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Tremendous research efforts are developed to suppress the reverse dark current (Jd) and enhance the responsivity of organic photodiodes (OPDs). The functional layers of traditional OPDs usually follow the principle of energy level alignment to make unobstructed photo‐carriers transport under reverse bias, but this inevitably leads to a large forward Jd. Herein, a universal strategy is proposed to manipulate the carrier dynamics and effectively suppress the forward Jd of OPDs, that is, tuning the energy level and electron traps of the anode interface layers (AILs). The bandgap and electron traps of typical organometallic chelate AIL (PEIE‐Co) can be well controlled by adjusting the component ratio of PEIE and metal ions. The wide bandgap increases the carrier injection barrier under reverse and forward bias, endowing OPD with a much lower Jd; the electron traps induce hole tunneling injection by capturing photo‐generated electrons under forward bias, thereby enabling the photomultiplication effect. The obtained OPD exhibits photoconductive/photomultiplication working mode at reverse/forward bias and the specific detectivity approaches ≈1013/1012 Jones, showing promise for adaptively detecting faint and strong light. This study presents an intelligent strategy to achieve dual‐mode OPDs, paving the way for the multifunctional development of photodetectors. [ABSTRACT FROM AUTHOR]

Hydrogen fuel produced from water splitting using sunlight remains one of the cleanest and most sustainable energy sources. However, photoelectrochemical hydrogen production faces several challenges, including poor sunlight absorption, deficient charge carrier lifetime and transfer rates, and very sluggish kinetics of the water oxidation half‐reaction. To address these challenges, researchers have concentrated on enhancing the efficiency of photoelectrochemical water splitting. A critical factor in this enhancement is a deeper understanding of the fundamental processes involved in photoabsorption and the subsequent oxidation evolution reaction. The advent of ultrafast lasers has enabled the detailed tracking of charge carriers within materials after sunlight absorption, including their separation and migration to the surface for water oxidation. Ultrafast transient absorption spectroscopy is a powerful technique that allows real‐time observation of the behavior of excited states and charge carriers on femtosecond to nanosecond timescales. Recent studies utilizing ultrafast transient absorption spectroscopy are explored to investigate the dynamics of charge carriers in photoelectrochemical water splitting, providing insights into the mechanisms that lead to the design of enhanced photoanodes with improved efficiency. [ABSTRACT FROM AUTHOR]

Due to the extraordinary chemical and physical properties, ternary chalcogenides with wide‐range bandgap, unique structures, and exceptional photoelectric properties have attracted great interest recently. Among them, Chromium thiophosphate (CrPS4) is a promising semiconductor with excellent optical, mechanical, and electronic properties and has a lower lattice symmetry resulting in significant in‐plane anisotropy. Here, the polarization‐dependent nonlinear optical (NLO) responses of CrPS4 flake are investigated. Based on polarization‐resolved Raman spectroscopy, which identifies the crystal axis of sample, the anisotropic linear absorption spectra, I‐scan technique, and pump‐probe measurements are utilized to investigate the optical absorption information of CrPS4. The polarization‐dependent NLO responses are found originating from the anisotropic optical transition probability. And the polar optical phonons play an important role in carrier relaxation processes. Furthermore, CrPS4‐based all‐optical switches are proposed, which can modulate the transmittance of signal light (CW laser) by changing the polarization of switch light (fs laser). This work provides insights into polarization‐dependent CrPS4‐based devices, which is important to promote the potential applications in optical modulator, versatile optoelectronic detection, and so on. [ABSTRACT FROM AUTHOR]

Water motion‐induced energy harvesting has emerged as a prominent means of facilitating renewable electricity from the interaction between nanostructured materials and water over the past decade. Despite the growing interest, comprehension of the intricate solid–liquid interfacial phenomena related to solid state physics remains elusive and serves as a hindrance to enhancing energy harvesting efficiency up to the practical level. Herein, the study introduces the energy harvester by utilizing inversion on the majority charge carrier in graphene materials upon interaction with water molecules. Specifically, various metal electrode configurations are employed on reduced graphene oxide (rGO) to unravel its distinctive charge carriers that experience the inversion in semiconductor type upon water contact, and exploit this characteristic to leverage the efficacy of generated electricity. Through the strategic arrangement of the metal electrodes on rGO membrane, the open‐circuit voltage (Voc) and short‐circuit current (Isc) have exhibited a remarkable augmentation, reaching 1.05 V and 31.6 µA, respectively. The demonstration of effectively tailoring carrier dynamics via electrode configuration expands the practicality by achieving high power density and elucidating how the water‐induced carrier density modulation occurs in 2D nanomaterials. [ABSTRACT FROM AUTHOR]

The current efficiency records for generating green hydrogen via solar water splitting are held by indium phosphide (InP)‐based photo‐absorbers, protected by TiO2 layers grown through atomic layer deposition (ALD). InP is also a leading material for photonic integrated circuits and computing, where ultrafast near‐surface behavior is key. A previous study described electronic pathways at the phosphorus‐rich (P‐rich) surface of p‐doped InP(100) using time‐resolved two‐photon photoemission (tr‐2PPE) spectroscopy. Here, the intricate electron pathways of the P‐rich InP surface modified with ALD‐deposited TiO2 are explored. Photoexcited bulk InP electrons migrate through a bulk‐to‐surface transition cluster of states and surface states and inject into the TiO2 conduction band (CB). Energy levels and occupation dynamics of CB states in P‐rich InP and TiO2 adlayers are observed, with discrete states preserved up to 10 nm TiO2 deposition. Thermalization lifetimes of excited electrons > 0.8 eV above the InP conduction band minimum (CBM) are preserved for layer thicknesses up to 2.5 nm. Annealing at 300 °C to achieve crystalline TiO2 reconstructions destroys interfacial states, affecting charge transfer. These observations enable innovative engineering of the P‐rich InP/TiO2 heterointerface, opening new possibilities for studying hot‐carrier extraction, adsorbate effects, surface plasmons, and improving photovoltaic and PEC water‐splitting devices. [ABSTRACT FROM AUTHOR]

Bias‐switchable dual‐mode organic photodiodes (OPDs) that integrate photovoltaic and photomultiplication modes are recently developed and shown prospects in complex light‐intensity applications. Yet, the device physics that focuses on carrier dynamics is still a challenge and needs to be further explored. Herein, dual‐mode OPDs are developed through interface layer manipulation, that is, introducing cathode interface layers (typically, ZnxO:D149) with deep energy levels and abundant bulk defects and an anode interface layer of thermally‐evaporated ZnO (e‐ZnO) with a wide bandgap. Under reverse bias, ZnxO:D149 forms a barrier wall to effectively block external holes and maintain the photovoltaic mode of the OPDs. Under forward bias, the capturing effect of ZnxO:D149 and blocking effect of e‐ZnO help to reduce the dark current; when under illumination, defect traps capture photo‐generated holes, eliminating the barrier traps and promoting unobstructed injection of external carriers to achieve photomultiplication effect. The typical device delivers high specific detectivity (>1012 Jones) and fast response (<40 µs), and exhibits disparate external quantum efficiency in two operating modes, showing promise for simultaneously detecting faint and strong light. This general strategy for preparing dual‐mode OPDs is compatible with CMOS processing technology and meets the miniaturization and integration requirements of next‐generation detection systems. [ABSTRACT FROM AUTHOR]

The quality of organic thin films critically influences carrier dynamics in organic semiconductors. In neat/doped films, even tiny voids can be penetrated by water or oxygen molecules to create charge‐trap states called water/oxygen‐induced traps that significantly hinder carrier mobility. While the water/oxygen‐induced traps in non‐doped films and crystalline states have been investigated comprehensively, there is a lack of thorough examination regarding their properties in the doped state. Therefore, there is a high demand for a molecular design strategy that effectively modulates the molecular stacking behavior in doped films and practical devices and enhances the quality of these films. Herein, we proposed a versatile molecular design principle that enables the formation of “nano‐cluster” structures on both the surface and interior of doped films in target molecule 10‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐1′‐(4‐fluorophenyl)‐10H‐spiro[acridine‐9,9′‐xanthene] (DspiroO‐F‐TRZ), which is modified with a fluorophenyl group. These “nano‐cluster” structures exhibit more uniform shapes within doped films and effectively reduce defective state densities while enhancing carrier injection and transport properties, ultimately improving device performance. Notably, TADF‐OLED based on DspiroO‐F‐TRZ demonstrates nearly twice as much efficiency as its control counterpart due to contributions from ′nano‐cluster′ structure enhancements toward improved electroluminescence performance. [ABSTRACT FROM AUTHOR]

Strain engineering, as a non‐chemical tuning knob, can enhance the performance of semiconductor devices. Here, an efficient manipulation of light emission is revealed in thin‐layered 2D perovskite strongly correlated to layer numbers of [PbI6]4− octahedron (n) and [C6H5(CH2)2NH3]2(CH3NH3)n‐1PbnI3n+1 (N) by applying uniaxial strains (ɛ) via bending the flexible substrate. As <n> increases from 1 to 3, an efficient light emission redshift (ɛ from −0.97% to 0.97%) is observed from bandgap shrinkage, and the shrinkage rate increases from 1.97 to 10.38 meV/%, which is attributed to the predominant uniaxial intralayer deformation due to the anisotropy of the [PbI6]4− octahedron lattice strain. Conversely, as <N> increases from 7 to 48 for n = 3, the deformation related to bandgap shrinkage rate is more prominent in small‐N flakes (<N> ≈ 7, 15.2 meV/%) but is easily offset in large‐N flakes (<N> ≈ 48, 7.7 meV/%). This anisotropic lattice deformation, meanwhile, inevitably modulates the carrier recombination dynamics of [C6H5(CH2)2NH3]2(CH3NH3)n‐1PbnI3n+1, which is essential for the development of highly efficient photoelectronic devices. [ABSTRACT FROM AUTHOR]

Layered materials exhibit different electronic and phonon properties along in-plane and out-of-plane directions; existing studies focus on their in-plane behaviors, and the influence of such anisotropies on the dynamics of photocarriers and phonons is unknown. Here, we fabricate layered PdSe2 crystals with flat edge surfaces and compare the time-resolved ultrafast spectroscopies on their basal and edge surfaces. Pronounced differences in the transient reflection spectroscopies reveal the inconsistent photocarrier and phonon dynamics behaviors on the two surfaces: the slow hot carrier relaxation process is accelerated and the thermoelasticity-induced longitudinal coherent acoustic phonon oscillation completely vanishes on the edge surface, as compared with the basal surface. Theoretical analysis reveals that the inconsistent hot carrier dynamics originate from the anisotropic properties of low-energy phonons in PdSe2, and the absence of phonon oscillation on the edge surface results from the wavevector-limited sensitivity of acoustic B1u mode. Moreover, polarization-dependent spectroscopies indicate the diverse optical anisotropies beyond the in-plane of PdSe2. This work provides a new method to explore unique physical properties and modulate the optical anisotropy of layered materials. [ABSTRACT FROM AUTHOR]

Charge‐transfer (CT) cocrystals have attracted continuous interest for their promising optical and optoelectronic applications. To improve the performance of this class of material, a CT cocrystal with long‐lived CT excitons is highly desired. Herein, the development of a pyrene‐doped trans‐1,2‐diphenylethylene‐1,2,4,5‐tetracyanobenzene ternary cocrystal (named P‐TS‐TC) is reported. Compared to the undoped binary cocrystals (without pyrene), P‐TS‐TC exhibits a two times longer CT exciton lifetime (≈60.2 ns), and thus 8.8‐ and 16.6‐times improvement in photocurrent response and photocatalytic H2 evolution activity. By using transient photoluminescence spectroscopy, it is uncovered that the absorbed photon energy in P‐TS‐TC is localized to a lower energy CT state through an efficient energy transfer process (≈389.8 ps) between two co‐existing CT states. The CT exciton lifetime is prolonged due to the weakened CT coupling strength, as a result of the enlarged donor‐acceptor distance and the change of geometric structure. The result is expected to inspire the design of cocrystals with manipulatable CT exciton properties and to promote the potential application of multi‐component CT cocrystals. [ABSTRACT FROM AUTHOR]

As a new group of van der Waals (vdWs) ferroic materials, transition metal dioxydihalides MO2X2 (M: Mo, W; X: halogen) with a perovskite‐like structure are theoretically predicted to exhibit intriguing physics and versatile ferroic characteristics, which is not achieved experimentally as far as it is known. In this work, the robust ferroelasticity in vdWs WO2I2 with the switching strain as low as ≈0.3%, accompanied with the striped optical contrast between adjacent domains, spot splitting of selected area electron diffraction (SAED) patterns at domain wall, and 90° domain wall is demonstrated. With the aid of ab‐initio calculations, the origin of ferroelasticity in WO2I2 is unveiled, where the imaginary phonon mode in the high‐symmetry paraelastic phase leads to the spontaneous displacement of W atom away from the center of the [WO4I2] octahedron, resulting in the switchable spontaneous strain under an external strain field. Moreover, transient absorption microscopy (TAM) measurements demonstrate that the diffusion of photogenerated carriers is significantly hindered by the ferroelastic domain walls. This study provides deep insights into the ferroic order and domain wall in perovskite‐like vdWs MO2X2 for new physics and functionalities. [ABSTRACT FROM AUTHOR]

As a new group of van der Waals (vdWs) ferroic materials, transition metal dioxydihalides MO2X2 (M: Mo, W; X: halogen) with a perovskite‐like structure are theoretically predicted to exhibit intriguing physics and versatile ferroic characteristics, which is not achieved experimentally as far as it is known. In this work, the robust ferroelasticity in vdWs WO2I2 with the switching strain as low as ≈0.3%, accompanied with the striped optical contrast between adjacent domains, spot splitting of selected area electron diffraction (SAED) patterns at domain wall, and 90° domain wall is demonstrated. With the aid of ab‐initio calculations, the origin of ferroelasticity in WO2I2 is unveiled, where the imaginary phonon mode in the high‐symmetry paraelastic phase leads to the spontaneous displacement of W atom away from the center of the [WO4I2] octahedron, resulting in the switchable spontaneous strain under an external strain field. Moreover, transient absorption microscopy (TAM) measurements demonstrate that the diffusion of photogenerated carriers is significantly hindered by the ferroelastic domain walls. This study provides deep insights into the ferroic order and domain wall in perovskite‐like vdWs MO2X2 for new physics and functionalities. [ABSTRACT FROM AUTHOR]

With strong electron–phonon coupling, self‐trapped excitons (STEs) are typically formed in perovskite materials, and radiative recombination of STEs can produce broadband emission with large Stokes shifts. STEs are essential to further improve the optoelectronic properties of materials. Surprisingly, 2D system is the edge case, with low even no self‐trapping barriers, leading to effortless formation of STEs. In this work, 2D strontium titanate (SrTiO3) with defects is prepared using supercritical carbon dioxide (SC CO2) and its carrier transport and transition are studied. The appearance of wide photoinduced positive absorption signals in the femtosecond transient absorption spectra is direct evidence for the formation of STEs. The presence of STEs is further supported by the increased Stokes shift and full width at half maximum in the steady‐state photoluminescence spectra. [ABSTRACT FROM AUTHOR]

ZnTeSe quantum dots (QDs), recognized as promising eco-friendly blue electroluminescent emitters, remain under-explored in light-emitting diode (LED) applications. Here, to elucidate the operation and degradation mechanisms of ZnTeSe blue QD-LEDs, stacked ZnTeSe QD layers with discernable luminescence are designed by varying Te doping concentrations, and the recombination zones (RZs) of the blue QD-LEDs are investigated. The RZs are identified near the hole-transport layer (HTL), confirmed by angular-dependent electroluminescence measurements and optical simulations. In addition, in order to investigate carrier dynamics in the process of recombination, the transient electroluminescence (tr-EL) signals of the dichromatic QD-LEDs are analyzed. As a result, it is inferred that the RZ initially formed near the electron-transport layer (ETL) due to the high injection barriers of electrons. However, due to the fast electron mobility, the RZ shifts toward the HTL as the operating current increases. After the device lifetime tests, the RZ remains stationary while the photoluminescence (PL) corresponding to the RZ undergoes a substantial decrease, indicating that the degradation is accelerated by the concentrated RZ. Thus this study contributes to a deeper understanding of the operational mechanisms of ZnTeSe blue QD-LEDs. [ABSTRACT FROM AUTHOR]

In semiconductor‐based microlasers, the generated excited carriers can affect both the optical absorption coefficient and refractive index, influencing the lasing behavior and device performance. In this study, the lasing behavior of individual MAPbBr3 microrods pumped by femtosecond laser pulses is investigated using an ultrafast optical Kerr gating technique. By analyzing the spectral and temporal evolution of the lasing behavior, it is found that the band‐filling (BF) effect can cause a significant change in the refractive index of the material, which results in an unfavorable red‐shift and broadening of the resonant modes, deteriorating the laser linewidth and quality factor. The hot carrier cooling process can provide a buffer for alteration of the energy level occupation state, resulting in a small transient refractive index change and slight red‐shift. These results offer insights into the lasing behavior driven by photogenerated carrier dynamics and provide an optimization strategy for semiconductor‐based microlasers. [ABSTRACT FROM AUTHOR]

Abstract Water motion‐induced energy harvesting has emerged as a prominent means of facilitating renewable electricity from the interaction between nanostructured materials and water over the past decade. Despite the growing interest, comprehension of the intricate solid–liquid interfacial phenomena related to solid state physics remains elusive and serves as a hindrance to enhancing energy harvesting efficiency up to the practical level. Herein, the study introduces the energy harvester by utilizing inversion on the majority charge carrier in graphene materials upon interaction with water molecules. Specifically, various metal electrode configurations are employed on reduced graphene oxide (rGO) to unravel its distinctive charge carriers that experience the inversion in semiconductor type upon water contact, and exploit this characteristic to leverage the efficacy of generated electricity. Through the strategic arrangement of the metal electrodes on rGO membrane, the open‐circuit voltage (Voc) and short‐circuit current (Isc) have exhibited a remarkable augmentation, reaching 1.05 V and 31.6 µA, respectively. The demonstration of effectively tailoring carrier dynamics via electrode configuration expands the practicality by achieving high power density and elucidating how the water‐induced carrier density modulation occurs in 2D nanomaterials.

The performance and stability of perovskite solar cells (PSCs) are dominated by the electron and hole transport dynamics, which are highly desired to be characterized in the working devices. Nevertheless, for the opaque PSCs, the hole transport layer (HTL) in n–i–p devices or the electron transport layer (ETL) in p–i–n devices are typically buried beneath the metal electrode, which makes it difficult to simultaneously characterize the charge transport dynamics from both sides of the working devices. In this work, for the first time, the charge transport dynamics of the working devices from both the front and rear side of semitransparent PSCs (ST‐PSCs) is characterized via femtosecond transient reflection spectroscopy (FS‐TRS). A significant enhancement of the hole transport and negligible change of the electron transport is observed when the perovskite/HTL interface is treated by 2‐chloro‐phenethylammonium iodide (2‐Cl‐PEAI). A champion power conversion efficiency (PCE) of 23.3% (certified 22.3%) is achieved, which is the highest certified PCE for ST‐PSCs up to date. The ST‐PSCs maintained more than 90% of its initial efficiency after 2000 h of maximum power point tracking (MPPT). Additionally, the ST‐PSCs are implemented in 4‐terminal (4‐T) perovskite/passivated emitter rear silicon cell (PERC), reaching a simulated output power of 30.8 mW cm−2. [ABSTRACT FROM AUTHOR]

Monolayer two-dimensional transition metal dichalcogenide (TMD)-based materials have become one of the ideal platforms for the study of multibody interactions due to their rich excitonic complexes. The coupling between optical nanocavity and material has become an important means for manipulating the optical properties of materials, but there are few studies on the coupling of nanocavities and the multi-body effect in materials. In this study, we investigate the optical properties of silver nanodisk (Ag ND) arrays covering a monolayer WS2. In the experimental sample, we observed a ~114.3-fold photoluminescence enhancement of charged biexciton in the heterostructure region, as compared to the monolayer WS2 region, a value which is much higher than those for exciton (~2.2-fold) and trion (~16.4-fold), a finding which is attributed to the Fano resonant coupling between monolayer WS2 and the Ag ND. By means of time-resolved spectroscopy, we studied the carrier dynamics in the hybrid system. Our findings reveal that resonant coupling promotes the formation and radiation recombination processes of the charged biexciton, significantly reducing the radiative recombination lifetime by ~15-fold, which is much higher than the measurement in exciton (~2-fold). Our results provide an opportunity to understand the multibody physics of coupling with nanocavities, which could facilitate the application of multi-body excitons in the fields of light-emitting devices and lasers, etc. [ABSTRACT FROM AUTHOR]

In this study, the electronic band structure and the dynamics of the excited carriers in the formamidinium lead bromide (FAPbBr3) perovskite are investigated by combining the information obtained from steady‐state absorption, photoluminescence, femtosecond transient absorption spectroscopy, photoelectron spectroscopy, and density functional theory calculations. A detailed description of the electronic transitions in the UV–vis energy range has been provided, giving an estimation of the exciton binding energy (40 ± 5 meV), the transition energy from the first valence band (VB1) to the conduction band (CB1) (electronic bandgap at 2.37 ± 0.01 eV) and assigning the broad peak at ≈3.4 eV to the transition from the second innermost valence band (VB2) to CB1. The temporal dynamics of the excited carriers involved in these transitions are investigated using different excitation energies and carrier densities. Different trends are observed in the dynamics of the transient signals associated with the VB1→CB1 (PB1) and VB2→CB1 (PB2) transitions. As the carrier density increased, PB1 exhibited a slowing down of its rise time, while PB2 showed an acceleration attributed to the thermalization of the excited holes in VB2. These valuable findings have the potential to unlock new strategies aimed at maximizing the efficiencies and performance of perovskite‐based solar cell devices. [ABSTRACT FROM AUTHOR]

CsPbI3 quantum dots (QDs) hold tremendous promise for quantum emitters, but they undergo a considerable energy loss when excited above their optical bandgap, which impedes the utilization of high‐energy photons. Different surface modification strategies have been proposed to improve the phase stability of CsPbI3 QDs, however, little progress has been made to realize high photoluminescence quantum yield (PLQY) with high‐energy photon excitation. Here, a non‐resonant barrier excitation (NRBE) mechanism in conjugated aromatic tetraphenylporphyrin (H2TPP)‐modified CsPbI3 QDs is presented, which enables a high PLQY in the high‐energy excitation regime as well as enhanced phase stability. Particularly, the proposed H2TPP ligand possesses adequate energy depth needed to realize NRBE in CsPbI3 QDs, which allows efficient charge injection from organic ligands to the inorganic core. As a result, the H2TPP‐modified CsPbI3 QDs exhibit enhanced light absorption, large Stokes shift, and near‐unity red emission when excited above the optical bandgap. The findings provide new insights into the ligand design strategies for improving optoelectronic properties. [ABSTRACT FROM AUTHOR]

With strong electron–phonon coupling, self‐trapped excitons (STEs) are typically formed in perovskite materials, and radiative recombination of STEs can produce broadband emission with large Stokes shifts. STEs are essential to further improve the optoelectronic properties of materials. Surprisingly, 2D system is the edge case, with low even no self‐trapping barriers, leading to effortless formation of STEs. In this work, 2D strontium titanate (SrTiO3) with defects is prepared using supercritical carbon dioxide (SC CO2) and its carrier transport and transition are studied. The appearance of wide photoinduced positive absorption signals in the femtosecond transient absorption spectra is direct evidence for the formation of STEs. The presence of STEs is further supported by the increased Stokes shift and full width at half maximum in the steady‐state photoluminescence spectra.

InP/ZnS core/shell quantum dots (QDs) have been widely applied in many fields due to their excellent luminescence performance. In this study, the correlation between the ZnS shell thickness and the ultrafast carrier dynamics in InP/ZnS QDs was investigated by using femtosecond time-resolved transient absorption spectroscopy and time-resolved photoluminescence techniques. We found that a thicker ZnS shell could confine more carriers to the InP core, thus effectively weakening the interactions between the carriers in the core and the interfacial defect states; this further enhanced the quantum yields and photoluminescence properties in InP/ZnS QDs.

Two-dimensional (2D) layered materials exhibit strong light-matter interactions, remarkable excitonic effects, and ultrafast optical response, making them promising for high-speed on-chip nanophotonics. Recently, significant attention has been directed towards anisotropic 2D materials (A2DMs) with low in-plane crystal symmetry. These materials present unique optical properties dependent on polarization and direction, offering additional degrees of freedom absent in conventional isotropic 2D materials. In this review, we discuss recent progress in understanding the fundamental aspects and ultrafast nanophotonic applications of A2DMs. We cover structural characteristics and anisotropic linear/nonlinear optical properties of A2DMs, including well-studied black phosphorus and rhenium dichalcogenides, as well as emerging quasi-one-dimensional materials. Then, we discuss fundamental ultrafast anisotropic phenomena occurring in A2DMs, such as polarization-dependent ultrafast dynamics of charge carriers and excitons, their direction-dependent spatiotemporal diffusion, photo-induced symmetry switching, and anisotropic coherent acoustic phonons. Furthermore, we review state-of-the-art ultrafast nanophotonic applications based on A2DMs, including polarization-driven active all-optical modulations and ultrafast pulse generations. This review concludes by offering perspectives on the challenges and future prospects of A2DMs in ultrafast nanophotonics.

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) can be improved through the concurrent strategies of enhancing charge transfer and passivating defects. Graphite carbon nitride (g-C3N4) has been demonstrated as a promising modifier for optimizing energy level alignment and reducing defect density in PSCs. However, its preparation process can be complicated. A simple one-step calcination approach was used in this study to prepare g-C3N4-modified TiO2 via the incorporation of urea into the TiO2 precursor. This modification simultaneously tunes the energy level alignment and passivates interface defects. The comprehensive research confirms that the addition of moderate amounts of g-C3N4 to TiO2 results in an ideal alignment of energy levels with perovskite, thereby enhancing the ability to separate and transfer charges. Additionally, the g-C3N4-modified perovskite films exhibit an increase in grain size and crystallinity, which reduces intrinsic defects density and extends charge recombination time. Therefore, the g-C3N4-modified PSC achieves a champion PCE of 20.00%, higher than that of the control PSC (17.15%). Our study provides a systematic comprehension of the interfacial engineering strategy and offers new insights into the development of high-performance PSCs. [ABSTRACT FROM AUTHOR]

In the past 20 years, perovskite-related research has attracted wide attention. The related research into two-dimensional/quasi-two-dimensional perovskite has propelled the research of perovskite materials to a new height. To improve the properties of quasi-2D perovskite, improve the stability of materials, and achieve specific functions, using different types, volumes, and lengths of organic spacers is an essential method. In this paper, quasi-2D perovskites with EDA (ethylene diammonium), PDA (1,3-propanediammonium), and BDA (1,4-butanediammonium) (m = 2–4) as organic spacers were prepared, and the effects of different organic spacers on the 2D perovskite were investigated. The results show that the length of the organic spacer significantly impacts the perovskite's properties. A shorter organic spacer can effectively reduce the quantum confinement and dielectric confinement in perovskite. It should be noted that if the organic spacer is too short, the stability of the quasi-2D perovskite will be greatly reduced. [ABSTRACT FROM AUTHOR]

Low-dimensional perovskite (PVK) materials have attracted significant research interest, because of their quantum-confined effect, tunable band gap structures, and higher stability than that of three-dimensional (3D) PVKs. In semiconductor optoelectronic devices, high speed and small size are closely interlinked. The development of high-speed devices requires researchers to fully understand the properties of materials, especially the dynamic processes such as carrier recombination, separation, and transport, which often play a crucial role in the performance of devices. As an indispensable part of dynamic research, spin relaxation is also of great significance in studying the properties of materials and explore possible applications. Lead halide PVK materials have strong spin-orbit coupling (SOC), which provides a basis for information storage and processing by using spin degrees of freedom. Therefore, studying the carrier and spin dynamics of low-dimensional PVKs is an effective way to understand the internal properties of low-dimensional PVKs clearly. This paper summarizes the latest research progress on the ultrafast carrier and spin dynamics in low-dimensional PVKs, to comprehensively understand their carrier and spin behaviors and present an outlook for relevant studies in this area. [ABSTRACT FROM AUTHOR]

Overall water splitting (OWS) using semiconductor photocatalysts is a promising method for solar fuel production. Achieving a high quantum efficiency is one of the most important prerequisites for photocatalysts to realize high solar‐to‐fuel efficiency. In a recent study (Nature2020, 58, 411–414), a quantum efficiency of almost 100 % has been achieved in an aluminum‐doped strontium titanate (SrTiO3 : Al) photocatalyst. Herein, using the SrTiO3 : Al as a model photocatalyst, we reveal the criteria for efficient photocatalytic water splitting by investigating the carrier dynamics through a comprehensive photoluminescence study. It is found that the Al doping suppresses the generation of Ti3+ recombination centers in SrTiO3, the surface band bending facilitates charge separation, and the in situ photo‐deposited Rh/Cr2O3 and CoOOH co‐catalysts render efficient charge extraction. By suppressing photocarrier recombination and establishing a facile charge separation and extraction mechanism, high quantum efficiency can be achieved even on photocatalysts with a very short (sub‐ns) intrinsic photocarrier lifetime, challenging the belief that a long carrier lifetime is a fundamental requirement. Our findings could provide guidance on the design of OWS photocatalysts toward more efficient solar‐to‐fuel conversion. [ABSTRACT FROM AUTHOR]

The electronic structure of halide perovskites is central to their carrier dynamics, enabling the excellent optoelectronic performance. However, the experimentally resolved transient absorption spectra exhibit large discrepancies from the commonly computed electronic structure by density functional theory. Using pseudocubic CsPbI3 as a prototype example, here, it is unveiled with both ab initio molecular dynamics simulations and transmission electron microscopy that there exists pronounced dynamical lattice distortion in the form of disordered instantaneous octahedral tilting. Rigorous first‐principles calculations reveal that the lattice distortion substantially alters the electronic band structure through renormalizing the band dispersions and the interband transition energies. Most notably, the electron and hole effective masses increase by 65% and 88%, respectively; the transition energy between the two highest valence bands decreases by about one half, agreeing remarkably well with supercontinuum transient‐absorption measurements. This study further demonstrates how the resulting electronic structure modulates various aspects of the carrier dynamics such as carrier transport, hot‐carrier relaxation, Auger recombination, and carrier multiplication in halide perovskites. The insights provide a pathway to engineer carrier transport and relaxation via lattice distortion, enabling the promise to achieve ultrahigh‐efficiency photovoltaic devices. [ABSTRACT FROM AUTHOR]

Herein, highly luminescent eco‐friendly CuGaS2/ZnS (CGS/ZnS) and CuGaInS2/ZnS (CGIS/ZnS) core/shell quantum dots (QDs) are rationally prepared for luminescent solar concentrator (LSC) application. It is demonstrated that the optical properties of these core/shell QDs can be tailored by engineering the ZnS shell thickness, leading to large Stokes shifts and high‐photoluminescence quantum yields up to 94.6%. As‐synthesized core/shell QDs with optimized optical properties are employed to fabricate LSCs (5 × 5 × 0.5 cm3) using glasses as waveguides, wherein the individual CGS/ZnS and CGIS/ZnS QD‐based LSCs, respectively, exhibit an optical efficiency (ηopt) of ≈3.26% and 6.53% under AM1.5G illumination (100 mW cm−2). Remarkably, a tandem QDs‐LSC integrated via vertical stacking of the top yellow‐emitting CGS/ZnS QDs‐LSC and bottom red‐emitting CGIS/ZnS QDs‐LSC delivers an optical efficiency (ηopt) as high as 9.94%, which is, respectively, ≈3 and 1.5 times higher than the individual QDs‐LSCs and is comparable to various best‐reported QDs‐LSCs. The results indicate that environment‐benign I–III–VI2 core/shell QDs with engineered optical properties and LSC architectural design are promising to develop future cost‐effective and high‐performing building‐integrated photovoltaics. [ABSTRACT FROM AUTHOR]

The excellent luminescence properties and structural dynamics driven by the stereoactivity of the lone pair in a variety of low‐dimensional ns2 metal halides have attracted growing investigations for optoelectronic applications. However, the structural and photophysical aspects of the excited state associated with the lone pair expression are currently open questions. Herein, zero‐dimensional Sn‐based halides with static stereoactive 5 s2 lone pairs are selected as a model system to understand the correlations between the distinctive lone pair expression and the excited‐state structural relaxation and charge carrier dynamics by continuous lattice manipulation. Lattice compression drives 5 s2 lone pair active switching and self‐trapped exciton (STE) redistribution by suppressing excited‐state structural deformation of the isolated SnBr42− units. Our results demonstrate that the static expression of the 5 s2 lone pair results in a red broadband triplet STE emission with a large Stokes shift, while its dynamic expression creates a sky‐blue narrowband emission dominated by the radiative recombination of singlet STEs. Our findings and the photophysical mechanism proposed highlight the stereochemical effects of lone pair expression in controlling light emission properties and offer constructive guidelines for tuning the optoelectronic properties in diverse ns2 metal halides. [ABSTRACT FROM AUTHOR]

Although doping organic fluorophores into host matrix is a common way to obtain high‐efficiency organic light‐emitting diodes (OLEDs), trapped charges are often observed on dopant molecules that are considered to be detrimental to the further enhancement of their photoelectric performances. Surprisingly, trapped charges with a super‐long lifetime of >2 h are detected in doped OLEDs operated at 20 K, which has never been discovered previously in the literature. However, the observations demonstrate that the longer lifetimes of these trapped charges are primarily governed by the larger spacing distances between trapped electrons and holes, rather than being solely determined by the well‐accepted energy‐level depth of trap states. More amazingly, compared with the device driven only by a conventional constant voltage source, a tenfold enhancement in light emission is achieved in low‐temperature operational doped OLEDs powered by an alternating positive and negative pulse voltage, because the applied negative pulse voltage can assist these super‐long‐lived trapped electrons and holes to detrap and then recombine with each other for producing enhanced light‐emission. Therefore, this study clarifies the dynamic behaviors of trapped charges and paves the pathway for obtaining high‐performance OLEDs working in low‐temperature circumstances such as outer‐space or aerospace fields. [ABSTRACT FROM AUTHOR]

[Display omitted] The p-i heterojunction imbedded underneath the perovskite layer plays a vital role in determining the efficiency and stability of inverted perovskite solar cells (PSCs). We found that poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA) suffers from the severely chain entanglement resulting in poor contact with perovskite. In this work, PTAA layer was treated by poly[(2,6-(4,8-bis(5-(2-ethylhexylthio)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5-b′] dithiophene))- alt -(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl) benzo[1′,2′-c:4′,5′-c′] dithiophene-4,8-dione)] (PBDB- T -SF) diluted solution in chlorobenzene. PBDB- T -SF, which contains dual carbonyl groups in its backbones and suitable electronic levels, can spontaneously fill the voids in chlorobenzene-washed PTAA (nano-PTAA). This not only promotes the work function of the substrate but also strengthens the coherence between perovskite and the substrate. Blade coated PSC (0.09 cm2) containing PBDB- T -SF (s-PSCs) realized a power conversion efficiency (PCE) of 21.83 %. After aging for more than 2000 h, s-PSCs maintains 88 % of the initial efficiency which is only 59 % for the control devices. [ABSTRACT FROM AUTHOR]

Metal halide perovskites (MHPs) have attracted considerable attentions as promising candidates for next-generation optoelectronic devices, such as light-emitting diode (LED), owing to their outstanding photophysical properties. Nanostructuring is considered an essential approach to facilitate the bright emission of MHPs, which entails an increase in the surface domain that can directly affect the carrier dynamics. However, a comprehensive understanding of the surface termination effect on the photodynamic properties of MHPs is still lacking. Herein, we systematically investigate the effect of surface termination on the carrier recombination dynamics of CsPbBr3 using ab-initio non-adiabatic molecular dynamics simulations. We found separate localizations of electron and hole carriers in the vicinity of the more and less coordinated inorganic polyhedral, respectively, which can be explained by the energy level changes associated with the modifications in Pb–Br bond lengths and their anharmonicity. This leads to the spatial separation of charge carriers, which retards the radiative kinetics more than the non-radiative one, reducing the photoluminescence quantum yield (PLQY). We further found that the homogenous linewidth is broadened upon introduction of surface terminations. Thus, our study suggests a possible LED-performance degradation mechanism due to surface termination, and thereby proposes guidelines for enhancing the light-emission properties of nanostructured MHPs. [ABSTRACT FROM AUTHOR]

The exceptional thermoelectric properties of PbTe are believed to be associated with the incipient ferroelectricity of this material, which is caused by strong electron–phonon coupling that connects phononic and electronic dynamics. Here, we have used terahertz time-domain spectroscopy measurements to generate complex permittivity spectra for a set of epitaxially grown PbTe thin films with thicknesses between 100 nm and 500 nm at temperatures from 10  K  to 300  K. Using a Drude–Lorentz model, we retrieved the physical parameters of both the phononic and electronic contributions to the THz permittivity. We observed a strong decrease, or softening, of the transverse optical phonon mode frequency with decreasing temperature, determining a thickness-independent negative ferroelectric-transition critical temperature, while we found a thickness-dependent anharmonic phonon decay lifetime. The electronic contribution to the permittivity was larger in thinner films, and both the carrier density and mobility increased with decreasing temperature in all films. Finally, we detected a thickness-dependent longitudinal optical phonon mode frequency, indicating the presence of plasmon–phonon coupling. [ABSTRACT FROM AUTHOR]

Bi‐based nanomaterials have unique physicochemical and structural properties, allowing them to respond to visible light effectively. The Bi‐based semiconductors can be used for degradation of pollutants, reduction of CO2, nitrogen fixation, and decomposition of water, having the strong potential to address environmental pollution and energy shortages. However, the large‐scale application is still limited by the separation and transfer of electron–hole pairs. Herein this review article, carrier dynamics are studied from three perspectives, including structural modulation, atomic conformational reorganization, and surface modification. The relevant mechanisms of different strategies are mainly highlighted, the variations of carrier motion or lifetime are explored, and the progress in practical studies is elucidated. The structural modulation strategy is first introduced, including different dimensional structures and heterogeneous junctions. Further, the atomic conformational reorganization strategy is presented, including Bi‐rich strategies, doping, and vacancy defects. Additionally, the surface modification strategy is outlined, including facet engineering, loading of semiconductor cocatalysts, precious metals, or single atoms. Finally, these strategies are summarized for regulating carrier dynamics and the outlook with the main research trend, providing more inspiration for designing desirable Bi‐based semiconductors. [ABSTRACT FROM AUTHOR]

Sn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb‐based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p‐doping characteristics and possess abundant vacancy defects, which result in under‐optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic "electron and defect compensation" strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol %) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p‐type to weak p‐type (i.e. up‐shifting the Fermi level by ∼0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02 %, which is ∼46 % higher than that of control device (9.56 %). Notably, a record‐high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb‐based analogues (∼0.30 V). [ABSTRACT FROM AUTHOR]

The charge transport in quasi‐2D perovskites limits their applications despite the superior stability and optoelectronic properties. Herein, a novel strategy is proposed to enhance the charge transport by regulating 3D perovskite phase in quasi‐2D perovskite films. The carbohydrazide (CBH) as an additive is introduced into (PEA)2MA3Pb4I13 precursors, which slows down the crystallization process and improves the phase ratio and crystal quality of the 3D phase. This structure change results in a significant improvement in charge transport and extraction, leading to the device demonstrating an almost 100% internal quantum efficiency, a peak responsivity of 0.41 A W−1, and a detectivity of 1.31 × 1012 Jones at 570 nm under 0 V bias. Furthermore, the air and moisture stability of (PEA)2MA3Pb4I13 films is not deteriorated but gets significantly improved due to the better crystal quality and the passivation of defects by the residual CBH molecule. This work demonstrates a strategy for improving the charge transport properties of quasi‐2D perovskites and also sheds light on solving the stability issue of 3D perovskite films via the proper passivation or additives, which will inspire the fast development of the perovskite community. [ABSTRACT FROM AUTHOR]

Understanding the effect of additive on the interfacial charge‐carrier transfer dynamics is very crucial to obtaining highly efficient perovskite solar cells (PSCs). Herein, we designed a simple additive, dimethyl oxalate (DO), functioning as an effective defect passivator of perovskite grain boundaries via the coordination interaction between the carbonyl (C=O) and the exposed Pb2+. The modification with DO produces pinhole‐free and compact perovskite films, enhancing the transportation capability of carriers. As a consequence, the DO‐treated PSCs exhibited a power conversion efficiency (PCE) of 22.19%, which is significantly higher than that of the control device without additive (19.58%). More importantly, detailed transient absorption characterization reveals that the use of additive can decrease the hot‐carrier cooling dynamics, improve the carrier transfer, and eliminate nonradiative recombination in PSCs. This present work provides a profound understanding the additives effect on the carrier dynamics in PSCs toward the Shockley−Queisser limit. [ABSTRACT FROM AUTHOR]

All-inorganic lead halide perovskite nanocrystals (NCs) have been widely investigated as highly promising optical gain materials due to their compelling electrical and optical properties. Although many efforts have been carried out, a deep understanding of perovskite NC vertical-cavity surface-emitting lasers (VCSELs) is elusive, which is very important in the development of photoelectronic integrated circuits. Along these lines, in this work, a low lasing threshold (22 μJ/cm2) single-mode VCSEL consisting of CsPbBr3 NCs film and two distributed Bragg reflectors was successfully constructed. The CsPbBr3 NCs were synthesized by using the supersaturated recrystallization method. Interestingly, benefiting from the strong coupling between the active layer and the optical field in the cavity, a single-mode lasing at 527 nm was demonstrated under femtosecond optical pumping. The carrier dynamics of the perovskite NC VCSEL was also thoroughly investigated by performing pump intensity-dependent time-resolved photoluminescence measurements. The typical gain-switching phenomenon was observed with an ultrafast decay of the laser pulse of ∼10 ps. Our work provides valuable insights for the implementation of the CsPbBr3 NC VCSEL for various optoelectronic applications.

The charge carrier formation and transport in the pristine polymers as well as in the polymer–fullerene blend is still a hot topic of discussion for the scientific community. In the present work, the carrier generation in some prominent organic molecules has been studied through ultrafast transient absorption spectroscopy. The identification of the exciton and polaron lifetimes of these polymers has led to device performance-related understanding. In the Energy Gap Law, the slope of the linear fit gradient (γ) of lifetimes vs. bandgap are subjected to the geometrical rearrangements experienced by the polymers during the non-radiative decay from the excited state to the ground state. The value of gradient (γ) for excitons and polarons is found to be −1.1 eV−1 and 1.14 eV−1, respectively. It suggests that the exciton decay to the ground state is likely to involve a high distortion in polymer equilibrium geometry. This observation supports the basis of Stokes shift found in the conjugated polymers due to the high disorder. It provides the possible reasons for the substantial variation in the exciton lifetime. As the bandgap becomes larger, exciton decay rate tends to reduce due to the weak attraction between the holes in the HUMO and electron in the LUMO. The precise inverse action is observed for the polymer–fullerene blend, as the decay of polaron tends to increase as the bandgap of polymer increases.

Monolayer two-dimensional transition metal dichalcogenide (TMD)-based materials have become one of the ideal platforms for the study of multibody interactions due to their rich excitonic complexes. The coupling between optical nanocavity and material has become an important means for manipulating the optical properties of materials, but there are few studies on the coupling of nanocavities and the multi-body effect in materials. In this study, we investigate the optical properties of silver nanodisk (Ag ND) arrays covering a monolayer WS2. In the experimental sample, we observed a ~114.3-fold photoluminescence enhancement of charged biexciton in the heterostructure region, as compared to the monolayer WS2 region, a value which is much higher than those for exciton (~2.2-fold) and trion (~16.4-fold), a finding which is attributed to the Fano resonant coupling between monolayer WS2 and the Ag ND. By means of time-resolved spectroscopy, we studied the carrier dynamics in the hybrid system. Our findings reveal that resonant coupling promotes the formation and radiation recombination processes of the charged biexciton, significantly reducing the radiative recombination lifetime by ~15-fold, which is much higher than the measurement in exciton (~2-fold). Our results provide an opportunity to understand the multibody physics of coupling with nanocavities, which could facilitate the application of multi-body excitons in the fields of light-emitting devices and lasers, etc.

Abstract The electronic structure of halide perovskites is central to their carrier dynamics, enabling the excellent optoelectronic performance. However, the experimentally resolved transient absorption spectra exhibit large discrepancies from the commonly computed electronic structure by density functional theory. Using pseudocubic CsPbI3 as a prototype example, here, it is unveiled with both ab initio molecular dynamics simulations and transmission electron microscopy that there exists pronounced dynamical lattice distortion in the form of disordered instantaneous octahedral tilting. Rigorous first‐principles calculations reveal that the lattice distortion substantially alters the electronic band structure through renormalizing the band dispersions and the interband transition energies. Most notably, the electron and hole effective masses increase by 65% and 88%, respectively; the transition energy between the two highest valence bands decreases by about one half, agreeing remarkably well with supercontinuum transient‐absorption measurements. This study further demonstrates how the resulting electronic structure modulates various aspects of the carrier dynamics such as carrier transport, hot‐carrier relaxation, Auger recombination, and carrier multiplication in halide perovskites. The insights provide a pathway to engineer carrier transport and relaxation via lattice distortion, enabling the promise to achieve ultrahigh‐efficiency photovoltaic devices.

The metal halide perovskite CH3NH3PbI3 (MAP) can be applied as the shell layer of lead sulfide quantum dots (PbS QDs) for improving solar power conversion efficiency. However, basic physics for this PbS core/MAP shell QD system is still unclear and needs to be clarified to further improve efficiency. Therefore, in this study, we investigate how MAP shell thickness affects device performance and dynamics of charge carriers for PbS QD-sensitized solar cells. Covering the PbS QDs with the MAP shell layers of an appropriate thickness around 0.34 nm greatly suppresses charge carrier recombination at surface defects along with improved carrier transport to neighboring oxide and polymer layers. Therefore, this MAP shell thickness provides the highest open-circuit voltage, short-circuit current density, and fill factor for solar cells. Overall power conversion efficiencies of these solar cells reached about 4.1%, which is about six-fold higher than that for solar cells without MAP (about 0.7%). Additionally, use of the MAP shell layers can prevent oxidation of PbS QDs and, therefore, makes a degradation of solar cell performance slower in air.

Abstract The charge transport in quasi‐2D perovskites limits their applications despite the superior stability and optoelectronic properties. Herein, a novel strategy is proposed to enhance the charge transport by regulating 3D perovskite phase in quasi‐2D perovskite films. The carbohydrazide (CBH) as an additive is introduced into (PEA)2MA3Pb4I13 precursors, which slows down the crystallization process and improves the phase ratio and crystal quality of the 3D phase. This structure change results in a significant improvement in charge transport and extraction, leading to the device demonstrating an almost 100% internal quantum efficiency, a peak responsivity of 0.41 A W−1, and a detectivity of 1.31 × 1012 Jones at 570 nm under 0 V bias. Furthermore, the air and moisture stability of (PEA)2MA3Pb4I13 films is not deteriorated but gets significantly improved due to the better crystal quality and the passivation of defects by the residual CBH molecule. This work demonstrates a strategy for improving the charge transport properties of quasi‐2D perovskites and also sheds light on solving the stability issue of 3D perovskite films via the proper passivation or additives, which will inspire the fast development of the perovskite community.