteristics of theGMO:Bi,Yb Nanophosphor. In this study, we focused on theGMO:Bi,Yb nanophosphor due to its strong luminescenceintensity and possibility up to long wavelengths. Figure 8adisplays the in-line transmittance spectra for the PDMS/GMO:Bi,Yb film, as well as the PDMS film put onto quartzglass as the reference. The PDMS film exhibited hightransmittance, approximately 92%, while it showed a reductionof transmittance from near-UV (NUV) to the NIR region ofFigure 6. UV−vis-NIR diffuse reflectance spectra and Tauc plot(inset) of Gd2Mo1−xWxO6:Bi,Yb (blue: x = 0, light blue: x = 0.2,green: x = 0.5).Figure 7. (a) Normalized PLE spectra (λem= 975 nm), (b) PLspectra, and (c)decay curve (λex= 360 nm, λem= 975 nm) forGd2Mo1−xWxO6:Bi,Yb (blue: x = 0, light blue: x = 0.2, and green: x =0.5).The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c04814J. Phys. Chem. C 2024, 128, 20360−2036820365https://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig7&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c04814?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asapproximately 1% compared to quartz. This reduction shouldnot significantly impact conversion efficiency, although strongabsorption was observed in the wavelength region below 250nm due to the intrinsic absorption of PDMS. It suggests thatthe GMO:Bi,Yb nanophosphor with absorption over 250 nmwould be beneficial if using PDMS as a polymer resin tomitigate the effects of absorption of PDMS. Compared to thePDMS film, the PDMS/GMO:Bi,Yb film displayed a reductionin transmittance of 2−4% in the visible to NIR region due toscattering and absorption by GMO:Bi,Yb nanophosphors.Additionally, the transmittance in the NUV region around 300nm decreased further by approximately 10% due to the LMCTtransition in GMO:Bi,Yb. Therefore, it was anticipated that thePDMS/GMO:Bi,Yb film would enhance the properties of c-Sisolar cells. Figure S12 shows a photograph of the PDMS/GMO:Bi,Yb film, which was highly transparent due to theabsorption of GMO:Bi,Yb only in the UV region and itsnanoscale size with an average thickness of 312 μm. Figure 8billustrates the current−voltage (I−V) characteristic of c-Si withand without a film. The conversion efficiency (η) of bare c-Si,PDMS, and PDMS/GMO:Bi,Yb film was 5.47, 4.71, and5.01%, respectively. The PDMS film led to a 0.8% decrease inη value compared to the bare state, attributed to the scatteringof the light, indicating that film deposition conditions, such asfilm thickness, are necessary to be optimized. Although there isstill room for improvement in the deposition conditions, the ηvalue of the PDMS/GMO:Bi,Yb film is 0.3% higher than thatof the PDMS film. This improvement was attributed to thecontribution of the GMO:Bi,Yb nanophosphor. Moreover, theconversion efficiency was evaluated by varying the concen-trations of the GMO:Bi,Yb chloroform solution. The η valuesfor 0.5, 1.0, and 1.5 mg/mL were found to be 4.94, 5.01, and4.42%, respectively, with the highest efficiency observed at aconcentration of 1.0 mg/mL.4. CONCLUSIONSIn this research, RE2MoO6:Bi,Yb (RE = Gd, Y, Lu) andGd2Mo1−xWxO6:Bi,Yb (x = 0−0.5) nanophosphors weresuccessfully synthesized using a solvothermal reaction method.The RE2MoO6:Bi,Yb nanophosphors exhibited peaks corre-sponding to the Yb3+ and Bi3+ ions in XPS spectra, suggestingsuccessful substitution in the host matrix. The TEM imagesrevealed that all samples had spherical nanoparticles with adiameter of approximately 50 nm, and no difference wasobserved among them. Furthermore, the RE2MoO6:Bi,Ybnanophosphor exhibited absorption in the UV region and highreflectivity in the visible region. A broad excitation bandattributed to the CT transition between the Bi 6s/O 2p andMo 4d orbitals was observed. Additionally, upon excitation at364 nm, the RE2MoO6:Bi,Yb nanophosphor displayed NIRluminescence around 1000 nm due to the 4f-4f transition ofYb3+ ions, and Gd2MoO6:Bi,Yb exhibited the highestluminescence intensity. By tuning the chemical compositionsof Gd2Mo1−xWxO6:Bi,Yb nanophosphors, the band edge inreflectance spectra shifted toward a shorter wavelength withincreasing x values, and also the blue shift of peak position wasobserved in the PLE spectra. The PL intensity at 975 under363 nm excitation was the highest in Gd2Mo0.5W0.5O6:Bi,Yb.The PDMS/GMO:Bi,Yb film showed a 0.5% increase inconversion efficiency compared to the PDMS film, indicatingthat the GMO:Bi,Yb nanophosphor played a significant role inthe enhancement. This study suggests that other phosphorsalso improve the conversion efficiency of c-Si solar cells, andthe combination of GMO:Bi,Yb nanophosphors could helpaddress global energy issues.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814.XPS spectra, XANES spectra, Rietveld refinement, Taucplot, EXAFS, PL spectra, and photographs (PDF)■ AUTHOR INFORMATIONCorresponding AuthorTakuya Hasegawa − Institute of Multidisciplinary Research forAdvanced Material (IMRAM), Tohoku University, Sendai980-8577, Japan; orcid.org/0000-0002-6170-5632;Email: hase@tohoku.ac.jpAuthorsTaisei Hangai − Institute of Multidisciplinary Research forAdvanced Material (IMRAM), Tohoku University, Sendai980-8577, Japan; orcid.org/0009-0002-8165-0568Jian Xu − International Center for Young Scientists (ICYS),National Institute for Materials Science (NIMS), Tsukuba,Ibaraki 305-0044, Japan; orcid.org/0000-0002-1040-5090Takayuki Nakanishi − Advanced Phosphor Group, NationalInstitute for Materials Science (NIMS), Tsukuba, Ibaraki305-0044, Japan; orcid.org/0000-0003-3412-2842Takashi Takeda − Advanced Phosphor Group, NationalInstitute for Materials Science (NIMS), Tsukuba, Ibaraki305-0044, Japan; orcid.org/0000-0003-2510-4562Tomoyo Goto − SANKEN (The Institute of Scientific andIndustrial Research), Osaka University, Ibaraki, Osaka 567-0047, Japan; Institute for Advanced Co-Creation Studies,Osaka University, Suita, Osaka 565-0871, Japan;orcid.org/0000-0003-1362-6750Yasushi Sato − Department of Chemistry, Faculty of Science,Okayama University of Science, Okayama 700-0005, Japan;orcid.org/0000-0001-5132-0301Figure 8. (a) Transmittance spectra of quartz, PDMS film, andPDMS/GMO:Bi,Yb film. (c) I−V curve for c-Si solar cells with thePDMS film and c-Si/PDMS/GMO:Bi,Yb film.The Journal of Physical Chemistry C pubs.acs.org/JPCC Articlehttps://doi.org/10.1021/acs.jpcc.4c04814J. Phys. Chem. C 2024, 128, 20360−2036820366https://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.4c04814/suppl_file/jp4c04814_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takuya+Hasegawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-6170-5632mailto:hase@tohoku.ac.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Taisei+Hangai"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0009-0002-8165-0568https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jian+Xu"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-1040-5090https://orcid.org/0000-0002-1040-5090https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takayuki+Nakanishi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-3412-2842https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Takeda"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-2510-4562https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomoyo+Goto"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-1362-6750https://orcid.org/0000-0003-1362-6750https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yasushi+Sato"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-5132-0301https://orcid.org/0000-0001-5132-0301https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ayahisa+Okawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.jpcc.4c04814?fig=fig8&ref=pdfpubs.acs.org/JPCC?ref=pdfhttps://doi.org/10.1021/acs.jpcc.4c04814?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asAyahisa Okawa − Institute of Multidisciplinary Research forAdvanced Material (IMRAM), Tohoku University, Sendai980-8577, JapanShu Yin − Institute of Multidisciplinary Research for AdvancedMaterial (IMRAM), Tohoku University, Sendai 980-8577,Japan; Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan;orcid.org/0000-0002-5449-4937Complete contact information is available at:https://pubs.acs.org/10.1021/acs.jpcc.4c04814NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported by research granted from TheMurata Science Foundation; JSPS KAKENHI (grant numbers20K15106, 22K05264, and 20H00297); Feasibility Study forYoung Researchers (FS) in “Crossover Alliance to Create theFuture with People, Intelligence and Materials”, MEXT, Japan;the establishment of university fellowships towards the creationof science technology innovation, grant JPMJFS2102; andNIMS Joint Research Hub Program. 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