Quantum-Cutting Luminescent Solar Concentrators Using Ytterbium

Dec 10, 2018 - We introduce and demonstrate the concept of quantum-cutting luminescent solar concentrators (QC-LSCs) using Yb3+-doped perovskite ...
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Quantum Cutting Luminescent Solar Concentrators Using Ytterbium Doped Perovskite Nanocrystals Xiao Luo, Tao Ding, Xue Liu, Yuan Liu, and Kaifeng Wu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03966 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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Concentrators Using Ytterbium Doped Perovskite Nanocrystals Xiao Luo†, Tao Ding†, Xue Liu, Yuan Liu, and Kaifeng Wu* State Key Laboratory of Molecular Reaction Dynamics and Dynamics Research Center for Energy and Environmental Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China

KEYWORDS: Quantum cutting; Luminescent solar concentrators; Solar energy; Doped nanocrystals; Ytterbium doping

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Abstract: We introduce and demonstrate the concept of quantum-cutting luminescent solar concentrators (QC-LSCs) using Yb3+-doped perovskite nanocrystals. These NCs feature a photoluminescence quantum yield approaching 200% and virtually zero self-absorption loss of PL photons, defining a new upper limit of 150% for the internal optical efficiency (ηint) of LSCs that is almost independent of LSC sizes. An unoptimized 25 cm2 QC-LSC fabricated from Yb3+-doped CsPbCl3 NCs already displayed an ηint of 118.1±6.7% that is 2-fold higher than previous records using Mn2+-doped quantum dots (QDs). If using CsPbClxBr3-x NCs capable of absorbing ~7.6% of solar photons, the projected external optical efficiency (ηext) of QC-LSCs can exceed 10% for >100 cm2 devices which still remains a big challenge in the field. The advantage of QC-LSCs over conventional QD-LSCs becomes especially obvious with increasing LSC sizes, which is predicted to exhibit more than 4-fold efficiency enhancement in the case of window size (1 m2) devices. TOC Figure

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Luminescent Solar Concentrators (LSCs) are large-area sunlight collectors that absorb incident solar photons, emit photoluminescence (PL) photons and waveguide these photons by total internal reflection to the device edges to drive attached photovoltaic (PV) cells.1 The ratio between edge-emitted photons and absorbed solar photons defines the internal optical efficiency (ηint) of an LSC, while that between edge-emitted photons and incident solar photons defines the external optical efficiency (ηext). ηint and ηext are related by: ηext = ηint × ηabs, with ηabs being the LSC absorption efficiency for solar photons.2 Common luminophores for LSCs such as organic dyes3-5 and colloidal quantum dots (QDs)6-13 often suffer from reabsorption loss of PL photons along the waveguiding path, lowering the ηint of an LSC. Mn2+-doped QDs using Mn2+-dopant emission have been reported for virtually “zero-reabsorption-loss” LSCs,2,

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as have organic dyes using triplet emission.16 PL

quantum yields (ηPL) of these luminophores, however, are typically ≤ 80%, and therefore the ηint of an LSC has never exceeded the limit defined by the product of light trapping efficiency (ηtrap; ~75%) and ηPL which is ~60%.2 Note that ηtrap of ~75% is determined by the refractive index of the waveguide (~1.5 for common glass materials and polymers such as polymethyl methacrylate) and that of the air (~1) . Quantum-cutting is a peculiar optical phenomenon referring to the emission of two low-energy photons by absorbing one high-energy photon.17 It offers a disruptive solution to exceeding the above limit for ηint. Specifically, by emitting two photons at a wavelength far from the absorption edge, the reabsorption loss can be eliminated and meanwhile the PL QY is doubled. In the ideal situation, LSCs fabricated from quantum-cutting materials can attain an LSC-size-independent ηint of 150% (75% × 200%). Here we apply recently reported 3 ACS Paragon Plus Environment

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Yb3+-doped CsPbCl3 NCs18-20 to demonstrate the concept of quantum-cutting LSCs (QC-LSCs). The PL QY of these NCs reaches 164%, enabling a 25 cm2 LSC with an ηint of 118.1%. As a result, despite the weak absorption of CsPbCl3 NCs towards sunlight (ηabs ≤ 3.1%), the QC-LSC attains an ηext approaching 3.7%. If we use Yb3+-doped CsPbClxBr3-x NCs capable of absorbing ~7.6% of solar photons, the projected ηext of QC-LSCs can exceed 10% for >100 cm2 devices which still remains a big challenge for LSCs based on either organic dyes or inorganic QDs. We note that Yb3+-doped CsPbCl3 NCs have been applied as down converters to enhance the efficiency of solar cells in a recent work.20 However, LSCs have radically different scopes from down converters. LSCs can significantly reduce the cost of expensive PV technologies (such as III-V PVs) by allowing for attaching only small amounts of PV cells onto the edges of LSCs. They can also enable novel-concept PV technologies such as semitransparent solar windows and solar sidings.4, 21

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Figure 1. (a) PL spectra (solid lines with shading) of undoped (purple) and Yb3+-doped (wine) CsPbCl3 NCs excited with a 365 nm light source. Top inset shows their absorption spectra. Bottom inset is a TEM image of doped NCs. (b) Total (wine), face (red) and edge (orange) emissions measured for a 5 cm × 5cm QC-LSC using Yb3+-doped CsPbCl3 NCs. The ηint of this LSC is measured to be 118.1±6.7%. Top inset is the picture of the LSC under sunlight; bottom inset shows the edge emission from the LSC under UV illumination and 4 ACS Paragon Plus Environment

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with a 570 nm long-pass filter (right) taken using a cell phone (MI® 8 SE) camera. Yb3+-doped CsPbCl3 NCs were synthesized using a literature method;18 see Supporting Information (SI) for details. Fig. 1a bottom inset shows the transmission electron microscopy (TEM) image of Yb3+-doped CsPbCl3 nanocubes with edge lengths of 15.8±1.8 nm; the undoped ones are shown in Fig. S1. The Yb3+ atomic concentration was determined to be ~6% using energy dispersive X-ray spectroscopy (EDX). The absorption spectra of doped and undoped CsPbCl3 NCs are almost the same (Fig. 1a top inset), while their PL spectra (excited at 365 nm) are radically different (Fig. 1a). Due to effective excitation energy transfer from CsPbCl3 host to Yb3+-dopants, the band edge PL of NCs is strongly quenched in the doped sample and is replaced by a strong, sharp PL band at ~990 nm arising from the 2F5/2 → 2F7/2 f-f emission of Yb3+-dopants.18 The PL QY of the dopant emission is measured to be 164±7% using a calibrated integrating sphere (SI), an indication of quantum-cutting, that is, excitation of two dopant excited states by one host exciton. As measured by transient absorption spectroscopy (Fig. S2), the quantum-cutting process occurs on a picosecond timescale, consistent with previous reports.18, 22 Yb3+-doped CsPbCl3 NCs were incorporated into LSCs by coating NC-polymethyl methacrylate (PMMA) mixtures as thin-films onto borosilicate glass substrates using doctor-blading or spin-coating methods (SI). As demonstrated previously, doctor-blading is particularly suited for large-size LSCs.12 The PL QY of NCs is retained in the film. Fig. 1b top inset displays the picture of a 5 cm × 5 cm × 0.2 cm (thickness) LSC under sunlight, showing a perfectly transparent appearance that is well suited for the purpose of building-integrated solar windows. By applying UV illumination in combination with a 570 5 ACS Paragon Plus Environment

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nm long-pass filter, strong near-infrared (IR) emissions from the edges can be seen using a cell phone camera. The performance of this QC-LSC was evaluated using an integrating sphere method; see SI for details. As established by previous works, this method enables detailed insights into the efficiency loss mechanisms of LSCs by allowing for differentiating total, face and edge emissions from LSCs (Fig. 1b). The ratio between the edge and total emissions of the QD-LSC (ηedge) is 72±1%, which is slightly lower than the ideal light trapping efficiency of 75% due to weak scattering loss in the LSC.2 The ηint of this LSC is then calculated to be 118.1±6.7% using ηint = ηPL × ηedge. Measurements on another independently prepared LSC give the same result (Fig. S3). This ηint value sets a record that is more than 2-fold higher than the previous one (~58%).2 Based on the geometric gain factor (G) of the LSC (ratio between face and edge areas; ~6.25 for the 5× 5 × 0.2 cm LSC), we can calculate the internal concentration factor: Cint = Gηint ~7.4. Since the maximum solar photon absorption efficiency (ηabs) attainable with CsPbCl3 NCs is only 3.1%, the ηext of the LSC is 3.7%, which is not high compared to previous QD-LSCs with similar sizes.2, 6-8, 12, 23 The external concentration factor (Cext) for incident solar photons is therefore also limited to ~0.2.

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Figure 2. (a) Extrapolated ηint for square-shaped QC-LSCs with edge lengths from 1 to 100 cm and the same thickness of 0.2 cm (wine solid line). The red circle is the one measured in Figure 1. ηint for Mn2+-doped QD-LSCs adapted from previous work is also shown for comparison (yellow dashed line).2 Inset is a 30 cm long QC-LSC slab. (b) Extrapolated ηext for square-shaped QC-LSCs with edge lengths from 1 to 100 cm for solar photon absorption efficiency (ηabs) of 3.1% (orange solid line), 7.6% (red solid line) and for ηabs of 7.6% and ηPL of 200% (wine solid line). ηext for CuInSe2 QD-LSCs adapted from previous work is also shown for comparison (purple dashed line).2 The following part of this paper makes projections for the performance of QC-LSCs by adjusting device parameters such as LSC sizes, ηabs and ηPL. First, using a previously established scaling law (SI),2 we can project the ηint of LSC with larger sizes. In practice, the LSC-size can be readily scaled up using the doctor-blading method; for example, Fig. 2a inset shows the picture of a 30-cm-long LSC slab. Fig. 2a shows the extrapolated ηint for square-shaped LSCs with the same thickness of 0.2 cm. According to this extrapolation, even a 100 cm QC-LSC can retain an ηint of 71%, which is much higher than the previous record achieved using Mn2+-doped QD-LSCs (~41%).2 As mentioned above, the ηext of CsPbCl3-based QC-LSCs is limited by their low ηabs. However, the advantage of reabsorption-free LSCs becomes more obvious for larger-size LSCs. According to our extrapolation, a 100 cm CsPbCl3-based QC-LSC should have an ηext of 2.1% (Fig. 2b), which 7 ACS Paragon Plus Environment

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is already superior to our previous LSCs using near-IR CuInSe2 QDs (ηext ~ 1.5% for 100 cm device).2 Next, we make projections for the scenario of improved solar photon absorption efficiency. This should be achievable by using CsPbClxBr3-x NCs with an absorption onset at ~480 nm that still allows for efficient quantum-cutting according to a recent report.22 In this case, ηabs can be improved to ~7.6%. Assuming the same ηPL of 164% and the same size-dependence of ηint as in Fig. 2a, ηext can reach 9.0% for 5 cm QC-LSC (Fig. 2b). Note that, however, improved sunlight absorption is achieved at the expense of reduced visible transmittance. An absorption onset at ~480 nm would lead to yellowish LSCs that are not well suited for solar windows but may find alternative applications such as solar sidings and solar panels. Finally, if ηPL of Yb3+-doped CsPbClxBr3-x NCs is further optimized to approaching the theoretical limit of 200%,22 ηext can reach 10.6% for 10 cm QC-LSC (Fig. 2b). Note that ηext exceeding 10% for >100 cm2 devices has been a big challenge for LSCs using QDs. For example, our previous work using near-IR CuInSe2 QDs (absorption onset at ~800 nm) with a reasonably high ηPL of ~72% achieved an ηext of 5.8% for 100 cm2 LSC.2 Another work using CuInS2 QDs with similar absorption onset but a very high ηPL of ~91% reported an ηext of 8.1% for 100 cm2 LSC.23 Therefore, arguably, it is difficult even for nearly “perfect” near-IR QDs to display an ηext > 10% for > 100 cm2 LSC due to severe efficiency loss to reabsorption in large-area devices. According to our extrapolation in Fig. 2b, a 1 m2 window-size QC-LSC can still maintain an ηext of 6.3% that is more than 4-fold higher than that of our previous device of the same area using near-IR CuInSe2 QDs.2 8 ACS Paragon Plus Environment

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In addition to enabling a higher optical efficiency, QC-LSCs using Yb3+-doped NCs should couple better with silicon photovoltaics (PV) than typical near-IR-emitting LSCs to deliver a higher solar-to-electricity power conversion efficiency for a complete LSC-PV system. This is because the sharp emission of Yb3+-dopants at 990 nm matches perfectly with the peak external quantum efficiency (EQE) of silicon solar cells, whereas CuInS2 or CuInSe2 QDs, for example, have broad emission spanning the lower region of the EQE curve.7 Although we haven’t tested complete coupled LSC-PV systems, we follow the procedure in ref. 11 to predict the flux gain (FG) of Si PVs attached to square-shaped QC-LSCs (thickness of 0.2 cm) based on CsPbClxBr3-x NCs with ηabs of 7.6% and ηPL of 200%. FG can be viewed as the effective enhancement factor for photocurrent of Si PVs in coupled systems as compared to stand-alone PVs (Fig. 3 inset): FG = qGηext, where q is the spectral reshaping factor defined as the ratio between the EQE of Si PVs averaged over the LSC PL spectrum and that averaged over the whole solar spectrum; see SI for calculation details. According to this calculation, for a 100 cm QC-LSC (corresponding to G = 125), FG can reach ~14 (Fig. 3) that is higher than all types of QD-LSCs of the same G reported in ref. 11. A higher FG (~27) was predicted for 100 cm square-shaped LSCs (G ~ 150) based on CuInS2/CdS core/shell QDs in ref. 24, which was enabled the high ηPL (~86%) and, in particular, the broad-band solar absorption (onset at ~700 nm) of these QDs. Note, however, this projection was made for solution-like, scattering-free LSCs. In real LSC devices based on very similar QDs but including scattering losses,25 an FG of only ~2.6 was predicted for an LSC with G = 100.

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In conclusion, we introduced the concept of quantum-cutting LSCs (QC-LSC) using Yb3+-doped perovskite NCs. A prototype QC-LSC of 5 cm × 5cm fabricated from Yb3+-doped CsPbCl3 NCs was demonstrated, with its internal optical efficiency reaching 118.1±6.7% that is 2-fold higher than previous records. The external optical efficiency of QC-LSCs was predicted to be able to exceed 10% for > 100 cm2 area by using CsPbClxBr3-x NCs to enhance absorption efficiency for solar photons. QC-LSCs are also an ideal choice to implement as top-layer LSCs in tandem LSCs, as they absorb very limited sunlight but can deliver the highest ever efficiency out of the absorbed light.

AUTHOR INFORMATION Corresponding Author* [email protected]

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge financial supports from the Strategic Pilot Science and Technology Project of Chinese Academy of Sciences (XDA21010206), the Dalian City Foundation for Science and Technology Innovation (2018J12GX051) and the National Natural Science Foundation of China (21773239). We also thank Prof. Yu Zhang from Jilin University for providing samples for initial absorption and PL measurements. 11 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information. Figures S1-S3; sample preparations; LSC fabrications and measurements; LSC efficiency calculation and extrapolation; Flux gain (FG) calculation and extrapolation. REFERENCES: 1.

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