${\mathrm{Yb}}^{3+}$ speciation and energy-transfer dynamics in quantum-cutting ${\mathrm{Yb}}^{3+}$-doped $\mathrm{CsPb}{\mathrm{Cl}}_{3}$ perovskite nanocrystals and single crystals

${Yb}^{3 +}$ speciation and energy-transfer dynamics in quantum-cutting ${Yb}^{3 +}$ -doped $CsPb {Cl}_{3}$ perovskite nanocrystals and single crystals

Joo Yeon D. Roh, Matthew D. Smith, Matthew J. Crane, Daniel Biner, Tyler J. Milstein, Karl W. Krämer, and Daniel R. Gamelin

Phys. Rev. Materials 4, 105405 – Published 27 October 2020

Abstract

${Yb}^{3 +}$ -doped inorganic metal-halide perovskites $(Y b^{3 +} : CsPb X_{3}$ , $X = Cl, Br)$ have recently been discovered to display highly efficient quantum cutting, in which the energy from individual blue or UV photons absorbed by the material is reemitted in the form of pairs of near-infrared photons by ${Yb}^{3 +}$ dopants. Experimental photoluminescence quantum yields approaching 200% have been reported. As the first quantum-cutting materials that combine such high-photoluminescence quantum yields with strong, broadband absorption in the visible, these materials offer unique opportunities for enhancing the efficiencies of solar technologies. Little is known about the fundamental origins of this quantum cutting, however. Here, we describe variable-temperature and time-resolved photoluminescence studies of ${Yb}^{3 +} : CsPb {Cl}_{3}$ in two disparate forms–colloidal nanocrystals and macroscopic single crystals. Both forms show very similar spectroscopic properties, demonstrating that quantum cutting is an intrinsic property of the ${Yb}^{3 +} : CsPb X_{3}$ composition itself. Diverse ${Yb}^{3 +}$ speciation is observed in both forms by low-temperature photoluminescence spectroscopy, but remarkably, quantum cutting is dominated by the same specific ${Yb}^{3 +}$ species in both cases. Time-resolved photoluminescence measurements provide direct evidence of the previously hypothesized intermediate state in the quantum-cutting mechanism. This intermediate state mediates relaxation from the photogenerated excited state of the perovskite to the emissive excited state of ${Yb}^{3 +}$ , and hence is of critical mechanistic importance. At room temperature, this intermediate state is populated within a few picoseconds and has a decay time of only ∼7 ns in both nanocrystalline and single-crystal ${Yb}^{3 +} : CsPb {Cl}_{3}$ . The mechanistic implications of these observations are discussed. These results provide valuable information about characteristics of this unique quantum cutter that will aid its optimization and application in solar technologies.

Received 7 July 2020
Revised 28 September 2020
Accepted 1 October 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.105405

Physics Subject Headings (PhySH)

Defects Electronic structure Luminescence

Energy materials Nanoparticles Photovoltaic absorbers Single crystal materials Solar cells

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Joo Yeon D. Roh ¹, Matthew D. Smith¹, Matthew J. Crane ¹, Daniel Biner², Tyler J. Milstein ¹, Karl W. Krämer ², and Daniel R. Gamelin ^1,*

¹Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA
²Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland

^*gamelin@chem.washington.edu

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Vol. 4, Iss. 10 — October 2020

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Figure 1
(a) TEM image of 4.3% ${Yb}^{3 +} : CsPb {Cl}_{3}$ NCs and (b) a photograph of a 2% ${Yb}^{3 +} : CsPb {Cl}_{3}$ SC. Tick marks indicate mm spacings. (c) Powder XRD data collected for the same ${Yb}^{3 +} : CsPb {Cl}_{3}$ NCs (red) and SC (blue). Reference indices (black) are shown for the cubic high-temperature form of $CsPb {Cl}_{3}$ (PDF 73–692), which is stable in bulk above ∼37–47 °C [32]. Bulk $CsPb {Cl}_{3}$ adopts the orthorhombic ${GdFeO}_{3}$ -type structure (space group Pnma) at room temperature [31], giving rise to additional small reflections that are not visible on the scale of panel (c).
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Figure 2
(a) 5 K PL spectra of ${Yb}^{3 +} : CsPb {Cl}_{3}$ NCs (red, $[{Yb}^{3 +}] = 1.7 %$ ) and the SC sample from Fig. 1 (blue, $[{Yb}^{3 +}] = 2 %$ ), normalized to the peak at $\sim 9960 {cm}^{- 1}$ . Inset: The first set of peaks at highest energies in both samples at 5 K, showing the dominant 0′→0 and 1′→0 (hot-band) crystal-field components of the ${Yb}^{3 +} {}^{2}F_{5 / 2} \to {}^{2}F_{7 / 2}$ transition as well as a series of smaller peaks from other ${Yb}^{3 +}$ species. Variable-temperature PL spectra of (b) the NCs and (c) the SC from panel (a) measured at 15 K (blue), 80 K (green), and 245 K (red). $λ_{ex} = 375 nm$ for all panels. The spectrum in (b) includes markers denoting the periodic structure attributed to vibronic coupling, both on the Stokes and anti-Stokes (hot-band) sides of the first intense maximum. (d) Schematic illustration of ${Yb}^{3 +}$ f-f energy levels, including the low-symmetry splitting of $Γ_{8}$ components. The idealized reduced site symmetry is labeled as $D_{2 h}$ . The solid and dashed black arrows show the ${}^{2}F_{5 / 2} \to {}^{2}F_{7 / 2}$ 0′→0 and 1′→0 crystal-field transitions labeled in panel (a), respectively. The gray arrows indicate other low-temperature crystal-field origins that have not been conclusively identified in the spectrum.
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Figure 3
Representative room-temperature TRPL data collected for (a) NCs ( $[{Yb}^{3 +}] = 1.7 %$ ) and (b) a SC $([{Yb}^{3 +} = 2 % (nom .)$ ) of ${Yb}^{3 +} : {CsPbCl}_{3}$ . Inset: First 100 ns of the room-temperature TRPL trace, showing a distinct rise at short times. The red curve plots the experimental instrument response function (IRF). $λ_{ex} = 404 nm$ , $λ_{em} = 985 nm$ .
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Figure 4
Variable-temperature TRPL traces measured for (a), (b) NCs and (c), (d) a SC of ${Yb}^{3 +} : CsPb {Cl}_{3}$ (1.7% and 2%(nom.) ${Yb}^{3 +}$ , respectively), from 5 K (blue) to room temperature (red). Temperatures: 5, 15, 30, 45, 60, 80, 100, 125, 150, 200, 250, 295 K. $λ_{ex} = 404 nm$ , $λ_{em} = 982 nm$ (at 5–150 K), 985 nm (at 200–295 K).
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Figure 5
Scatter plots summarizing the variable-temperature PL data in Figs. 2 and 4. (a), (d) Integrated NIR PL intensities (green) and decay times (black) for the ${Yb}^{3 +} : CsPb {Cl}_{3}$ NCs (a) and SC (d) plotted vs temperature. For the SC, $τ_{decay}$ reflects the average PL decay time obtained from biexponential fitting, as described by Eq. (2). (b), (c), (e), (f) Fast (red) and slow (blue) NIR PL rise times and amplitudes measured for the ${Yb}^{3 +} : CsPb {Cl}_{3}$ NCs (b), (c) and SC (e), (f), plotted vs temperature.
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Figure 6
Quantum-cutting energy transfer in ${Yb}^{3 +}$ -doped $CsPb {Cl}_{3}$ . Photoexcitation of $CsPb {Cl}_{3}$ (black up arrow) is followed by intense ${Yb}^{3 +}$ f-f luminescence (red down arrows) after quantum cutting. The majority of the quantum cutting proceeds following the blue arrows, via energy capture by a dopant-induced defect ( $k_{defect}$ ) and then simultaneous energy transfer to two ${Yb}^{3 +}$ dopants ( $k_{QC}$ ). This process takes <10 ns at room temperature and <50 ns at liquid-helium temperatures. A minority slow pathway is also observed at low temperatures that is proposed to involve temporary trapping and detrapping by native defects (green arrows). This process takes ∼20 to 50 μs at liquid-helium temperatures. Solid arrows indicate radiative processes and dashed arrows indicate nonradiative processes.
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Physical Review Materials

Yb3+ speciation and energy-transfer dynamics in quantum-cutting Yb3+-doped CsPbCl3 perovskite nanocrystals and single crystals

Joo Yeon D. Roh, Matthew D. Smith, Matthew J. Crane, Daniel Biner, Tyler J. Milstein, Karl W. Krämer, and Daniel R. Gamelin

Phys. Rev. Materials 4, 105405 – Published 27 October 2020

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${Yb}^{3 +}$ speciation and energy-transfer dynamics in quantum-cutting ${Yb}^{3 +}$ -doped $CsPb {Cl}_{3}$ perovskite nanocrystals and single crystals