Exciton transport in molecular organic semiconductors boosted by transient quantum delocalization
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Abstract Designing molecular materials with very large exciton diffusion lengths would remove some of the intrinsic limitations of present-day organic optoelectronic devices. Yet, the nature of excitons in these materials is still not sufficiently well understood. Here we present Frenkel exciton surface hopping, an efficient method to propagate excitons through truly nano-scale materials by solving the time-dependent Schrödinger equation coupled to nuclear motion. We find a clear correlation between diffusion constant and quantum delocalization of the exciton. In materials featuring some of the highest diffusion lengths to date, e.g. the non-fullerene acceptor Y6, the exciton propagates via a transient delocalization mechanism, reminiscent to what was recently proposed for charge transport. Yet, the extent of delocalization is rather modest, even in Y6, and found to be limited by the relatively large exciton reorganization energy. On this basis we chart out a path for rationally improving exciton transport in organic optoelectronic materials.Keywords:
Biexciton
Organic semiconductor
We report unusual excitonic absorption spectra with dip-shaped structures caused by exciton–biexciton coherent interactions in quantum dots. The exciton absorption spectrum is measured with the micro-photoluminescence excitation technique in a single InGaAs quantum dot. The spectrum changes from Lorenztian-shaped to dip-shaped with increasing excitation intensity. A theoretical analysis with the density matrix method reveals that exciton–biexciton coherent interactions create dressed states in the exciton–biexciton system and lead to the unusual absorption spectra.
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Photoluminescence excitation
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Abstract The investigation results of exciton, exciton-magnon and biexciton absorption by α-oxygen at T = 1.5 K are reported. It is shown that the doublet structure of exciton-magnon bands is due to anisotropy of the exciton and rnagnon bands; the doublet structure of the bands in the two-particle absorption results from the bound two-exciton states, the biexcitons, present in α-O2.
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Magnon
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Biexciton
J-aggregate
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We resolve the characteristic emission features of excitons in a single GaAs quantum wire using near-field photoluminescence spectroscopy. We report emission from both localized excitons and quasi-one-dimensional excitons that are delocalized over a length of up to several /spl mu/m.
Semiconductor nanostructures
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A delocalized exciton state is revealed by resonant Raman scattering in GaAs-AlGaAs multiple-quantum-well heterostructures where only lower energy, confined quasi-two-dimensional excitons had previously been observed. In spite of its extension across the abrupt GaAs-AlGaAs interface, the delocalized exciton remains a well-defined state inhomogeneously broadened to widths between 6 and 14 meV. We estimate an exciton binding energy greater than 2 meV. Scattering of the delocalized state by the exciton-optical phonon interaction results in transitions to both delocalized and quasi-two-dimensional localized excitons.
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Biexciton
Electro-absorption modulator
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We have investigated photoluminescence (PL) properties of GaAs (15 nm)/AlAs (15 nm) and GaAs (20 nm)/AlAs (20 nm) multiple quantum wells at 10 K under high density excitation conditions at excitation energies in the region of the fundamental excitons. It has been found that the PL due to exciton-exciton scattering, the so-called P emission, is observed with a threshold nature in addition to the appearance of the biexciton PL. The energy spacing between the P-PL band and the heavy-hole exciton depends on the layer thickness, which reflects the change of the exciton binding energy by the quantum size effect. The intensity of the biexciton-PL band is saturated by the appearance of the P-PL band. Both the exciton-exciton scattering process and the biexciton formation process require the collision of two excitons. Thus, the exciton-exciton scattering process prevents the formation of biexcitons, which leads to the saturation behavior of the biexciton-PL intensity. Furthermore, we have confirmed the existence of optical gain leading to stimulated emission due to the exciton-exciton scattering process with use of a variable-stripe-length method.
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Abstract Optical properties in (C 4 H 9 NH 3 ) 2 PbBr 4 single crystals under high density excitation have been investigated by photoluminescence (PL) spectroscopy. The PL bands associated with the Γ 1 , Γ 2 and Γ 5 excitons and the biexcitons have been observed. While the Γ 2 exciton and biexciton PL intensities are proportional to the 0.9 and 1.7 power of the excitation density, respectively, the Γ 5 exciton PL intensity shows the nonlinear dependence on the excitation density. The excitation‐density dependence of the PL bands is calculated by taking account of the dynamical change of the exciton and biexciton populations in an exciton‐biexciton system consisting of two exciton states and a biexciton state. The calculated results well explain the nonlinear dependence of the Γ 5 exciton PL intensity on the excitation density. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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Photoluminescence excitation
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A comprehensive experimental investigation of exciton and biexciton recombination in GaAs/AlGaAs quantum wells is presented. Exciton and biexciton recombination times are found to be 16 and 55 ps, respectively. A method of determining the dynamics of the exciton temperature is developed. It is shown that both exciton and biexciton recombination processes increase the exciton temperature by an amount as high as $\ensuremath{\sim}10$ K. These processes impose a new restriction on the possibility of exciton Bose-Einstein condensation and make impossible its achievement in a system of direct excitons in GaAs quantum wells even for resonantly excited exciton gas.
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The emission lines of biexciton and donor-bound biexciton are observed in a high-quality ZnO thin-film sample with time-resolved photoluminescence (TRPL) measurement. The TRPL intensity profiles reveal the formation sequence of various types of exciton. After free excitons are first generated, part of them is trapped by neutral donors to form donor-bound excitons. The other part contributes to the generation of biexcitons through free exciton scattering. Next, a donor-bound biexciton is generated through the trapping of a biexciton or two free excitons by a neutral donor or the trapping of a free exciton by a donor-bound exciton. Except donor-bound exciton, the relaxations of all other exciton states show two decay stages. Either the increasing or decreasing trends of the calibrated decay times in increasing the excitation power are well interpreted with a four-level model.
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