Exciton Mobility Research Articles

Enhanced UV Light Responsivity in <110>‐Oriented 2D Perovskites Realized by Pressure‐induced Ultrafast Exciton Transport

Two‐dimensional (2D) <100>‐oriented perovskites exhibit superior optoelectronic properties, offering significant potential in photovoltaic, light‐emitting, and photodetection applications. Nevertheless, their enlarged interlayer spacing restricts longitudinal carrier transport, thereby limiting its potential applications. While <110>‐oriented 2D perovskites provide a prospective solution with their compact interlayer spacing, their inherent structure, characterized by octahedra tilting, indirectly hinders carrier transport due to the generation of self‐trapped excitons (STEs) caused by strong electron‐phonon coupling. Here, we adeptly regulate the photoluminescence (PL) from STEs to free excitons (FEs) emission within the 2D <110>‐oriented (API)PbBr4 single crystal through structure optimization under pressure treatment. Besides, we observed anisotropic FE transport with an anisotropy ratio of 4.97. The exciton mobility reaches a peak of 93.6 cm2 V‐1s‐1 at 2.7 GPa, a value comparable to those of their three‐dimensional (3D) counterparts, which is attributed to a reduction in electron‐phonon coupling and exciton reduced mass. Additionally, this ultrafast exciton transport significantly enhances UV light responsivity, exhibiting an increase of approximately 5000 times at 2.7 GPa in comparison to ambient conditions. These findings highlight the prospective application of 2D <110>‐oriented perovskites in high‐performance optoelectronic devices through intrinsic structural modulation.

Enhanced UV Light Responsivity in -Oriented 2D Perovskites Realized by Pressure-induced Ultrafast Exciton Transport.

Two-dimensional (2D) <100>-oriented perovskites exhibit superior optoelectronic properties, offering significant potential in photovoltaic, light-emitting, and photodetection applications. Nevertheless, their enlarged interlayer spacing restricts longitudinal carrier transport, thereby limiting its potential applications. While <110>-oriented 2D perovskites provide a prospective solution with their compact interlayer spacing, their inherent structure, characterized by octahedra tilting, indirectly hinders carrier transport due to the generation of self-trapped excitons (STEs) caused by strong electron-phonon coupling. Here, we adeptly regulate the photoluminescence (PL) from STEs to free excitons (FEs) emission within the 2D <110>-oriented (API)PbBr4 single crystal through structure optimization under pressure treatment. Besides, we observed anisotropic FE transport with an anisotropy ratio of 4.97. The exciton mobility reaches a peak of 93.6 cm2V-1s-1 at 2.7 GPa, a value comparable to those of their three-dimensional (3D) counterparts, which is attributed to a reduction in electron-phonon coupling and exciton reduced mass. Additionally, this ultrafast exciton transport significantly enhances UV light responsivity, exhibiting an increase of approximately 5000 times at 2.7 GPa in comparison to ambient conditions. These findings highlight the prospective application of 2D <110>-oriented perovskites in high-performance optoelectronic devices through intrinsic structural modulation.

Exciton-Exciton Interactions in Van der Waals Heterobilayers

Exciton-exciton interactions are key to understanding nonlinear optical and transport phenomena in van der Waals heterobilayers, which emerged as versatile platforms to study correlated electronic states. We present a combined theory-experiment study of excitonic many-body effects based on first-principle band structures and Coulomb interaction matrix elements. Key to our approach is the explicit treatment of the fermionic substructure of excitons and dynamical screening effects for density-induced energy renormalization and dissipation. We demonstrate that dipolar blueshifts are almost perfectly compensated by many-body effects, mainly by screening-induced self-energy corrections. Moreover, we identify a crossover between attractive and repulsive behavior at elevated exciton densities. Theoretical findings are supported by experimental studies of spectrally narrow, mobile interlayer excitons in atomically reconstructed, h-BN-encapsulated MoSe2/WSe2 heterobilayers. Both theory and experiment show energy renormalization on a scale of a few meV even for high injection densities in the vicinity of the Mott transition. Our results revise the established picture of dipolar repulsion dominating exciton-exciton interactions in van der Waals heterostructures and open up opportunities for their external design. Published by the American Physical Society 2024

Impact of Packing Geometry on Excimer Characteristics and Mobility in Perylene Bisimide Polycrystalline Films.

Efficient exciton transport is essential for high-performance optoelectronics. Considerable efforts have been focused on improving the exciton mobility in organic materials. While it is feasible to improve mobility in organic systems by forming well-ordered stacks, the formation of trap states, particularly the lower-lying states referred to as excimers, remains a significant challenge to enhancing mobility. The mobility of excimer excitons intricately depends on the strength of excitonic coupling in terms of Förster-type diffusive exciton transfer processes. Given that the formation and mobility of excimer excitons are highly sensitive to molecular arrangements (packing geometries), conducting comprehensive investigations into the structure-property relationship in organic systems is crucial. In this study, we prepared three types of polycrystalline films of perylene bisimide (PBI) by varying substituents at the imide and bay positions, which allowed us to tailor the properties of excimer excitons and their mobility based on packing geometries and excitonic coupling strengths. By utilizing femtosecond transient absorption spectroscopy, we observed ultrafast excimer formation in the higher coupling regime, while in the lower coupling regime, the transition from Frenkel to excimer excitons occurs with a time constant of 500 fs. Under high pump-fluence, exciton-exciton annihilation processes occur, indicating the diffusion of excimer excitons. Intriguingly, employing a three-dimensional diffusion model, we derived a diffusion constant that is 3000 times greater in the high coupling regime than in the low coupling regime. To investigate the optoelectronic properties in the form of a bulk system, we fabricated n-type organic field effect transistors and obtained 8000 times higher mobility in the high coupling regime. Furthermore, photocurrent measurements enable us to investigate the charge carrier transport by mobile excimer excitons, suggesting a 230-fold improvement in external quantum efficiency with tightly packing PBI molecules compared to the low coupling regime. These findings not only offer valuable insights into optimizing organic materials for optoelectronic devices but also unveil the intriguing potential of exciton migration within excimers.

Superior Phonon-Limited Exciton Mobility in Lead-Free Two-Dimensional Perovskites.

Tin-based two-dimensional (2D) perovskites are emerging as lead-free alternatives in halide perovskite materials, yet their exciton dynamics and transport remain less understood due to defect scattering. Addressing this, we employed temperature-dependent transient photoluminescence (PL) microscopy to investigate intrinsic exciton transport in three structurally analogous Sn- and Pb-based 2D perovskites. Employing conjugated ligands, we synthesized high-quality crystals with enhanced phase stability at various temperatures. Our results revealed phonon-limited exciton transport in Sn perovskites, with diffusion constants increasing from 0.2 cm2 s-1 at room temperature to 0.6 cm2 s-1 at 40 K, and a narrowing PL line width. Notably, Sn-based perovskites exhibited greater exciton mobility than their Pb-based equivalents, which is attributed to lighter effective masses. Thermally activated optical phonon scattering was observed in Sn-based compounds but was absent in Pb-based materials. These findings, supported by molecular dynamics simulations, demonstrate that the phonon scattering mechanism in Sn-based halide perovskites can be distinct from their Pb counterparts.

Boosting exciton mobility approaching Mott-Ioffe-Regel limit in Ruddlesden−Popper perovskites by anchoring the organic cation

Exciton transport in two-dimensional Ruddlesden−Popper perovskite plays a pivotal role for their optoelectronic performance. However, a clear photophysical picture of exciton transport is still lacking due to strong confinement effects and intricate exciton-phonon interactions in an organic-inorganic hybrid lattice. Herein, we present a systematical study on exciton transport in (BA)2(MA)n−1PbnI3n+1 Ruddlesden−Popper perovskites using time-resolved photoluminescence microscopy. We reveal that the free exciton mobilities in exfoliated thin flakes can be improved from around 8 cm2 V−1 s−1 to 280 cm2V−1s−1 by anchoring the soft butyl ammonium cation with a polymethyl methacrylate network at the surface. The mobility of the latter is close to the theoretical limit of Mott-Ioffe-Regel criterion. Combining optical measurements and theoretical studies, it is unveiled that the polymethyl methacrylate network significantly improve the lattice rigidity resulting in the decrease of deformation potential scattering and lattice fluctuation at the surface few layers. Our work elucidates the origin of high exciton mobility in Ruddlesden−Popper perovskites and opens up avenues to regulate exciton transport in two-dimensional materials.

On Analytical Modeling of Hopping Transport of Charge Carriers and Excitations in Materials with Correlated Disorder.

Spatial-energy correlations strongly influence charge and exciton transport in weakly ordered media such as organic semiconductors and nanoparticle assemblies. Focusing on cases with shorter-range interparticle interactions, we develop a unified analytic approach that allows us to calculate the temperature and field dependence of charge carrier mobility in organic quadrupole glasses and the temperature dependence of the diffusion coefficient of excitons in quantum dot solids. We obtain analytic expressions for the energy distribution of hopping centers, the characteristic escape time of charge/exciton from the energy well stemming from energy correlations around deep states, and the size of the well. The derived formulas are tested with Monte Carlo simulation results, showing good agreement and providing simple analytic expressions for analysis of charge and exciton mobility in a broad range of partially ordered media.

Exciton localization and dynamics in GaNAsP nanowires

This work investigates exciton localization and dynamics in semiconductor GaNAsP nanowires (NWs) with varying nitrogen concentrations. Through detailed time-resolved photoluminescence studies, we identify a nitrogen composition-dependent difference in exciton transfer between localized states formed due to alloy disorder. With [N] = 0.1%, the localized states exhibit cluster-like, non-interacting behavior, whereas at [N] = 1.1%, a continuous band of localized states is observed. Additionally, the phosphorous incorporation in the NWs appears to enhance the exciton spatial confinement compared to behaviors observed in phosphorous-free GaNAs NWs, emphasizing the role of the alloy composition in the nature of exciton localization. Temperature is highlighted as a significant factor affecting exciton mobility, enabling efficient transfer between the localized states at higher temperatures. This, in turn, influences exciton lifetimes. Our findings, therefore, shed light on the nature of exciton dynamics in GaNAsP NWs, enriching our understanding of these materials and paving the way for their applications in optoelectronics.

How to recognize clustering of luminescent defects in single-wall carbon nanotubes.

Semiconducting single-wall carbon nanotubes (SWCNTs) are a promising material platform for near-infrared in vivo imaging, optical sensing, and single-photon emission at telecommunication wavelengths. The functionalization of SWCNTs with luminescent defects can lead to significantly enhanced photoluminescence (PL) properties due to efficient trapping of highly mobile excitons and red-shifted emission from these trap states. Among the most studied luminescent defect types are oxygen and aryl defects that have largely similar optical properties. So far, no direct comparison between SWCNTs functionalized with oxygen and aryl defects under identical conditions has been performed. Here, we employ a combination of spectroscopic techniques to quantify the number of defects, their distribution along the nanotubes and thus their exciton trapping efficiencies. The different slopes of Raman D/G+ ratios versus calculated defect densities from PL quantum yield measurements indicate substantial dissimilarities between oxygen and aryl defects. Supported by statistical analysis of single-nanotube PL spectra at cryogenic temperatures they reveal clustering of oxygen defects. The clustering of 2-3 oxygen defects, which act as a single exciton trap, occurs irrespective of the functionalization method and thus enables the use of simple equations to determine the density of oxygen defects and defect clusters in SWCNTs based on standard Raman spectroscopy. The presented analytical approach is a versatile and sensitive tool to study defect distribution and clustering in SWCNTs and can be applied to any new functionalization method.

Ab initio investigation on exciton-phonon interaction and exciton mobility in black phosphorene

<i>Ab initio</i> investigation on exciton-phonon interaction and exciton mobility in black phosphorene

Isolated Dimers Versus Solid-State Dimers of N-Heteropolycycles: Matrix-Isolation Spectroscopy in Concert with Quantum Chemistry.

In this work, matrix-isolation spectroscopy and quantum-chemical calculations are used together to analyse the structure and properties of weakly bound dimers of the two isomers benzo[a]acridine and benzo[c]acridine. Our measured experimental electronic absorbance spectra agree with simulated spectra calculated for the equilibrium structures of the dimers in gas-phase, but in contrast, disagree with the simulated spectra calculated for the structures obtained by optimising the experimental solid-state structures. This highlights the sensitivity of the electronic excitations with respect to the dimer structures. The comparison between the solid-state and gas-phase dimers shows how far the intermolecular interactions could change the geometric and electronic structure in a disordered bulk material or at device interfaces, imposing consequences for exciton and charge mobility and other material properties.

Enhanced Exciton Drift Transport through Suppressed Diffusion in One-Dimensional Guides.

Drift-diffusion dynamics is investigated in a one-dimensional (1D) exciton guide at room temperature. Spatial engineering of the exciton energy in a WSe2 monolayer is achieved using local strain to confine and direct exciton transport. An unexpected and massive deviation from the Einstein relation is observed and correlated to exciton capture by defects. We find that the capture reduces exciton temperature and diffusion so much that drift transport visibility improves to 38% as excitons traverse asymmetrically over regions with occupied defect states. Based on measurements over multiple potential gradients, we estimate the exciton mobility to be 169 ± 39 cm2/(eV s) at room temperature. Experiments at elevated exciton densities reveal that the exciton drift velocity monotonically increases with exciton density, unlike exciton mobility, due to contributions from nonequilibrium many-body effects.

Singlet-Triplet Energy Inversion in Carbon Nitride Photocatalysts.

Time-resolved EPR (TR-EPR) demonstrates the formation of well-defined spin triplet excitons in carbon nitride. This permits to experimentally probe the extent of the triplet wavefunction which delocalizes over several tri-s-triazine units. Analysis of the temperature dependence of the TR-EPR signal reveals the mobility of the triplet excitons. By employing monochromatic light excitation in the range 430-600 nm, the energy of the spin triplet is estimated to be ≈0.2 eV above the conduction band edge, proving that the triplet exciton lies above the corresponding singlet. Comparison between amorphous and graphitic forms establishes the singlet-triplet inversion as a general feature of carbon nitride materials.

Singlet‐Triplet Energy Inversion in Carbon Nitride Photocatalysts

AbstractTime‐resolved EPR (TR‐EPR) demonstrates the formation of well‐defined spin triplet excitons in carbon nitride. This permits to experimentally probe the extent of the triplet wavefunction which delocalizes over several tri‐s‐triazine units. Analysis of the temperature dependence of the TR‐EPR signal reveals the mobility of the triplet excitons. By employing monochromatic light excitation in the range 430–600 nm, the energy of the spin triplet is estimated to be ≈0.2 eV above the conduction band edge, proving that the triplet exciton lies above the corresponding singlet. Comparison between amorphous and graphitic forms establishes the singlet‐triplet inversion as a general feature of carbon nitride materials.

Kapitza-resistance-like exciton dynamics in atomically flat MoSe2-WSe2 lateral heterojunction

Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe2-WSe2) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices.

Enhanced photocatalytic H2 production independent of exciton dissociation in crystalline carbon nitride

The role of exciton dissociation in charge separation dominated by in-plane crystallinity has been rarely addressed, although it is the decisive step of polymeric carbon nitride (CN) photocatalysis. Here, taking ten kinds of crystalline carbon nitride (CCN) as examples, the relationship between exciton dissociation, carrier mobility, conductivity, charge separation and photocatalytic activity under different in-plane crystallinity has been investigated. Although the in-plane crystallinity dominates charge separation and photocatalytic activity, it does not significantly reduce excitons binding energy (EBE) or improve exciton dissociation efficiency (EDE), only decreasing by 1.5 meV and increasing by 2.3 %, respectively. It is the greatly improved carrier mobility and conductivity that are more responsible for the increased charge separation and photocatalytic activity than the slight change in EDE. To our knowledge, this is the first reported case of overall water splitting in CCN of poly(heptazine imide) structure, resulting from the optimized in-plane crystallinity.

Exciton annihilation in molecular aggregates suppressed through quantum interference.

Exciton-exciton annihilation (EEA), an important loss channel in optoelectronic devices and photosynthetic complexes, has conventionally been assumed to be an incoherent, diffusion-limited process. Here we challenge this assumption by experimentally demonstrating the ability to control EEA in molecular aggregates using the quantum phase relationships of excitons. We employed time-resolved photoluminescence microscopy to independently determine exciton diffusion constants and annihilation rates in two substituted perylene diimide aggregates featuring contrasting excitonic phase envelopes. Low-temperature EEA rates were found to differ by more than two orders of magnitude for the two compounds, despite comparable diffusion constants. Simulated rates based on a microscopic theory, in excellent agreement with experiments, rationalize this EEA behaviour based on quantum interference arising from the presence or absence of spatial phase oscillations of delocalized excitons. These results offer an approach for designing molecular materials using quantum interference where low annihilation can coexist with high exciton concentrations and mobilities.

Comparison of Frenkel and Excimer Exciton Diffusion in Perylene Bisimide Nanoparticles.

Exciton migration is an important process for light harvesting with organic systems and often the bottleneck. Especially the formation of trap states hinders the mobility considerably. Although excimer excitons are often referred to as traps, their mobility has been demonstrated while their nature is still unclear. Here, we compare the mobility of singlet and excimer excitons in nanoparticles consisting of the same type of perylene bisimide molecules. By changing the preparation conditions, nanoparticles with different intermolecular coupling strengths are prepared. Femtosecond transient absorption spectroscopy reveals the formation of excimer excitons from Frenkel excitons. The mobility of both exciton types is determined by evaluating exciton-exciton annihilation processes. In the lower coupling regime, singlet mobility is observed, whereas for stronger coupling the dynamics is dominated by a 10-fold increased excimer mobility. The excimer mobility can thus even be higher than the singlet mobility and is affected by the intermolecular electronic coupling.

Interactions between Fermi polarons in monolayer WS2

Interactions between quasiparticles are of fundamental importance and ultimately determine the macroscopic properties of quantum matter. A famous example is the phenomenon of superconductivity, which arises from attractive electron-electron interactions that are mediated by phonons or even other more exotic fluctuations in the material. Here we introduce mobile exciton impurities into a two-dimensional electron gas and investigate the interactions between the resulting Fermi polaron quasiparticles. We employ multi-dimensional coherent spectroscopy on monolayer WS2, which provides an ideal platform for determining the nature of polaron-polaron interactions due to the underlying trion fine structure and the valley specific optical selection rules. At low electron doping densities, we find that the dominant interactions are between polaron states that are dressed by the same Fermi sea. In the absence of bound polaron pairs (bipolarons), we show using a minimal microscopic model that these interactions originate from a phase-space filling effect, where excitons compete for the same electrons. We furthermore reveal the existence of a bipolaron bound state with remarkably large binding energy, involving excitons in different valleys cooperatively bound to the same electron. Our work lays the foundation for probing and understanding strong electron correlation effects in two-dimensional layered structures such as moiré superlattices.

The effect of positional disorder and the Beer-Lambert law in organic photovoltaics : A kinetic Monte Carlo simulation analysis.

It is urgent to address climate change by radically changing our energy sources. Organic photovoltaics (OPVs) are a competitive clean energy emerging technology and will undoubtedly have a market niche in a world that needs to take advantage of every possible type of renewable energy. Recent studies have brought relevant improvements on internal efficiency, focusing on two properties at the interface: energetic disorder and bending. However, how positional disorder affects internal efficiency is still an open question. Here, we show that positional disorder is desired at the interface, but only up to a threshold value of 0.2 nm for poly p-phenylene vinylene. Using a kinetic Monte Carlo simulator, we realized that not enough excitons were reaching the interface, and introduced the Beer-Lambert law of attenuance to correct it. Furthermore, we realized that the same disorder that facilitates charge separation at the interface diminishes exciton and charge mobility in bulk, so we propose here a new morphology for the active layer of OPVs. Our suggestion implicates in better overall performance, improving not just the internal but the overall cell efficiency.