Excitation Density Dependence Research Articles

Excitation Density Effects in the Luminescence Yield and Kinetics of MAPbBr3 Single Crystals

The luminescent Z-scan technique with time resolution is applied to the study of the luminescence properties of CH3NH3PbBr3 single crystals representative of the family of hybrid organic–inorganic lead perovskites successfully applied recently in photovoltaics and currently investigated as potential nanosecond scintillators. The third harmonic of Ti-sapphire laser (λ = 266 nm) with a pulse duration of 26 fs and 1 kHz frequency was applied for the luminescence excitation creating the charge carriers with the estimated density from 1017 to 1021 cm−3 in the temperature range from 13 to 300 K. Temperature and excitation density dependence of the luminescence yield and kinetics is interpreted with the consideration of the temperature-dependent binding of electrons and holes into excitons, a saturation of defects responsible for the non-radiative relaxation channel competing with exciton creation; absorption saturation resulting in the increased penetration depth of the excitation radiation and hence the increased contribution of the re-absorption.

Dynamics of collective modes in an unconventional charge density wave system BaNi2As2

BaNi2As2 is a non-magnetic analogue of BaFe2As2, the parent compound of a prototype pnictide high-temperature superconductor, displaying superconductivity already at ambient pressure. Recent diffraction studies demonstrated the existence of two types of periodic lattice distortions above and below the triclinic phase transition, suggesting the existence of an unconventional charge-density-wave (CDW) order. The suppression of CDW order upon doping results in a sixfold increase in the superconducting transition temperature and enhanced nematic fluctuations, suggesting CDW is competing with superconductivity. Here, we apply time-resolved optical spectroscopy to investigate collective dynamics in BaNi2As2. We demonstrate the existence of several CDW amplitude modes. Their smooth evolution through the structural phase transition implies the commensurate CDW order in the triclinic phase evolves from the high-temperature unidirectional incommensurate CDW, and may indeed trigger the structural phase transition. Excitation density dependence reveals exceptional resilience of CDW against perturbation, implying an unconventional origin of the underlying electronic instability.

Excitation-density and excess-energy dependence of ultrafast dynamics of photoexcited carriers in intrinsic bulk CdTe

In this study, ultrafast dynamics of photoexcited carriers in an intrinsic CdTe film are investigated deeply and systematically as a function of carrier density and excess energy using femtosecond-resolved pump–probe transient differential transmission spectroscopy. A dynamic model is developed based on an interband transition absorption model in direct bandgap semiconductors by introducing the dynamics of thermalization, cooling and recombination of photoexcited carriers into the interband transition absorption model. Then, the model is used to best fit the measured ultrafast dynamics to retrieve the initial temperature of thermalized electrons and time constants of the thermalization, cooling and recombination dynamic processes as a function of carrier density and excess energy. The increase with carrier density and the decrease with excess energy of thermalization time constant reveal that thermalization process of photoexcited carriers is dominated by carrier-phonon scatterings. We find for the first time that cooling of thermalized carriers depend strongly on carrier density and excess energy, and cooling time constant varies in a wide range from 0.2 to 5 ps. It is also found that carrier recombination in the intrinsic CdTe film is dominated by the radiative recombination of electron-hole pairs, rather than Auger recombination as reported in doped CdTe films. These new results are very important to understanding of microscopic mechanism of relaxation processes of photoexcited carriers and applications of CdTe-based optoelectronic devices.

Role of dark exciton states in the relaxation dynamics of bright 1s excitons in monolayer WSe2

Monolayer transition metal dichalcogenides (1L-TMDs) are excellent platforms for exciton physics. In tungsten-based 1L-TMDs, the existence of dark excitons at lower energy has important roles for bright exciton relaxation. However, the detailed relaxation dynamics from bright to dark excitons have not been revealed sufficiently. In this paper, we studied the rise dynamics of out-of-plane polarized photoluminescence (PL) from spin-forbidden dark excitons in monolayer WSe2. Under conditions of resonant excitation to the bright 1s excitons, PL from the spin-forbidden dark exciton has a finite rise time of a few tens of picoseconds, which suggests that intermediate states, probably hot indirect dark excitons, should play an important role in the relaxation pathway from the bright to the spin-forbidden dark excitons. The excitation density dependence indicates that exciton–exciton scattering should promote faster relaxation to the spin-forbidden dark excitons.

Competition between Oxygen Curing and Ion Migration in MAPbI3 Induced by Irradiation Exposure.

Organometal halide perovskites (OHPs) have been considered as promising materials for light-emission devices. However, the factors influencing the luminescent property of OHPs are intricate. It is not only affected by the intrinsic crystalline quality but also depends on the surrounding environment. Here we demonstrate that the luminescence of CH3NH3PbI3 (MAPbI3) is governed by light-irradiation-induced oxygen curing and vacancy-mediated ion migration. The luminescence increases under continuous irradiation because of the curing of iodine vacancies (VI) by oxygen. While, it decreases with enhanced ion migration, which would induce excess trap states. The existence of VI is proved by low-temperature photoluminescence (PL) spectra, the hysteresis effect in J-V curves, and the excitation density dependence of the PL lifetime. Different oxygen environments and applied biases are employed to control the degree of oxygen curving and ion migration. These results provide a perception of the correlation of the complicated influencing factors affecting the luminescence of OHPs.

Zero-field spin precession dynamics of high-mobility two-dimensional electron gas in persistent spin helix regime

We investigated the spin-orbit (SO) effective magnetic-field-induced spin precession of a high-mobility two-dimensional electron gas (2DEG) in a modulation-doped (001) GaAs/AlGaAs quantum well close to the persistent spin helix (PSH) state. The oscillating spin signal induced by the SO fields in zero external magnetic fields was clearly observed. When the photoexcited carrier density is sufficiently small compared with the 2DEG density, the spin precession length in the space domain was obtained by analyzing the precession frequency in the PSH regime. The excitation density dependence of the spin dynamics was reproduced well using the scattering time-dependent spin dynamics simulation. We revealed that the increase in the photoexcited carriers results in a transition from ballistic motion to diffusive motion, leading to a decrease in the spin-diffusion coefficient.

Comparative studies of optoelectrical properties of prominent PV materials: Halide perovskite, CdTe, and GaAs

We compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination and passivation, over multiple orders of photo-excitation density or carrier density appropriate for different applications. An analytic model is used to describe the excitation density dependence of PL intensity and extract the internal PL efficiency and multiple pertinent recombination parameters. A PL imaging technique is used to obtain carrier diffusion length without using a PL quencher, thus, free of unintended influence beyond pure diffusion. Our results show that perovskite samples tend to exhibit lower Shockley–Read–Hall (SRH) recombination rate in both bulk and surface, thus higher PL efficiency than the inorganic counterparts, particularly under low excitation density, even with no or preliminary surface passivation. PL lineshape and diffusion analysis indicate that there is considerable structural disordering in the perovskite materials, and thus photo-generated carriers are not in global thermal equilibrium, which in turn suppresses the nonradiative recombination. This study suggests that relatively low point-defect density, less detrimental surface recombination, and moderate structural disordering contribute to the high PV efficiency in the perovskite. This comparative photovoltaics study provides more insights into the fundamental material science and the search for optimal device designs by learning from different technologies.

Strong exciton-photon coupling in organic microcavity electroluminescence devices with thiophene/phenylene co-oligomer derivatives

Organic electroluminescence (EL) devices with surface-emitting microcavity structure were fabricated using vapor-deposited films of thiophene/phenylene co-oligomer derivatives, aiming at polariton formation under electrical excitation. Owing to strong coupling between excitons and photons in the device, we demonstrated the formation of cavity polaritons with a large Rabi-splitting energy (2ħΩ = 250 meV). Furthermore, efficient relaxation into the bottom of lower polariton branch was confirmed from angle-resolved EL spectra. In excitation-density dependence, the luminance of microcavity EL devices showed a superlinear increase with an elevation of current density indicating a contribution of triplet excitons.

Incoherent phonon population and exciton-exciton annihilation dynamics in monolayer WS2 revealed by time-resolved Resonance Raman scattering.

Atomically thin layer transition metal dichalcogenides have been intensively investigated for their rich optical properties and potential applications on nano-electronics. In this work, we study the incoherent phonon and exciton population dynamics in monolayer WS2 by time-resolved Resonance Raman scattering spectroscopy. Upon excitation of the exciton transition, both Stokes and anti-Stokes scattering strength of the optical and the longitudinal acoustic two phonon modes exhibit large reduction. Based on the assumption of quasi-equilibrium distribution, the hidden phonon population dynamics is retrieved, which shows an instant build-up and a relaxation lifetime of ∼4 ps at the exciton density ∼1012cm-2. A phonon temperature rises of ∼20 K was identified due to the exciton excitation and relaxation. The exciton relaxation dynamics extracted from the transient vibrational Raman response shows strong excitation density dependence, signaling an important bi-molecular contribution to the decay. These results provide significant knowledge on the thermal dynamics after optical excitation, enhance the understanding of the fundamental exciton dynamics in two-dimensional transition metal materials, and demonstrate that time-resolved Resonance Raman scattering spectroscopy is a powerful method for exploring quasi-particle dynamics in optical materials.

Generation processes of superfluorescence of biexcitons in CuCl quantum dots by one- and two-photon resonant excitation

We investigated the influence of excitation processes of a biexciton on the generation of superfluorescence, which is pulsed radiation from coherently coupled two-level systems, from biexcitons in CuCl quantum dots. Two excitation processes of biexcitons were examined: resonant two-photon excitation of biexcitons, and resonant one-photon excitation of excitons. The excitation density dependence of the time-resolved photoluminescence was measured. The results showed that for one-photon excitation, a stronger peak intensity, longer delay time and wider pulse width were observed compared to resonant two-photon excitation. The high density of the excited dots in the one-photon excitation process results in the strong pulse, while an initial population of excitons results in a suppression of coherent coupling.

Excitation Density Dependent Photoluminescence Quenching and Charge Transfer Efficiencies in Hybrid Perovskite/Organic Semiconductor Bilayers

AbstractThis study addresses the dependence of charge transfer efficiency between bilayers of methylammonium lead iodide (MAPI3) with PC61BM or poly(3,4‐ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) charge transfer layers on excitation intensity. It analyzes the kinetic competition between interfacial electron/hole transfer and charge trapping and recombination within MAPI3 by employing a range of optical measurements including steady‐state (SS) photoluminescence quenching (PLQ), and transient photoluminescence and absorption over a broad range of excitation densities. The results indicate that PLQ measurements with a typical photoluminescence spectrometer can yield significantly different transfer efficiencies to those measured under 1 Sun irradiation. Steady‐state and pulsed measurements indicate low transfer efficiencies at low excitation conditions (<5E + 15 cm−3) due to rapid charge trapping and low transfer efficiencies at high excitation conditions (>5E + 17 cm−3) due to fast bimolecular recombination. Efficient transfer to PC61BM or PEDOT:PSS is only observed under intermediate excitation conditions (≈1 Sun irradiation) where electron and hole transfer times are determined to be 36 and 11 ns, respectively. The results are discussed in terms of their relevance to the excitation density dependence of device photocurrent generation, impact of charge trapping on this dependence, and appropriate methodologies to determine charge transfer efficiencies relevant to device performance.

Efficient Photo- and Electroluminescence by Trap States Passivation in Vacuum-Deposited Hybrid Perovskite Thin Films.

Methylammonium lead iodide (MAPI) has excellent properties for photovoltaic applications, although it typically shows low photoluminescence quantum yield. Here, we report on vacuum-deposited MAPI perovskites obtained by modifying the methylammonium iodide (MAI) to PbI2 ratio during vacuum deposition. By studying the excitation density dependence of the photoluminescence lifetime, a large concentration of trap states was deduced for the stoichiometric MAPI films. The use of excess MAI during vacuum processing is capable of passivating these traps, resulting in luminescent films which can be used to fabricate planar light-emitting diodes with quantum efficiency approaching 2%.

Quantitative characterization of highly efficient correlated photon-pair source using biexciton resonance.

A high efficiency method for the generation of correlated photon pairs accompanied by reliable means to characterize the efficiency of that process is needed in the study of entangled states, which have important potential applications in quantum information and quantum communication. In this study, we report the first characterization of the efficiency of generation of correlated photon pairs emitted from a CuCl single crystal using the biexciton-resonance hyper-parametric scattering (RHPS) method which is the highly efficient method of generation of correlated photon pairs. In order to characterize the generation efficiency and signal-to-noise ratio of correlated photon pairs using this method, we investigated the pump power dependence on the photon counting rate and coincidence counting rate under resonant excitation. The pump power dependence shows that the power characteristic of the photon counting rates changes from linear to quadratic dependence of the pump power. This behavior represents a superposition of contributions from correlated photon pairs and non-correlated photons. The analysis of the pump power dependence shows that one photon-pair is produced by a pump pulse with 2 x 106 photons. Moreover, the generation efficiency of this method obtained by calculating the number of generated photon pairs per pump power is comparable to that of several methods based on the χ(3) parametric process.

Below‐gap emission bands in undoped GaN and its excitation density dependence

AbstractThe below‐gap emission bands of undoped GaN grown by MOCVD on sapphire substrate have been investigated by photoluminescence technique. The intensity of yellow luminescence decreases due to the saturation of deep states with increasing excitation power that reflects the contribution of deep states at actual device operating condition. The excitation induced blue‐shift of emission energy rationalizes the DAP characteristics. The integrated photoluminescence intensity of each band is analysed experimentally and theoretically by a power law model, and the reasonable agreement between experiment and model calculation justify our model consideration. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Role of Edge Dislocations on the Red Luminescence of ZnO Films Deposited by RF‐Sputtering

The existence of extended defects (i.e., dislocations) in inorganic semiconductors, such as GaN or ZnO, responsible for broad emission peaks in photoluminescence analysis remains unresolved. The possible assignments of these luminescence bands are still matter of discussion. In this study, two different zinc oxide samples, grown under different oxygen partial pressures and substrate temperatures, are presented. Epitaxial and structural properties were analysed by means of X‐ray diffraction and transmission electron microscopy techniques. They confirm that the layers are single‐phase with a good crystalline quality. Nevertheless, a different density of threading dislocations, with a higher contribution of edge dislocations, was found. Photoluminescence spectroscopy has been used to investigate the optical properties. The steady state luminescence spectra performed at 14 K evidenced the donor bound exciton recombination and deep green and red emission bands. The red band with a maximum at 1.78 eV was found to be stronger in the sample grown at lower oxygen pressure which also shows higher density of threading dislocations. From the temperature and excitation density dependence of the red band, a donor acceptor pair recombination model was proposed, where hydrogen and zinc vacancies are strong candidates for the donor and acceptor species, respectively.

Excitation density dependence of radiative and nonradiative recombination lifetimes in InGaN/GaN multiple quantum wells

AbstractThe optical properties of InGaN/GaN multiple quantum wells (MQWs) have been studied by means of photoluminescence (PL) and time‐resolved photoluminescence (TRPL) spectroscopy. The radiative and nonradiative recombination lifetimes were evaluated as a function of the excitation energy density. The radiative recombination lifetime decreased and subsequently reached a nearly constant value with increasing excitation energy density, which was attributed to screening of internal electric fields by photoexcited carriers. On the other hand, the nonradiative recombination lifetime increased and subsequently decreased with increasing excitation energy density. The initial increase in the nonradiative recombination lifetime could be attributed to saturation of nonradiative recombination centers with photoexcited carriers. The nonradiative recombination lifetime was found to decrease to a considerably weaker extent than that expected for the Auger recombination process of free carriers. This indicated that the decrease in the internal quantum efficiency (IQE) at high carrier densities could not be explained only by the Auger recombination process of free carriers.

Effect of exciton oscillator strength on upconversion photoluminescence in GaAs/AlAs multiple quantum wells

We report upconversion photoluminescence (UCPL) in GaAs/AlAs multiple quantum wells. UCPL from the AlAs barrier is caused by the resonant excitation of the excitons in the GaAs well. When the quantum well has sufficient miniband width, UCPL is hardly observed because of the small exciton oscillator strength. The excitation-energy and excitation-density dependences of UCPL intensity show the exciton resonant profile and a linear increase, respectively. These results demonstrate that the observed UCPL caused by the saturated two-step excitation process requires a large number of excitons.

Nonlinear photoluminescence properties of trions in hole-doped single-walled carbon nanotubes

We studied the excitation density dependence of photoluminescence (PL) spectra of excitons and trions (charged excitons) in hole-doped single-walled carbon nanotubes. We found that the PL intensity of trions exhibited a strong nonlinear saturation behavior as the excitation density increased, whereas that of excitons exhibited a weak sublinear behavior. The strong PL saturation of trions is attributed to depletion of doped holes that are captured by excitons in the formation processes. Moreover, the effective radiative lifetime of a trion was evaluated to be approximately 20 ns.

Kinetic Monte Carlo Simulations of Scintillation Processes in NaI(Tl)

Developing a comprehensive understanding of the processes that govern the scintillation behavior of inorganic scintillators provides a pathway to optimize current scintillators and allows for the science-driven search for new scintillator materials. Recent experimental data on the excitation density dependence of the light yield of inorganic scintillators presents an opportunity to incorporate parameterized interactions between excitations in scintillation models and thus enable more realistic simulations of the nonproportionality of inorganic scintillators. Therefore, a kinetic Monte Carlo (KMC) model of elementary scintillation processes in NaI(Tl) is developed in this paper to simulate the kinetics of scintillation for a range of temperatures and Tl concentrations as well as the scintillation efficiency as a function of excitation density. The ability of the KMC model to reproduce available experimental data allows for elucidating the elementary processes that give rise to the kinetics and efficiency of scintillation observed experimentally for a range of conditions.

Mechanism of excitonic dephasing in layered InSe crystals

The dephasing and lifetime of excitons in InSe layered crystals are carefully measured using three-pulse, four-wave mixing and two-dimensional Fourier transform (2DFT) spectroscopy. We obtain a detailed picture of the mechanism of excitonic dephasing in this layered material. The 2DFT spectra reveal contributions from the coherent excitation of continuum states, whereas the temperature dependence provides a detailed description of the phonon-exciton interactions and the low temperature limit of the homogeneous linewidth. The excitation density dependence reveals excitation-induced dephasing due to exciton-exciton scattering. The temperature-dependent homogeneous linewidth is dominated by the interaction of excitons with the $\ensuremath{\sim}$113 cm${}^{\ensuremath{-}1}$ out-of-plane phonon mode.