Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites

Abstract

Emission characteristics of metal halide perovskites play a key role in the current widespread investigations into their potential uses in optoelectronics and photonics. However, a fundamental understanding of the molecular origin of the unusual blueshift of the bandgap and dual emission in perovskites is still lacking. In this direction, we investigated the extraordinary photoluminescence behavior of three representatives of this important class of photonic materials, that is, CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CH(NH₂)₂PbBr₃, which emerged from our thorough studies of the effects of temperature on their bandgap and emission decay dynamics using time-integrated and time-resolved photoluminescence spectroscopy. The low-temperature (<100 K) photoluminescence of CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ reveals two distinct emission peaks, whereas that of CH(NH₂)₂PbBr₃ shows a single emission peak. Furthermore, irrespective of perovskite composition, the bandgap exhibits an unusual blueshift by raising the temperature from 15 to 300 K. Density functional theory and classical molecular dynamics simulations allow for assigning the additional photoluminescence peak to the presence of molecularly disordered orthorhombic domains and also rationalize that the unusual blueshift of the bandgap with increasing temperature is due to the stabilization of the valence band maximum. Our findings provide new insights into the salient emission properties of perovskite materials, which define their performance in solar cells and light-emitting devices.

Keywords: Hybrid perovskites; density functional theory; luminescence; photoluminescence; photovoltaics.

Fig. 1. Temperature-dependent emission characteristics of CH₃NH₃PbI₃ (fluence = 2 μJ/cm²).

(A) Normalized PL intensity of CH₃NH₃PbI₃ as a function of temperature recorded from 15 to 300 K (spectra have been vertically shifted for clarity). (B) Position of the PL peaks corresponding to the low energy and the orthorhombic and tetragonal phases of CH₃NH₃PbI₃ as a function of temperature. (C) FWHM of the PL peaks corresponding to the low energy and the orthorhombic and tetragonal phases of CH₃NH₃PbI₃ as a function of temperature. Green solid line shows the fitting obtained by taking into account the temperature-independent inhomogeneous broadening (Γ₀) and the interaction between charge carriers and longitudinal optical phonons (LO-phonons), as described by the Fröhlich Hamiltonian. (D) Absolute intensity of PL spectra corresponding to the low-energy emission peak and the orthorhombic and tetragonal phases of CH₃NH₃PbI₃ as a function of temperature from 15 to 300 K.

Fig. 2. Fluence-dependent emission characteristics of CH₃NH₃PbI₃ recorded at 15 and 300 K.

(A) PL spectra of the low- and high-energy emission peaks as a function of fluence recorded at 15 K. (B) Position of the low- and high-energy emission peaks as a function of fluence recorded at 15 K. (C) Intensity of the low- and high-energy emission peaks as a function of fluence recorded at 15 K. (D) PL spectra of the tetragonal phase as a function of fluence recorded at 300 K. (E) Position of the tetragonal emission peak as a function of fluence recorded at 300 K. (F) Intensity of the tetragonal emission peak as a function of fluence recorded at 300 K.

Fig. 3. Time-resolved PL of CH₃NH₃PbI₃ performed at 15 and 300 K.

(A) Fluence-dependent time-resolved PL of the low-energy emission peak recorded at 15 K. (B) Charge carrier lifetime (τ₁₀) in the low-energy emission peak decreases with increasing fluence recorded at 15 K. (C) Fluence-dependent time-resolved PL of the high-energy emission peak (orthorhombic phase) recorded at 15 K. (D) Charge carrier lifetime (τ₁, faster component) in the high-energy emission peak (orthorhombic phase) increases with increasing fluence recorded at 15 K. (E) Fluence-dependent time-resolved PL of the tetragonal phase recorded at 300 K. (F) Charge carrier lifetime (τ₁₀) decreases with increasing fluence in the tetragonal phase recorded at 300 K. (τ₁₀, time at which the maximum PL intensity decreases by a factor of 10).

Fig. 4. Temperature-dependent emission characteristics of CH₃NH₃PbBr₃ (A to C) and CH(NH₂)₂PbBr₃ (D to F) perovskite (fluence = 3 μJ/cm²).

(A) Normalized PL intensity of CH₃NH₃PbBr₃ as a function of temperature. (B) FWHM of the low- and high-energy emission peaks of CH₃NH₃PbBr₃ as a function of temperature. (C) Position of the low- and high-energy emission peaks of CH₃NH₃PbBr₃ as a function of temperature. (D) Normalized PL intensity of CH(NH₂)₂PbBr₃ as a function of temperature. (E) FWHM of the PL peak of CH(NH₂)₂PbBr₃ as a function of temperature. (F) Position of the PL peak of CH(NH₂)₂PbBr₃ as a function of temperature. [Note that because of better structural stability, CH(NH₂)₂PbBr₃ was chosen over CH(NH₂)₂PbI₃.]

Fig. 5. Classical MD simulations.

(A) Snapshots extracted from the classical MD simulations at (a) 100 K and (b) 300 K. Panels (c), (d), and (e) show the configurations of the samples used in the first-principles electronic structure calculations of the MA-ordered and MA-disordered orthorhombic and the tetragonal systems, respectively. Periodic boundary conditions are applied to improve the visualization. (B) E_g as a function of the pseudocubic lattice parameter, a = $\sqrt[3]{\mathit{V}}$ (V, volume per stoichiometric unit) for the MA-ordered (black symbols) and MA-disordered (red symbols) orthorhombic systems and the tetragonal system (blue symbols). E_g of the orthorhombic systems is computed starting from the computational equilibrium lattice. Subsequently, the lattice is isotropically expanded over a range of ~0.2 Å. The E_g of the tetragonal phase is computed over the same range as a. Open and filled symbols are introduced to improve readability, highlighting the change in the E_g across the phase transition. For the orthorhombic systems, filled symbols refer to a lattice parameter over a range of ~0.05 Å, consistent with the literature (29). Filled symbols for the tetragonal system are used in the complementary range.