Coupled Electronic and Anharmonic Structural Dynamics for Carrier Self-Trapping in Photovoltaic Antimony Chalcogenides

Dynamic Exciton Polarons Enabling Dual-Mode White-Light Emission with Tunable Color Temperatures in 2D Hybrid Lead Bromides

White-light emission (WLE) with tunable chromaticity and correlated color temperature (CCT) is critical for lighting, display, and sensing applications. While recent two-dimensional (2D) lead halide perovskites have emerged as promising single-component WLE materials, their application is hindered by constrained and low CCT due to dominant localized exciton (LE) emission. Here, we report a dynamic exciton polaron mechanism in ⟨100⟩-oriented 2D lead bromides, (CnH2n+4N2)PbBr4 (n = 5, 7, 9, 11), enabling intrinsic dual-mode WLE with widely tunable CCT. By combining ultrafast transient absorption spectroscopy and thermodynamic analyses, we reveal a double-well potential energy landscape driving the dynamic equilibrium between band-edge exciton (BE) and self-trapped LE states, and thus the formation of a dynamic exciton polaron. By elongating the ligand from n = 5 to 11, the self-trapping barrier decreases from 23.3 ± 1.3 to 10.1 ± 0.8 meV, and the trapping depth increases from 3.8 ± 0.4 to 14.7 ± 1.8 meV due to enhanced exciton-phonon coupling, which shifts the BE/LE emission ratio and tunes CCT from 21 000 K (bluish cold white) to 5100 K (reddish warm white). The dynamic exciton polaron with correlated BE and LE exhibits matched radiative lifetimes, ensuring stable dual-mode WLE without spectral distortion. Our work establishes that the dynamic exciton polaron, combined with ligand engineering, acts as a general principle for designing single-component multimode WLE materials with tailored chromatic properties, advancing their potential in efficient lighting and multifunctional optoelectronics.

Structural and electronic features enabling delocalized charge-carriers in CuSbSe2

Inorganic semiconductors based on heavy pnictogen cations (Sb3+ and Bi3+) have gained significant attention as potential nontoxic and stable alternatives to lead-halide perovskites for solar cell applications. A limitation of these novel materials, which is being increasingly commonly found, is carrier localization, which substantially reduces mobilities and diffusion lengths. Herein, CuSbSe2 is investigated and discovered to have delocalized free carriers, as shown through optical pump terahertz probe spectroscopy and temperature-dependent mobility measurements. Using a combination of theory and experiment, the critical enabling factors are found to be: 1) having a layered structure, which allows distortions to the unit cell during the propagation of an acoustic wave to be relaxed in the interlayer gaps, with minimal changes in bond length, thus limiting deformation potentials; 2) favourable quasi-bonding interactions across the interlayer gap giving rise to higher electronic dimensionality; 3) Born effective charges not being anomalously high, which, combined with the small bandgap ( ≤ 1.2 eV), result in a low ionic contribution to the dielectric constant compared to the electronic contribution, thus reducing the strength of Fröhlich coupling. These insights can drive forward the rational discovery of perovskite-inspired materials that can avoid carrier localization.

Intrinsic Self-Trapped Excitons in Graphitic Carbon Nitride

Graphitic carbon nitrides (g-C3N4) as low-cost, chemically stable, and ecofriendly layered semiconductors have attracted rapidly growing interest in optoelectronics and photocatalysis. However, the nature of photoexcited carriers in g-C3N4 is still controversial, and an independent charge-carrier picture based on the band theory is commonly adopted. Here, by performing transient spectroscopy studies, we show characteristics of self-trapped excitons (STEs) in g-C3N4 nanosheets including broad trapped exciton-induced absorption, picosecond exciton trapping without saturation at high photoexcitation density, and transient STE-induced stimulated emissions. These features, together with the ultrafast exciton trapping polarization memory, strongly suggest that STEs intrinsically define the nature of the photoexcited states in g-C3N4. These observations provide new insights into the fundamental photophysics of carbon nitrides, which may enlighten novel designs to boost energy conversion efficiency.

Transient Photoinduced Pb2+ Disproportionation for Exciton Self-Trapping and Broadband Emission in Low-Dimensional Lead Halide Perovskites

Low-dimensional lead halide perovskites with broadband emission hold great promise for single-component white-light-emitting (WLE) devices. The origin of their broadband emission has been commonly attributed to self-trapped excitons (STEs) composed of localized electronic polarization with a distorted lattice. Unfortunately, the exact electronic and structural nature of the STE species in these WLE materials remains elusive, hindering the rational design of high-efficiency WLE materials. In this study, by combining ultrafast transient absorption spectroscopy and ab initio calculations, we uncover surprisingly similar STE features in two prototypical low dimensional WLE perovskite single crystals: 1D (DMEDA)PbBr4 and 2D (EDBE)PbBr4, despite of their different dimensionalities. Photoexcited excitons rapidly localize to intrinsic STEs within ∼250 fs, contributing to the white light emission. Crucially, STEs in both systems exhibit characteristic absorption features akin to those of Pb+ and Pb3+. Further atomic level theoretical simulations confirm photoexcited electrons and holes are localized on the Pb2+ site to form Pb+- and Pb3+-like species, resembling transient photoinduced Pb2+ disproportionation. This study provides conclusive evidence on the key excited state species for exciton self-trapping and broadband emission in low dimensional lead halide WLE perovskites and paves the way for the rational design of high-efficiency WLE materials.

Ultrafast Spontaneous Localization of a Jahn-Teller Exciton Polaron in Two-Dimensional Semiconducting CrI3 by Symmetry Breaking

The excited state species and properties in low-dimensional semiconductors can be completely redefined by electron-lattice coupling or a polaronic effect. Here, by combining ultrafast broadband pump-probe spectroscopy and first-principles GW and Bethe-Salpeter equation calculations, we show semiconducting CrI₃ as a prototypical 2D polaronic system with characteristic Jahn-Teller exciton polaron induced by symmetry breaking. A photogenerated electron and hole in CrI₃ localize spontaneously in ∼0.9 ps and pair geminately to a Jahn-Teller exciton polaron with elongated Cr-I octahedra, large binding energy, and an unprecedentedly small exciton-exciton annihilation rate constant (∼10^-20 cm³ s^-1). Coherent phonon dynamics indicates the localization is mainly triggered by the coherent nuclear vibration of the I-Cr-I out-of-plane stretch mode at 128.5 ± 0.1 cm^-1. The excited state Jahn-Teller exciton polaron in CrI₃ broadens the realm of 2D polaron systems and reveals the decisive role of coupled electron-lattice motion on excited state properties and exciton physics in 2D semiconductors.

Band versus Polaron: Charge Transport in Antimony Chalcogenides

Antimony sulfide (Sb₂S₃) and selenide (Sb₂Se₃) are emerging earth-abundant absorbers for photovoltaic applications. Solar cell performance depends strongly on charge-carrier transport properties, but these remain poorly understood in Sb₂X₃ (X = S, Se). Here we report band-like transport in Sb₂X₃, determined by investigating the electron-lattice interaction and theoretical limits of carrier mobility using first-principles density functional theory and Boltzmann transport calculations. We demonstrate that transport in Sb₂X₃ is governed by large polarons with moderate Fröhlich coupling constants (α ≈ 2), large polaron radii (extending over several unit cells), and high carrier mobility (an isotropic average of >10 cm² V^-1 s^-1 for both electrons and holes). The room-temperature mobility is intrinsically limited by scattering from polar phonon modes and is further reduced in highly defective samples. Our study confirms that the performance of Sb₂X₃ solar cells is not limited by intrinsic self-trapping.