arXiv subjects

Thomas C. Rossi

Publications and source records attributed to Thomas C. Rossi.

3 recordsLinked to original sources

arXiv2026

Single Best Fits Can Be Misleading: Resolving Common Trapping Signatures across FA--Cs Perovskites

Identifying defects responsible for non-radiative recombination in metal-halide perovskites remains challenging, with no common defect signatures established across compositions. Here, we combine fluence-dependent time-resolved photoluminescence, full Shockley--Read--Hall modelling and Bayesian posterior inference across an FA$_{1-x}$Cs$_x$PbI$_3$ compositional series. We show that trap occupation governs carrier dynamics and defect-parameter identifiability in semiconducting materials: weakly occupied states exhibit electron-capture-coefficient--trap-density degeneracies, whereas trap filling lifts them. In our perovskite systems, a non-degenerate signature $Tβ$, characterized by a strongly asymmetric $β_p/β_n$ capture-coefficient ratio, occurs across all compositions, suggesting substantial trap filling is a general feature of perovskite carrier dynamics. We further identify $Tα$ as a shared, device-performance-limiting non-radiative recombination channel, while a shallow $Tε$ signature unique to FAPbI$_3$ produces the largest steady-state non-radiative recombination rate and is consistent with the previously observed high stacking-fault density of this composition. This framework therefore provides a basis for comparing trapping signatures across different semiconducting materials at room temperature.

cond-mat.mtrl-sci↗

arXiv2026

Light-Stabilized Metastable Electronic State in NiO with Enhanced Orbital Hybridization

Light-driven control of electronic structure in correlated metal oxides offers new opportunities for optimizing materials used in photovoltaic and photoelectrochemical technologies. We show that photoexcitation of NiO, a prototypical transparent semiconductor and hole-transport material, across its charge-transfer gap produces a long-lived metastable state with enhanced Ni 3d-O 2p orbital hybridization. We characterize this state using Ni K-edge X-ray absorption spectroscopy, which probes how structural and electronic changes affect the unoccupied p density of states during continuous and pulsed ultraviolet excitation. Under pulsed excitation, high carrier densities of approximately 10^20 per cubic centimeter generate a state with a lifetime of approximately 600 picoseconds, in which enhanced hybridization coexists with lattice heating. By contrast, continuous ultraviolet irradiation at much lower carrier densities of approximately 10^13 per cubic centimeter stabilizes a similar electronic state with negligible lattice heating, demonstrating that its formation is not solely thermally driven. First-principles DFT+U+V calculations attribute the spectral changes to stronger Ni 3d-O 2p hybridization, which alters the unoccupied Ni 4p states probed by dipole-allowed K-edge transitions. We attribute this change to the dynamic screening of on-site electronic correlations following photoexcitation, which redistributes the charge density. Because orbital hybridization governs carrier transport and charge-transfer energetics, our results identify photoinduced screening as a mechanism for dynamically tuning correlated oxides and suggest new design principles for optoelectronic materials.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast dynamic Coulomb screening of X-ray core excitons in photoexcited semiconductors

Ultrafast X-ray spectroscopy has been revolutionized in recent years due to the advent of fourth-generation X-ray facilities. In solid-state materials, core excitons determine the energy and line shape of absorption features in core-level spectroscopies such as X-ray absorption spectroscopy. The screening of core excitons is an inherent many-body process that can reveal insight into charge-transfer excitations and electronic correlations. Under non-equilibrium conditions such as after photoexcitation, however, core-exciton screening is still not fully understood. Here we demonstrate the dynamic Coulomb screening of core excitons induced by photoexcited carriers by employing X-ray transient absorption (XTA) spectroscopy with picosecond time resolution. Our interpretation is supported by state-of-the-art ab initio theory, combining constrained and real-time time-dependent density functional theory with many-body perturbation theory. Using ZnO as an archetypal wide band-gap semiconductor, we show that the Coulomb screening by photoexcited carriers at the Zn K-edge leads to a decrease in the core-exciton binding energy, which depends nonlinearly on both the excitation density and the distribution of photoexcited carriers in reciprocal space. The effect of Coulomb screening dominates over Pauli blocking in the XTA spectra. We show that dynamic core-exciton screening is also observed at other X-ray absorption edges and theoretically predict the effect of core-exciton screening on the femtosecond time scale for the case of ZnO, a major step towards hard X-ray excitonics. The results have implications for the interpretation of ultrafast X-ray spectra in general and their use in tracking charge carrier dynamics in complex materials on atomic length scales.

cond-mat.mtrl-sci↗