Photoexcitation

About: Photoexcitation is a research topic. Over the lifetime, 5874 publications have been published within this topic receiving 134733 citations.

Charge Transfer Exciton and Spin Flipping at Organic-Transition-Metal Dichalcogenide Interfaces.

Abstract: Two-dimensional transition-metal dichalcogenides (TMD) can be combined with other materials such as organic small molecules to form hybrid van der Waals heterostructures. Because of different properties possessed by these two materials, the hybrid interface can exhibit properties that cannot be found in either of the materials. In this work, the zinc phthalocyanine (ZnPc)–molybdenum disulfide (MoS2) interface is used as a model system to study the charge transfer at these interfaces. It is found that the optically excited singlet exciton in ZnPc transfers its electron to MoS2 in 80 fs after photoexcitation to form a charge transfer exciton. However, back electron transfer occurs on the time scale of ∼1–100 ps, which results in the formation of a triplet exciton in the ZnPc layer. This relatively fast singlet–triplet transition is feasible because of the large singlet–triplet splitting in organic materials and the strong spin–orbit coupling in TMD crystals. The back electron transfer would reduce the yield...

Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron–Phonon Coupling

Abstract: Unusual photophysical properties of organic-inorganic hybrid perovskites have not only enabled exceptional performance in optoelectronic devices, but also led to debates on the nature of charge carriers in these materials. This study makes the first observation of intense terahertz (THz) emission from the hybrid perovskite methylammonium lead iodide (CH3 NH3 PbI3 ) following photoexcitation, enabling an ultrafast probe of charge separation, hot-carrier transport, and carrier-lattice coupling under 1-sun-equivalent illumination conditions. Using this approach, the initial charge separation/transport in the hybrid perovskites is shown to be driven by diffusion and not by surface fields or intrinsic ferroelectricity. Diffusivities of the hot and band-edge carriers along the surface normal direction are calculated by analyzing the emitted THz transients, with direct implications for hot-carrier device applications. Furthermore, photogenerated carriers are found to drive coherent terahertz-frequency lattice distortions, associated with reorganizations of the lead-iodide octahedra as well as coupled vibrations of the organic and inorganic sublattices. This strong and coherent carrier-lattice coupling is resolved on femtosecond timescales and found to be important both for resonant and far-above-gap photoexcitation. This study indicates that ultrafast lattice distortions play a key role in the initial processes associated with charge transport.

Charge Separation and Recombination at Polymer–Fullerene Heterojunctions: Delocalization and Hybridization Effects

Abstract: We address charge separation and recombination in polymer/fullerene solar cells with a multiscale modeling built from accurate atomistic inputs and accounting for disorder, interface electrostatics and genuine quantum effects on equal footings. Our results show that bound localized charge transfer states at the interface coexist with a large majority of thermally accessible delocalized space-separated states that can be also reached by direct photoexcitation, thanks to their strong hybridization with singlet polymer excitons. These findings reconcile the recent experimental reports of ultrafast exciton separation ("hot" process) with the evidence that high quantum yields do not require excess electronic or vibrational energy ("cold" process), and show that delocalization, by shifting the density of charge transfer states toward larger effective electron-hole radii, may reduce energy losses through charge recombination.

Photoemission and electron states at clean surfaces

Abstract: Photoemission energy distribution spectra of clean metal and semiconductor surfaces are interpreted in terms of emission from electronic states characteristic of both the surface and 'infinite' bulk crystal. The photoexcitation matrix element is shown to depend on the degree of localization of the final-state wavefunction at the surface. Using the concept that the emitted electron is represented by an incoming plane wave, spectral regimes and experimental conditions are defined which are expected to give enhanced surface sensitivity. Experimental results are presented to illustrate the nature of the surface and volume photoelectric effect in nearly-free-electron metals, semiconductors, and transition metals.

Excited-State Self-Trapping and Ground-State Relaxation Dynamics in Poly(3-hexylthiophene) Resolved with Broadband Pump–Dump–Probe Spectroscopy

Abstract: Broadband femtosecond transient absorption spectroscopy is used to explore the mechanisms underlying excited-state and ground-state exciton relaxation in poly(3-hexylthiophene) (P3HT) solution. We focus on the picosecond spectral shifts in the ground and excited states of P3HT, using pump–probe (PP) and pump–dump–probe (PDP) techniques to investigate exciton relaxation mechanisms. Excited-state PP signals resolved a dynamic stimulated emission Stokes shift and ground-state reorganization; PDP signals resolved a blue-shifting nonequilibrium ground-state bleach. Initial structural reorganization is shown to be faster in the excited state. Ground-state reorganization is shown to be dependent on dump time, with later times resulting in relatively more population undergoing slow (∼20 ps) reorganization. These observations are discussed in the context of structural relaxation involving small-scale ( 1 ps) planarization of thiophene groups following photoexcitation. Excited-state and grou...