Unusual Temperature Dependence of Bandgap in 2D Inorganic Lead-Halide Perovskite Nanoplatelets

Abstract

Understanding the origin of temperature-dependent bandgap in inorganic lead-halide perovskites is essential and important for their applications in photovoltaics and optoelectronics. Herein, it is found that the temperature dependence of bandgap in CsPbBr₃ perovskites is variable with material dimensionality. In contrast to the monotonous redshift ordinarily observed in bulk-like CsPbBr₃ nanocrystals (NCs), the bandgap of 2D CsPbBr₃ nanoplatelets (NPLs) exhibits an initial blueshift then redshift trend with decreasing temperature (290-10 K). The Bose-Einstein two-oscillator modeling manifests that the blueshift-redshift crossover of bandgap in the NPLs is attributed to the significantly larger weight of contribution from electron-optical phonon interaction to the bandgap renormalization in the NPLs than in the NCs. These new findings may gain deep insights into the origin of bandgap shift with temperature for both fundamentals and applications of perovskite semiconductor materials.

Keywords: blueshift-redshift crossover; inorganic lead-halide perovskites; material dimensionality; temperature-dependent bandgap.

Figure 1

Schematic illustration of the unusual blueshift‐redshift crossover of bandgap with temperature in CsPbBr₃ 2‐ML NPLs, which is tentatively attributed to the trade‐off between the opposite contributions of electron‐acoustic phonon and electron‐optical phonon interactions to the bandgap renormalization.

Figure 2

a) TEM images of CsPbBr₃ NCs (upper) and 2‐ML NPLs (lower), respectively. b) XRD patterns of CsPbBr₃ NCs and 2‐ML NPLs. Absorption (black circles) and PL emission spectra (shadow) of c) CsPbBr₃ NCs and d) 2‐ML NPLs films recorded at 290 K, respectively. The fitting results based on Elliott's theory of Wannier excitons are plotted by red lines; the violet and pink dashed lines represent the contributions from 1s excitonic peak and the lowest band of continuum transition to the absorption near the band edge (area marked by grey dash‐dot line), respectively.

Figure 3

a) Normalized PL emission spectra a) of CsPbBr₃ NCs and b) 2‐ML NPLs films measured over the temperature range of 10–290 K under excitation at 365 nm. Insets: PL peak energy position versus temperature (left) and fit of PL emission spectrum at 290 K by line‐shape function (right). c) Wavelength‐dependent PL lifetimes of CsPbBr₃ NCs (upper) and 2‐ML NPLs films (lower) at 77 K with the 375‐nm ps‐pulsed laser as the excitation source. d) PL peak decay curves of the CsPbBr₃ NCs (upper) and 2‐ML NPLs (lower) films at 290 K and their corresponding fits by power‐law model, displayed in the semi‐logarithmic scale. Inset: the enlarged fitted decay curve spanning over the middle to caudal segment in the double logarithmic scale.

Figure 4

a–b) Absorption spectra (left) of a) CsPbBr₃ NCs and b) 2‐ML NPLs films, respectively, measured over the temperature range of 10–290 K with a temperature interval of 20 K, and the corresponding E _g and E _1s values (right) extracted through fitting the absorption spectra by Elliot model. c,d) Bandgap energy of c) CsPbBr₃ NCs and d) 2‐ML NPLs as a function of temperature from 10 to 290 K (blue circle), and their corresponding fits by the Bose–Einstein two‐oscillator model (red line), consisting of the contributions of electron‐acoustic phonon (violet dashed line) and electron‐optical phonon (orange dashed line) interactions to the bandgap shift with temperature. The horizontal grey dotted line denotes the unrenormalized bandgap energy E ₀.