Ultrafast Charge Transfer Processes Research Articles

A new charge transfer pathway in the MoSe2-WSe2 heterostructure under the conditions of B-excitons being resonantly pumped.

Most transition metal dichalcogenide (TMD) heterostructures (HSs) exhibit a type II band alignment, leading to a charge transfer process accompanied by the transfer of spin-valley polarization and spontaneous formation of interlayer excitons. This unique band structure facilitates achieving a longer exciton lifetime and extended spin-valley polarization lifetime. However, the mechanism of charge transfer in type II TMD HSs is not fully comprehended. Here, the ultrafast charge transfer process is studied in MoSe2-WSe2 HS via valley-solved broadband pump-probe spectroscopy. Under the conditions of B-excitons of WSe2 and MoSe2 being resonantly pumped, a new charge transfer pathway through the higher energy state associated with the B-exciton is found. Meanwhile, the holes (electrons) in the WSe2 (MoSe2) layer of MoSe2-WSe2 HS produce obvious spin-valley polarization even under the condition of B-exciton of WSe2 (MoSe2) being resonantly pumped, and the lifetime can reach tens of ps, which is in stark contrast to the absence of A-exciton spin-valley polarization in monolayer WSe2 (MoSe2) under the same pumping condition. The results deepen the insight into the charge transfer process in type II TMD HSs and show the great potential of TMD HSs in the application of spin-valley electronics devices.

Probing electron and lattice dynamics by ultrafast electron microscopy: Principles and applications

AbstractMicroscale charge and energy transfer is an ultrafast process that can determine the photoelectrochemical performance of devices. However, nonlinear and nonequilibrium properties hinder our understanding of ultrafast processes; thus, the direct imaging strategy has become an effective means to uncover ultrafast charge and energy transfer processes. Due to diffraction limits of optical imaging, the obtained optical image has insufficient spatial resolution. Therefore, electron beam imaging combined with a pulse laser showing high spatial–temporal resolution has become a popular area of research, and numerous breakthroughs have been achieved in recent years. In this review, we cover three typical ultrafast electron beam imaging techniques, namely, time‐resolved photoemission electron microscopy, scanning ultrafast electron microscopy, and ultrafast transmission electron microscopy, in addition to the principles and characteristics of these three techniques. Some outstanding results related to photon–electron interactions, charge carrier transport and relaxation, electron–lattice coupling, and lattice oscillation are also reviewed. In summary, ultrafast electron beam imaging with high spatial–temporal resolution and multidimensional imaging abilities can promote the fundamental understanding of physics, chemistry, and optics, as well as guide the development of advanced semiconductors and electronics.

Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures

Heterostructures composed of the intrinsic magnetic topological insulator MnBi2Te4 and its nonmagnetic counterpart Bi2Te3 host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi2Te4 and MnBi4Te7 following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi2Te4 and Bi2Te3 surface terminations of MnBi4Te7, revealing an instantaneous occupation of states associated with the Bi2Te3 surface layer followed by carrier extraction into the adjacent MnBi2Te4 layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi2Te4 septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi2Te4-based van der Waals compounds.

Interfacial Charge Transfer Processes in Perovskite-based Materials

Perovskite-based materials have gained significant attention in the last few years both in nanotechnology and optoelectronic applications. The interface between perovskites and electron (or hole) transport layers (ETL or HTL) plays a critical role in the charge transport properties in perovskite-based devices which eventually govern the final efficiency of the device. Therefore, it is extremely important to understand the interfacial charge transfer/transport processes in these materials. In this minireview, we summarize the ultrafast interfacial charge transfer processes from perovskites to electron/hole quenchers and highlight the importance of the surface coupling of such quenchers on the charge transfer and solar cell efficiency. A few examples of ETL and HTL and their effect on the device performance have been discussed. Therefore, the review will provide a platform to understand the importance of interfacial charge transfer processes and their effect on the final device efficiency in perovskite-based materials.

Charge Transfer Dynamics in MoSe2/hBN/WSe2 Heterostructures

Ultrafast charge transfer processes provide a facile way to create interlayer excitons in directly contacted transition metal dichalcogenide (TMD) layers. More sophisticated heterostructures composed of TMD/hBN/TMD enable new ways to control interlayer exciton properties and achieve novel exciton phenomena, such as exciton insulators and condensates, where longer lifetimes are desired. In this work, we experimentally study the charge transfer dynamics in a heterostructure composed of a 1 nm thick hBN spacer between MoSe2 and WSe2 monolayers. We observe the hole transfer from MoSe2 to WSe2 through the hBN barrier with a time constant of 500 ps, which is over 3 orders of magnitude slower than that between TMD layers without a spacer. Furthermore, we observe strong competition between the interlayer charge transfer and intralayer exciton-exciton annihilation processes at high excitation densities. Our work opens possibilities to understand charge transfer pathways in TMD/hBN/TMD heterostructures for the efficient generation and control of interlayer excitons.

Ultrafast Charge and Proton Transfer in Doubly Ionized Ammonia Dimers.

We investigate the ultrafast energy and charge transfer processes between ammonia molecules following ionization reactions initiated by electron impact. Exploring ionization-induced processes in molecular clusters provides us with a detailed insight into the dynamics using experiments in the energy domain. We ionize the ammonia dimer with 200 eV electrons and apply the fragment ions coincident momentum spectroscopy and nonadiabatic molecular dynamics simulations. We identify two mechanisms leading to the doubly charged ammonia dimer. In the first one, a single molecule is ionized. This initiates an ultrafast proton transfer process, leading to the formation of the NH2+ + NH4+ pair. Alternatively, a dimer with a delocalized charge is formed dominantly via the intermolecular Coulombic decay, forming the NH3+·NH3+ dication. This dication further dissociates into two NH3+ cations. The ab initio calculations have reproduced the measured kinetic energy release of the ion pairs and revealed the dynamical processes following the double ionization.

Distinguishing Ultrafast Energy Transfer in Atomically Thin MoS2 /WS2 Heterostructures.

Van der Waals semiconducting heterostructures, known as stacks of atomically thin transition-metal dichalcogenide (TMD) layers, have recently been reported as new quantum materials with fascinating optoelectronic properties and novel functionalities. These discoveries are significantly related to the interfacial carrier dynamics of the excited states. Carrier dynamics have been reported to be predominantly driven by the ultrafast charge transfer (CT) process; however, the energy transfer (ET) process remains elusive. Herein, the ET process in MoS2 /WS2 heterostructures via transient absorption microscopy is reported. By analyzing the ultrafast dynamics using various MoS2 /WS2 interfaces, an ET rate of ≈240 fs is obtain, which is not trivial to the CT process. This study elucidates the role of the ET process in interfacial carrier dynamics and provides guidance for engineering interfaces for optoelectronic and quantum applications of TMD heterostructures.

(Invited) Kinetics of Dye-Sensitized Water Oxidation Catalysis in Natural Sunlight

The couplings of ultrafast electronic excitation, charge injection and charge transfer processes to the catalytic cycles involved in water oxidation using molecular dye-catalyst photoelectrosynthesis systems are examined using detailed stochastic chemical kinetics simulations. By modeling the full timescale from subpicoseconds to many minutes and assuming illumination by the sun (AM 1.5), we have gained insights to how 4-photon catalytic cycle works when stimulated by relatively rare interactions with light. In this talk the model will be described, and its predictions for how catalyst and dye state populations evolve over time and how the flow of the catalytic cycle works will be discussed.

Deciphering Photoinduced Charge Transfer Dynamics in a Cross-Linked Graphene-Dye Nanohybrid.

The search for synthetic materials that mimic natural photosynthesis by converting solar energy into other more useful forms of energy is an ever-growing research endeavor. Graphene-based materials, with their exceptional electronic and optical properties, are exemplary candidates for high-efficiency solar energy harvesting devices. High photoactivity can be conveniently achieved by functionalizing graphene with small molecule organic semiconductors whose band-gaps can be tuned by structural modification, leading to interactions between the π-conjugated electronic systems in both the semiconductor and graphene. Here we investigate the ultrafast transient optical properties of a cross-linked graphene–dye (diphenyl-dithiophenediketopyrrolopyrrole) nanohybrid material, in which oligomers of the organic semiconductor dye are covalently bound to a random network of few-layer graphene flakes, and compare the results to those obtained for the reference dye monomer. Using a combination of ultrafast transient absorption and two-dimensional electronic spectroscopy, we provide substantial evidence for photoinduced charge transfer that occurs within 18 ps in the nanohybrid system. Notably, subpicosecond photoinduced torsional relaxation observed in the constituent dye monomer is absent in the cross-linked nanohybrid system. Through density functional theory calculations, we compare the competing effects of covalent bonding, increasing conjugation length, and the presence of multiple graphene flakes. We find evidence that the observed ultrafast charge transfer process occurs through a superexchange mechanism in which the oligomeric dye bridge provides virtual states enabling charge transfer between graphene–dye covalent bond sites.

Interfacial charge and energy transfer in van der Waals heterojunctions

AbstractVan der Waals heterojunctions are fast‐emerging quantum structures fabricated by the controlled stacking of two‐dimensional (2D) materials. Owing to the atomically thin thickness, their carrier properties are not only determined by the host material itself, but also defined by the interlayer interactions, including dielectric environment, charge trapping centers, and stacking angles. The abundant constituents without the limitation of lattice constant matching enable fascinating electrical, optical, and magnetic properties in van der Waals heterojunctions toward next‐generation devices in photonics, optoelectronics, and information sciences. This review focuses on the charge and energy transfer processes and their dynamics in transition metal dichalcogenides (TMDCs), a family of quantum materials with strong excitonic effects and unique valley properties, and other related 2D materials such as graphene and hexagonal‐boron nitride. In the first part, we summarize the ultrafast charge transfer processes in van der Waals heterojunctions, including its experimental evidence and theoretical understanding, the interlayer excitons at the TMDC interfaces, and the hot carrier injection at the graphene/TMDCs interface. In the second part, the energy transfer, including both Förster and Dexter types, are reviewed from both experimental and theoretical perspectives. Finally, we highlight the typical charge and energy transfer applications in photodetectors and summarize the challenges and opportunities for future development in this field.

Charge Transfer Properties of Heterostructures Formed by Bi2 O2 Se and Transition Metal Dichalcogenide Monolayers.

Atomically thin bismuth oxyselenide (Bi2 O2 Se) exhibits attractive properties for electronic and optoelectronic applications, such as high charge-carrier mobility and good air stability. Recently, the development of Bi2 O2 Se-based heterostructures have attracted enormous interests with promising prospects for diverse device applications. Although the electrical properties of Bi2 O2 Se-based heterostructures have been widely studied, the interlayer charge transfer in these heterostructures remains elusive, despite its importance in harnessing their emergent functionalities. Here, a comprehensive experimental investigation on the interlayer charge transfer properties of two heterostructures formed by Bi2 O2 Se and representative transition metal dichalcogenides (namely, WS2 /Bi2 O2 Se and MoS2 /Bi2 O2 Se) is reported. Kelvin probe force microscopy is used to measure the work functions of the samples, which are further employed to establish type-II band alignment of both heterostructures. Photoluminescence quenching is observed in each heterostructure, suggesting high charge transfer efficiency. Time-resolved and layer-selective pump-probe measurements further prove the ultrafast interlayer charge transfer processes and formation of long-lived interlayer excitons. These results establish the feasibility of integrating 2D Bi2 O2 Se with other 2D semiconductors to fabricate heterostructures with novel charge transfer properties and provide insight for understanding the performance of optoelectronic devices based on such 2D heterostructures.

Theoretical Study on Understanding the Effects of Core Structure and Energy Level Tuning on Efficiency of Nonfullerene Acceptors in Organic Solar Cells

AbstractNonfullerene acceptors (NFAs) are a new focus in organic photovoltaics (OPVs), and continue to progress upon the drawbacks of many fullerene‐based electron acceptors. The aim of this work is to identify some important parameters that influence the efficiency of NF‐acceptors in OPVs. These results provide an enhanced understanding of the effect of the NFAs core structure (electron rich/poor group) on the photophysical and optoelectronic properties. In addition, the effect of the small ΔLUMO value (the subtle difference in energy between LUMO+1 and LUMO orbitals of NFAs; LUMO=lowest unoccupied molecular orbitals) on ultrafast charge transfer and charge separation processes in OPVs, recently identified as a key factor for all top rated high performing NFAs, is studied. So far, ΔLUMO‐based theoretical studies are limited to individual NFAs; here, for the first time, the authors have extended to the respective donor/acceptor complexes as well. Finally, based on the first‐principles density functional theory calculations with the seven reported NFAs, , P3HT, and the seven newly modeled donor–acceptor complexes, this study sheds light on important factors that will provide trends and guidelines for further rational design of more efficient NF‐acceptors for OPVs.

Nonlinear optical properties of a new panchromatic series of water-soluble bicationic push-pull fluorophores

Six water-soluble bications, characterized by a symmetric A+-π-D-π-A+ structure, were synthesized and comprehensively studied for their absorption and fluorescence properties, and their nonlinear optical response. Generally, the photobehavior exhibited by the six systems was found to be dependent on the molecular dimensionality and conjugation, unveiling the role of different electron-donor (dimethoxy-phenylene vs bithiophene) and electron-acceptor (methyl-pyridinium, methyl-quinolinum, and pyrimidin-pyridinium) substituents bound in different positions (ortho/para). Steady-state measurements revealed this series as a panchromatic family of dyes with noticeable absorption and emission abilities, covering the entire visible spectrum - from the near-ultraviolet to the near-infrared spectral region. Femtosecond-resolved spectroscopy disclosed the presence of ultrafast intramolecular charge transfer processes, as also predicted by TD-DFT calculations, from the central electron-rich moiety of the molecule towards the two lateral positively charged units. Further and for the first time, a solvatochromic method was successfully applied to this kind of quadrupolar double-charged compounds in order to estimate their static and dynamic second-order hyperpolarizability coefficients. Thanks to their appreciable fluorescence capability, significant Two-Photon Absorption cross-sections were also measured for all six dyes by means of the Two-Photon Excited Fluorescence technique. These findings, together with their good water solubility, designate these eye-catching fluorophores as potential candidates for bioimaging applications.

Electric Field Tunable Ultrafast Interlayer Charge Transfer in Graphene/WS2 Heterostructure.

Van der Waals heterostructures composed of two-dimensional materials offer an unprecedented control over their properties and have attracted tremendous research interest in various optoelectronic applications. Here, we study the photoinduced charge transfer in graphene/WS2 heterostructure by time-dependent density functional theory molecular dynamics. Our results show that holes transfer from graphene to WS2 two times faster than electrons, and the occurrence of interlayer charge transfer is found correlated with vibrational modes of graphene and WS2. It is further demonstrated that the carrier dynamics can be efficiently modulated by external electric fields. Detailed analysis confirms that the carrier transfer rate at heterointerface is governed by the coupling between donor and acceptor states, which is the result of the competition between interlayer and intralayer relaxation processes. Our study provides insights into the understanding of ultrafast interlayer charge transfer processes in heterostructures and broadens their future applications in photovoltaic devices.

Transparent and Colorless Dye-Sensitized Solar Cells Exceeding 75% Average Visible Transmittance.

Most photovoltaic (PV) technologies are opaque to maximize visible light absorption. However, see-through solar cells open additional perspectives for PV integration. Looking beyond maximizing visible light harvesting, this work considers the human eye photopic response to optimize a selective near-infrared sensitizer based on a polymethine cyanine structure (VG20-Cx) to render dye-sensitized solar cells (DSSCs) fully transparent and colorless. This peculiarity was achieved by conferring to the dye the ability to strongly and sharply absorb beyond 800 nm (S0–S1 transition) while rejecting the upper S0–Sn contributions far in the blue where the human retina is poorly sensitive. When associated with an aggregation-free anatase TiO2 photoanode, the selective NIR-DSSC can display 3.1% power conversion efficiency, up to 76% average visible transmittance (AVT), a value approaching the 78% AVT value of a standard double glazing window while reaching a color rendering index (CRI) of 92.1%. The ultrafast and fast charge transfer processes are herein discussed, clarifying the different relaxation channels from the dye monomer excited states and highlighting the limiting steps to provide future directions to enhance the performances of this nonintrusive NIR-DSSC technology.

Understanding ultrafast charge transfer processes in SnS and SnS2: using the core hole clock method to measure attosecond orbital-dependent electron delocalisation in semiconducting layered materials

Electron delocalisation times are significantly faster for SnS than for SnS2. Ultrafast times, as low as 30 attoseconds, were measured thanks to the application of the core hole clock method.

Atomistic insight into the significantly enhanced photovoltaic cells of monolayer GaTe2 via two-dimensional van der Waals heterostructures engineering

Abstract Designing new van der Waals (vdW) heterostructures from various two-dimensional transition metal dichalcogenides (TMDs) materials shows outstanding properties, such as an ultrafast charge transfer process and strong interlayer interactions by combining the advantageous properties of the different TMD materials. In this study, using the density functional theory method, we systemically investigate the optical property, band alignment, electronic structures, interface charge transfer, mechanical properties and stability of MTe2/GaTe2 (M = Mo and W) vdW heterostructures as promising photovoltaic solar cells materials. In this work, gallium telluride and MTe2 were used as acceptors and donors in high-quality photovoltaic cells. The calculated binding energies suggest that they were energetically favourable and relatively easy to fabricate under suitable conditions. Moreover, the heterostructures possess exceptional characteristics of enhanced visible light absorption edge (∼104 cm−1), type-II band alignment and strong charge separation. The suitable band alignment leads to maximum power conversion efficiency (PCE) of 22.43 and 22.91%, respectively, which was quite promising for photovoltaic solar cells. The high PCE could be due to the internal built-in electric field at the MTe2/GaTe2 interface, which induces efficient separation of charge carriers. This work offers theoretical support for the design and prediction of next-generation low-cost, highly efficient and promising materials for solar device applications.

Combining Theory and Experiment for Understanding of Ultrafast Photoinduced Charge-Transfer Processes

This article gives a brief overview of the longstanding collaboration between our experimental spectroscopy group in Geneva and the theoretical group lead by Prof. A.I. Ivanov in Volgograd. This fruitful collaboration resulted in a significant increase of our understanding of the dynamics of several ultrafast chargetransfer processes in the condensed phase.

A Projective Method for the Calculation of Excited-State Electronic Coupling: Isolating Charge Transfer/Recombination Processes in Organic Photovoltaics.

Electronic coupling between excited states is a vital parameter required to describe ultrafast energy and charge transfer processes that occur in photoresponsive organic materials. In such systems, short-range Coulombic, exchange, overlap, and configuration interaction effects must all be accounted for. Although a number of methods are available, the evaluation of coupling between arbitrary excited states remains challenging. In this contribution, a flexible and scalable method for the calculation of short-range electronic coupling between excited states is developed. Excitation- or charge-localized states are projected onto the adiabatic states of a dimeric molecular system using an efficient wave function overlap algorithm. In addition to correctly treating Coulombic, exchange, and overlap contributions, the inclusion of multistate interactions is inherent in the procedure. The method is then used to disentangle excitation energy transfer, charge transfer, and charge recombination processes in donor/acceptor systems relevant to organic photovoltaics, with a view toward the development of material design principles. Calculations were performed within single-excitation frameworks, but the scheme has the potential to be extended to multireference/higher-order excitation quantum-chemical methods.

Ultrafast hole transfer mediated by polaron pairs in all-polymer photovoltaic blends

The charge separation yield at a bulk heterojunction sets the upper efficiency limit of an organic solar cell. Ultrafast charge transfer processes in polymer/fullerene blends have been intensively studied but much less is known about these processes in all-polymer systems. Here, we show that interfacial charge separation can occur through a polaron pair-derived hole transfer process in all-polymer photovoltaic blends, which is a fundamentally different mechanism compared to the exciton-dominated pathway in the polymer/fullerene blends. By utilizing ultrafast optical measurements, we have clearly identified an ultrafast hole transfer process with a lifetime of about 3 ps mediated by photo-excited polaron pairs which has a markedly high quantum efficiency of about 97%. Spectroscopic data show that excitons act as spectators during the efficient hole transfer process. Our findings suggest an alternative route to improve the efficiency of all-polymer solar devices by manipulating polaron pairs.