Bright circularly polarized photoluminescence in chiral layered hybrid lead-halide perovskites

Abstract

Hybrid perovskite semiconductor materials are predicted to lock chirality into place and encode asymmetry into their electronic states, while softness of their crystal lattice accommodates lattice strain to maintain high crystal quality with low defect densities, necessary for high luminescence yields. We report photoluminescence quantum efficiencies as high as 39% and degrees of circularly polarized photoluminescence of up to 52%, at room temperature, in the chiral layered hybrid lead-halide perovskites (R/S/Rac)-3BrMBA₂PbI₄ [3BrMBA = 1-(3-bromphenyl)-ethylamine]. Using transient chiroptical spectroscopy, we explain the excellent photoluminescence yields from suppression of nonradiative loss channels and high rates of radiative recombination. We further find that photoexcitations show polarization lifetimes that exceed the time scales of radiative decays, which rationalize the high degrees of polarized luminescence. Our findings pave the way toward high-performance solution-processed photonic systems for chiroptical applications and chiral-spintronic logic at room temperature.

Fig. 1.. Structure of layered (S/Rac)-3BrMBA₂PbI₄ perovskites.

(A and B) Crystal structure of (S)-3BrMBA₂PbI₄ and Rac-3BrMBA₂PbI₄ single-crystal structures. (C) Chemical structures of chiral organic enantiomers (R/S)-3BrMBA. (D) XRD patterns of chiral (R/S)-3BrMBA₂PbI₄ and Rac-3BrMBA₂PbI₄ thin films. Compared with racemic cations, chiral spacer cations (R/S)-3BrMBA induce structure distortions in the orientation of organic spacers, as well as in inorganic layers, resulting in chiral crystal lattices. a.u., arbitrary units.

Fig. 2.. Comparison of optical properties for chiral (R/S)-3BrMBA₂PbI₄ and Rac-3BrMBA₂PbI₄.

(A) Linear absorption, (B) CD, and (C) linear PL spectra of chiral layered Ruddlesden-Popper hybrid perovskite thin films. (D to F) Circularly polarized PL (CPL) spectra of (R/S)-3BrMBA₂PbI₄ and Rac-3BrMBA₂PbI₄ single crystals at room temperature. Chiral (R)- and (S)-3BrMBA₂PbI₄ show similar linear optical features but with opposite CD signals, while the racemic sample shows no CD signals and is characterized by red-shifted absorption and PL bands. CPL results were obtained under linear excitation at 405 nm, and σ⁺ and σ⁻ signals were detected simultaneously on different regions of the detector. Chiral (R) and (S) crystals exhibit opposite CPL signals, while racemic samples show identical σ⁺ and σ⁻ signals, confirming that crystal chirality induces chiroptical properties.

Fig. 3.. Charge carrier dynamics of chiral perovskite thin films.

(A and D) Linearly polarized TA maps, (B and E) time-resolved differential transmission spectra, (C) recombination kinetics collected at the GSB regions, and (F) TCSPC kinetics of (R)-3BrMBA₂PbI₄ and (R)-4BrMBA₂PbI₄ thin films, respectively. TA spectra were collected under 385-nm excitation with a pump fluence of ~1 μJ/cm², while TCSPC measurements were performed with photoexcitation at 405 nm and excitation fluence of ~10 μJ/cm².

Fig. 4.. CTA spectroscopy of chiral (R)-3BrMBA₂PbI₄ perovskites, under 385-nm excitation with a pump fluence of ~2.5 μJ/cm² (nanosecond regime) and ~50 μJ/cm² (microsecond regime).

(A and B) CTA spectra and kinetics showing the difference between four circularly polarized configurations. The different polarizations of pump and probe pulses are indicated σ⁺ (right-handed) or σ⁻ (left-handed). The spectra shown in (A) are averaged over the initial 10 ps. Decay kinetics are extracted over the gray spectral range in (A), and the polarization relaxation kinetics between black and red curves are calculated and plotted as blue triangles. (C) Comparison of TA kinetics with linear pump and circularly polarized probes showing net circular polarization. (D) Long-term CTA kinetics obtained from the PIA region between 490 and 510 nm. At room temperature, chiral (R)-3BrMBA₂PbI₄ shows different depolarization kinetics for σ⁺ and σ⁻ states, even under linear polarization of optical excitation.

Fig. 5.. Electronic structure of layered (Rac/S)-3BrMBA₂PbI₄ perovskites.

(A and C) DFT computed band structures of Rac-3BrMBA₂PbI₄ and (S)-3BrMBA₂PbI₄, respectively. In-plane hole (m_h∥) and electron (m_e∥) effective masses are computed at the valence and conduction band edges. (B) Partial charge density taken at the VBM and CBM. (D) Spin textures computed for the (S) structure in the (k_x, k_y) plane for the inner and outer branches of the conduction bands.