Fraction Of Excitons Research Articles

Quantum efficiency of ambipolar light-emitting polymer field-effect transistors

The emission characteristics and external quantum efficiencies of ambipolar polymer light-emitting field-effect transistors are investigated as a function of applied voltage, current density, and ratio of hole to electron mobility. Green-emitting poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) with balanced electron and hole mobilities and red-emitting poly((9,9-dioctylfluorene)-2,7- diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) with strongly unbalanced hole and electron mobilities as semiconducting and emissive polymers are compared. The current-voltage and light output characteristics of the two types of light-emitting transistors were found to be fundamentally alike independent of mobility ratio. Device modeling allowing for a single (Langevin-type) charge recombination mechanism was able to reproduce the device characteristics for both cases but could not replicate the experimentally observed dependence of external quantum efficiency on current density. The increase of quantum efficiency with current density up to a saturation value could be indicative of a trap-assisted nonradiative decay mechanism at the semiconductor-dielectric interface. Optical output modeling confirmed that the maximum external quantum efficiency of F8BT light-emitting transistors of 0.8% is consistent with complete recombination of all charges and a singlet exciton fraction of 25%.

Carrier density dependence of nonresonant carrier tunneling in GaAs double quantum wells – effect of exciton and free carrier thermodynamic

AbstractNonresonant carrier tunneling dynamics in GaAs/AlGaAs double quantum wells as a function of carrier density was investigated by using time‐resolved pump and probe measurements. At low temperatures, tunneling time increases with increasing carrier density. At high temperatures, no dependence of tunneling time on carrier density was shown. This unique behavior is well explained by the thermodynamic effect be‐ tween excitons and free carriers as a function of carrier density on the tunneling process. At low temperatures, a larger fraction of excitons at a higher carrier density, which have a relatively slow tunneling time in contrast to free electrons, leads to a slow tunneling time. The independence of the tunneling time at high temperatures is attributed to the insensitivity of the exciton population to the increased carrier density. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Extrafluorescent electroluminescence in organic light-emitting devices

Organic light-emitting devices (OLEDs) are a promising technology for flat-panel displays and solid-state lighting. While OLED efficiencies have increased dramatically in recent years, further progress is complicated by the fact that the vast majority of organic materials are fluorescent and therefore emit only from molecular excited states ('excitons') with spin 0, or 'singlet' spin symmetry. Here, we demonstrate the ability to manipulate the fraction of excitons which form as singlets in fluorescent materials by altering the OLED structure. We insert a mixing layer that affects only charge-transfer (CT) states, which are the precursors to excitons. As a result, we triple the singlet fraction and the efficiency of the red fluorophore DCM2. We term fluorescence enhanced by CT spin mixing 'extrafluorescence', and show that its origin is in part an inversion of the usual energetic ordering of the singlet and triplet CT states.

Dissociation of excitons in organic light-emitting diodes

In this paper, the dissociation of excitons in both doped and undoped organic light-emitting diodes is investigated in detail by means of electric-field-induced photoluminescence quenching. The results show that the doped devices demonstrate lower quenching than that of undoped device. The reason is that the narrower energy band gap of guest molecules compared to that of the host molecules. In doped devices the increasing concentration of the guest molecules leads to a decrease in dissociation of excitons. The reason is that the increasing of the guest molecules concentration can result in an increase in the fraction of excitons residing on the guest molecules. Besides, the decreasing energy band gap of guest molecules can also make lower quenching in the doped devices.

Many-body dynamics and exciton formation studied by time-resolved photoluminescence

The dynamics of exciton and electron-hole plasma populations is studied via time-resolved photoluminescence after nonresonant excitation. By comparing the peak emission at the exciton resonance with the emission of the continuum, it is possible to experimentally identify regimes where the emission originates predominantly from exciton and/or plasma populations. The results are supported by a microscopic theory which allows one to extract the fraction of bright excitons as a function of time.

Time-dependent photocurrent of a CdTe nanoparticle film under the above-gap illumination

Time-dependent photocurrent of a solid film of 1-thioglycerol-capped CdTe nanoparticles under the illumination of the 325-nm wavelength light is characterized to investigate the transport mechanism of photo-generated charge carriers in this nanoparticle film. Under the illumination of the above-gap light, photocurrent rises rapidly, and subsequently it decays or rises slowly, depending on the magnitude of bias voltage. A careful investigation into the variation in the magnitude of the current measured as a function of time while the light was switched on and off periodically reveals that rapidly and slowly respondent photocurrents overlap in the time-dependent photocurrent. Charge carriers contributing to the rapidly and slowly respondent photocurrents are electrons and holes separated from a fraction of excitons excited by the above-gap light. The transport behaviors of these charge carriers may explain the monotonously decay and slow rising of the photocurrent after its rapidly rising under the illumination for the solid film at unbiased and biased voltage, respectively.

Triplet Formation In Polyfluorene Devices

The fraction of excitons formed as singlets governs the efficiency of a polymer light‐emitting diode (LED) (see Figure). By measuring absorptions due to triplet excitons in working poly(9,9‐dioctylfluorene) devices, the singlet formation probability is calculated. A lower limit of 58 % is found, significantly larger than that predicted statistically. In devices based on the copolymer F8BT, no triplet exciton absorption can be detected.

Spin-Dependent Recombination in PPV and Polyfluorene LEDs

AbstractIn a polymer light-emitting diode, the fraction of excitons formed as singlets is of crucial importance in determining the quantum efficiency. We have shown that it is possible to measure excited state absorptions due to triplet excitons and polarons in working polymer LEDs, and we are able to quantify the triplet generation rate by measuring the strength of the triplet absorption. Here, we show that by careful study of singlet emission and triplet absorption in an LED based on a poly(p-phenylenevinylene) derivative we can obtain an accurate value of 83±7% for the singlet formation probability, significantly higher than the value of 25% predicted by simple spin statistics. We extend these measurements to devices based on poly(dioctyl-fluorene), where we find similarly high values for the singlet formation probability. In devices using the polyfluorene copolymer F8BT, we find that the triplet absorption is extremely small, consistent with even higher singlet formation probabilities.

Excitonic absorption and gain in ZnSe

We present calculations of the linear optical absorption/gain in ZnSe including the relevant many-body effects which comply with a correct description of position and linewidth of the exciton and of the transition from gain to absorption at the chemical potential. The influence of an excitonic fraction is included considering the chemical equilibrium of bound e–h pairs and free carriers by a T-matrix approach. For temperatures from 40 upto 200 K, we found that increasing the excitation the chemical potential comes close to the excitonic absorption peak but does not cross it, as long as the latter survives. Moreover, there is only a minor fraction of excitons under gain conditions. We deduce, that the gain mechanism is governed by many-body correlations of a strongly Coulomb-correlated electron–hole plasma.

Polariton-polariton scattering in semiconductor microcavities: Experimental observation of thresholdlike density dependence

Polariton-polariton scattering in semiconductor microcavities has been studied by time-integrated, degenerate four-wave mixing using spectrally compressed pulses. We find a thresholdlike behavior for the density dependence of the scattering rate \ensuremath{\gamma} in the lower polariton branch. At low densities scattering is suppressed due to the small density of final states for the scattering process. At higher density \ensuremath{\gamma} increases as excitonlike states become accessible for the scattering and the scattering strength only depends on the exciton fraction of the initial states. The threshold density depends on the detuning between the polariton branches. The experimental findings are in qualitative agreement with calculations using the self-consistent Born approximation.

Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO2

Time resolved photoluminescence (PL) decays have been measured for Si nanocrystals embedded in silicon dioxide. The nanocrystals were formed by implanting 40 keV Si ions into a 1000 Å thick film of thermally grown SiO2, followed by thermal annealing at 1000–1200 °C. The observed luminescence, peaking at 700–850 nm, is compared to similar measurements performed on porous Si emitting in the same wavelength range. The results show that the PL from the nanocrystals exhibits a stretched exponential decay with characteristic decay time τ in the range 10–150 μs and dispersion factor β in the range 0.7–0.8. Both parameters are, however, higher for the nanocrystals compared to those of porous Si indicating superior passivation of the nanocrystals in the SiO2 matrix. Evidence is also presented for a single exponential behavior at the decay end suggesting a remaining fraction of excitons in isolated nanocrystals. We attribute the highly nonlinear dose dependence of the PL yield to a nucleation process for the nanocrystals and a more curved decay line shape for higher ion doses to a higher crystal density, promoting excitonic migration to nearby nanocrystals. These observations provide strong evidence that the origin of the stretched exponential line shape of the PL decay results from migration and trapping of excitons in a system of randomly distributed and interconnected nanocrystals with a dispersion in size.

Excitonic singlet-triplet ratio in a semiconducting organic thin film

A technique is presented to determine the spin statistics of excitons formed by electrical injection in a semiconducting organic thin film. With the aid of selective addition of luminescent dyes, we generate either fluorescence or phosphorescence from the archetype organic host material aluminum tris (8-hydroxyquinoline) $({\mathrm{Alq}}_{3}).$ Spin statistics are calculated from the ratio of fluorescence to phosphorescence in the films under electrical excitation. After accounting for varying photoluminescent efficiencies, we find a singlet fraction of excitons in ${\mathrm{Alq}}_{3}$ of $(22\ifmmode\pm\else\textpm\fi{}3)%.$

Exciton dynamics in thin [formula omitted] quantum wells grown by MBE

We report high-resolution, picosecond laser spectroscopy measurements of the relaxation of lowest-energy heavy-hole excitons in GaAs quantum wells grown with and without growth interruptions. In both cases excitons relax by losing potential energy in a diffusion motion driven by potential fluctuations in the quantum-well plane. In the quantum wells grown without growth interruptions the low-energy shift of exciton lines is comparable with the width of an inhomogeneously broadened line and shows two-step decay with a slow exponential component characteristic for exciton localization at well interfaces. In the growth-interrupted quantum wells in which the size of islands with constant well thickness is large compared with exciton diameter we observe splitting of the heavy-hole transitions into the multiplets of narrow lines corresponding to one monolayer difference in the well width. The energy shifts of each line in this case amount to only a fraction of the width of individual lines (or there is no shift at all) suggesting the interisland migration of excitons mediated by acoustic phonon scattering as being responsible for exciton relaxation. Again a two-step decay of the luminescence is observed at low temperatures (2 K). Temperature-dependent measurements show that at higher temperatures luminescence decay becomes governed by a single exponential as expected for delocalized excitons (no energy shift is observed during the exciton decay time). This allowed us to study directly intrinsic properties of excitons in quantum wells, i.e. to determine the lifetime of k I = 0 excitons, which is a fundamental parameter of the system and has been assessed by many theories. The experimentally determined, from the present work, values of the radiative lifetime are 24.4 ps for a 13 ML thick well and 21.8 ps for a 17 ML well. These values agree very well with theoretical estimates of Andreani et al. ( Solid State Commun., 77 (1991) 641). The effective lifetimes measured as a time to decay to 1 /e of the value of the maximum of PL intensity are considerably longer than the radiative lifetime, since in thermal equilibrium only a small fraction of excitons occupy the states with k I < k 0 which can decay radiatively.

Exchange interaction of excitons in GaAs heterostructures.

The circular polarization of cw photoluminescence as a function of applied magnetic field has been measured at 1.8 K for excitons in a series of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As and GaAs/AlAs type-I quantum wells having well widths between 2.5 and 8.0 nm. The results show evidence of level crossings, which have been analyzed to give the short-range, spin-dependent exchange interaction. The associated experimental exchange splittings between optically allowed and nonallowed exciton states are of the order of 0.15 meV for the narrowest wells and fall monotonically with increasing width. Simulations of the data using solutions of rate equations for the exciton-level populations show that the optically nonallowed exciton states lie below the allowed states and give insight into the field dependence of the spin-relaxation rates of excitons and of holes and electrons in an exciton. Calculations of the enhancement of exchange interaction due to carrier confinement relative to the bulk value give agreement between the quantum-well and bulk-exchange values, and confirm that exchange is particularly sensitive to barrier height and other details of the structure. The data also show evidence for eigenstate-polarization changes, which indicate a much smaller zero-field splitting due to departure of the quantum-well symmetry from the ideal ${\mathit{D}}_{2\mathit{d}}$. The simulation indicates that only a fraction (up to 20%) of excitons in these samples actually experiences a nonideal, distorted environment.

Exciton radiative decay and homogeneous broadening in CdTe/Cd0.85Mn0.15Te multiple quantum wells

Exciton resonances are observed in the room-temperature absorption and photoluminescence (PL) spectra of CdTe/${\mathrm{Cd}}_{0.85}$${\mathrm{Mn}}_{0.15}$Te multiple-quantum-well structures. Temperature-dependent absorption linewidth and PL decay measurements are reported. Above 45 K, exciton-LO-phonon scattering determines the homogeneous linewidth (${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$). High excitation intensities were used in the PL decay measurements so that delocalized excitons were created. The PL decay time is found to be a measure of the exciton radiative decay time between 45 and 145 K and it increases linearly with ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$/r(T), where r(T) is the fraction of excitons with transition frequencies in ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$. From 170 K nonradiative processes dominate the decay so that the PL decay time decreases with temperature.

Host-Sensitized Luminescence of KI : Tl under an Applied Electric Field

Effect of an electric field on the host-sensitized luminescence of Kl : Tl is studied at room temperature. Contribution of free carriers to the mechanism of the host sensitization is estimated form an abrupt reduction in the intensity of Tl + emission which is induced by an application of a dc electric field. Under optical excitation in the interband absorption region, absorbed energy is transferred from the host KI to a Tl + center by free carriers. On the contrary, the contribution of free carriers to the process of the energy transfer is small in the case of excitation in the exciton band. A small fraction of excitons seems to decompose through exciton-exciton collision. It is suggested that the Tl + emission stimulated in the region of exciton absorption band is caused not only by direct excitation of D state but also due to the energy transfer by excitons.