Greener Luminescent Solar Concentrators with High Loading

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Greener Luminescent Solar Concentrators with High Loading Contents Based on In-Situ Cross-Linked Carbon Nano-Dots for Enhancing SolarEnergy Harvesting and Resisting Concentration-Induced Quenching Maria Jessabel Aviles Talite, Hsiu-Ying Huang, Yao-Hsuan Wu, Princess Genevieve Sena, Kun-Bin Cai, Tzu-Neng Lin, Ji-Lin Shen, Wu-Ching Chou, and Chi-Tsu Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10618 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 15, 2018

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Greener Luminescent Solar Concentrators with High Loading Contents Based on In-Situ Cross-Linked Carbon Nano-Dots for Enhancing Solar-Energy Harvesting and Resisting Concentration-Induced Quenching Maria Jessabel Talite 1, Hsiu-Ying Huang 2, Yao-Hsuan Wu3, Princess Genevieve Sena, 2, Kun-Bin Cai 2, Tzu-Neng Lin2, Ji-Lin Shen2, Wu-Ching Chou1* and Chi-Tsu Yuan 2,3* 1

Department of Electrophysics, National Chiao Tung University, Hsinchu, 300 Taiwan 2

3

Department of Physics, Chung Yuan Christian University, Taoyuan, 320 Taiwan

Department of Nanotechnology, Chung Yuan Christian University, Taoyuan, 320 Taiwan

Abstract A luminescent solar concentrator (LSC) is composed of loaded luminophores and a waveguide that can be employed to harvest and concentrate both direct and diffused sunlight for promising applications in solar windows. Thus far, most of efficient LSCs still relied on the heavy-metal-containing colloidal quantum dots (CQDs) dispersed into a polymer matrix with a very low loading (typically < 1 wt%). Such low-loading constraint is required to mitigate the concentration-induced quenching (CIQ) and maintain high optical quality and film uniformity, but this would strongly reduce the light-absorbing efficiency. To address all issues, greener LSCs with high loading concentration were prepared by in-situ cross-linking organosilane-functionalized carbon nano-dots (Si-CNDs) and their photophysical properties relevant to LSC operation were studied. The PL emission is stable and does not suffer from severe CIQ effect for cross-linked Si-CNDs even with 25 wt% loadings, thus exhibiting high solid-state quantum yields (QYs) up to 45 ± 5% after the calibration of the reabsorption losses. Furthermore, such LSCs can still hold high optical quality and film uniformity, leading to low scattering losses and high internal quantum efficiency of ~22%. However, the reabsorption losses need to be further addressed to realize large-area LSCs based on earth-abundant, cost-effective CNDs. Key words: greener luminescent solar concentrators, cross-linked carbon nano-dots,

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high loading concentration, concentration-induced quenching

INTRODUCTION Colloidal quantum dots (CQDs) have drawn tremendous interest due to their unique photophysical properties, including tunable absorption/emission wavelengths, narrow and pure emission spectrum, and high photo-stability, that have been commercialized for light-emitting materials in display backlights and light-conversion nano-phosphors 1-3

. Unfortunately, most matured CQDs contain heavy metals and frequently need to

be prepared in the hazardous organic solvent based on a complex hot-injection method. It would be beneficial to replace those CQDs by "greener" alternatives that can be directly fabricated using a facile, mass-production method without involving toxic elements 4. In addition to materials toxicity issues, those CQDs also suffer from concentration-induced quenching (CIQ) in the solid state for the utilization in light-conversion nano-phosphors and luminescent solar concentrators (LSCs)5. Luminescent carbon nano-dots (CNDs) can be prepared using cost-effective, earth-abundant, eco-friendly precursors based on a facile hydrothermal method and exhibit some unique photophysical properties, thus are promising "greener" alternatives to compete or even replace conventional heavy-metal-containing CQDs in serving as light-emitting materials6-8. Similarly, both PL-QY reduction effects, namely CIQ and reabsorption restrict the efficiency of the CNDs in light-emitting materials9-13. It is well-known that the mechanism for concentration-induced quenching is attributed to dipole-dipole mediated resonance energy transfer, known as Forster-resonance energy transfer (FRET), whose efficiency strongly depends on the separation distance between the donors and acceptors14. As a result, a facile method to avoid such solid-state CIQ effect is to disperse the QDs by a solid matrix9,11. To this end, organic polymers such as hydrophilic PVA and hydrophobic PMMA were

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generally used for water-soluble and organic-soluble QDs15-19. However, the loading concentration of QDs in an organic polymer matrix must keep low (typically 10 would be significantly declined by the reabsorption effect. Surface-state engineering in Si-CNDs. As demonstrated previously, despite low CIQ effect, the reabsorption losses would significantly degrade the practical performance for large-area LSCs with high loadings. As previously shown in Figure S5, the reabsorption effect is mainly due to the spectral overlap between optical absorption of the surface states and PL emission

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of the core states. According to the PLE spectra monitored at the core-state emission and surface-state emission, as already shown in Figure 1b, we found that the core-state emission is mainly due to the n-π* excitation, while the surface-state emission can be induced by both n-π* excitation and surface-state excitation but with distinct PL-QYs. It implies that the surface states in cross-linked CNDs play negative roles in LSC operation in two aspects: the reabsorption of core-state emission and harvesting core-state excitation but with lower PL-QY. As a result, it is necessary to mitigate the influences of the surface states on both excitation and emission processes. Surface states of the CNDs have been modified by several strategies, for example, surface reduction/oxidation by strong reductants / oxidants and surface passivation 40,55

.

Here, a simple approach was tested to modify the electronic states of Si-CNDs with low loadings (1 wt%) by adjusting the medium pH values, as shown in Figure S9.As increasing the pH value, the PL intensity was increased accompanied with a spectral blue-shift of PL emission. We think more detailed investigation on the surface-engineering of Si-CNDs are needed in the future to further optimize the photophysical properties of Si-CNDs for developing greener, efficient LSCs.

CONCLUSION In conclusion, greener LSCs with high loading contents were prepared by in-situ cross-linking organosilane-functionalized CNDs, which can enhance light-absorbing efficiency while resisting concentration-induced quenching. The LSCs with 25 wt% loading exhibit high solid-state PL-QYs of ~45% after the calibration of the reabsorption effects. In addition, they still hold high optical quality and film uniformity, thus maintaining high edge-emission efficiency. The IQE and EQE values for the LSCs with G~10 are ~ 22% and ~12% at the excitation wavelength of ~354

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nm. However, the reabsorption effect need to be improved for realizing large-area, greener LSCs based on cost-effective, earth-abundant CNDs.

EXPERIMENTAL SECTION Materials. Citric acid monohydrate (CA), N-(3-(trimethoxysilyl)propyl) ethylene diamine (KH-792) and petroleum ether were purchased from Sigma-Aldrich. All of these reagents were of analytical grade and used without further treatment. Syntheses of Organosilane-Functionalized CNDs. A solution of silane-coupling agent KH-792 (10 ml) was gradually added into citric acid (500 mg). The resultant solution was mixed vigorously, and then transferred into a reaction vial and heated at 200°C in a microwave reactor (Monowave 300) for 3 hours. Petroleum Ether (45 ml) was consequently added to the resulting mixture and transferred to a proper vessel and spun at high speed (6000 rpm, 5 mins) using a laboratory centrifuge. The follow-on supernatant was disregarded and strained off. Another 45 ml of Petroleum Ether were added to the remainder of the solution and centrifuged again for 5 min then subsequently degassed for 5 min. As a final procedure, the as-prepared solution was filtrated using a syringe filter (0.22 µm). Fabrication of Cross-Linked Si-CND Thin-Film LSCs. To guarantee high-quality films free of contaminants and impurities, the glass substrates used were initially cleaned by ethanol and acetone bath in ultrasonic for 10 min, rinsed in de-ionized water and dried before further use. The Si-CNDs with different loading contents from 1 wt % to 75 wt% were prepared by changing the weight ratio between Si-CNDs and KH792. Those samples were uniformly coated onto the glass substrates (dimension: 30 mm × 30 mm × 3mm ) using a spin coater (3000 rpm, 30 seconds) then oven-dried for 5 min. This procedure was repeated 5

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multiple periods until the cross-linked Si-CNDs thin-films were obtained.

Characterization. Steady-state PL emission, PL excitation and time-resolved PL measurements were carried out based on a multi-functional spectrometers equipped with an integrating sphere (FluoTime 300). A xenon lamp or diode laser were used as the excitation sources, and the PL emission was collected by a PMT detector with the calibration according to the wavelength-response function of the detector. For absolute PL-QY measurement, the whole-range spectra covering both excitation and emission regions were measured for both experimental and reference samples placed within the integrating sphere. By comparing both spectra, the ratio between the number of photons emitted and the number of photons absorbed, the PL-QYs can be derived 56-58. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2010 high resolution transmission electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was conducted using Thermo Fisher Scientific K-Alpha X-ray photoelectron spectrometer. The UV–Vis absorption spectrum was recorded through a high resolution UV-visible double-beam spectrophotometer with variable spectral bandwidth and a photomultiplier tube (PMT) detector (JASCO V-750 UV–Vis spectrophotometer). A laser-scanning confocal microscope system (Micro Time 200) was used to obtain the PL image of the cross-linked Si-CNDs deposited on the glass substrates. The Raman spectrum was recorded using a Raman Microscope (HORIBA Scientific, iHR320) with a 532 nm laser wavelength. The height profile was investigated using an atomic force microscope (AFM, XE-100 PARK). The X-ray diffraction (XRD) patterns was recorded in the range of 10 ° ~ 65 ° on a Bruker D8 Advance Eco using Cu Ka radiation (k = 1.54 Å) at 40 kV and 25 mA.

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ASSOCIATED CONTENT Supporting Information The PL and PL-QYs spectra of diluted Si-CNDs. TEM imaging, AFM imaging and additional optical characterization data.

AUTHOR INFORMATION Corresponding Author *E-mail:[email protected] (Chi-Tsu Yuan) and [email protected] (Wu-Ching Chou).

ORCID Chi-Tsu Yuan:0000-0003-3790-9376 Maria Jessabel Talite and Hsiu-Ying Huang equally contributed to this work. Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the ministry of science and technology Taiwan under the grant

number,

MOST 104-2112-M-033-003-MY3,

107-2112-M-033 -004 -MY3.

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107-2112-M-033-007

and

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Multi-Doped Carbon Dots with Ratiometric pH Sensing Properties for Monitoring Enzyme Catalytic Reactions. Chem. Commun. 2016, 52, 9247-9250. (52) Coropceanu, I.; Bawendi, M. G. Core/Shell Quantum Dot Based Luminescent Solar Concentrators with Reduced Reabsorption and Enhanced Efficiency. Nano Lett. 2014, 14, 4097-4101. (53) Klimov, V. I.; Baker, T. A.; Lim, J.; Velizhanin, K. A.; McDaniel, H. Quality Factor of Luminescent Solar Concentrators and Practical Concentration Limits Attainable with Semiconductor Quantum Dots. ACS Photonics 2016, 3, 1138-1148. (54) Bergren, M. R.; Makarov, N. S.; Ramasamy, K.; Jackson, A.; Guglielmetti, R.; McDaniel, H. High-Performance CuInS2 Quantum Dot Laminated Glass Luminescent Solar Concentrators for Windows. ACS Energy Lett. 2018, 3, 520-525. (55) Songnan, Q.; Ding, Z.; Di, L.; Wenyu, J.; Pengtao, J.; Dong, H.; Lei, L.; Haibo, Z.; Dezhen, S. Toward Efficient Orange Emissive Carbon Nanodots through Conjugated sp2-Domain Controlling and Surface Charges Engineering. Adv. Mater. 2016, 28, 3516-3521. (56) Würth, C.; Grabolle, M.; Pauli, J.; Spieles, M.; Resch-Genger, U. Relative and Absolute Determination of Fluorescence Quantum Yields of Transparent Samples. Nat. Protoc. 2013, 8, 1535. (57) D.O., F.; J.J., M.; A.J., P.; D.D., P.; N.P., K.; G.A., O. Measurement of Absolute Photoluminescence Quantum Yields Using Integrating Spheres – Which Way to Go? Laser Photonics Rev. 2012, 6, 802-806. (58) Yuichiro, K.; Hiroyuki, S.; Chihaya, A. Simple Accurate System for Measuring Absolute Photoluminescence Quantum Efficiency in Organic Solid-State Thin Films. Jan. J. Appl. Phys. 2004, 43, 7729.

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Figure 1.(a) Absorbance and PL emission spectrum for as-prepared, diluted Si-CNDs and the corresponding photograph under UV irradiation and (b) PLE spectrum detected at ~450 nm and ~530 nm.

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Figure 2.(a) Excitation-wavelength-dependent PL spectra for as-prepared, diluted Si-CNDs and (b) PL spectra at different loading concentrations of Si-CNDs liquid.

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Figure 3. (a) FTIR spectrum of the Si-CNDs ; (b-d) The XPS analysis results (C 1s, N 1s, O 1s) of Si-CNDs ; (e) XRD pattern of the Si-CNDs and (f) Raman spectrum of the Si-CNDs.

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Figure 4. (a) Photographs for cross-linked Si-CNDs with different loading contents under ambient-light and UV-light illumination; (b) Optical absorbance and (c) PL emission spectra for cross-linked Si-CNDs with different loading contents.

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Figure 5. (a) Time-resolved decay profiles for cross-linked CNDs with different loading contents and (b) Laser-scanning confocal PL imaging with the sizes of 20 × 20µm 2 for cross-linked Si-CNDs with 25 wt% loading contents.

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Figure 6. (a) PL spectra for Si-CND thin films with 25 wt% loadings and reabsorption-correction referenced samples, and the photographs of the referenced samples and (b) PL stability testing under UV irradiation for cross-linked Si-CNDs.

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Figure 7. (a) PL emission spectra for cross-linked Si-CNDs with 25 wt% loadings collected from all surfaces, facial surfaces, and edge surfaces and (b) A plot of external quantum efficiency as a function of the geometric gains of the LSCs obtained by analytical models.

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TOC graphic

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Greener luminescent solar concentrators (LSCs) based on earth-abundant, cross-linked organosilanefunctionalized carbon nanodots (Si-CNDs) 341x121mm (96 x 96 DPI)

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