Bottleneck-Free Hot Hole Cooling in CH3NH3PbI3 Revealed by Femtosecond XUV Absorption

Femtosecond carrier cooling in the organohalide perovskite semiconductor CH3NH3PbI3 is measured using extreme ultraviolet (XUV) and optical transient absorption spectroscopy. XUV absorption between 44 eV and 58 eV measures transitions from the I 4d core to the valence and conduction bands and gives distinct signals for hole and electron dynamics. The core-to-valence-band signal directly maps the photoexcited hole distribution and provides a quantitative measurement of the hole temperature. The combination of XUV and optical probes reveals that upon excitation at 400 nm, the initial hole distribution is 3.5 times hotter than the electron distribution. At an initial carrier density of 1.4×1020 cm-3 both carriers are subject to a hot phonon bottleneck, but at 4.2×1019 cm-3 the holes cool to less than 1000 K within 400 fs. This result places significant constraints on the use of organohalide perovskites in hot-carrier photovoltaics.<br /

Nonequilibrium Dynamics of Electron Emission from Cold and Hot Graphene under Proton Irradiation

Characteristic properties of secondary electrons emitted from irradiated two-dimensional materials arise from multi-length and multi-time-scale relaxation processes that connect the initial nonequilibrium excited electron distribution with their eventual emission. To understand these processes, which are critical for using secondary electrons as high-resolution thermalization probes, we combine first-principles real-time electron dynamics with irradiation experiments. Our data for cold and hot proton-irradiated graphene show signatures of kinetic and potential emission and generally good agreement for electron yields between experiment and theory. The duration of the emission pulse is about 1.5 fs, which indicates high time resolution when used as a probe. Our newly developed method to predict kinetic energy spectra shows good agreement with electron and ion irradiation experiments and prior models. We find that the lattice temperature significantly increases secondary electron emission, whereas electron temperature has a negligible effect

Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes

Vitrimers are an important class of materials offering advantages over conventional thermosets due to their self-healing properties and reprocessability. Vitrimers are ideal candidate materials for solid polymer electrolytes because their viscoelasticity and conductivity can be independently tuned by salt addition in distinct ways from linear polymer electrolytes while further providing resistance to lithium dendrite propagation. In this work, the chemical and physical properties of vinylogous urethane (VU) vitrimers were characterized by using a combination of experiments and simulations to develop molecular design rules for controlling material properties. A series of VU vitrimers containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt were synthesized by precisely controlling the VU cross-linking density using defined linker lengths of ethylene glycol (xEG, x = 2, 3, 4, 6, or 12), thereby enabling control over the dynamic bond-to-EG ratio. Viscoelastic measurements show that the characteristic relaxation time τ* of VU vitrimers containing salt decreased by a factor of ∼70 relative to neutral vitrimers due to Li-ion coordination and catalysis of VU bond exchange. Stress relaxation times and shear moduli decrease with lower cross-linking densities in VU vitrimers. Solid-state 7Li NMR further reveals that VU vitrimers with longer linker lengths prefer lithium-ethylene oxide (Li-EO) solvation, whereas shorter linkers cannot sufficiently solvate the cation, and Li-VU coordination is preferred. Density functional theory (DFT) simulations were used to elucidate the dominant binding mode of Li-ion interaction as a function of linker length. The preferential partitioning of Li at the VU site leads to an order of magnitude decrease in stress relaxation times with a negligible impact on the conductivity after normalizing to the glass transition temperature Tg. Interestingly, our results show universal behavior for Tg-normalized ionic conductivity data regardless of linker length. Overall, this work provides new avenues for orthogonal tuning of bulk dynamics, recyclability, and conductivity in vitrimer electrolytes

Carrier-Specific Femtosecond XUV Transient Absorption of PbI₂ Reveals Ultrafast Nonradiative Recombination

Femtosecond carrier recombination in PbI₂ is measured using tabletop high-harmonic extreme ultraviolet (XUV) transient absorption spectroscopy and ultrafast electron diffraction. XUV absorption from 45 to 62 eV measures transitions from the iodine 4d core level to the conduction-band density of states. Photoexcitation at 400 nm creates separate and distinct transient absorption signals for holes and electrons, separated in energy by the 2.4 eV band gap of the semiconductor. The shape of the conduction band, and therefore the XUV absorption spectrum, is temperature-dependent, and nonradiative recombination converts the initial electronic excitation into thermal excitation within picoseconds. Ultrafast electron diffraction (UED) is used to measure the lattice temperature and confirm the recombination mechanism. The XUV and UED results support a second-order recombination model with a rate constant of 2.5 × 10^–9 cm³/s