High Exciton Concentrations Research Articles

Exciton annihilation in molecular aggregates suppressed through quantum interference.

Exciton-exciton annihilation (EEA), an important loss channel in optoelectronic devices and photosynthetic complexes, has conventionally been assumed to be an incoherent, diffusion-limited process. Here we challenge this assumption by experimentally demonstrating the ability to control EEA in molecular aggregates using the quantum phase relationships of excitons. We employed time-resolved photoluminescence microscopy to independently determine exciton diffusion constants and annihilation rates in two substituted perylene diimide aggregates featuring contrasting excitonic phase envelopes. Low-temperature EEA rates were found to differ by more than two orders of magnitude for the two compounds, despite comparable diffusion constants. Simulated rates based on a microscopic theory, in excellent agreement with experiments, rationalize this EEA behaviour based on quantum interference arising from the presence or absence of spatial phase oscillations of delocalized excitons. These results offer an approach for designing molecular materials using quantum interference where low annihilation can coexist with high exciton concentrations and mobilities.

Reducing efficiency roll-off of phosphorescent organic light emitting diodes by using phosphor assisted energy funneling

Though phosphorescent emitters have been widely applied in commercialized organic light-emitting diodes (OLEDs), most of them always suffer from severe efficiency roll-off because high exciton concentration at high current density would cause aggravation of triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) effect. Herein, an energy funneling system were formed in phosphorescent OLEDs (PhOLEDs) to relieve the exciton accumulation and further suppress the quenching effects by utilizing a phosphorescent emitter as assistant host. By simply adding blue phosphors to green and red devices, the EQEs of green and red devices at the brightness of 10000 cd m −2 were remarkably improved from 12.8% to 11.1%–20.2% and 22.4% for green and red OLEDs, respectively. The results presented in this work are quite beneficial to OLED display and lighting industry. • The reasons for the sharp decline in efficiency of PhOLED under high brightness are carefully analyzed. • An energy funneling system was proposed for improving the efficiency under high current density. • The EQEs at the brightness of 10000 cd/m2 were remarkably improved from 12.8% to 11.1%–20.2% and 22.4% for green and red PhOLEDs, respectively.

Specific quantum mechanical solution of difference equation of hyperbolic type

Difficulties connected to solving difference equations of hyperbolic type were analyzed in this work and discussed in detail. The results are compared to those of the standard wave equation and certain similarities were established. The method of solving the equation is generalized by means of kernel expanded into separable polynomials. The analysis was inspired by some new ideas concerning quantization of time. Two examples are given: excitons and phonons in thin crystalline films. The advanced methodology of Green’s function method and the application of this new methodology resulted in a set of interesting conclusions concerning thin film properties. The significance of the obtained spatial dependence of exciton concentration was discussed and it was concluded, on the basis of the found spatial dependence of exciton concentration, that such boundary conditions of a thin molecular film which lead to high exciton concentrations can be determined. It was also concluded that thin films possess high superconductive properties, that physical characteristics of thin films are spatially dependent and that the spatial dependence can be the basis for widening the field of application of nanostructures.

The Possibility of Achieving Extra High Exciton Concentration in Thin Molecular Films

The Possibility of Achieving Extra High Exciton Concentration in Thin Molecular Films

Proposal of structures possessing high exciton concentration

The possibility of achievement of high exciton concentrations is analyzed. It is shown that high concentrations can be achieved in a three-layer thin molecular film due to the autoreduction processes taking place in it. Shortly, the appearance of high concentrations is the consequence of boundary conditions in film and of the magnitude of matrix elements of dipol–dipol interactions. The autoreduction takes place in the cases when matrix elements characterizing exciton transfer are less than statistical matrix elements. Based on numerical analysis, it was found that optical quanta concentrations of a three-layer film can achieve values of about 5×10 −2. The structures possessing so high concentration do not exist in nature, thus they have to be synthesised. For the current state of nanotechnology, it is not a problem. Fortunately achieving high concentrations requires only certain ratios of relevant characteristics of the film with a two-level exciton scheme, but not their single values.

Influence of the miniband on emission mechanism in Zn1−xCdxSe/ZnSe quantum wells

The stimulated emission has been investigated in Zn1−xCdxSe/ZnSe quantum wells with a constant sample width of 60 Å and barrier widths from 47 Å to 500 Å. A redshift of the stimulated emission peak with increasing excitation intensity was observed in samples with barrier widths narrower than 350 Å, and the stimulated emission mechanism in these samples is found to be dominated by exciton-exciton scattering. In contrast, no redshifts were observed for the sample with a barrier width of 500 Å, and the stimulated emission in this sample appears to be dominated by biexcitons. Time-resolved spectroscopy measurements were also performed on samples with barrier width of 47 and 500 Å below the threshold of stimulated emission, and it was found that the exciton lifetime decreases with increasing exciton energy for both of the samples. With a ten times smaller barrier width in the 47 Å sample, the lifetime of high energy exciton is significantly shorter than that of the other sample, while the low energy lifetime becomes longer. This implies that with a narrower barrier width of the quantum wells the excitons move faster to lower energy regions. The narrower the barrier, the easier it is for excitons to move through the miniband. This freedom of movement increases the exciton concentration at low energies in localized areas in the wells. High exciton concentration made exciton-exciton scattering more probable and reduce the stimulated emission threshold value.

Difference equations in condensed matter physics and their application to exciton systems in thin molecular films

The main goal of this work is the development of the correct methodology of calculation of the Green's function of broken symmetry structures. The two methods were analysed and it was concluded that the direct method with the use of the translational operators is not convenient for the analyses of the exciton properties in thin molecular films. The second proposed method, consisting in translation of the difference equation with spatially variable parameter into the system of difference equations with constant parameters, was applied to thin molecular film and the solution of this system, being Green's function depending on two spatial indices, separately, was found exactly. With help of this Green's function the spatial dependence of exciton concentration was found. The significance of the obtained spatial dependence of exciton concentration is discussed and it was concluded that on the basis of the found spatial dependence of exciton concentration can be determined such boundary conditions of thin molecular film which lead to high exciton concentrations and consequently to the possibility of construction of high power lasers.

Excited-state photoelectron spectroscopy of excitons inC60and photopolymerizedC60films

Laser-excited states in films of pristine ${\mathrm{C}}_{60}$ and photopolymerized ${\mathrm{C}}_{60}$ $(pp\ensuremath{-}{\mathrm{C}}_{60})$ prepared in ultrahigh vacuum have been characterized in situ with pump-probe photoelectron spectroscopy using both synchrotron radiation and picosecond laser sources. Photoelectron spectra of singlet ${(S}_{1})$ and triplet ${(T}_{1})$ excitons overlap because of vibrational broadening in the photoemission final state. The spectra have been individually isolated in $pp\ensuremath{-}{\mathrm{C}}_{60}$ with time-resolved methods and are split by 0.33 eV. Signals from pristine ${\mathrm{C}}_{60}$ are weaker but are spectroscopically similar. The origin for exciton transitions for both ${\mathrm{C}}_{60}$ and $pp\ensuremath{-}{\mathrm{C}}_{60}$ is found to be properly located near the maximum of the highest occupied molecular orbital. Increasing excitation density favors ${T}_{1}$ production over ${S}_{1}$ such that, at high exciton concentrations, ${T}_{1}$ states predominate, even at times much shorter than the unimolecular intersystem crossing time, measured to be \ensuremath{\sim}2.5 ns for $pp\ensuremath{-}{\mathrm{C}}_{60}$ at 81 K. A weaker photoemission band located \ensuremath{\sim}0.5 eV above ${S}_{1}$ is also observed and is attributed to charge carriers and/or charge-transfer excitons.

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions.

Exciton-exciton annihilation in poly ( p-phenylenevinylene) films

We have measured the saturation of fluorescence intensity with increasing excitation intensity and the time dependence of fluorescence intensity decay at high excitation intensities in poly ( p-phenylenevinylene) films. The experimental results are consistent with the hypothesis that exciton-exciton annihilation reduces the exciton lifetime at high exciton concentrations and that the exciton diffusion coefficient is quite high. We conclude that the lowest energy electronic transition creates a Frenkel exciton rather than a free-carrier pair in a band-to-band transition.

Dielectric-metal phase transition in a system of interacting charge-transfer excitons

We demonstrate that strong dipole-dipole repulsion of 1D and 2D charge transfer excitons (CTE) can induce, at sufficiently high exciton concentrations, an electron dielectric-metal phase transition.

Pump—probe spectroscopy and exciton dynamics of J aggregates at high pump intensities

We have measured the picosecond pump—probe spectra of BIC J aggregates using a spectral continuum for the probe. The excitation pulses were intense (10 15−2×10 16 photons/cm 2) and created many excitons per aggregate. The observed dynamics are intensity dependent and reflect exciton—exciton annihilation processes. In addition, the zero-time spectrum is also intensity dependent, probably due to a reduction of k space at high exciton concentrations.

Dielectric function for semiconductors with a high exciton concentration

Abstract If polaritons are present in a crystal, one can create biexcitons by one-photon absorption. We show that for such systems the dielectric formalism allows a calculation of the electromagnetic response and the renormalized polariton spectrum in a very simple way.

Kinematical interaction of excitons and Davydov splitting

The molecular crystal with a complex lattice is analysed for low as well as high exciton concentrations. It was shown that apart from normal exciton levels the same number of kinematical levels exist in the system. The kinematical levels are shifted with respect to the normal levels and consequently, have the same splitting. The order parameter of the system at high concentrations is determined.

Exciton Luminescence in Semiconductors. Surface Recombination and Space‐Charge‐Layer Effects

AbstractThe effect of surface conditions on excitonic luminescence of wide‐band gap semiconductors like CdS in a situation when excitons are created by the binding of electron—hole pairs is considered. The surface contribution is considered by the velocity of surface exciton annihilation, the effective rate of electron and hole surface recombination, and by surface band bending. The analysis of continuity equations for electrons, holes, and excitons in line with Poisson's equation is carried out. The above set of equations is found to have analytical solutions in the following simple situations: a) at low exciton concentrations when both transfer and annihilation of the excitation energy are determined by the motion and recombination of electrons and holes; b) intermediate exciton concentrations when excitation energy transfer is realized by electron—hole diffusion while its annihilation is due mainly to the excitonic channel; c) high exciton concentrations (in respect to the minor carrier concentrations) when the diffusion as well as the annihilation of a excitation energy is realized by the excitonic channel. A new physical picture of the appearence of “dead” layer in excitonic luminescence based on the enhanced electron—hole recombination near the semiconductor surface, particularly, in conditions of a strong tunneling ionisation of excitons in surface electric fields is proposed.

Luminescence from the Excited States of Excitons in CdS and CdSe Single Crystals

AbstractThe photoluminescence in “pure” crystals of CdS and CdSe atT  1.4, 4.2, and 77 °K and high exciton concentration has been investigated. The correlation between arising (having mainly threshold character) of luminescence from the excited states of excitons A(n  2, 3) and B(n  1) and the existence of a fine structure, related to radiative recombination due to inelastic collisions between excitons (P, P2, P3, P4, and PB−lines) is observed. It is shown that with increasing excitation the “blue” shift of P‐line is twice than that for the A(n  1)‐line. The analysis of the problem using kinetic equations leads to conclusions about the possibility of stimulated enlargement (similarly to laser generation) of exciton concentration in their excited states due to Auger‐throwing of the excitons into these states at luminescence of P‐lines. The numerical estimations correspond to the experimental condition and the agreeement to the theory with experiment permitsto conclude that the observable luminescence from the excited states is dueto the stimulated generation of excitons in these states.

Simple Model for Some Dense Magnetic Exciton Systems

The (triplet‐state) TCNQ ion radical salts and also Würster's blue perchlorate exhibit deviations from the normal behavior of typical molecular solids. These systems are believed to contain triplet excitons which are restricted to essentially a unidirectional propagation. A simple phenomenological model is presented which attempts to describe the gross aspects of the observed anomalous behavior in terms of the relatively high exciton concentrations indicated. The model derives its interesting features from a quadratic dependence of the free energy on the excitation density (exciton concentration); the presence of such a term is made plausible by considerations of exciton—exciton and exciton—lattice interactions. The model accounts for the occurrence of phase transitions similar to those observed in (φ3PCH3)+(TCNQ)2— and Würster's blue perchlorate and indicates that under appropriate conditions one may achieve unusual magnetic phenomena in molecular solids.