Time-resolved Terahertz Spectroscopy Research Articles

Charge Generation Dynamics in Organic Photovoltaic Blends under One-Sun-Equivalent Illumination Detected by Highly Sensitive Terahertz Spectroscopy.

Organic photovoltaic (OPV) devices attain high performance with nonfullerene acceptors by utilizing the synergistic dual channels of charge generation that originate from excitations in both the donor and acceptor materials. However, the specific intermediate states that facilitate both channels are subject to debate. To address this issue, we employ time-resolved terahertz spectroscopy with improved sensitivity (ΔE/E < 10-6), enabling direct probing of charge generation dynamics in a prototypical PM6:Y6 bulk heterojunction system under one-sun-equivalent excitation density. Charge generation arising from donor excitations is characterized with a rise time of ∼9 ps, while that from acceptor excitations shows a rise time of ∼18 ps. Temperature-dependent measurements further reveal notably distinct activation energies for these two charge generation pathways. Additionally, the two channels of charge generation can be substantially manipulated by altering the ratio of bulk to interfaces. These findings strongly suggest the presence of two distinct intermediate states: interfacial and intramoiety excitations. These states are crucial in mediating the transfer of electrons and holes, driving charge generation within OPV devices.

Charge Transfer in Graphene-MoS2 Vertical Heterostructures Tuned by Stacking Order and Substrate-Introduced Electric Field.

Vertical van der Waals heterostructures composed of graphene (Gr) and transition metal dichalcogenides (TMDs) have created a fascinating platform for exploring optical and electronic properties in the two-dimensional limit. Numerous studies have focused on Gr/TMDs heterostructures to elucidate the underlying mechanisms of charge-energy transfer, quasiparticle formation, and relaxation following optical excitation. Nevertheless, a comprehensive understanding of interfacial charge separation and subsequent dynamics in graphene-based heterostructures remains elusive. Here, we have investigated the carrier dynamics of Gr-MoS2 heterostructures (including Gr/MoS2 and MoS2/Gr stacking sequences) grown on a fused silica substrate under varying photoexcitation energies by comprehensive ultrafast means, including time-resolved terahertz (THz) spectroscopy, THz emission spectroscopy, and transient absorption spectroscopy. Our findings highlight the impact of the substrate electric field on the efficiency of modulating the interfacial charge transfer (CT). Specifically, the optical excitation in Gr/MoS2 generates thermal electron injection from the graphene layer into the MoS2 layer with photon energy well below A-exciton of MoS2, whereas the interfacial CT in the MoS2/Gr is blocked by the electric field of the substrate. In turn, photoexcitation of the A exciton above leads to hole transfer from MoS2 to graphene, which occurs for both Gr-MoS2 heterostructures with opposite stacking orders, resulting in the opposite orientations of the interfacial photocurrent, as directly demonstrated by the out-of-phase THz emission. Moreover, we demonstrate that the recombination time of interfacial exciton is approximately ∼18 ps, whereas the defect-assisted interfacial recombination occurs on a time scale of ∼ns. This study provides valuable insights into the interplay between interfacial CT, substrate effects, and defect engineering in Gr-TMDs heterostructures, thereby facilitating the development of next-generation optoelectronic devices.

Tailoring Ultrafast Near-Band Gap Photoconductive Response in GeS by Zero-Valent Cu Intercalation.

Zero-valent intercalation of atomic metals into the van der Waals gap of layered materials can be used to tune their electronic, optical, thermal, and mechanical properties. Here, we report the impact of intercalating ∼3 atm percent of zero-valent copper into germanium sulfide (GeS). Advanced many-body calculations predict that copper introduces quasi-localized intermediate band states, and time-resolved THz spectroscopy studies demonstrate that those states have prominent effects on the photoconductivity of GeS. Cu-intercalated GeS exhibits a faster rise of transient photoconductivity and a shorter lifetime of optically injected carriers following near-gap excitation with 800 nm pulses. At the same time, Cu intercalation improves free carrier mobility from 1100 to 1300 cm2 V-1 s-1, which we attribute to the damping of acoustic phonons observed in Brillouin scattering and consequent reduction of phonon scattering.

Exciton formation dynamics at the SiO2/Si interface

Excitons can operate as carriers for energy transduction in optoelectronics, and engineering their dynamics is of great interest. Here, we employ time-resolved terahertz spectroscopy to analyze exciton formation dynamics as a function of temperature for a (100) N-type silicon substrate passivated by native SiO2. By analyzing the frequency-resolved complex conductivity as a function of temperature we resolve the photophysics for the formation of free carriers into excitons. Notably, we observe a relatively long-lived ~300 ps transient population of free carriers at temperatures well below the Mott transition (4 K). We rationalize this by transient photophysics at the SiO2/N-type-Si surface, where holes localize and release under high injection conditions due to a transient surface photovoltage effect. We believe our results have implications for the design of excitonic-based electronic applications operating at cryogenic temperatures and accessed optically.

Dominant Role of Hole Transport Pathway in Achieving Record High Photoconductivity in Two-Dimensional Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) with mobile charges have attracted significant attention due to their potential applications in photoelectric devices, chemical resistance sensors, and catalysis. However, fundamental understanding of the charge transport pathway within the framework and the key properties that determine the performance of conductive MOFs in photoelectric devices remain underexplored. Herein, we report the mechanisms of photoinduced charge transport and electron dynamics in the conductive 2D M-HHTP (M=Cu, Zn or Cu/Zn mixed; HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) MOFs and their correlation with photoconductivity using the combination of time-resolved terahertz spectroscopy, optical transient absorption spectroscopy, X-ray transient absorption spectroscopy, and density functional theory (DFT) calculations. We identify the through-space hole transport mechanism through the interlayer sheet π-π interaction, where photoinduced hole state resides in HHTP ligand and electronic state is localized at the metal center. Moreover, the photoconductivity of the Cu-HHTP MOF is found to be 65.5 S m-1 , which represents the record high photoconductivity for porous MOF materials based on catecholate ligands.

(Keynote) Recent Insights on Carrier Generation, Transport, and Extraction in Metal Oxide Photoelectrodes

After the absorption of light, the next steps in any artificial photosynthetic system are the generation, transport, and extraction of photoexcited charge carriers. These steps are well understood in solid state systems based on traditional semiconductors, such as Si and III-V absorbers, resulting in quantum efficiencies close to one for optimized devices. For metal oxide-based absorbers, which have attracted much interest in the past decades because of their relatively good (photo)chemical stability and low cost, the reported efficiencies tend to be much lower. This is often attributed to recombination at defects. While these may indeed adversely affect the efficiency, there are several other fundamental loss mechanisms in metal oxides that are not always fully appreciated. For example, certain metal ions exhibit localized d-d transitions that do not result in mobile charge carriers, resulting in a photogeneration yield < 1. Recent work showed that it is possible to quantify the fraction of mobile photoexcited carriers, from which an upper limit of the achievable photocurrent can be derived [1]. Losses can also occur in the form of recombination or carrier localization during transport. The chance of reaching the interface is often expressed as the carrier diffusion length, which in turn depends on the carrier lifetime and its mobility. Convenient contact-free methods to determine these parameters are time-resolved microwave conductivity (TRMC) and time-resolved THz spectroscopy (TRTS) [2]. Here, one of the challenges is to determine whether the decay in photoconductivity is due to a decay in carrier concentration (i.e., recombination), carrier mobility (trapping, polaron formation), or both. We recently developed a general analysis method for determining the diffusion length which is valid for time-dependent mobilities as well as time-dependent lifetimes [3]. Intringuingly, this method revealed a carrier diffusion length of only 15 nm for BiVO4, which is significantly shorter than previously reported values by us and others. The only way this short diffusion length can be reconciled with the high photocurrent densities reported for this material is by assuming field-assisted charge separation. The role of the electric field is, however, poorly understood in the photoelectrochemistry and photocatalysis communities [4]. In a system without externally applied bias, the electric field does not separate the charges; instead, charge separation is driven by the presence of selective contacts. During my presentation I will review these concepts and discuss how a better understanding of loss mechanisms, the role of the electric field, and selective contacts may help us design more efficient metal oxide-based photoelectrodes.

Bulky cation hinders undesired secondary phases in FAPbI3 perovskite solar cells

Interfacial treatments with bulky cations over 3D lead halide perovskites (LHPs) have enabled power conversion efficiencies beyond 25 %. However, the mechanisms behind these improvements still need to be better understood. In this work, we focus on a specific type of perovskite, formamidinium lead iodide (FAPI), without additives, and demonstrate that phenethylammonium iodide (PEAI) can prevent the transformation of the FAPI perovskite into undesirable hexagonal non-perovskite phases when exposed to air. We use various analytical techniques to understand how the organic salt interacts with the 3D perovskite absorber. Through grazing incidence wide-angle X-ray scattering analysis, we study the structure at the surface and bulk, observing that hexagonal FAPI phases dominate the phase composition in FAPI films without surface treatments. Time-resolved photoluminescence (TRPL) and terahertz (THz) spectroscopy show an enhancement in the carrier lifetime and photoconductivity, respectively, when the FAPI films are coated with PEAI. In solar cells, introducing PEAI enables power conversion efficiencies of up to 20.2 % and open circuit voltages (VOC) of up to 1.14 V, which are among the highest reported values for a FAPI solar cells without additional A-site cations.

Regulating Terahertz Photoconductivity in Two-Dimensional Materials

Two-dimensional materials represented by graphene have attracted extensive interest owing to the unique layer-dependent physical properties that are tunable with various external fields. In addition, by stacking two or more 2D materials together, a new material with the desired properties can be tailored and designed. Fully understanding the dynamical photoconductive response in 2D materials is uttermost important to design and develop the advanced optoelectronic devices. Terahertz (THz) time-domain spectroscopy (TDS) and time-resolved THz spectroscopy are powerful spectroscopic tools with the advantages of being contact-free and noninvasive, which have been widely used to study the photoconductivity (PC) of 2D materials. In this review, firstly, we provide a short introduction of the 2D materials and THz spectroscopy, and then a brief introduction of the experimental setup and experimental data analysis based on time-resolved THz spectroscopy are presented. After that, we overview the latest progress on the regulation of the THz PC that includes: (1) regulating the THz PC of graphene (Gr) and transition metal dichalcogenide (TMD) thin films with oxygen adsorption; (2) regulating the THz PC of Gr and Gr/TMDs heterostructures by electric gating and a built-in field introduced by a substrate; (3) regulating the THz PC of Gr/TMD heterostructures via optical gating; and (4) we overview the latest progress on the observation of elementary excitations in 2D materials with THz PC spectra following optical excitation and THz PC regulation via the photoexcitation of quasi-particles. Finally, we conclude the review and present a short overview of future research directions.

Optical Pump Terahertz Probe (OPTP) and Time Resolved Terahertz Spectroscopy (TRTS) of emerging solar materials

Photoconductivity is the crucial benchmark to assess the potential of any emerging material for future solar applications. Many optical techniques, like transient absorption and photoluminescence, explore bound electron states and provide indirect access to photoconductivity. Direct current (DC) measurements under solar simulation determine the total performance of a novel solar device. While this technique has a clear appeal, it involves electrical contacts, causing contact resistance, which impacts the measured conductivity. Furthermore, DC measurements do not provide any insight into ultrafast effects and the photophysics defining a novel material. Terahertz (THz) spectroscopy presents a contact-free technique to measure photoconductivity on a sub-ps time scale. These measurements can be performed on as-synthesized sample materials, including powders. The ultrafast time resolution informs us of trapping dynamics and reveals what physical processes limit the carrier lifetime in a novel material. Additionally, complex conductivity can be measured at THz frequencies. THz-conductivity and photoconductivity shed light on scattering effects, providing a road map toward minimizing these effects. However, THz spectroscopy is less intuitive than widely used DC measurements, and the interpretation of THz-results is more challenging. This tutorial aims to familiarize the reader with the main THz techniques used to explore emerging materials. We will illustrate how carrier lifetimes can be extracted from optical pump THz probe measurements. We will guide the reader through the process of extracting accurate photoconductivities from time resolved THz spectroscopy measurements and present the most commonly used models to describe the underlying physics. We will then discuss the difference between sample and material parameters and highlight potential pitfalls. The tutorial concludes with a perspective view on the ever evolving field of optical pump-THz probe spectroscopy of emerging materials.

Ultrafast activation of the double-exchange interaction in antiferromagnetic manganites

In doped manganite systems, strong electronic correlations result in rich phase diagrams where electron delocalization strongly affects the magnetic order. Here, we employ a femtosecond all-optical pump-probe scheme to impulsively photodope the antiferromagnetic parent manganite system CaMnO3 and unveil the formation dynamics of a long-range ferromagnetic state. We resonantly target intense charge transfer electronic transitions in CaMnO3 to photodope the system and probe the subsequent dynamics of both charges and spins using a unique combination of time-resolved terahertz spectroscopy and time-resolved magneto-optical Faraday measurements. We demonstrate that photodoping promotes a long-lived population of delocalized electrons and induces a net magnetization, effectively promoting ferromagnetism resulting from light-induced carrier-mediated short-range double-exchange interactions. The picosecond set time of the magnetization, much longer than the electron timescale, and the presence of an excitation threshold are consistent with the formation of ferromagnetic patches in an antiferromagnetic background.

Observation of an Exciton-Plasma Transition in a Molecular Semiconductor.

The nonfullerene electron acceptors (NFAs) for organic solar cells are attracting intense research efforts due to their impressive performance. Understanding the temporal evolution of the excited states in NFAs is essential to gain insights into the working mechanism of these state-of-the-art devices. Here we characterized the photoconductivities of a neat Y6 film and a Y6:PM6 blend film using time-resolved terahertz spectroscopy. Three different types of excited states were identified based on their distinct terahertz responses, i.e., plasma-like carriers, weakly bound excitons, and spatially separated carriers. Under high-intensity excitation, the many-body interaction of excitons in the Y6 film leads to the plasma-like state, giving rise to a terahertz response characteristic for a dispersive charge transport. This transient state decays quickly into exciton gas due to fast Auger annihilation. Under low-intensity excitation, only isolated excitons are created and the plasma state is absent.

Ultrafast Long-Distance Electron-Hole Plasma Expansion in GaAs Mediated by Stimulated Emission and Reabsorption of Photons.

Electron-hole plasma expansion with velocities exceeding c/50 and lasting over 10ps at 300K was evidenced by time-resolved terahertz spectroscopy. This regime, in which the carriers are driven over >30 μm is governed by stimulated emission due to low-energy electron-hole pair recombination and reabsorption of the emitted photons outside the plasma volume. At low temperatures a speed of c/10 was observed in the regime where the excitation pulse spectrally overlaps with emitted photons, leading to strong coherent light-matter interaction and optical soliton propagation effects.

Transient gap generation in BaFe2As2 driven by coherent lattice vibrations.

Iron-based superconductors provide a rich platform to investigate the interplay between unconventional superconductivity, nematicity, and magnetism. The electronic structure and the magnetic properties of iron-based superconductors are highly sensitive to the pnictogen height. Coherent excitation of the phonon by femtosecond laser directly modulates the pnictogen height, which has been used to control the physical properties of iron-based superconductors. Previous studies show that the driven phonon resulted in a transient increase of the pnictogen height in BaFeAs, favoring an enhanced Fe magnetic moment. However, there are no direct observations on either the enhanced Fe magnetic moments or the enhanced spin-density wave (SDW) gap. Here, we use time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFeAs in the phonon-driven state. Below the SDW transition temperature, we observe a transient gap generation at early-time delays. A similar transient feature is observed in the normal state up to room temperature.

Free Electron Dynamics and Intrinsic Mobility in Ga2O3 Revealed by Transient Terahertz Conductivity

Here, we investigate the photoconductivity of gallium oxide thin films at different temperatures using time-resolved terahertz spectroscopy. The photogenerated electrons in the conduction band show a monoexponential decay, implying a first-order electron depopulation mechanism. The electron lifetime increases with rising temperature, and this trend coincides with the temperature dependence of the electron mobility rather than diffusion coefficient, suggesting that electron-hole recombination is determined by directional electron drift instead of random diffusion. The electron mobilities extracted from the transient terahertz conductivity are substantially greater than the previously reported Hall mobilities over a wide temperature range, and this is probably because the electron drift in response to the terahertz field is immune from scattering with macroscopic defects. Thus, the mobilities measured here may represent the intrinsic limit of the electron mobility in gallium oxide crystals. Our finding suggests that the current Hall mobility in this wide bandgap semiconductor is still far below the limit, and the long-range electron transport can be further increased by improving the crystalline quality.

Observation of negative terahertz photoconductivity in strongly correlated electron-doped CaMnO3 thin film

Electron-doped Ca0.96Ce0.04MnO3 (CCMO) possesses a unique band structure and exhibits a giant topological Hall effect contrary to other correlation-driven manganites known for insulator-to-metal transition, magnetoresistance, complex magnetic order, etc. The interaction mechanisms among the fundamental entities and their dynamical evolutions responsible for this unusual topological phase are yet to be understood. Here, we employ time-averaged and sub-picosecond time-resolved terahertz (THz) spectroscopy to explore the low-energy steady-state and ultrafast carrier dynamics, respectively, to unravel the complexity of charge carriers during their transition from a non-equilibrium state to the ground state in CCMO thin film. The THz optical conductivity confirms the presence of dichotomic charge carriers, i.e. heavy and light carriers throughout the temperature range of 15–300 K. A rare observation of both positive and negative photoconductivities along with a sharp crossover between the two resolved to a few picoseconds of illumination confirms the formation of polaron with a lifetime of a few nanoseconds. These optical evidences of dichotomic charge carriers, along with manipulation of the sign of photoconductivity induced by dynamics of related quasiparticles could facilitate a new mechanism for ultrafast optoelectronic switching devices.

Ultrafast carrier dynamics in GeSn thin film based on time-resolved terahertz spectroscopy

We measure the time-resolved terahertz spectroscopy of GeSn thin film and studied the ultrafast dynamics of its photo-generated carriers. The experimental results show that there are photo-generated carriers in GeSn under femtosecond laser excitation at 2500 nm, and its pump-induced photoconductivity can be explained by the Drude–Smith model. The carrier recombination process is mainly dominated by defect-assisted Auger processes and defect capture. The first- and second-order recombination rates are obtained by the rate equation fitting, which are (2.6 ± 1.1) × 10−2 ps−1 and (6.6 ± 1.8) × 10−19 cm3⋅ps−1, respectively. Meanwhile, we also obtain the diffusion length of photo-generated carriers in GeSn, which is about 0.4 μm, and it changes with the pump delay time. These results are important for the GeSn-based infrared optoelectronic devices, and demonstrate that GeSn materials can be applied to high-speed optoelectronic detectors and other applications.

Conjugated Nonplanar Copper-Catecholate Conductive Metal–Organic Frameworks via Contorted Hexabenzocoronene Ligands for Electrical Conduction

Conductive metal-organic frameworks (c-MOFs) with outstanding electrical conductivities and high charge carrier mobilities are promising candidates for electronics and optoelectronics. However, the poor solubility of planar ligands greatly hinders the synthesis and widespread applications of c-MOFs. Nonplanar ligands with excellent solubility in organic solvents are ideal alternatives to construct c-MOFs. Herein, contorted hexabenzocoronene (c-HBC) derivatives with good solubility are adopted to synthesize c-MOFs. Three c-MOFs (c-HBC-6O-Cu, c-HBC-8O-Cu, and c-HBC-12O-Cu) with substantially different geometries and packing modes have been synthesized using three multitopic catechol-based c-HBC ligands with different symmetries and coordination numbers, respectively. With more metal coordination centers and increased charge transport pathways, c-HBC-12O-Cu exhibits the highest intrinsic electrical conductivity of 3.31 S m-1. Time-resolved terahertz spectroscopy reveals high charge carrier mobilities in c-HBC-based c-MOFs, ranging from 38 to 64 cm2 V-1 s-1. This work provides a systematic and modular approach to fine-tune the structure and enrich the c-MOF family with excellent charge transport properties using nonplanar and highly soluble ligands.

Probing photocarrier dynamics of pressurized graphene using time-resolved terahertz spectroscopy

Graphene hosts intriguing photocarrier dynamics such as negative transient terahertz (THz) photoconductivity, high electron temperature, benefiting from the unique linear Dirac dispersion. In this work, the pressure effects of photocarrier dynamics of graphene have been investigated using in situ time-resolved THz spectroscopy in combination with diamond anvil cell exceeding 9 GPa. We find that the negative THz conductivity maintains in our studied pressure range both for monolayer and bilayer graphene. In particular, the amplitude of THz photoconductivity in monolayer graphene manifests an extraordinary dropping with pressure, compared with that from the counterparts such as bulk silicon and bilayer graphene. Concomitantly, the time constant is reduced with increasing pressure, highlighting the pressure-induced hot carrier cooling. The pressure dependence of photocarrier dynamics in monolayer graphene is likely related with the enhancement of the interfacial coupling between diamond surface and sample, allowing for the activity of new electron–phonon scattering. Our work is expected to provide an impetus for the studies of high-pressure THz spectroscopy of two-dimensional materials.

Contactless determination and parametrization of charge-carrier mobility in silicon as a function of injection level and temperature using time-resolved terahertz spectroscopy

Here, we analyze in a non-contact fashion charge carrier mobility as a function of injection level and temperature in silicon by time resolved THz spectroscopy (TRTS) and parametrize our data by the classical semi-empirical models of Klaassen and Dorkel & Leturcq. Our experimental results are in very good agreement with the pioneering works of Krausse and D\"annhauser analyzing this phenomena by employing contact-based methods. This agreement, that validates our methodology, can only be achieved by considering charge carrier diffusion effects following above bandgap near-surface pump photo-excitation of the sample. From our results, obtained over a large range of injection levels, we conclude that the model of Klaassen is the best on describing the collected data at room temperature. Furthermore, we analyze by TRTS the dependence of charge carrier mobility with temperature for a fixed injection level. Once more, the parametrization made by the classical model of Klaassen describe our data appropriately even without the necessity of applying any fitting parameters (just with the charge carrier density as an input). In this respect, our work supports the validity of the model and parametrization proposed by Klaassen, and also illustrate how TRTS can be reliably employed for the quantitative determination of mobility in semiconductors as a function of key parameters as injection level and temperature.

Thickness-dependent Auger scattering in a single WS2 microcrystal probed with time-resolved terahertz near-field microscopy.

Time-resolved terahertz (THz) spectroscopy has been shown as a powerful technique to non-invasively determine the charge carrier properties in photoexcited semiconductors. However, the long wavelengths of terahertz radiation reduce the applicability of this technique to large samples. Using THz near-field microscopy, we show THz measurements of the lifetime of 2D single exfoliated microcrystals of transition metal dichalcogenides (WS2). The increased spatial resolution of THz near-field microscopy allows spatial mapping of the evolution of the carrier lifetime, revealing Auger assisted surface defect recombination as the dominant recombination channel. THz near-field microscopy allows for the non-invasive and high-resolution investigation of material properties of 2D semiconductors relevant for nanoelectronic and optoelectronic applications.