Hot Exciton Relaxation Research Articles

Dynamics of Hot Exciton Relaxation in Conjugated Polymer Chain

Hot excitons are formed after photoexcitation of conjugated polymer chains. Hot excitons relax and convert into cold excitons with the aid of vibrational modes. In this study, the dynamics of such conversion is studied within the strong coupling regime. It has been found that the magnitudes of electronic coupling integrals for hot exciton relaxation are mostly due to exchange interactions between the interacting units. During relaxation, hot excitons oscillate back and forth between two different sites until they lose their extra energy. The time step for each oscillation has been found as small as 0.3 fs. It has also been found that photoexcited states in conjugated polymer chains do not necessarily localize at their initial location formed. Monte-Carlo simulations show that hot excitons can sustain their coherent motion along the conjugated backbone to some extent before total relaxation.

Highly stable CsPbBr3/ PMA perovskite nanocrystals for improved optical performance

In this study, to address the stability issues, we synthesized a CsPbBr3-coated poly (maleic anhydride-alt-1-octadecene) (CsPbBr3/PMA) using a modified hot-injection method. The CsPbBr3/PMA perovskite nanocrystals (PNCs) exhibited effective green emission at 522 nm with an improved photoluminescence quantum yield (86.8 %) compared to traditional CsPbBr3 PNCs (54.2 %). The ligands in the polymer coating can bond with the uncoordinated Pb and Br ions on the surface of PNCs to minimize surface defects and avoid exposure to the external environment, enhancing the stability of the perovskites. Time-resolved photoluminescence spectra showed longer lifetimes for CsPbBr3/PMA PNCs, while transient absorption measurements provided valuable insights into the intraband hot-exciton relaxation and recombination. We demonstrate the potential application of CSPbBr3/PMA in a down-conversion white-light-emitting diode (LED) by coupling green CsPbBr3/PMA and red K2SiF6:Mn4+ phosphor-coated glass slides onto a 455-nm blue GaN LED. The white LED produced a white light with the International Commission on Illumination color coordinates of (0.323, 0.345), luminous efficiency of 58.4 lm/W, and color rendering index of 83.2. The fabricated, white-LED system obtained a wide color gamut of 125.3 % of the National Television Standards Committee and 98.9 % of Rec. 2020. The findings demonstrate that CsPbBr3/PMA can be an efficient down-conversion material for white LEDs and backlighting.

Ultrafast Dynamics of Valley-Polarized Excitons in WSe2 Monolayer Studied by Few-Cycle Laser Pulses.

We report on the experimental investigation of the ultrafast dynamics of valley-polarized excitons in monolayer WSe2 using transient reflection spectroscopy with few-cycle laser pulses with 7 fs duration. We observe that at room temperature, the anisotropic valley population of excitons decays on two different timescales. The shorter decay time of approximately 120 fs is related to the initial hot exciton relaxation related to the fast direct recombination of excitons from the radiative zone, while the slower picosecond dynamics corresponds to valley depolarization induced by Coloumb exchange-driven transitions of excitons between two inequivalent valleys.

Breaking Phonon Bottlenecks through Efficient Auger Processes in Perovskite Nanocrystals.

The hot phonon bottleneck has been under intense investigation in perovskites. In the case of perovskite nanocrystals, there may be hot phonon bottlenecks as well as quantum phonon bottlenecks. While they are widely assumed to exist, evidence is growing for the breaking of potential phonon bottlenecks of both forms. Here, we perform state-resolved pump/probe spectroscopy (SRPP) and time-resolved photoluminescence spectroscopy (t-PL) to unravel hot exciton relaxation dynamics in model systems of bulk-like 15 nm nanocrystals of CsPbBr3 and FAPbBr3, with FA being formamidinium. The SRPP data can be misinterpreted to reveal a phonon bottleneck even at low exciton concentrations, where there should be none. We circumvent that spectroscopic problem with a state-resolved method that reveals an order of magnitude faster cooling and breaking of the quantum phonon bottleneck that might be expected in nanocrystals. Since the prior pump/probe methods of analysis are shown to be ambiguous, we perform t-PL experiments to unambiguously confirm the existence of hot phonon bottlenecks as well. The t-PL experiments reveal there is no hot phonon bottleneck in these perovskite nanocrystals. Ab initio molecular dynamics simulations reproduce experiments by inclusion of efficient Auger processes. This experimental and theoretical work reveals insight on hot exciton dynamics, how they are precisely measured, and ultimately how they may be exploited in these materials.

Synergistic Control of Hot Exciton Relaxation and Exciton–Polaron Dispersion Using a Polaron Retarder for Long Lifetime in Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diodes

AbstractThe operational stability of thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs) is an essential property to utilize their efficient triplet harvesting ability and reduce the usage of expensive metal complexes in commercial applications. In this study, a device strategy for extending the device lifetime using the synergistic effects of the hot exciton relaxation and polaron dispersion is demonstrated. By doping a polaron retarder to a conventional TADF‐emitting layer, the lifetime of the blue TADF device is enhanced by 98% at an initial luminance of 1000 cd m−2. Kinetic modeling describing the device degradation indicates that the concurrent contribution via the suppression of exciton and polaron interactions and hot exciton‐related degradation are the origin of the lifetime enhancement. It is demonstrated that the introduction of the polaron retarder finely fits to the ground and that the excited energetic relationship of the host–dopant structure simultaneously improves the operation lifetime and device efficiency without sacrificing other device characteristics.

Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized Carrier-Induced Stark and Phonon Bottleneck Effects.

Using femtosecond transient absorption (fs-TA), we investigate the hot exciton relaxation dynamics in strongly confined lead iodide perovskite nanoplatelets (NPLs). The large quantum and dielectric confinement leads to discrete excitonic transitions and strong Stark features in the TA spectra. This prevents the use of conventional relaxation analysis methods extracting the carrier temperature or measuring the buildup of the band-edge bleaching. Instead, we show that the TA spectral line shape near the band-edge reflects the state of the system, which can be used to probe the exciton cooling dynamics. The ultrafast hot exciton relaxation in one- to three- monolayer-thick NPLs confirms the absence of intrinsic phonon bottleneck. However, excitation fluence-dependent measurements reveal a hot phonon bottleneck effect, which is found to be independent of the nature of the internal cations but strongly affected by the ligands and/or sample surface state. Together, these results suggest a role of the surface ligands in the cooling process.

(Invited) Photocatalytic Water Splitting Using SWCNT/TiO2 Nanohybrids

Water splitting photocatalysts based on semiconducting single-walled carbon nanotubes (s-SWCNTs) have been attracted much attention from the viewpoint of solar fuel production.[1-7] Since s-SWCNTs have chirality dependent bandgap covering a majority of the solar spectrum (400 – 2000 nm), s-SWCNTs are expected for an efficient solar-light absorber.[8] However, upon photoirradiation, the large exciton binding energy in s-SWCNTs prevents mobile carrier generation. Then, in order to make a particulate photocatalyst consisting of s-SWCNTs, an electron-extracting material is necessary. In this context, we have developed CNT-photocatalysts that contain a s-SWCNT/C60 coaxial heterojunction fabricated by a physical modification of s-SWCNTs using fullerodendron.[4] Here, the C60 acts as the electron-extracting material to generate a charge-separated state, s-SWCNT•+/C60 •–, followed by electron transfer to a co-catalyst that produces hydrogen from water. Z-scheme photocatalyst system employing BiVO4 as an oxygen evolution photocatalyst (OEP) and s-SWCNT/C60 coaxial hybrids as a hydrogen evolution photocatalyst (HEP) produced H2 and O2 from water, of which solar-to-hydrogen conversion efficiency (STH) was 0.089%.[7] We conducted an H2 evolution reaction using s-SWCNT/C60/Ru(III) upon monochromatic irradiation at 570, 650, and 680 nm, which are E22 absorption of (6,5), (7,5), and (8,3)tubes, respectively, and estimated the external quantum yields to be 0.22% for (6,5), 0.16% for (7,5), and 0.70% for (8,3). These EQY values were considerably lower than that using 1000-nm-light (E11 absorption of (6,5) and (8,3)tubes) irradiation, 12.8%.[7] The result might be attributed to branching pathways from the higher exciton (E22 excitation) to other than the lower exciton (E11 excitation). Blackburn and co-workers reported such a branching ratio is crucial for the PL quantum yield of s-SWCNTs.[9] Cui and co-workers describe E22 excitation makes interfacial electrotransfer inhibited in the (6,5)tube/C60 heterojunction, although ultrafast electron transfer process can be seen in E11 excitation by a DFT-based nonadiabatic dynamics simulation.[10] In this context, direct electron extraction from E22 excitation, which is faster than other branching pathways, is expected to improve the photocatalytic activity of CNT-photocatalysts. Recently, Blackburn reported higher-energy second excitonic s-SWCNT transitions produce more photocurrent by the use of submonolayer coverages of s-SWCNTs on single crystal TiO2 electrode, demonstrating carrier injection rates are competitive with fast hot-exciton relaxation processes.[11] These backgrounds prompt us to investigate a CNT-photocatalyst containing the s-SWCNT/TiO2 heterojunction. This paper describes fabrication of s-SWCNT/TiO2/Pt nanohybrids and their photocatalytic activity for H2 evolution from water. Interestingly, their EQYs of H2 evolution reaction using E22 excitation, 12% for (6,5)tube/TiO2/Pt and 43% for (8,3)tube/TiO2/Pt is much higher than those of (6,5)tube/C60/Ru(III) and (8,3)tube/C60/Ru(III), 0.22% and 0.70%.

Hot exciton relaxation in coupled ultra-thin CdTe/ZnTe quantum well structures

The photoluminescence (PL) and PL excitation (PLE) spectra of CdTe/ZnTe asymmetric double quantum well (QW) structures are studied on a series of samples containing two CdTe layers with nominal thicknesses of 2 and 4 monolayers (ML) in the ZnTe matrix. The samples differ in the thickness of the ZnTe spacer between CdTe QWs which is 45, 65 and 75 ML. It has been found that at above-barrier excitation the PL from a shallow QW at sufficiently weak excitation intensities is determined by recombination of hot excitons. It is shown that under these conditions, when PL is excited by lasers with different wavelengths, the ratio of the PL intensities from shallow and deep QWs decreases exponentially with an increase of the initial kinetic energy of hot excitons. It is found that energy relaxation of hot excitons with LO phonon emission determine the shape of the PLE spectrum of shallow QW in the range of exciton kinetic energies up to more than 20 LO phonons above ZnTe bandgap. We have shown that the results obtained are well described by the model of charge and energy transfer between QWs.

Hot exciton transport in WSe2 monolayers

We experimentally demonstrate hot exciton transport in h-BN encapsulated WSe2 monolayers via spatially and temporally resolved photoluminescence measurements at room temperature. We show that the nonlinear evolution of the mean squared displacement of the non-resonantly excited hot exciton gas is primarily due to the relaxation of its excess kinetic energy and is characterized by a density-dependent fast expansion that converges to a slower, constant rate expansion. We also observe saturation of the hot exciton gas' expansion rate at high excitation densities due to the balance between Auger-assisted hot exciton generation and the phonon-assisted hot exciton relaxation processes.

Enhanced Dissociation of Hot Excitons with an Applied Electric Field under Low-Power Photoexcitation in Two-Dimensional Perovskite Quantum Wells.

The dependence of photoluminescence (PL) on excitation power and the effect of an external electric field have been studied for a two-dimensional (2D) perovskite (C4H9NH3)2PbI4 thin-film sample. The efficiency of dissociation of hot excitons to produce free carriers was enhanced with a small excitation power because the relaxation of hot excitons to cold emissive excitons was slow, indicating that the thermal energies of hot carriers can be utilized in solar cells under weak photoirradiation. The dissociation was notably enhanced with an applied electric field, resulting in efficient field-induced quenching of the PL. The present results shed light on an application of 2D perovskite materials to photovoltaic (PV) devices with dim radiation, e.g., for indoor PV applications; the concept of electric field-assisted solar cells might be applicable to next-generation solar cells.

On the absence of a phonon bottleneck in strongly confined CsPbBr3 perovskite nanocrystals.

In traditional solar cells, photogenerated energetic carriers (so-called hot carriers) rapidly relax to band edges via emission of phonons, prohibiting the extraction of their excess energy above the band gap. Quantum confined semiconductor nanocrystals, or quantum dots (QDs), were predicted to have long-lived hot carriers enabled by a phonon bottleneck, i.e., the large inter-level spacings in QDs should result in inefficient phonon emissions. Here we study the effect of quantum confinement on hot carrier/exciton lifetime in lead halide perovskite nanocrystals. We synthesized a series of strongly confined CsPbBr3 nanocrystals with edge lengths down to 2.6 nm, the smallest reported to date, and studied their hot exciton relaxation using ultrafast spectroscopy. We observed sub-ps hot exciton lifetimes in all the samples with edge lengths within 2.6-6.2 nm and thus the absence of a phonon bottleneck. Their well-resolved excitonic peaks allowed us to quantify hot carrier/exciton energy loss rates which increased with decreasing NC sizes. This behavior can be well reproduced by a nonadiabatic transition mechanism between excitonic states induced by coupling to surface ligands.

Optically Generated Free-Carrier Collection from an All Single-Walled Carbon Nanotube Active Layer.

Semiconducting single-walled carbon nanotubes' (SWCNTs) broad absorption range and all-carbon composition make them attractive materials for light harvesting. We report photoinduced charge transfer from both multichiral and single-chirality SWCNT films into atomically flat SnO2 and TiO2 crystals. Higher-energy second excitonic SWCNT transitions produce more photocurrent, demonstrating carrier injection rates are competitive with fast hot-exciton relaxation processes. A logarithmic relationship exists between photoinduced electron-transfer driving force and photocarrier collection efficiency, becoming more efficient with smaller diameter SWCNTs. Photocurrents are generated from both conventional sensitization and in the opposite direction with the semiconductor under accumulation and acting as an ohmic contact with only the p-type nanotubes. Finally, we demonstrate that SWCNT surfactant choice and concentration play a large role in photon conversion efficiency and present methods of maximizing photocurrent yields.

Hot Exciton Relaxation and Exciton Trapping in Single-Walled Carbon Nanotube Thin Films

Time-resolved two-photon photoemission spectroscopy (TR-TPPE) was employed to investigate the hot exciton relaxation and exciton trapping dynamics in semiconductive, single-walled carbon nanotube thin films. Compared to other conventional optical ultrafast spectroscopy techniques, the TR-TPPE can temporally resolve both the energy and the population of optically excited excitons, which enables unambiguous identification of hot and relaxed band-edge exciton states. It is found that hot excitons populated by photons with energies above the band gap lose most of their excess energy within the first 100 fs after photoexcitation. Unlike isolated nanotubes, the generation of multiple excitons per absorbed photon is not observed for an excitation energy as high as five times the optical band gap. This difference is attributed to the different dielectric environment and the presence of intertube interaction in nanotube thin films. The subsequent population decay and energy relaxation dynamics of the band-edge exc...

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).

Exciton Relaxation Dynamics in Photo-Excited CsPbI3 Perovskite Nanocrystals.

The exciton relaxation process of CsPbI3 perovskite nanocrystals (NCs) has been investigated by using transient absorption (TA) spectroscopy. The hot exciton relaxation process is confirmed to exist in the CsPbI3 NCs, through comparing the TA data of CsPbI3 NCs in low and high energy excitonic states. In addition, the Auger recombination and intrinsic decay paths also participate in the relaxation process of CsPbI3 NCs, even the number of exciton per NC is estimated to be less than 1. Excitation intensity-dependent TA data further confirms the existence of Auger recombination. Meanwhile, the spectral data also confirms that the weight of hot exciton also increase together with that of Auger recombination at high excitation intensity when CsPbI3 NCs in high energy excitonic states.

Hot exciton relaxation in multiple layers CdSe/ZnSe self-assembled quantum dots separated by thick ZnSe barriers

We have studied PL and PLE spectra of two samples (A and B) of MBE grown CdSe/ZnSe asymmetric double quantum wells with different amount of deposited CdSe layers separated by 14 nm ZnSe barrier. It has been found that PLE spectra of the states forming short wavelength side of the PL spectra of both deep and shallow QWs of the sample A as well as that of deep QW of the sample B demonstrate oscillating structure in the spectral ranges corresponding to exciton states of self-assembled quantum dots only. Meanwhile PLE spectra of the short wavelength states of shallow QW the sample B revealed pronounced oscillating structure with energy period of ZnSe LO phonon under excitation with photons in a wide energy range both in the regions of quantum-dot states and in that of free states in the ZnSe barrier. In these spectra creating of excitons with kinetic energies more than 0.3 eV was observed which considerably exceed the exciton binding energy as well as LO phonon energy (both appr. 0.03 eV). It has been concluded that oscillating structure of the PLE spectra arises due to cascade relaxation of hot excitons. We discuss the model which explains these experimental findings.

Time-Domain Ab Initio Simulation of Energy Transfer in Double-Walled Carbon Nanotubes

The exact mechanism of photoinduced hot exciton relaxation in semiconducting double-walled carbon nanotubes (DWNTs) is impossible to determine based solely on the results of the time-resolved spectroscopy. While optical transitions can be resolved with a high accuracy, dark and charge transfer states remain undetected. The time-dependent density functional theory combined with nonadiabatic molecular dynamics simulation allows us to take into account all the possible relaxation pathways, including dipole-forbidden dark and charge transfer states. We show that exciton energy transfer in semiconducting DWNTs strongly depends on geometry fluctuation of the comprising tubes. Driven by low-frequency, radial breathing modes, it affects the relative alignment of energy levels, leads to level crossings, and opens new relaxation channels. Our simulations indicate that crossover transitions and charge transfer states affect the energy transfer dynamics in DWNTs, and largely govern the transfer rates in the system. These findings should be applicable to other nanoscale systems.

Excitonic states and energy relaxation in low-dimension CdSe/ZnSe structures with isolated ZnCdSe islands

Photoluminescence (PL) and PL excitation (PLE) spectra of structures containing MBE-grown CdSe layers with a nominal thickness tCdSe of 1.4–2.7 monolayers (MLs) in a ZnSe matrix have been studied. It is shown that the main features of the PLE spectra in structures with different amounts of deposited CdSe can be described in terms of two different models: in structures with tCdSe exceeding approximately 1.7 ML, the properties of electronic states correspond to the frequently used model of the quantum well formed by an inhomogeneous ZnCdSe solid solution with nanosize inclusions constituted by planar ZnCdSe islands whose composition is strongly enriched with CdSe, whereas the properties of electronic states in structures with tCdSe < 1.6 ML can be described in the model of isolated uncoupled ZnCdSe nanoislands. It was found that the main channel by which emitting states are occupied under excitation with photons in a wide energy range both in the region of quantum-dot states and in that of free states in the ZnSe barrier is the cascade relaxation of hot excitons, with emission of longitudinal optical phonons. Other properties of electronic states in nano-objects of this kind are discussed.

Photoluminescence and exciton resonances over the scattered light in multiphonon spectra of resonant scattering in the CdTe/ZnTe superlattices with narrow quantum wells

Photoluminescence and Raman scattering spectra in CdTe/ZnTe heterostructures and superlattices with narrow quantum wells (4.8–9.2 A) in a temperature range of 5–300 K have been measured. The temperature dependences of the intensity of exciton luminescence in isolated quantum wells have been studied, and the thermal activation energies associated with the effective barriers for electrons and holes have been determined. In CdTe/ZnTe heterostructures, the binding energies of an exciton with a heavy hole have been determined as functions of the quantum well width. The multiphonon Raman spectra that exhibit distinctive features, such as the weak intensity of nLO phonon lines of ZnTe (n 2), and the multiple participation in scattering of acoustic LA phonons with large wave vector, have been investigated. The results have been explained based on the concept of the relaxation of hot excitons over the exciton band.

Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals

Height time hyperspectral near IR probing providing broad-band coverage is employed on PbSe nanocrystals, uncovering spectral evolution following high energy photo-excitation due to hot exciton relaxation and recombination. Separation of single, double and triple exciton state contributions to these spectra is demonstrated, and the mechanisms underlying the course of spectral evolution are investigated. In addition no sign of MEG was detected in this sample up to a photon energy 3.7 times that of the band gap.