Anti-Stokes photoluminescence from ${\mathrm{CsPbBr}}_3$ nanostructures embedded in a ${\mathrm{Cs}}_4{\mathrm{PbBr}}_6$ crystal

Lead halide perovskites possess high photoluminescence (PL) efficiency and strong electron-phonon interactions, and therefore optical cooling using up-conversion PL has been expected. We investigate anti-Stokes PL from green-luminescent ${\mathrm{Cs}}_4{\mathrm{PbBr}}_6$, whose origin is attributable to ${\mathrm{CsPbBr}}_3$ nanostructures embedded in a ${\mathrm{Cs}}_4{\mathrm{PbBr}}_6$ crystal. Because of the high transparency, low refractive index, and high stability of ${\mathrm{Cs}}_4{\mathrm{PbBr}}_6$, the green PL displays high external quantum efficiency without photodegradation. Time-resolved PL spectroscopy reveals the excitonic behaviors in the recombination process. The shape of the PL spectrum is almost independent of excitation photon energy, which means that the spectral width is determined by homogeneous broadening. We demonstrate that the phonon-assisted process dominates the Urbach tail of optical absorption and anti-Stokes PL at room temperature. Anti-Stokes PL is observed down to 70 K. We determine the temperature dependence of the Urbach energy and estimate the strength of the electron-phonon coupling. Our spectroscopic data show that ${\mathrm{CsPbBr}}_3$ nanostructures have potentially useful features for optical cooling.

Figure 1

(a) Energy diagram of ${\mathrm{CsPbBr}}_3$ and ${\mathrm{Cs}}_4{\mathrm{PbBr}}_6$ (top panel) and schematic of ${\mathrm{CsPbBr}}_3$ nanostructures in a ${\mathrm{Cs}}_4{\mathrm{PbBr}}_6$ crystal (bottom panel). (b) Photographs of a $\mathrm{CsPb}{\mathrm{Br}}_3/{\mathrm{Cs}}_4\mathrm{Pb}{\mathrm{Br}}_6$ crystal under room light and UV lamp excitation.

Figure 2

(a) Optical absorption and PL spectra of $\mathrm{CsPb}{\mathrm{Br}}_3/{\mathrm{Cs}}_4\mathrm{Pb}{\mathrm{Br}}_6$ (top panel) and ${\mathrm{CsPbBr}}_3$ bulk crystals (bottom panel). (b) EQE as a function of the excitation power density for the $\mathrm{CsPb}{\mathrm{Br}}_3/{\mathrm{Cs}}_4\mathrm{Pb}{\mathrm{Br}}_6$ crystal. Excitation photon energy was 3.1 eV. (c) Dependence of the PL intensity on the exposure time for the $\mathrm{CsPb}{\mathrm{Br}}_3/{\mathrm{Cs}}_4\mathrm{Pb}{\mathrm{Br}}_6$ (red data) and the ${\mathrm{CsPbBr}}_3$ bulk crystal (green data) under continuous-wave photoexcitation in air at room temperature (excitation intensity: $2\mathrm{kW}/\mathrm{c}{\mathrm{m}}^2$; photon energy: 3.0 eV). The PL intensities are normalized to the values at the beginning of the experiment. Solid curves are guides for the eye.

Figure 3

(a) 2D streak data of $\mathrm{CsPb}{\mathrm{Br}}_3/{\mathrm{Cs}}_4\mathrm{Pb}{\mathrm{Br}}_6$ under excitation at 2.99 eV $(10\mu \mathrm{J}/\mathrm{c}{\mathrm{m}}^2)$. (b) Time-resolved PL spectra at different times after excitation. Solid curves are fits to a pseudo-Voigt function. (c) Excitation-fluence dependence of the PL decay profile monitored at 2.39 eV $(2\mathrm{nJ}/\mathrm{c}{\mathrm{m}}^2\text{--}1\mu \mathrm{J}/\mathrm{c}{\mathrm{m}}^2)$. The broken curves are fits to a triple-exponential function. The inset shows the PL dynamics on a longer timescale. (d) Effective lifetime and PL intensity just after excitation as a function of $N_{\mathrm{ph}}$. We fit ${\tau }_{\mathrm{eff}}$ data to function $1/\left(a+b{N}_{\mathrm{ph}}\right)$.

Figure 4

(a) PLE map of $\mathrm{CsPb}{\mathrm{Br}}_3/{\mathrm{Cs}}_4\mathrm{Pb}{\mathrm{Br}}_6$. (b) PL spectra under different excitation photon energies. Solid curves are fits to a pseudo-Voigt function. (c) Integrated PL intensity ($I_{\mathrm{PLE}}$) as a function of excitation photon energy ($E_{\mathrm{ex}})$. The solid line is a fit to Eq. (2). The inset shows the excitation-fluence dependence of the integrated anti-Stokes PL intensity under 2.34-eV excitation. The solid line indicates the linear dependence.

Figure 5

(a) PL and (b) PLE spectra at different temperatures. Blue and red curves in (a) represent the spectra obtained under excitation at 2.53 and 2.33 eV, respectively. The solid lines in (b) are fits to Eq. (2), and the evaluated Urbach energy $E_U$ is provided in each figure. (c) The closed circles show $E_U$, $E_{\mathrm{PL}}$, and the PL linewidth as a function of the sample temperature. The Urbach energies derived from the PL spectrum using the van Roosbroeck–Shockley relation are shown by the open squares. The solid curve in the upper panel is a fit to Eq. (4), from which we obtained $\hslash {\omega }_{\mathrm{ph}}=3\mathrm{meV}$. The dashed curves in the lower panel are fits to Eqs. (S3) and (S4) in the Supplemental Material [39].