A Strategy for Tuning Electron-Phonon Coupling and Carrier Cooling in Lead Halide Perovskite Nanocrystals

Abstract

Perovskites have been recognized as a class of promising materials for optoelectronic devices. We intentionally include excessive Cs⁺ cations in precursors in the synthesis of perovskite CsPbBr₃ nanocrystals and investigate how the Cs⁺ cations influence the lattice strain in these perovskite nanocrystals. Upon light illumination, the lattice strain due to the addition of alkali metal Cs⁺ cations can be compensated by light-induced lattice expansion. When the Cs⁺ cation in precursors is about 10% excessive, the electron-phonon coupling strength can be reduced by about 70%, and the carrier cooling can be slowed down about 3.5 times in lead halide perovskite CsPbBr₃ nanocrystals. This work reveals a new understanding of the role of Cs⁺ cations, which take the A-site in ABX₃ perovskite and provide a new way to improve the performance of perovskites and their practical devices further.

Keywords: carrier cooling; electron–phonon coupling; light–induced lattice expansion; perovskites.

Figure 1

The XRD and XPS results of the perovskite CsPbBr₃ nanocrystal samples. (a) General XRD results of samples S1–S5. (b) Plane (100) XRD result of samples S1–S5. (c) The relationship of the distance of planes (100) series with a ratio of Cs/Pb in precursors. (d–f) The binding energy of Cs–3d, Pb–4f, and Br–3d from stoichiometric samples S1 to nonstoichiometric samples S5.

Figure 2

PL spectra of the perovskite CsPbBr₃ samples. (a) The PL spectra mapping of sample S5. (b) The peak energy of PL dependent on sample temperature. (c) The FWHM of PL spectra dependent on sample temperature. (d) The relationship of $E_{g0}$ with a ratio of Cs/Pb in precursors. (e) The relationship of electron–phonon coupling with the concentration of Cs⁺ cations in precursors.

Figure 3

PL decay and TAS results of perovskite CsPbBr₃ samples. (a) The PL decay of samples S1–S5. (b) The relationship between average PL lifetime and concentration of Cs⁺ cations in precursors. (c) The TAS result of nonstoichiometric sample S3. (d) Several representative TAS of nonstoichiometric sample S3.

Figure 4

Schematic illustration of the lattice strain and compensation of light–induced lattice expansion in perovskite nanocrystal. (a) PL spectrum of stoichiometric sample S1. (b) PL spectrum of nonstoichiometric sample S3 excited by low power. (c) PL spectrum of nonstoichiometric sample S3 excited by high power. There is obviously a higher energy peak (2.43 eV) absent, which can be explained by the light–induced lattice expansion. (d) Schematic illustration of the bandgap of stoichiometric sample. (e) Schematic illustration of the bandgap of nonstoichiometric sample. (f) Schematic illustration of extra–intra–band level in the bandgap of the stoichiometric sample by light–induced lattice expansion. (g) Light–induced lattice expansion reduced lattice strain in perovskite nanocrystal.

Figure 5

Cooling of carriers in perovskite CsPbBr₃ samples. (a) The PB decay of samples S1–S5. (b) The relationship between hot–carrier cooling and concentration of Cs⁺ cations in precursors. (c) Schematic illustration of hot–carrier relaxation.

Figure 6

Schematic illustration of the crystallization kinetics via rationale adjustment of the concentration of alkaline cation.