Photoexcitation

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

Exciton Dissociation Dynamics in Model Donor-Acceptor Polymer Heterojunctions: I. Energetics and Spectra

Abstract: In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band off-set in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band off-set at the heterojunction, the exciton will remain the lowest energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the off-set is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases.

Photogenerated Intrinsic Free Carriers in Small-molecule Organic Semiconductors Visualized by Ultrafast Spectroscopy.

Abstract: Confirmation of direct photogeneration of intrinsic delocalized free carriers in small-molecule organic semiconductors has been a long-sought but unsolved issue, which is of fundamental significance to its application in photo-electric devices. Although the excitonic description of photoexcitation in these materials has been widely accepted, this concept is challenged by recently reported phenomena. Here we report observation of direct delocalized free carrier generation upon interband photoexcitation in highly crystalline zinc phthalocyanine films prepared by the weak epitaxy growth method using ultrafast spectroscopy. Transient absorption spectra spanning the visible to mid-infrared region revealed the existence of short-lived free electrons and holes with a diffusion length estimated to cross at least 11 molecules along the π-π stacking direction that subsequently localize to form charge transfer excitons. The interband transition was evidenced by ultraviolet-visible absorption, photoluminescence and electroluminescence spectroscopy. Our results suggest that delocalized free carriers photogeneration can also be achieved in organic semiconductors when the molecules are packed properly.

Density-dependent electron scattering in photoexcited GaAs in strongly diffusive regime

Abstract: In a series of systematic optical pump–terahertz probe experiments, we study the density-dependent electron scattering rate in photoexcited GaAs in the regime of strong carrier diffusion. The terahertz frequency-resolved transient sheet conductivity spectra are perfectly described by the Drude model, directly yielding the electron scattering rates. A diffusion model is applied to determine the spatial extent of the photoexcited electron-hole gas at each moment after photoexcitation, yielding the time-dependent electron density, and hence the density-dependent electron scattering time. We find that the electron scattering time decreases from 320 to 60 fs, as the electron density changes from 1015 to 1019 cm−3.

Synthesis, Crystal Structure and Green Luminescence in Zero-Dimensional Tin Halide (C8H14N2)2SnBr6.

Abstract: Organic-inorganic hybrid metal halides with broad-band emission are currently receiving an increasing interest for their unique light emission properties. Here we report a novel lead-free zero-dimensional (0D) tin halide, (C8H14N2)2SnBr6, in which isolated [SnBr6]4- octahedrons are cocrystallized with organic cations, 1,3-bis(aminomethyl)benzene (C8H14N22+). Upon photoexcitation, the bulk crystals exhibit broad-band green emission peaking at 507 nm with a full width at half-maximum (fwhm) of 82 nm (0.395 eV), a Stokes shift of 157 nm (1.09 eV), and a photoluminescence quantum yield (PLQY) of 36 ± 4%. Combined structural analysis and density functional theory (DFT) calculations indicate that the excited state structural distortion of [SnBr6]4- octahedral units account for the formation of this green emission. The relatively small Stokes shift and narrow fwhm of the emission are hence caused by the reduced distortion of [SnBr6]4- octahedrons and rigid molecular structure. The discovery of lead-free (C8H14N2)2SnBr6 and insight into the mechanism of green emission provide an essential platform toward unveiling the relationship between structure and property for 0D metal halide perovskites.

Time-resolved photoexcitation of the superconducting two-gap state in MgB2 thin films.

Abstract: Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is governed by the sub-ps electron-phonon (e-ph) relaxation present at all temperatures, the few-ps e-ph process well pronounced below 70 K, and the sub-ns superconducting relaxation below T(c). The amplitude of the superconducting component versus temperature follows the superposition of the isotropic dirty gap and the three-dimensional pi gap dependences, closing at two different T(c) values. The time constant of the few-ps relaxation exhibits a double divergence at temperatures corresponding to the T(c)'s of the two gaps.