Where Chirality Matters in Perovskite-Based Circularly Polarized Light Emission

Abstract

Circularly polarized photoluminescence (CPL) adds an optical degree of freedom for emerging applications in sensing, information processing, counterfeiting security, and quantum communications. Developing emitters that directly generate CPL without external polarizing optics or magnetic fields is therefore of considerable interest. Halide perovskites are particularly attractive in this context owing to their high radiative efficiency, strong spin–orbit coupling, and structural flexibility. Incorporating enantiopure organic cations into low-dimensional hybrid perovskites can transfer molecular chirality to the extended metal–halide crystal lattice through asymmetric hydrogen bonding and lattice distortion, producing helicity-dependent absorption and emission near the excitonic band edge. Early studies therefore focused on enhancing circular dichroism (CD) and CPL emission by tuning organic-cation packing, octahedral tilting, and local distortions. In this Account, we examine a key question: where does chirality matter along the excitation-to-emission pathway? We first describe low-dimensional chiral perovskites as structurally well-defined model platforms for establishing structure–property relationships. We then discuss how chirality contributes to CPL emission from two closely related perspectives. At the transition level, emission dissymmetry is described by rotational strength arising from interference between the electric and magnetic transition dipole moments. Structural chirality can alter orbital mixing, parity character, and local coordination, thereby modifying the transition-dipole characteristics of excitonic states. At the electronic-structure level, strong spin–orbit coupling can facilitate transition pathway governed by spin-dependent optical selection rules, through which nonequilibrium spin populations can yield unequal emission intensities for opposite photon helicities. More recently, the chirality-induced spin selectivity (CISS) effect has provided a promising pathway in which spin-polarized carriers are generated in a chiral component and injected into a spatially distinct achiral emitter. Spin-light-emitting diodes (LEDs) and chiral-shell/achiral-core nanocrystals have consequently demonstrated that chirality need not be intrinsic to the emissive lattice when spin information is preserved until radiative recombination. Thus, inherent structural chirality alone is not a sufficient descriptor of CPL emission. These findings expand the design space from intrinsically chiral emitters to functionally separated architectures in which spin selection and light emission can be independently engineered. We highlight several directions required to translate these mechanistic concepts into predictive design rules. Quantitative evaluation of the electric and magnetic transition dipole moments is needed to establish how crystallographic symmetry determines rotational strength. Chiral perovskite nanocrystals provide additional design variables─including morphology, ligand binding, facet exposure, strain, and quantum or dielectric confinement─for systematically perturbing emissive-state characteristics. For spin-mediated emission, band alignment, interfacial coupling, carrier-transfer dynamics, and spin relaxation must be considered together; compositionally graded quasi-two-dimensional perovskites may provide favorable energy landscapes for spin-preserving transport and efficient CPL emission. Finally, CPL emitters should be evaluated using integrated metrics that combine dissymmetry with emission efficiency, brightness, stability, angular dependence, and reproducibility while excluding contributions from linear anisotropy. Determining where and how chirality influences the transition-dipole characteristics of emissive states and nonequilibrium spin populations could provide a basis for the rational design of perovskite-based CPL emitters that simultaneously achieve high helicity selectivity and emission efficiency.