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This letter reports a time resolved pump-probe reflectance spectroscopic study on moiré excitons in a twisted monolayer WS2/WSe2 heterostructure. By probing at the resonant energies of intralayer excitons, we observed their distinct temporal tracks under the influence of interlayer excitons, which we attribute to the discrepancy in spatial distribution of the intralayer excitons in different layers. We also observed that intralayer moiré excitons in WSe2 layer differ at decay rate, which reflects different locations of Wannier-like and charge-transfer intralayer excitons in a moiré cell. We concluded that the interlayer moiré excitons form within a few picoseconds and have the lifetime exceeding five nanoseconds. Our results provide insights into the nature of moiré excitons and the strain's significant impact on their behaviour in twisted heterostructures, which could have important implications for the development of novel optoelectronic devices. [ABSTRACT FROM AUTHOR]

Up‐conversion of triplet into singlet exciton in the emitting layer is believed to be one of the ways that thermally activated delayed fluorescent (TADF) materials may employ to reduce triplet exciton density hence preventing device quenching by triplet excitons. Yet, two donor‐acceptor type molecules; 5‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐12‐(3‐(triphenylsilyl) phenyl)‐5,12‐dihydroindolo[3,2‐a]carbazole (SiPhCzTrz) and 5‐phenyl‐12‐(4‐(4‐phenyl‐6‐(3‐(triphenylsilyl) phenyl)‐1,3,5‐triazin‐2‐yl)pheny‐l)‐5,12‐dihydroindolo [3,2‐a]carbazole (SiTrzPhCz), which exhibited different TADF properties depending on the relative positions of their electron donor unit (PhCz) and electron acceptor unit (Trz), show opposite behaviors. These materials are used as sensitizer in phosphorescent solution‐processed organic light emitting diodes (s‐OLEDs) showing moderately high current efficiencies of 19.3 and 20 cd/A, respectively. SiTrzPhCz exhibits stronger TADF properties compared to SiPhCzTrz; however, these stronger TADF characteristics lead to a more pronounced efficiency roll‐off, mainly due to the longer residence time of excitons in SiTrzPhCz, leading to exciton quenching. In contrast, due to their twisted structures, the efficiency roll‐off is efficiently suppressed, particularly for SiTrzPhCz, when both materials are used as hosts. Their twisted structures promote aggregate‐induced emission and prevent aggregation‐caused quenching. Further analysis of exciton dynamics reveals faster decay rate for both singlet and triplet densities in SiPhCzTrz compared to SiTrzPhCz, indicated by its higher fast prompt emission, kFRET and knrT. [ABSTRACT FROM AUTHOR]

Layered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin‐orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher‐energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low‐temperature magneto‐optical measurements with sophisticated many‐particle theory, the origin of the bright exciton emission in perovskites is elucidated by tracking the thermalization of dark and bright excitons under a magnetic field. The unexpectedly high emission is clearly attributed to a pronounced phonon‐bottleneck effect, considerably slowing down the relaxation toward the energetically lowest dark states. It is demonstrated that this bottleneck can be tuned by manipulating the bright‐dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications. [ABSTRACT FROM AUTHOR]

The extreme confinement and reduced screening in monolayer transition metal dichalcogenides (TMDs) leads to the appearance of tightly bound excitons which can also couple to free charges, forming trions, owing to strong Coulomb interactions. Low temperatures and encapsulation in hexagonal boron nitride (hBN) can narrow the excitonic linewidth, approaching the regime of homogeneous broadening, mostly dominated by the radiative decay. Ultrafast spectroscopy is a perfect tool to study exciton formation and relaxation dynamics in TMD monolayers. However, high‐quality hBN‐encapsulated structures have usually lateral sizes of the order of a few micrometers, calling for the combination of high spatial and temporal resolution in pump–probe experiments. Herein, a custom broadband pump–probe optical microscope is used to measure the ultrafast dynamics of neutral and charged excitons in high‐quality hBN‐encapsulated monolayer MoS2 at 8 K. Neutral excitons exhibit a narrow linewidth of 7.5 meV, approaching the homogeneous limit, which is related to the fast recombination time of ≈130 fs measured in pump–probe. Moreover, markedly different dynamics of the trions over the neutral ones are observed. The results provide novel insights on the exciton recombination processes in TMD monolayers, paving the way for exploring the ultrafast behavior of excitons and their many‐body complexes in TMD heterostructures. [ABSTRACT FROM AUTHOR]

The operational lifetime of blue organic light‐emitting diodes (OLEDs) is still insufficient for practical applications in lighting and display. One type of blue organic emitting materials with hybridized local and charge‐transfer (HLCT) process are beneficial in achieving high‐efficiency OLEDs through "hot exciton" channel by harnessing high‐lying triplet (Tn) excitons. However, the operational lifetime of the resulting blue OLEDs is rarely studied and understood. In this article, the aging properties of blue fluorescent OLEDs based on an HLCT material (2‐(4‐(10‐(3‐(9H‐carbazol‐9‐yl)phenyl)anthracen‐9‐yl)phenyl)‐1‐phenyl‐1H‐phenanthro[9,10‐d]imidazole) (PAC) are systematically investigated by exciton dynamics calculation and transient EL experiments. It is experimentally and theoretically revealed that whether the reverse intersystem crossing (hRISC) process from high‐lying excited triplet to singlet in HLCT materials is completely effective determines the device degradation. A fluorescent emitter is doped into PAC host to accelerate the hRISC process, thus enhancing device operational lifetime to reach T75 = 110 ± 2 h (time to 75% of initial luminance) under 1000 cd m−2. This work provides inspirations to investigate the stability of blue fluorescent OLEDs based on HCLT materials and further enhance the operational lifetime. [ABSTRACT FROM AUTHOR]

Exciton dynamics in ReS2 are rich and complicated due to its unique lattice and band structures. With femtosecond pump‐probe spectroscopy, the wavelength‐dependent exciton dynamics in bulk ReS2 are studied at room temperature, and the effects from polarization and stacking order are also considered. Signatures from both bright and dark excitons are observed, where the lifetime at bright exciton levels is only several picoseconds, while it is about tens of picoseconds at dark exciton levels. The extremely long up‐/down‐going trend at selective wavelengths is attributed to the transition between different dark exciton levels. Two types of coherent dynamics are observed when the laser wavelength is scanned through different exciton levels: coherent optical stark effect and exciton‐exciton coherent interaction, both occurring within the laser pulse width. Exciton dynamics show a similar trend for the sample with AA stacking order, regardless of polarization. For AB stacking, dark excitons for Exciton II and III are missing when probe pulse polarization is perpendicular to b‐axis, which is attributed to the polarization and stacking‐order dependent band structure. Studying the rich exciton dynamics in bulk ReS2 at room temperature not only has scientific significance, but also facilitates the development of high‐performance excitonic devices. [ABSTRACT FROM AUTHOR]

Exciton dynamics in ReS2 are rich and complicated due to its unique lattice and band structures. With femtosecond pump‐probe spectroscopy, the wavelength‐dependent exciton dynamics in bulk ReS2 are studied at room temperature, and the effects from polarization and stacking order are also considered. Signatures from both bright and dark excitons are observed, where the lifetime at bright exciton levels is only several picoseconds, while it is about tens of picoseconds at dark exciton levels. The extremely long up‐/down‐going trend at selective wavelengths is attributed to the transition between different dark exciton levels. Two types of coherent dynamics are observed when the laser wavelength is scanned through different exciton levels: coherent optical stark effect and exciton‐exciton coherent interaction, both occurring within the laser pulse width. Exciton dynamics show a similar trend for the sample with AA stacking order, regardless of polarization. For AB stacking, dark excitons for Exciton II and III are missing when probe pulse polarization is perpendicular to b‐axis, which is attributed to the polarization and stacking‐order dependent band structure. Studying the rich exciton dynamics in bulk ReS2 at room temperature not only has scientific significance, but also facilitates the development of high‐performance excitonic devices. [ABSTRACT FROM AUTHOR]

The rate constants that describe the decays of excited states (free charge carriers, excitons) are important parameters for perovskite materials. Typically, these rates are determined using detectors with high time‐resolution to record the time‐dependent photoluminescence on the nanosecond scale. Herein, a method is applied that uses two excitation pulses with a variable delay and an inexpensive detector capable of measuring only the quasisteady state photoluminescence. Based on how the time‐integrated photoluminescence varies as a function of the delay time between the two excitation pulses, the rate constants for exciton–exciton annihilation, monomolecular exciton decay, bimolecular free charge carrier recombination, and monomolecular trapping of free charge carriers can be extracted. To demonstrate the method quasi‐2D perovskites are investigated as they are known to exhibit excited‐state populations that can be exciton dominated, free charge carrier dominated, or a mixture of the two. The method leads to unique curves and accurate extracted parameters in all cases. The introduced method of determining excited‐state lifetimes can be used with detectors that only have a low temporal resolution. This opens the path to the spatial mapping of excited‐state dynamics using standard cameras which would be attractive for quality control of photovoltaic layers. [ABSTRACT FROM AUTHOR]

Light-emitting diodes (LEDs) based on perovskite materials are a new group of devices that are currently undergoing rapid development. A significant fraction of these devices is based on quasi-2D perovskites fabricated with large organic cations. In this work, we describe the ultrafast scale dynamics in a quasi-2D PEA2(Rb0.6Cs0.4)2Pb3Br10 perovskite material with an excess of RbBr, which was previously used to fabricate blue-emitting perovskite LEDs. The results obtained using transient absorption spectroscopy are consistent with the assumption that the carrier dynamics in this material are dominated by excitons, most of which decay by exciton–exciton annihilation when high-intensity excitation is used. Furthermore, a slow energy transfer between different quasi-2D domains taking place within 50 ps was observed. The content of the RbBr did not show any strong influence on the observed dynamics. Our results show that the exciton–exciton annihilation proceeds much faster in thin (n = 2) quasi-2D domains than in thick (n ≥ 4) domains. This finding implies that perovskites with high-n, quasi-2D domains are preferable for efficient perovskite lasers and bright perovskite LEDs. [ABSTRACT FROM AUTHOR]

Triplet–triplet annihilation (TTA) up‐conversion is an effective way to utilize triplet excitons in organic light‐emitting diodes (OLEDs). However, the parameters characterizing the triplet excitons and relevant TTA process in OLEDs under working conditions have not been quantified. Here, an in situ method is established to map these parameters for further ascertaining their impact on device efficiency. The physical parameters, including triplet recombination rate, TTA rate, typical current JTTA, and saturated ratio, can be in situ quantified by transient electroluminescence technique. The expression of JTTA shows that minimizing the triplet quenching and maximizing the TTA rate are effective ways to lower JTTA. While highly efficient devices require a lower JTTA. Guided by these criteria, the device efficiency is promoted by weakening the triplet quenching via blending two materials. These investigations establish an in situ method to quantify the physical parameters that allow identifying the useful TTA materials and optimizing the design of device structures. [ABSTRACT FROM AUTHOR]

Solid‐state films exhibiting thermally activated delayed fluorescence (TADF) can offer high internal electroluminescence (EL) quantum efficiency, even in a simplified device structure. However, the exciton dynamics in solid‐state TADF films, particularly for the unexpected exciton loss processes such as concentration quenching, have not yet been clarified. The dynamics of photoexcited electrons in the TADF process of a 2,4,5,6‐Tetra (9H‐carbazol‐9‐yl) isophthalonitrile (4CzIPN) solid film are observed via time‐resolved photoelectron emission microscopy (TR‐PEEM) and the results are compared with the conventional time‐resolved photoluminescence (TR‐PL) technique. The initial decay process of the photoexcited electrons probed via TR‐PEEM is thoroughly traced in the TR‐PL signal, while unusual long‐lived electrons are detected only through the use of TR‐PEEM. These results indicate that the excitons of 4CzIPN spontaneously dissociate into free carriers within the exciton lifetime, which seems to be a common process that contributes to the exciton loss in polar organic solids. [ABSTRACT FROM AUTHOR]

In recent years, a variety of studies have investigated in great depth the details of singlet fission mechanisms in terms of electronic energies and couplings. The nature and properties of the triplet‐pair intermediate state or the role of charge separated configurations (CT states) have been extensively explored with the help of theoretical models, ab initio calculations, and experimental characterization. On the other hand, much less is known about the involvement of the localized double electron excitation, that is the D state. In organic conjugated molecules, the D state can be typically described as the HOMO‐to‐LUMO promotion of two electrons, which makes it inactive to photoabsorption (dark state). In the present study, we tackle the possible roles of the D state by means of electronic structure calculations and the simulation of singlet fission dynamics. Our results suggest that, indeed, the D state can have a significant impact on singlet fission. On one hand, if it lies energetically below the optical exciton it is able to drive the system to singlet fission through direct coupling or via a CT‐mediated path. The D state can also act as a mediator in the formation of the triplet‐pair state from initial photoactivation. These results should motivate researchers in the field to include this state in their analysis, and help to clarify electronic mechanisms in singlet fission materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • Exciton dynamics includes exciton generation, diffusion, dissociation and recombination. • Recent research progress of exciton dynamics in photo(electro)catalysis is overviewed. • Characterization techniques of exciton dynamics in photo(electro)catalysis are summarized. • Modulating strategies of exciton dynamics in photo(electro)catalysis are discussed. • Conclusions and outlooks of exciton dynamics in photo(electro)catalysis are presented. The fundamental study on photo(electro)catalytic reactions is central for the potential application in solving energy and environmental issues. However, the exciton effect, which affects their photo(electro)catalytic performance especially in organic and low-dimensional inorganic materials, is less explored. Therefore, it is necessary to investigate the exciton dynamics involved in the photo(electro)catalytic process, and the strategies for modulating exciton dynamics are also needed for improving the photo(electro)catalytic performance. In this review, we introduce the exciton dynamics and main exciton characterization techniques, and summarize the mechanisms for modulating exciton dynamics in photo(electro)catalysis. Finally, current challenges and future possible developments for exciton dynamics in the photo(electro)catalytic process are discussed. We hope this review will provide insights on the exciton dynamics in the photo(electro)catalysis, thus inspiring more researches on the materials with strong exciton effect in photo(electro)catalytic applications. [ABSTRACT FROM AUTHOR]

2D metal halide semiconductors have been intensively studied in the past few years due to their unique optical properties and potential for new‐generation photonic devices. Despite the large number of recent works, this class of materials is still in need of further understanding due to their complex structural and optical characteristics. In this work, a molecular‐level explanation for the dual band emission in the 2D (C4H9NH3)2PbI4 in its bulk form is presented, demonstrating that this feature is caused by a strong exciton–phonon coupling. Temperature‐dependent photoluminescence with Raman and IR spectroscopies reveals that vibrations involving the C‐NH3+ butylammonium polar head are responsible for this exciton–phonon coupling. Additionally, experimental shifts in the mean phonon frequencies coupled with the electronic excitation, combined with a theoretical model, show that these vibrational modes present a soft‐mode behavior in the phase transition of this material. [ABSTRACT FROM AUTHOR]

The singlet fission (SF) dynamics of pentacene linear aggregate models are investigated using the quantum master equation method by focusing on the Frenkel excitonic (FE) coupling effects on the SF rate and double triplet (TT) yield as well as on their aggregate size dependences. It is found that for the dimer model, unrealistically large FE couplings are needed to provide significant effects on the SF dynamics, while for the larger aggregate models a realistic FE coupling causes significant variations in the SF dynamics: as increasing the aggregate size, the SF rate rapidly increases, attains the maximum at 8‐mer (~3 times enhancement as compared to the non‐FE‐coupling case) and then decreases, approaching a stationary value after 12‐mer, although the stationary TT yield at 20‐mer remains slightly smaller than that in the non‐FE‐coupling case. These features are explained based on the relative relaxation factors between the adiabatic exciton states. The present results contribute to constructing the design guidelines for highly efficient SF aggregates. © 2018 Wiley Periodicals, Inc. The singlet fission (SF) dynamics of pentacene linear aggregates are investigated using the quantum master equation method by focusing on the Frenkel excitonic (FE) coupling effects on the SF rate and double triplet yield as well as on their aggregate size dependences. It is found that a realistic FE coupling causes significant size‐dependent variations in the SF rate, which are explained by the relative relaxation factor analysis. [ABSTRACT FROM AUTHOR]

For the past three decades, the field of quantum dot research has been expanding enormously, owing to the unique dynamics of charge carriers in these tiny crystals. This review provides an overview of charge carrier dynamics in photoactivated quantum dots. Specifically, it deals with the exciton fine structure, multiple exciton generation and the origin and control of photoluminescence intermittency in cadmium and lead chalcogenide quantum dots. These properties are discussed within the framework of spectroscopy and microscopy of single and multiple exciton states. Finally, this review touches on the photoluminescence- and multiple exciton- based applications of quantum dots, such as lasing and high efficiency solar cells. [ABSTRACT FROM AUTHOR]

The performance of polymer light emitting diodes (PLEDs) degrades due to exciton quenching at the interface with charge injection layers and electrodes. We investigate the photo-physics of singlet excitons in Poly (9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT) conjugated polymer interfaced with various commonly used hole and electron injection layers. Absolute, steady-state and transient photoluminescence (PL) studies are carried out on pristine F8BT film and films with injection layer/F8BT to understand the role of injection layers on exciton quenching. Exciton quenching by the charge injection layers is treated by accounting for both exciton diffusion and the non-radiative transfer of energy to the charge injection layer. The non-radiative transfer of energy is modelled using dipole-dipole interaction theory coupled with diffusion of excitons, from which we obtain the exciton capture radius ( x 0 ) in the range of 1–7 nm. We also correlate x 0 with PL decay time (τ) using the relation τ α 1/ x 0 3 . The steady-state PL yield for each case also shows correlation with the PL decay lifetime. This study provides interesting insight on the selection criterion for injection layer to be used in PLEDs for minimizing optical losses while preserving the electronic injection properties. [ABSTRACT FROM AUTHOR]

Monolayer transition-metal dichalcogenides (TMDCs) have recently emerged as promising candidates for advanced photonic and valleytronic applications due to their unique optoelectronic properties. However, the low luminescence efficiency of monolayer TMDCs has significantly hampered their use in these fields. Here it is reported that the photoluminescence efficiency of monolayer WS2 can be remarkably enhanced up to fourfold through the fluorination, surpassing the reported performance of molecular and/or electrical doping methods. Its degree is easily controlled by changing the fluorine plasma duration time and can also be reversibly tuned via additional hydrogen plasma treatment, allowing for its versatile tailoring for interfacial band alignment and customized engineering. The striking photoluminescence improvement occurs via a substantial transition of trions to excitons as a result of the strong electron affinity of fluorine dopants, and the fluorination enables unprecedented detection of n-type NH3 gas in WS2 due to changed excitonic dynamics showing excellent sensitivity (at least down to 1.25 ppm). This work provides valuable strategies and insights into exciton physics in monolayer TMDCs, opening up avenues toward highly-efficient 2D light emitters, photovoltaics, nanosensors, and optical interconnects. [ABSTRACT FROM AUTHOR]

In natural light harvesting systems, the sequential quantum events of photon absorption by specialized biological antenna complexes, charge separation, exciton formation and energy transfer to localized reaction centers culminates in the conversion of solar to chemical energy. A notable feature in these processes is the exceptionally high efficiencies ( $$>$$ 95 %) at which excitation is transferred from the illuminated protein complex site to the reaction centers. The high speed of excitation propagation within a system of interwoven biomolecular network structures, is yet to be replicated in artificial light harvesting complexes. A clue to unraveling the quantum puzzles of nature may lie in the observations of long lived coherences lasting several picoseconds in the electronic spectra of photosynthetic complexes, which occurs even in noisy environmental baths. However the exact nature of the association between the high energy propagation rates and strength of quantum coherences remains largely unsolved. A number of experimental and theoretical studies have been devoted to unlocking the links between quantum processes and information protocols, in the hope of finding the answers to nature's puzzling mode of energy propagation. This review presents recent developments in quantum theories, and links information-theoretic aspects with photosynthetic light-harvesting processes in biomolecular systems. There is examination of various attempts to pinpoint the processes that underpin coherence features arising from the light harvesting activities of biomolecular systems, with particular emphasis on the effects that factors such non-Markovianity, zeno mechanisms, teleportation, quantum predictability and the role of multipartite states have on the quantum dynamics of biomolecular systems. A discussion of how quantum thermodynamical principles and agent-based modeling and simulation approaches can improve our understanding of natural photosynthetic systems is included. [ABSTRACT FROM AUTHOR]

The exciton recombination processes in a series of elastically strained GaAsBi epilayers are investigated by means of time-integrated and time-resolved photoluminescence at T = 10 K. The bismuth content in the samples was adjusted from 1.16% to 3.83%, as confirmed by high-resolution X-ray diffraction (HR-XRD). The results are well interpreted by carrier trapping and recombination mechanisms involving the Bi-related localized levels. Clear distinction between the localized and delocalized regime was observed in the spectral and temporal photoluminescence emission. [ABSTRACT FROM AUTHOR]

Much progress has been made in using hematite (α-Fe2O3) as a potentially practical and sustainable material for applications such as solar-energy conversion and photoelectrochemical (PEC) water splitting; however, recent studies have shown that the performance can be limited by a very short charge-carrier diffusion length or exciton lifetime. In this study, we performed ultrafast studies on hematite nanoparticles of different shapes to determine the possible influence of particle shape on the exciton dynamics. Nanorice, multifaceted spheroidal nanoparticles, faceted nanocubes, and faceted nanorhombohedra were synthesized and characterized by using SEM and XRD techniques. Their exciton dynamics were investigated by using femtosecond transient absorption (TA) spectroscopy. Although the TA spectral features differ for the four samples studied, their decay profiles are similar, which can be fitted with time constants of 1-3 ps, approximately 25 ps, and a slow nanosecond component extending beyond the experimental time window that was measured (2 ns). The results indicate that the overall exciton lifetime is weakly dependent on the shape of the hematite nanoparticles, even though the overall optical absorption and scattering are influenced by the particle shape. This study suggests that other strategies need to be developed to increase the exciton lifetime or to lengthen the exciton diffusion length in hematite nanostructures. [ABSTRACT FROM AUTHOR]

The poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blending is one of the most intensively studied active material systems for bulk-heterojunction (BHJ) organic photovoltaic devices. For the active layer with BHJ structure, the charge tranfer (CT) and exciton dissociation processes between P3HT and PCBM are the important initial steps to form free charge carriers in the active layer. To compare the CT related species from the transient absorption dynamics in P3HT:PCBM blend systems with different orderings of the polymer, we investigated the charge carrier dynamics in P3HT:PCBM blend films with regiorandom and regioregular polymer structures using femtosecond transient absorption spectroscopy. [ABSTRACT FROM PUBLISHER]

Abstract: A method of generation of powerful pulses of ultraviolet and soft X-ray coherent radiation is considered. It consists in employing superradiant indirect exciton recombination in large enough high quality diamond samples. The theory of this transient process is developed and its dynamics is simulated. In result it is shown that this method may probably provide the generation of picosecond pulses of coherent electromagnetic field with peak powers larger than 30W at wavelength 235nm. [Copyright &y& Elsevier]

Abstract: Exciton dynamics in a chain ring consisting of closely spaced nanocrystals has been studied with a simple resonant dipole–dipole interaction model. We show that the electronic excitation population probability, through the resonant energy transport, propagates like a wave packet in the chain and it interferes with itself at each node of the ring. [Copyright &y& Elsevier]

The two-exciton dynamics in a dendritic molecular aggregate composed of monomers coupled with each other by the dipole–dipole interaction is performed to investigate the effect of two-exciton states on the exciton migration using the master equation approach. Although the spatial unique feature of exciton migration from the periphery to the core in the one-exciton model is also observed in the two-exciton model, about 36% of such exciton migration is found to be carried out by the relaxation among two-exciton states in the present model. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]

The energy relaxation kinetics and the structure of the J-aggregates of water-soluble porphyrin 5,10,15,20-tetrasulphonatophenyl porphine (TPPS4) were investigated in aqueous medium by means of time-resolved fluorescence spectroscopy and confocal laser-scanning fluorescence microscopy. The excitation of the J-aggregates, at excitation intensities higher than ∼1015 photons/cm2 per pulse, results in a remarkable decrease of the fluorescence quantum yield and in the appearance of an additional, non-exponential energy relaxation channel with a decay constant that depends on the excitation intensity. This relaxation mechanism was attributed to the exciton single–singlet annihilation. The exciton lifetime in the absence of the annihilation was calculated to be ∼150 ps. Using exciton annihilation theory, the exciton migration within the J-aggregates could be characterized by determining the exciton diffusion constant (1.8±0.9) 10−3 cm2/s and the hopping time (1.2±0.6) ps. Using the experimental data, the size of the J-aggregate could be evaluated and was seen to yield at least 20 TPPS4 molecules per aggregate. It was shown by means of confocal fluorescence laser scanning microscopy that TPPS4 does self-associate in polyvinyl alcohol (PVA) at acidic pH forming molecular macro-assemblies on a scale of ∼1 μm in PVA matrices. [Copyright &y& Elsevier]

Nanostructures of diluted magnetic semiconductors were fabricated to study novel magneto-optical properties that are derived from quantum confined band electrons interacting with magnetic ions. Quantum dots (QDs) of Cd0.97Mn0.03Se were grown by the self-organization on a ZnSe substrate layer. QDs of Zn0.69Cd0.23Mn0.08Se and quantum wires (QWRs) of Cd0.92Mn0.08Se and Zn0.69Cd0.23Mn0.08Se were fabricated by the electron beam lithography. A single quantum well (QW) of ZnTe/Zn0.97Mn0.03Te and double QWs of Cd0.95Mn0.05Te–CdTe were grown by molecular beam epitaxy. Magneto-optical properties and the formation and relaxation dynamics of excitons were investigated by ultrafast time-resolved photoluminescence (PL) spectroscopy. Excitons in these nanostructures were affected by the low-dimensional confinement effects and the interaction with magnetic ion spins. The exciton luminescence of the Cd0.97Mn0.03Se QDs shows the confined exciton energy due to the dot size of 4–6 nm and also shows marked increase of the exciton lifetime with increasing the magnetic field. The QDs of Zn0.69Cd0.23Mn0.08Se fabricated by the electron beam lithography display narrow exciton PL spectrum due to the uniform shape of the dots. The exciton luminescence from the QWRs of Cd0.92Mn0.08Se and Zn0.69Cd0.23Mn0.08Se shows the influence of the one-dimensional confinement effect for the exciton energy and the luminescence is linearly polarized parallel to the wire direction. The transient PL from the ZnTe/Zn0.97Mn0.03Te QWs displays, by the magnetic field, the level crossing of the exciton spin states of the nonmagnetic and magnetic layers and the spatial spin separation for the excitons. Cd0.95Mn0.05Te–CdTe double QWs show the injection of the spin polarized excitons from the magnetic well to the nonmagnetic QW. [Copyright &y& Elsevier]

Two-exciton migration dynamics of a molecular aggregate is performed using the density matrix approach. Exciton–exciton correlation is shown to cause an oscillatory behavior in the one-exciton population dynamics. Such a feature is found to be well explained by the bypass transition from the ground to a one-exciton forbidden state via a two-exciton state. [ABSTRACT FROM AUTHOR]

InGaN multiple quantum well laser diode (LD) wafer that lased at 400 nm was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit. However, the wells exhibited a Stokes-like shift (SS) of 49 meV and an effective localization depth E0 was estimated by time-resolved photoluminescence (TRPL) measurement to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells is estimated to be as small as 286 kV/cm, SS is considered to originate from an effective bandgap inhomogeneity. Because the well thickness fluctuation was insufficient to produce SS or E0, the exciton localization is considered to be an intrinsic phenomenon in InGaN material. Indeed, bulk cubic In0.1Ga0.9N, which does not suffer any polarization field or thickness fluctuation effect, exhibited an SS of 140 meV at 77 K and similar TRPL results. The origin of the localization is considered to be due to the large bandgap bowing and In clustering in InGaN material. Such shallow and low density localized states are leveled by injecting high density carriers under the lasing conditions for the 400 nm LDs. [Copyright &y& Elsevier]

Non-fullerene type acceptors (NFA) have gained attention owing to their spectral extension that enables efficient solar energy capturing. For instance, the solely NFA-mediated absorbing region contributes to the photovoltaic power conversion efficiency (PCE) as high as ~30%, in the case of the solar cells comprised of fluorinated materials, PBDB-T-2F and ITIC-4F. This implies that NFAs must be able to serve as electron donors, even though they are conventionally assigned as electron acceptors. Therefore, the pathways of NFA-originated excitons need to be explored by the spectrally resolved photovoltaic characters. Additionally, excitation wavelength dependent transient absorption spectroscopy (TAS) was performed to trace the nature of the NFA-originated excitons and polymeric donor-originated excitons separately. Unique origin-dependent decay behaviors of the blend system were found by successive comparing of those solutions and pristine films which showed a dramatic change upon film formation. With the obtained experimental results, including TAS, a possible model describing origin-dependent decay pathways was suggested in the framework of reaction kinetics. Finally, numerical simulations based on the suggested model were performed to verify the feasibility, achieving reasonable correlation with experimental observables. The results should provide deeper insights in to renewable energy strategies by using novel material classes that are compatible with flexible electronics. [ABSTRACT FROM AUTHOR]

Abstract: The helical self-assembly of MOPV4 oligomers dissolved in dodecane solution has a large effect on the experimental fluorescence spectra. To describe the change in fluorescence upon aggregation, we combine a statistical mechanical theory describing the helical self-assembly with a tight-binding model to account for the delocalization of the exciton. We find our combined model to describe the red-shift of the main fluorescence peak upon aggregation qualitatively. [Copyright &y& Elsevier]

We provide an extensive and systematic investigation of exciton dynamics in droplet epitaxial quantum dots comparing the cases of (311)A, (001), and (111)A surfaces. Despite a similar s-shell exciton structure common to the three cases, the absence of a wetting layer for (311)A and (111)A samples leads to a larger carrier confinement compared to (001), where a wetting layer is present. This leads to a more pronounced dependence of the binding energies of s-shell excitons on the quantum dot size and to the strong anti-binding character of the positive-charged exciton for smaller quantum dots. In-plane geometrical anisotropies of (311)A and (001) quantum dots lead to a large electron-hole fine interaction (fine structure splitting (FSS) ∼100 μeV), whereas for the three-fold symmetric (111)A counterpart, this figure of merit is reduced by about one order of magnitude. In all these cases, we do not observe any size dependence of the fine structure splitting. Heavy-hole/light-hole mixing is present in all the studied cases, leading to a broad spread of linear polarization anisotropy (from 0 up to about 50%) irrespective of surface orientation (symmetry of the confinement), fine structure splitting, and nanostructure size. These results are important for the further development of ideal single and entangled photon sources based on semiconductor quantum dots. [ABSTRACT FROM AUTHOR]

Abstract: Dynamics of the photo-excited excitons in CdSe colloidal quantum dots was investigated by degenerate pump-probe measurement. Under resonant excitation, the lowest energy state of an electron-hole pair dominates two-step decay. The fast initial decay and long-term decay were attributed to the intrinsic Auger recombination and radiative recombination, respectively. As increasing the excited exciton density, a significant change of the later decay was seen before the absorption bleaching. As our nanocrystal is the case of strong confinement, in which an excitonic Bohr radius is smaller than the dot size, the biexciton-involved recombination is suggested to dominate at high carrier densities. Optical gain was measured by the variable stripe length method and broad gain spectrum was obtained due to the recombination of many excited levels between biexciton and exciton states. This result also supports biexcitons play an important role in the strong confinement regime at high carrier densities. [Copyright &y& Elsevier]

Droplet epitaxy allows the efficient fabrication of a plethora of 3D, III–V-based nanostructures on different crystalline orientations. Quantum dots grown on a (311)A-oriented surface are obtained with record surface density, with or without a wetting layer. These are appealing features for quantum dot lasing, thanks to the large density of quantum emitters and a truly 3D lateral confinement. However, the intimate photophysics of this class of nanostructures has not yet been investigated. Here, we address the main optical and electronic properties of s-shell excitons in individual quantum dots grown on (311)A substrates with photoluminescence spectroscopy experiments. We show the presence of neutral exciton and biexciton as well as positive and negative charged excitons. We investigate the origins of spectral broadening, identifying them in spectral diffusion at low temperature and phonon interaction at higher temperature, the presence of fine interactions between electron and hole spin, and a relevant heavy-hole/light-hole mixing. We interpret the level filling with a simple Poissonian model reproducing the power excitation dependence of the s-shell excitons. These results are relevant for the further improvement of this class of quantum emitters and their exploitation as single-photon sources for low-density samples as well as for efficient lasers for high-density samples. [ABSTRACT FROM AUTHOR]

It is well known that one of the major challenges facing white organic light‐emitting diodes (WOLEDs) has been to achieve high efficiency, low efficiency roll‐off, and high color rendering index (CRI) simultaneously. A simple charge‐ and exciton‐confining emitting layer structure formed by employing a blue emitting p/n heterojunction, and combined with red, yellow, and green phosphorescent ultrathin layers, is presented, thus realizing the stable utilization of 100% excitons. The resulting hybrid WOLEDs exhibit the maximum forward‐viewing external quantum efficiency and power efficiency of 23.4% and 50.4 lm W−1, and keep them at 20.3% and 31.2 lm W−1 at 1000 cd m−2, showing high efficiency and low efficiency roll‐off. Furthermore, the device also shows extremely stable electroluminescent spectra in the whole luminance range with a high CRI of 93 and the Commission Internationale de L'Eclairage coordinates of (0.49, 0.43) at 1000 cd m−2, which is perfect warm white for indoor lighting. The achievement of the excellent comprehensive performances is the best result reported to date among the conventional blue fluorescence‐based hybrid WOLEDs possessing such high CRI, which may be a new starting point for further research on high‐performance hybrid WOLEDs. [ABSTRACT FROM AUTHOR]