Time-resolved Photoluminescence Data Research Articles

The use of He buffer gas for moderating the plume kinetic energy during Nd:YAG-PLD growth of EuxY2−xO3 phosphor films

We have investigated the He buffer gas process of moderating the kinetic energy of the pulsed laser deposition (PLD) plume during EuxY2−xO3 phosphor film growth. When using a neodymium yttrium aluminum garnet laser for PLD thin film growth, the kinetic energy of the ablation plumes can be high enough to cause the formation of point defects in the film. The buffer gas pressure is an important process parameter in PLD film growth. We find that the presence of the He buffer gas reduces the kinetic energy of the laser deposition plume through many low-angle collisions in the gas phase by a factor of 7 without reducing the deposition rate. This is because He is much lighter than any of the elements in the plume and it does not affect the composition of the oxide films. Consequently, the resputtering of the Y2O3 film surface by the plume was significantly suppressed in the presence of the He gas moderator, leading to a decrease of the defect density in the Y2O3 films. The improvement of the film quality was verified by a systematic analysis of time-resolved photoluminescence (PL) data for EuxY2−xO3 composition–gradient films. The PL lifetime and intensity of Eu0.2Y1.8O3, which shows the highest PL intensity, increased by 13.3% and 36.4%, respectively, when the He gas moderation process was used. The He buffer gas process is applicable to the PLD growth of the other oxide materials as well, where the reduction of the kinetic energy of the plume would bring the PLD process closer to the molecular beam epitaxy growth condition.

Photo‐Carrier Lifetime in Binary and Ternary Heterostructures of Transition Metal Dichalcogenides

Binary and ternary transition metal dichalcogenide (TMD) heterostructures, MSe2 and MSeS (where M = Mo or W), are investigated as future building blocks for high‐performance device applications with a tailorable electronic bandgap. For light‐emitting device applications, investigating the decay time of the material is a prerequisite. Herein, time‐resolved photoluminescence (TRPL) of binary and ternary TMD heterostructures is measured. A tri‐exponential function is adopted to fit the TRPL data. Short, medium, and long decay times are <50 ps, ≈400 ps, and >400 ps, respectively. Except for monolayer MoSe2 and MoSeS, only two PL decay time components are observed with a large contribution from short decay time photocarriers (>80%). Bilayer TMDs exhibit a much shorter PL decay time, which could be attributed to the indirect nature of excitons and different dielectric environments, compared with that of monolayer TMDs. Using the center‐of‐mass (CM) method, a PL decay time map is successfully obtained for binary and ternary TMD heterostructures. This allows the qualitative visualization of the decay time distribution within the heterostructure or among other materials. This technique could be applied to assess the decay time of materials used in high‐performance light‐emitting devices.

Machine learning for enhanced semiconductor characterization from time-resolved photoluminescence

<h2>Summary</h2> To enhance the accuracy and effectiveness of optoelectronic characterization, Bayesian inference has been applied to the statistical analysis of time-resolved photoluminescence (TRPL) data with large-scale graphics-processing unit (GPU)-based simulations of electron dynamics. A simulated TRPL dataset, derived from a CH₃NH₃PbI_3-xCl_x perovskite absorber, was used as a case study. From a power scan, Bayesian inference extracts values of the (ambipolar) carrier mobility, free-carrier density, radiative recombination rate, and the overall lifetime of the nonradiative recombination processes. Analysis of the experimental TRPL data yields similar parameter values. The independent contributions of bulk and surface recombination can be distinguished via the introduction of an additional sample thickness. Further experiments separate the front and back surface recombination velocities and can distinguish the electron and hole mobilities. Ultimately, Bayesian inference enables a significant increase in the information yield from TRPL measurements.

Evaluating Recombination Mechanisms in RbF Treated Cu(In${}_\mathrm{x}$Ga$_\mathrm{1-x}$)Se$_{2}$ Solar Cells

Rubidium fluoride (RbF) postdeposition treatment (PDT) has been shown to improve the performance of Cu(In <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${}_\mathrm{x}$</tex-math></inline-formula> Ga <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_\mathrm{1-x}$</tex-math></inline-formula> )Se <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> (CIGS) photovoltaic devices. In this study, temperature-dependent current voltage (JVT) and time-resolved photoluminescence (TRPL) experiments were combined with modeling using the solar cell capacitance simulator (SCAPS) computer code to investigate the effect of the RbF PDT. Two devices, one as-deposited and one with RbF PDT, were deposited by a three stage coevaporation process. JVT measurements suggest the dominant recombination mechanism may be tunneling-enhanced recombination via bandtail states, but that defect states in the bandgap can also be important. RbF PDT is shown to decrease the characteristic energy of the bandtails. TRPL data show an increase in the minority carrier lifetime after RbF PDT, leading to an improved open-circuit voltage. SCAPS modeling indicates that the dominant recombination mechanism is dependent on the specific defect makeup of a device, suggesting that small changes in processing conditions can impact device behavior. This explains the observation that, for some devices, defect states in the gap dominate while others, as is the case here, appear to be dominated by bandtails.

Assignment of Core and Surface States in Multicolor-Emissive Carbon Dots.

It is important to reveal the luminescence mechanisms of carbon dots (CDs). Herein, CDs with two types of optical centers are synthesized from citric acid in formamide by a solvothermal method, and show high photoluminescence quantum yield reaching 42%. Their green/yellow emission exhibits pronounced vibrational structure and high resistance toward photobleaching, while broad red photoluminescence is sensitive to solvents, temperature, and UV-IR. Under UV-IR, the red emission is gradually bleached due to the photoinduced dehydration of the deprotonated surface of CDs in dimethyl sulfoxide, while this process is hindered in water. From the analysis of steady-state and time-resolved photoluminescence and transient absorption data together with density functional theory calculations, the green/ yellow emission is assigned to conjugated sp2 -domains (core state) similar to organic dye derivatives stacked within disk-shaped CDs; and the broad red emission-to oxygen-containing groups bound to sp2 -domains (surface state), whereas energy transfer from the core to the surface state can happen.

Time-Resolved Photoluminescence Spectroscopy of Semiconductor Nanocrystals and Other Fluorophores.

Time-resolved photoluminescence (TRPL) is a key technique for understanding the photophysics of semiconductor nanocrystals and light-emitting materials in general. This work is a primer for setting up and conducting TRPL on nanocrystals and related materials using single-photon-counting (SPC) systems. Basic sources of error in the measurement can be avoided by consideration of the experimental setup and calibration. The detector properties, count rate, the spectral response, reflections in optical setups, and the specific instrumentation settings for single photon counting will be discussed. Attention to these details helps ensure reproducibility and is necessary for obtaining the best possible data from an SPC system. The main aim of the protocol is to help a student of TRPL understand the experimental setup and the key hardware parameters one must generally comprehend in order to gain useful TRPL data in many common single-photon-counting setups. The secondary purpose is to serve as a condensed primer for the student of experimental time-resolved luminescence spectroscopy.

Enhancing efficiency of perovskite solar cells from surface passivation of Co2+ doped CuGaO2 nanocrystals

Before completely applying inorganic materials as hole transport materials (HTM) for perovskite solar cells (PSCs), modifying devices with inorganic oxides that have the potential as inorganic hole transporters is an effective way to improve device performance and stability. Co2+ doped CuGaO2 nanocrystals (Co-CuGaO2 NCs) with sizes about 20 nm are synthesized by hydrothermal method and used for surface passivation at the interface of perovskite (PVK)/2,2′,7,7′-Tetrakis[N,N-di (4-methoxyphenyl) amino]-9,9′-spirobifluorene (spiroOMeTAD). Co-CuGaO2 NCs have a larger bandgap with lower valance band compared with spiroOMeTAD, which is more beneficial to the conduction of holes and the blocking of electrons. Furthermore, the Co-CuGaO2 has a lower valance band energy compared with the original CuGaO2, which reduces the energy gap between Co-CuGaO2 and PVK. Co-CuGaO2 NCs fully cover the upper surface of PVK, which helps prevent direct contact between PVK and oxygen and moisture. The Co-CuGaO2 NCs surface passivation also gives better hole transport as revealed by the ultraviolet photoelectron spectroscopy (UPS), steady-state photoluminescence (PL), and time-resolved photoluminescence (TRPL) data. When the concentration of Co-CuGaO2 NCs solution is set to 7.5 mg mL−1, the device exhibits a best PCE of 20.39% and maintains 84.34% of the initial power conversion efficiency (PCE) after stored 30 days under air atmosphere with 15 ± 5% humidity.

Tuning the interfacial stoichiometry of InP core and InP/ZnSe core/shell quantum dots

We demonstrate fine-tuning of the atomic composition of InP/ZnSe quantum dots (QDs) at the core/shell interface. Specifically, we control the stoichiometry of both anions (P, As, S, and Se) and cations (In and Zn) at the InP/ZnSe core/shell interface and correlate these changes with the resultant steady-state and time-resolved optical properties of the nanocrystals. The use of reactive trimethylsilyl reagents results in surface-limited reactions that shift the nanocrystal stoichiometry to anion-rich and improve epitaxial growth of the shell layer. In general, anion deposition on the InP QD surface results in a redshift in the absorption, quenching of the excitonic photoluminescence, and a relative increase in the intensity of broad trap-based photoluminescence, consistent with delocalization of the exciton wavefunction and relaxation of exciton confinement. Time-resolved photoluminescence data for the resulting InP/ZnSe QDs show an overall small change in the decay dynamics on the ns timescale, suggesting that the relatively low photoluminescence quantum yields may be attributed to the creation of new thermally activated charge trap states and likely a dark population that is inseparable from the emissive QDs. Cluster-model density functional theory calculations show that the presence of core/shell interface anions gives rise to electronic defects contributing to the redshift in the absorption. These results highlight a general strategy to atomistically tune the interfacial stoichiometry of InP QDs using surface-limited reaction chemistry allowing for precise correlations with the electronic structure and photophysical properties.

Optical characteristics of type-II hexagonal-shaped GaSb quantum dots on GaAs synthesized using nanowire self-growth mechanism from Ga metal droplet

We report the growth mechanism and optical characteristics of type-II band-aligned GaSb quantum dots (QDs) grown on GaAs using a droplet epitaxy-driven nanowire formation mechanism with molecular beam epitaxy. Using transmission electron microscopy and scanning electron microscopy images, we confirmed that the QDs, which comprised zinc-blende crystal structures with hexagonal shapes, were successfully grown through the formation of a nanowire from a Ga droplet, with reduced strain between GaAs and GaSb. Photoluminescence (PL) peaks of GaSb capped by a GaAs layer were observed at 1.11 eV, 1.26 eV, and 1.47 eV, assigned to the QDs, a wetting-like layer (WLL), and bulk GaAs, respectively, at the measurement temperature of 14 K and excitation laser power of 30 mW. The integrated PL intensity of the QDs was significantly stronger than that of the WLL, which indicated well-grown GaSb QDs on GaAs and the generation of an interlayer exciton, as shown in the power- and temperature-dependent PL spectra, respectively. In addition, time-resolved PL data showed that the GaSb QD and GaAs layers formed a self-aligned type-II band alignment; the temperature-dependent PL data exhibited a high equivalent internal quantum efficiency of 15 ± 0.2%.

Light management using CsPbBr3 colloidal quantum dots for luminescent solar concentrators

CsPbBr3 colloidal quantum dots have been synthesized by hot-injection method showing spherical shape with an average diameter of ∼10.5 nm. UV–vis absorption of CsPbBr3 colloidal quantum dots shows a broad spectrum with an optical bandgap of ∼2.3682 eV. The steady-state photoluminescence measurement reveals a narrow emission peak at 2.352 eV with full-width at half maximum of 0.113 eV. Absolute photoluminescence quantum yield of colloidal quantum dots dispersed in poly(methyl methacrylate) was found to be 60 ± 1%. The time-resolved photoluminescence data recorded at 266 nm excitation were well fitted using a mono-exponential curve with a decay time of 25.36 (5) ns. A luminescent solar concentrator was fabricated using colloidal quantum dots in transparent poly(methyl methacrylate) polymer uniformly coated over glass substrate that shows an external optical conversion efficiency of ∼5.4% under one sun illumination. The experimental results presented in this manuscript reveals that luminescent solar concentrator prepared using colloidal CsPbBr3 quantum dots shows absorption in wide spectral range, high absorption coefficient, high photoluminescence quantum yield, high external optical conversion efficiency, and good photostability, thermal stability and long-term stability under ambient conditions and therefore are in many ways superior to the other luminescent materials explored for LSC devices.

ZnAl2O4 decorated Al-doped ZnO tetrapodal 3D networks: microstructure, Raman and detailed temperature dependent photoluminescence analysis.

3D networks of Al-doped ZnO tetrapods decorated with ZnAl2O4 particles synthesised by the flame transport method were investigated in detail using optical techniques combined with morphological/structural characterisation. Low temperature photoluminescence (PL) measurements revealed spectra dominated by near band edge (NBE) recombination in the UV region, together with broad visible bands whose peak positions shift depending on the ZnO : Al mixing ratios. A close inspection of the NBE region evidences the effective doping of the ZnO structures with Al, as corroborated by the broadening and shift of its peak position towards the expected energy associated with the exciton bound to Al. Both temperature and excitation density-dependent PL results pointed to an overlap of multiple optical centres contributing to the broad visible band, with the peak position dependent on the Al content. While in the reference sample the wavelength of the green band remained unchanged with temperature, in the case of the composites, the deep level emission showed a blue shift with increasing temperature, likely due to distinct thermal quenching of the overlapping emitting centres. This assumption was further validated by the time-resolved PL data, which clearly exposed the presence of more than one optical centre in this spectral region. PL excitation analysis demonstrated that the luminescence features of the Al-doped ZnO/ZnAl2O4 composites revealed noticeable changes not only in deep level recombination, but also in the material's bandgap when compared with the ZnO reference sample. At room temperature, the ZnO reference sample exhibited free exciton resonance at ∼3.29 eV, whereas the peak position for the Al-doped ZnO/ZnAl2O4 samples occurred at ∼3.38 eV due to the Burstein–Moss shift, commonly observed in heavily doped semiconductors. Considering the energy shift observed and assuming a parabolic conduction band, a carrier concentration of ∼1.82 ×1019 cm−3 was estimated for the Al-doped ZnO/ZnAl2O4 samples.

Deep Ultraviolet Luminescence Due to Extreme Confinement in Monolayer GaN/Al(Ga)N Nanowire and Planar Heterostructures.

We present experimental results confirming extreme quantum confinement in GaN/AlxGa1-xN (x = 0.65 and 1.0) nanowire and planar heterostructures, where the GaN layer thickness is of the order of a monolayer. The results were obtained from temperature- and excitation-dependent and time-resolved photoluminescence measurements. In the GaN/AlN nanowire heterostructure array sample, the measured emission peak at 300 K is ∼5.18-5.28 eV. This is in excellent agreement with the calculated optical gap of 5.23 eV and 160-260 meV below the calculated electronic gap of 5.44 eV, suggesting that the observed emission is excitonic in nature with an exciton binding energy of ∼160-260 meV. Similarly, in the monolayer GaN/Al0.65Ga0.35N planar heterostructure, the measured emission peak at 300 K is 4.785 eV and in good agreement with the calculated optical gap of 4.68 eV and 95 meV below the calculated electronic gap of 4.88 eV. The estimated exciton binding energy is 95 meV and in close agreement with our theoretical calculations. Excitation-dependent and time-resolved photoluminescence data support the presence of excitonic transitions. Our results indicate that deep-ultraviolet excitonic light sources and microcavity devices can be realized with heterostructures incorporating monolayer-thick GaN.

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

Summary Electron-transport-layer-free (ETL-free) device architectures are promising designs for perovskite photovoltaics because they offer simpler configurations, low cost, and convenience for versatile optoelectronics. However, the development of ETL-free photovoltaics is hindered by their low performance. Herein, we reveal that a low electron-injection rate at the ETL-free interface is responsible for the performance loss. Moreover, we demonstrate that improving carrier lifetimes in the perovskite films can remedy the poor carrier extraction at interfaces, enabling carrier collection efficiency in ETL-free photovoltaics to approach that in ETL-containing devices. Using perovskite films with microsecond carrier lifetimes, we obtained ETL-free devices with a power conversion efficiency (PCE) of 19.5%, nearly eliminated hysteresis, and good stability. Such a PCE value is comparable to that (20.7%) of the analogous ETL-containing photovoltaics. These results offer opportunities for ETL-free architecture designs in the perovskite photovoltaics family. More importantly, this research provides a general approach to improving the performance of photovoltaics with low-injection-rate interfaces.

Identifying the Real Minority Carrier Lifetime in Nonideal Semiconductors: A Case Study of Kesterite Materials

Time‐resolved photoluminescence (TRPL) is a powerful characterization technique to study carrier dynamics and quantify absorber quality in semiconductors. The minority carrier lifetime, which is critically important for high‐performance solar cells, is often derived from TRPL analysis. However, here it is shown that various nonideal absorber properties can dominate the TRPL signal making reliable extraction of the minority carrier lifetime not possible. Through high‐resolution intensity‐, temperature‐, voltage‐dependent, and spectrally resolved TRPL measurements on absorbers and devices it is shown that photoluminescence (PL) decay times for kesterite materials are dominated by minority carrier detrapping. Therefore, PL decay times do not correspond to the minority carrier lifetime for these materials. The lifetimes measured here are on the order of hundreds of picoseconds in contrast to the nanosecond lifetimes suggested by the decay curves. These results are supported with additional measurements, device simulation, and comparison with recombination limited PL decays measured on Cu(In,Ga)Se2. The kesterite material system is used as a case study to demonstrate the general analysis of TRPL data in the limit of various measurement conditions and nonideal absorber properties. The data indicate that the current bottleneck for kesterite solar cells is the minority carrier lifetime.

Hierarchical Corannulene-Based Materials: Energy Transfer and Solid-State Photophysics.

We report the first example of a donor–acceptor corannulene‐containing hybrid material with rapid ligand‐to‐ligand energy transfer (ET). Additionally, we provide the first time‐resolved photoluminescence (PL) data for any corannulene‐based compounds in the solid state. Comprehensive analysis of PL data in combination with theoretical calculations of donor–acceptor exciton coupling was employed to estimate ET rate and efficiency in the prepared material. The ligand‐to‐ligand ET rate calculated using two models is comparable with that observed in fullerene‐containing materials, which are generally considered for molecular electronics development. Thus, the presented studies not only demonstrate the possibility of merging the intrinsic properties of π‐bowls, specifically corannulene derivatives, with the versatility of crystalline hybrid scaffolds, but could also foreshadow the engineering of a novel class of hierarchical corannulene‐based hybrid materials for optoelectronic devices.

Time-resolved correlative optical microscopy of charge-carrier transport, recombination, and space-charge fields in CdTe heterostructures

From time- and spatially resolved optical measurements, we show that extended defects can have a large effect on the charge-carrier recombination in II–VI semiconductors. In CdTe double heterostructures grown by molecular beam epitaxy on the InSb (100)-orientation substrates, we characterized the extended defects and found that near stacking faults the space-charge field extends by 2–5 μm. Charge carriers drift (with the space-charge field strength of 730–1,360 V cm−1) and diffuse (with the mobility of 260 ± 30 cm2 V−1 s−1) toward the extended defects, where the minority-carrier lifetime is reduced from 560 ns to 0.25 ns. Therefore, the extended defects are nonradiative recombination sinks that affect areas significantly larger than the typical crystalline grains in II–VI solar cells. From the correlative time-resolved photoluminescence and second-harmonic generation microscopy data, we developed a band-diagram model that can be used to analyze the impact of extended defects on solar cells and other electronic devices.

Investigation of carrier dynamics in InAs/GaAsSb quantum dots with different silicon delta-doping levels

The optical properties of InAs quantum dots (QDs) embedded in a GaAsSb matrix with different delta (δ)-doping levels of 0, 2, 4, and 6 electrons per dot (e−/dot), incorporated to control the occupation of QD electronic states, are studied by photoluminescence (PL) spectroscopy. The time-resolved PL data taken at 10 K reveal that the increase of δ-doping density from 2 to 6 e−/dot decreases the recombination lifetime of carriers at ground states of the QDs from 996 ± 36 to 792 ± 19 ps, respectively. Furthermore, the carrier lifetime of the sample with 4 e−/dot is found to increase at a slower rate than that of the undoped sample as temperature increases above 70 K. An Arrhenius plot of the temperature dependent PL intensity indicates that the thermal activation energy of electrons in the QDs, required for carrier escape from the dot ground state to continuum state, is increased when the δ-doping density is high enough (>4 e−/dot). These results are attributed to the enhanced Coulomb interaction of electrons provided by the δ-doping, leading to reduced thermal quenching of the PL.

Nonlinear behavior of the emission in the periodic structure of InAs monolayers embedded in a GaAs matrix

We report time-resolved photoluminescence (TRPL) measurements performed at different temperatures for the Bragg structure containing 60 InAs monolayer-based quantum wells (QWs) periodically arranged in a GaAs matrix. TRPL data reveal an appearance of the additional superradiant (SR) mode originated from coherent collective interaction of QWs. The SR mode is not manifested in the case if a small number of QWs is excited, then only an exciton emission related to the InAs QWs dominates the PL spectrum. The SR mode demonstrates a superlinear dependence of the intensity and radiative decay rate on the excitation power and its intensity increases at elevated temperatures compared to the excitonic emission. The photoluminescence delay time is much shorter for the SR mode indicating that the relaxation of hot excitons can occur via stimulated scattering processes. The specific behavior of the SR emission can have a strong potential for different applications such as optical logic devices, superluminescent diodes, optical switches, and low-threshold lasers. Time-resolved photoluminescence image at low temperature for the Bragg structure consisting of InAs monolayer-based quantum wells (inset).

Yolk-Shell Nanocrystals for Efficient Photoelectrochemical Water Splitting

Here we demonstrate the practical use of yolk-shell nanocrystals for efficient photoelectrochemical water splitting. The samples were prepared by using Au-Cu2O core-shell nanocrystals as the growth template. By performing selenization treatment on Au-Cu2O accompanied with the Kirkendall effect, abundant voids were generated inside the structure, resulting in the formation of Au-Cu2Se yolk-shell nanocrystals. Steady-state photoluminescence spectra show that charge separation efficiency was particularly prevalent in yolk-shell nanocrystals. Time-resolved photoluminescence data further reveal that yolk-shell nanocrystals displayed significantly larger interfacial charge transfer rate constant than the core-shell counterpart. The samples were further employed as photocathode for photoelectrochemical water splitting to illustrate their applicability. Four different samples including Au-Cu2Se, Au-Cu2O, pure Cu2Se and pure Cu2O were compared to illustrate the applicability of yolk-shell nanocrystals. The yolk-shell nanocrystals were found to exhibit substantially higher photocurrent generation than the other three counterpart samples. Both the moveable Au particle core and the void-space structural feature have contributed to the resultant superior photoelectrochemical performance of Au-Cu2Se. The influence of void size and shell thickness on the water splitting efficiency of yolk-shell nanocrystals is also studied. The results show that large void space and thicker shell thickness may lead to higher photoelectrochemical efficiency.

Quantum Yield Heterogeneity among Single Nonblinking Quantum Dots Revealed by Atomic Structure-Quantum Optics Correlation.

Physical variations in colloidal nanostructures give rise to heterogeneity in expressed optical behavior. This correlation between nanoscale structure and function demands interrogation of both atomic structure and photophysics at the level of single nanostructures to be fully understood. Herein, by conducting detailed analyses of fine atomic structure, chemical composition, and time-resolved single-photon photoluminescence data for the same individual nanocrystals, we reveal inhomogeneity in the quantum yields of single nonblinking "giant" CdSe/CdS core/shell quantum dots (g-QDs). We find that each g-QD possesses distinctive single exciton and biexciton quantum yields that result mainly from variations in the degree of charging, rather than from volume or structure inhomogeneity. We further establish that there is a very limited nonemissive "dark" fraction (<2%) among the studied g-QDs and present direct evidence that the g-QD core must lack inorganic passivation for the g-QD to be "dark". Therefore, in contrast to conventional QDs, ensemble photoluminescence quantum yield is principally defined by charging processes rather than the existence of dark g-QDs.