Exciton Decay Rate Research Articles

An accurate measurement of the dipole orientation in various organic semiconductor films using photoluminescence exciton decay analysis.

In this study, we report an accurate and more reliable approach to estimate the dipole orientation of emitters especially phosphorescence, fluorescence and even thermally activated delayed fluorescence. The dipole orientation measurements are performed by examining the variation of the photoluminescence (PL) exciton decay rate from time-resolved PL and optical analysis. Our anisotropic dipole orientation results are consistent with those of previous reports. The studied measurement approach is very reliable and accurate to estimate the dipole orientation of any organic semiconductor materials regardless of whether they are doped or neat films.

Exploration of charge carrier delocalization in the iron oxide/CdS type-II heterojunction band alignment for enhanced solar-driven photocatalytic and antibacterial applications.

Recyclable magnetic photocatalysts of iron oxide (IO)/CdS core/shell nanocrystals (CSNCs) were prepared by a facile sequential one-pot method using 3, 3'-thiobispropanoic acid (TDP) as a bridge. The CSNCs showed redshift in absorption edge, decrease in the optical band gap, reduced exciton decay rates and increment in particle size. Quenching studies have been employed to understand the position of the electron/hole wave-functions at the IO/CdS interface. Antimicrobial tests have also been performed using broth tube dilution and disc diffusion methods against S. aureus. Additionally, photocatalytic properties of IO/CdS CSNCs have been evaluated for the decomposition of xylenol blue. In comparison with CdS quantum dots (QDs) and iron oxide nanoparticles (IONPs), the IO/CdS CSNCs showed improved photocatalytic and bactericidal activities. Finally, levels of oxidative damage to proteins and lipids were evaluated.

Interaction-induced photon blockade using an atomically thin mirror embedded in a microcavity

Narrow dark resonances associated with electromagnetically induced transparency play a key role in enhancing photon-photon interactions. The schemes realized to date relied on the existence of long-lived atomic states with strong van der Waals interactions. Here, we show that by placing an atomically thin semiconductor with ultra-fast radiative decay rate inside a \textcolor{black}{0D} cavity, it is possible to obtain narrow dark or bright resonances in transmission whose width could be much smaller than that of the cavity and bare exciton decay rates. While breaking of translational invariance places a limit on the width of the dark resonance width, it is possible to obtain a narrow bright resonance that is resilient against disorder by tuning the cavity away from the excitonic transition. Resonant excitation of this bright resonance yields strong photon antibunching even in the limit where the interaction strength is arbitrarily smaller than the non-Markovian disorder broadening and the radiative decay rate of the bare exciton. Our findings suggest that atomically thin semiconductors could pave the way for realization of strongly interacting photonic systems in the solid-state.

Tuning Exciton-Mn2+ Energy Transfer in Mixed Halide Perovskite Nanocrystals.

Doping nanocrystals (NCs) with luminescent activators provides additional color tunability for these highly efficient luminescent materials. In CsPbCl3 perovskite NCs the exciton-to-activator energy transfer (ET) has been observed to be less efficient than in II–VI semiconductor NCs. Here we investigate the evolution of the exciton-to-Mn2+ ET efficiency as a function of composition (Br/Cl ratio) and temperature in CsPbCl3–xBrx:Mn2+ NCs. The results show a strong dependence of the transfer efficiency on Br– content. An initial fast increase in the relative Mn2+ emission intensity with increasing Br– content is followed by a decrease for higher Br– contents. The results are explained by a reduced exciton decay rate and faster exciton-to-Mn2+ ET upon Br– substitution. Further addition of Br– and narrowing of the host bandgap make back-transfer from Mn2+ to the CsPbCl3–xBrx host possible and lead to a reduction in Mn2+ emission. Temperature-dependent measurements provide support for the role of back-transfer as the highest Mn2+-to-exciton emission intensity ratio is reached at higher Br– content at 4.2 K where thermally activated back-transfer is suppressed. With the present results it is possible to pinpoint the position of the Mn2+ excited state relative to the CsPbCl3–xBrx host band states and predict the temperature- and composition-dependent optical properties of Mn2+-doped halide perovskite NCs.

20‐4: Dipole Orientation Measurement Method by Time Resolved Photoluminescence

In this study, we report a new molecular dipole orientation measurement method by simply measuring photolumincence exciton decay rate with time resolved equipment in the film state. Our measurement method is very simple, universal and reliable to measure the dipole orientation in any OLED molecules.

Effect of various host characteristics on blue thermally activated delayed fluorescent devices

Abstract Two bipolar hosts consisting of phosphine oxide and carbazole, (9-phenyl-9H-carbazole-3,6-diyl)bis (diphenylphosphine oxide) (PPO2) and phenylbis (9-phenyl-9H-carbazol-3-yl)phosphine oxide (3DCPO) generally used as blue phosphorescence hosts are successively applied to blue thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). These hosts have suitable high triplet energy (T1) and bipolar characteristics expecting increase in TADF characteristics of dopant. The exciton decay rate and photoluminance quantum yield (PLQY) of the dopant according to the polarity of host are analyzed. It is shown that the host, PPO2, with appropriate polarity increased the TADF characteristics of the dopant and efficiency of OLED. The PPO2 based blue TADF OLED shows a high external quantum efficiency of 27.8% achieving enhanced efficiency roll-off characteristic.

Controlled Addition of Carbon Nanotube Defects and Their Role in Limiting Solar Cell Performance

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted significant attention as a photoactive component in thin film photovoltaics due to their strong absorptivity and high charge mobility. However, the external quantum efficiency (EQE) of s-SWCNT/C60 heterojunction solar cells is limited by poor exciton diffusion to the heterointerface. Exciton diffusion is believed to be strongly dependent on the length between defects, which quench excitons. Here, we systematically study the influence of defects on s-SWCNT/C60 planar heterojunction photovoltaic devices. We use covalent sidewall functionalization via diazonium chemistry to introduce sp3 defects to the s-SWCNTs at varying concentrations. We generate a defect-adding model to describe the position of defects on the surface of the nanotube, and use it to successfully reproduce the time-resolved exciton decay rate measured with transient absorption spectroscopy. We fabricate s-SWCNT/C60 heterojunction photovoltaic cells and characterize the EQE as a function of defect concentration. We develop a diffusion limited contact quenching Monte Carlo model to reproduce the EQE trends we observe. The model allows us to predict the behavior of theoretical s-SWCNT devices with varying length distribution and defect concentration, toward the limit of long length and zero defects. This work guides the field of nanotube photovoltaics and illuminates the promise and limitations of s-SWCNT solar cell devices.

Anisotropy-Induced Quantum Interference and Population Trapping between Orthogonal Quantum Dot Exciton States in Semiconductor Cavity Systems.

We describe how quantum dot semiconductor cavity systems can be engineered to realize anisotropy-induced dipole-dipole coupling between orthogonal dipole states in a single quantum dot. Quantum dots in single-mode cavity structures as well as photonic crystal waveguides coupled to spin states or linearly polarized excitons are considered. We demonstrate how the dipole-dipole coupling can control the radiative decay rate of excitons and form pure entangled states in the long time limit. We investigate both field-free entanglement evolution and coherently pumped exciton regimes, and show how a double-field pumping scenario can completely eliminate the decay of coherent Rabi oscillations and lead to population trapping. In the Mollow triplet regime, we explore the emitted spectra from the driven dipoles and show how a nonpumped dipole can take on the form of a spectral triplet, quintuplet, or a singlet, which has applications for producing subnatural linewidth single photons and more easily accessing regimes of high-field quantum optics and cavity-QED.

Controlling the exciton emission of gold coated GaAs–AlGaAs core–shell nanowires with an organic spacer layer

Excitons are the most prominent optical excitations and controlling their emission is an important step towards new optical devices. We have investigated the exciton emission from uncoated and gold/aluminum quinoline (Alq3) coated GaAs–AlGaAs–GaAs core–shell nanowires (NWs) using temperature-, intensity- and polarization dependent photoluminescence (PL). Plasmonic GaAs–AlGaAs–GaAs NWs with a ∼10 nm thick Au coating but without an Alq3 spacer layer reveal a significant reduction of the PL intensity of the exciton emission compared with the uncoated NW sample. Plasmonic NW samples with the same nominal Au coverage and an additional Alq3 interlayer of 3 or 6 nm thickness show a clearly stronger PL intensity which increases with rising Alq3 spacer thickness. Time-resolved (TR) PL measurements reveal an increase of the exciton decay rate by a factor of up to two with decreasing Alq3 spacer thickness suggesting the presence of Förster energy transfer from NW excitons to plasmon oscillations in the gold film. The weak change of the decay time, however, indicates that Förster energy-transfer is only partially responsible for the PL quenching in the gold coated NWs. The main reason for the reduction of the PL emission is attributed to a gold induced band-bending in the GaAs NW core which causes exciton dissociation. With increasing Alq3 spacer thickness the band-bending decreases leading to a reduction of the exciton dissociation and PL quenching. Our interpretation is supported by electron energy loss spectroscopy measurements which show a signal reduction and blue shift of defect (possibly EL2) transitions when gold particles are deposited on NWs compared with bare or Alq3 coated NWs.

Increase in exciton decay rate due to plane-to-plane interaction between cyanine thin films

We report an increase in exciton decay rates because of long-range interaction based on surface charge between cyanine thin films. The dependence of the decay rate on the spatial separation between the cyanine molecule layers shows that the rate is almost constant, which is different from the well-known energy transfer process. The rate is hardly affected by the fluctuation of the film thickness, which is an advantage of using cyanine or organic molecules.

Enhanced Local and Nonlocal Photoluminescence of Organic Rubrene Microrods using Surface Plasmon of Gold Nanoparticles: Applications to Ultrasensitive and Remote Biosensing

Nonlocal photoluminescence (PL) signal transfer through semiconducting nanostructures has been intensively studied for its potential applicability in photonic circuits, optical communications, and optical sensing. In this study, organic semiconducting rubrene microrods (MRs) were synthesized and hybridized with functionalized gold nanoparticles (Au-NPs) to optimize both their optical and biosensing properties. The steady-state local PL intensity of the rubrene MR was considerably enhanced by the Au-NPs’ hybridization due to the energy-transfer effect from the surface plasmon (SP) coupling. It was clearly observed that the nonlocal PL signal-transfer efficiency of rubrene/Au-NPs hybrid MRs drastically increased along crystalline axes with the aid of the SP effect. The coupling of exciton polaritons in the luminescent rubrene MR with the SP as well as the scattering effect contribute to the variation of the exciton decay rate, resulting in a change in the PL signal-transfer efficiency for the hybrid MRs. Th...

Radiative decay rate of excitons in square quantum wells: Microscopic modeling and experiment

The binding energy and the corresponding wave function of excitons in GaAs-based finite square quantum wells (QWs) are calculated by the direct numerical solution of the three-dimensional Schrödinger equation. The precise results for the lowest exciton state are obtained by the Hamiltonian discretization using the high-order finite-difference scheme. The microscopic calculations are compared with the results obtained by the standard variational approach. The exciton binding energies found by two methods coincide within 0.1 meV for the wide range of QW widths. The radiative decay rate is calculated for QWs of various widths using the exciton wave functions obtained by direct and variational methods. The radiative decay rates are confronted with the experimental data measured for high-quality GaAs/AlGaAs and InGaAs/GaAs QW heterostructures grown by molecular beam epitaxy. The calculated and measured values are in good agreement, though slight differences with earlier calculations of the radiative decay rate are observed.

Role of impurities in determining the exciton diffusion length in organic semiconductors

The design and performance of organic photovoltaic cells is dictated, in part, by the magnitude of the exciton diffusion length (LD). Despite the importance of this parameter, there have been few investigations connecting LD and materials purity. Here, we investigate LD for the organic small molecule N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine as native impurities are systematically removed from the material. Thin films deposited from the as-synthesized material yield a value for LD, as measured by photoluminescence quenching, of (3.9 ± 0.5) nm with a corresponding photoluminescence efficiency (ηPL) of (25 ± 1)% and thin film purity of (97.1 ± 1.2)%, measured by high performance liquid chromatography. After purification by thermal gradient sublimation, the value of LD is increased to (4.7 ± 0.5) nm with a corresponding ηPL of (33 ± 1)% and purity of (98.3 ± 0.8)%. Interestingly, a similar behavior is also observed as a function of the deposition boat temperature. Films deposited from the purified material at a high temperature give LD = (5.3 ± 0.8) nm with ηPL = (37 ± 1)% for films with a purity of (99.0 ± 0.3)% purity. Using a model of diffusion by Förster energy transfer, the variation of LD with purity is predicted as a function of ηPL and is in good agreement with measurements. The removal of impurities acts to decrease the non-radiative exciton decay rate and increase the radiative decay rate, leading to increases in both the diffusivity and exciton lifetime. The results of this work highlight the role of impurities in determining LD, while also providing insight into the degree of materials purification necessary to achieve optimized exciton transport.

Efficient Green Organic Light-Emitting Diodes by Plasmonic Silver Nanoparticles

The resonance effect between the localized surface plasmon mode and the excited excitons in organic materials can enhance the optical and electrical performances of organic light-emitting diodes (OLEDs). Here, we show an enhancement in the luminescence efficiency of a green OLED by employing silver nanoparticles without an optical microcavity effect. The enhanced photoluminescence and exciton decay rate support the idea that the improved luminescence efficiency is due to the increased spontaneous emission rate of the emitter. Our results indicate that the plasmonic near-field interaction of the light-matter can be a strong engineering tool for high-efficiency organic optoelectronic devices in the display industry.

Towards the design of efficient quantum dot light-emitting diodes by controlling the exciton lifetime.

Time-resolved photoluminescence and electroluminescence measurements were used to explore the emission characteristics of excitons in quantum dot light-emitting diodes (QD-LEDs). It is found that the lifetime of excitons in the QDs can be varied by adjusting the distance between the excitons and metal Al mirror, which is due to the effect of local density of optical states (LDOS) on the exciton decay rate. QD-LEDs with different hole transport layer (HTL) thickness, i.e., different distance between QDs and Al reflective anode, have been fabricated and it is found that the HTL thickness affects the device efficiency performance greatly, and the optimal HTL thickness for the red QD-LED (emission peak is at 621 nm) is 80 nm. These results shed light on the factors affecting the efficiency and efficiency roll-off in QD-LEDs, thus may provide a clue for high performance QD-LEDs.

The search for Bose–Einstein condensation of excitons in Cu2O: exciton-Auger recombination versus biexciton formation

Excitons in high-purity crystals of Cu2O undergo a density-dependent lifetime that opposes Bose–Einstein condensation (BEC). This rapid decay rate of excitons at a density n has generally been attributed to Auger recombination having the form , where A is an exciton-Auger constant. Various measurements of A, however, have reported values that are orders-of-magnitude larger than the existing theory. In response to this conundrum, recent work has suggested that excitons bind into excitonic molecules, or biexcitons, which are short-lived and expected to be optically inactive. Of particular interest is the case of excitons confined to a parabolic strain well—a method that has recently achieved exciton densities approaching BEC. In this paper we report time- and space-resolved luminescence data that supports the existence of short-lived biexcitons in a strain well, implying an exciton loss rate of the form with a biexciton capture coefficient C(T) proportional to , as predicted by basic thermodynamics. This alternate theory will be considered in relation to recent experiments on the subject.

Quasiparticle spectra and excitons of organic molecules deposited on substrates:G0W0-BSE approach applied to benzene on graphene and metallic substrates

We present an alternative methodology for calculating the quasiparticle energy, energy loss, and optical spectra of a molecule deposited on graphene or a metallic substrate. To test the accuracy of the method it is first applied to the isolated benzene (${\text{C}}_{6}{\text{H}}_{6}$) molecule. The quasiparticle energy levels and especially the energies of the benzene excitons (triplet, singlet, optically active and inactive) are in very good agreement with available experimental results. It is shown that the vicinity of the various substrates [pristine/doped graphene or (jellium) metal surface] reduces the quasiparticle highest occupied molecular orbital--lowest unoccupied molecular orbital (HOMO-LUMO) gap by an amount that slightly depends on the substrate type. This is consistent with the simple image theory predictions. It is even shown that the substrate does not change the energy of the excitons in the isolated molecule. We prove (in terms of simple image theory) that energies of the excitons are indeed influenced by two mechanisms which cancel each other. We demonstrate that the benzene singlet optically active (${E}_{1u}$) exciton couples to real electronic excitations in the substrate. This causes it substantial decay, such as $\ensuremath{\Gamma}\ensuremath{\approx}174$ meV for pristine graphene and $\ensuremath{\Gamma}\ensuremath{\approx}362$ meV for metal surfaces as the substrate. However, we find that doping graphene does not influence the ${E}_{1u}$ exciton decay rate.

Unexpectedly slow two particle decay of ultra-dense excitons in cuprous oxide

For an ultra-dense exciton gas in cuprous oxide (Cu2O), exciton–exciton interactions are the dominant cause of exciton decay. This study demonstrates that the accepted Auger recombination model overestimates the exciton decay rate following intense two photon excitation. Two exciton decay is relevant to the search for collective quantum behavior of excitons in bulk systems. These results suggest the existence of a new high density regime of exciton behavior.

Influence of exciton lifetime on charge carrier dynamics in an organic heterostructure

Interactions between charge carriers and excitons, as well as between excitons and optical cavity modes in organic optoelectronic devices are fundamental to their operational limits and chief in preventing the realization of certain phenomena, such as electrically pumped organic lasing. We uncovered a previously unreported phenomenon, wherein optical cavity-modulated exciton decay rate leads to a concomitant modulation in the electrical current of an archetypal NPD/Alq3 organic light emitting device operated in forward bias. The magnitude of this variation is sensitive to the local dielectric environment of the device and is found to be as large as 15%.

Temperature dependence of photoluminescence spectra in hole-doped single-walled carbon nanotubes: Implications of trion localization

We studied the dimensionality of excitons and trions (charged excitons) in hole-doped single-walled carbon nanotubes (SWNTs) by means of photoluminescence (PL) spectroscopy. We found that the temperature ($T$) dependence of the PL intensity of the trions and excitons is significantly different, reflecting their dimensionality. The PL intensity of the trions was almost unchanged for 5--300 K, whereas clear ${T}^{\ensuremath{-}1/2}$ dependence of the exciton PL intensity was observed, which reflects the characteristic radiative decay rate of one-dimensional excitons. The results suggest that the radiative decay rate of the trions is independent of the temperature, which is the manifestation of the localization of the trions in hole-doped SWNTs.