Enhanced charge-carrier dynamics and efficient solar-to-urea conversion on Si-based photocathodes.

Photoelectrochemical (PEC) coupling of CO2 and nitrate can provide a useful and green source of urea, but the process is affected by the photocathodes with poor charge-carrier dynamics and low conversion efficiency. Here, a NiFe diatomic catalysts/TiO2 layer/nanostructured n+p-Si photocathode is rationally designed, achieving a good charge-separation efficiency of 78.8% and charge-injection efficiency of 56.9% in the process of PEC urea synthesis. Compared with the electrocatalytic urea synthesis by using the same catalysts, the Si-based photocathode shows a similar urea yield rate (81.1 mg·h-1·cm-2) with a higher faradic efficiency (24.2%, almost twice than the electrocatalysis) at a lower applied potential under 1 sun illumination, meaning that a lower energy-consumption method acquires more aimed productions. Integrating the PEC measurements and characterization results, the synergistic effect of hierarchical structure is the dominating factor for enhancing the charge-carrier separation, transfer, and injection by the matched band structure and favorable electron-migration channels. This work provides a direct and efficient route of solar-to-urea conversion.

Electroluminescence as a Probe of Strong Exciton-Plasmon Coupling in Few-Layer WSe2.

The manipulation of coupled quantum excitations is of fundamental importance in realizing novel photonic and optoelectronic devices. We use electroluminescence to probe plasmon-exciton coupling in hybrid structures consisting of a nanoscale plasmonic tunnel junction and few-layer two-dimensional transition-metal dichalcogenide transferred onto the junction. The resulting hybrid states act as a novel dielectric environment that affects the radiative recombination of hot carriers in the plasmonic nanostructure. We determine the plexcitonic spectrum from the electroluminescence and find Rabi splittings exceeding 50 meV in the strong coupling regime. Our experimental findings are supported by electromagnetic simulations that enable us to explore systematically and in detail the emergence of plexciton polaritons as well as the polarization characteristics of their far-field emission. Electroluminescence modulated by plexciton coupling provides potential applications for engineering compact photonic devices with tunable optical and electrical properties.

Diffusion-Dominated Luminescence Dynamics of CsPbBr3 Studied Using Cathodoluminescence and Microphotoluminescence Spectroscopy.

Time-resolved or time-correlation measurements using cathodoluminescence (CL) reveal the electronic and optical properties of semiconductors, such as their carrier lifetimes, at the nanoscale. However, halide perovskites, which are promising optoelectronic materials, exhibit significantly different decay dynamics in their CL and photoluminescence (PL). We conducted time-correlation CL measurements of CsPbBr3 using Hanbury Brown-Twiss interferometry and compared them with time-resolved PL. The measured CL decay time was on the order of subnanoseconds and was faster than PL decay at an excited carrier density of 2.1 × 1018 cm-3. Our experiment and analytical model revealed the CL dynamics induced by individual electron incidences, which are characterized by highly localized carrier generation followed by a rapid decrease in carrier density due to diffusion. This carrier diffusion can play a dominant role in the CL decay time for undoped semiconductors, in general, when the diffusion dynamics are faster than the carrier recombination.

Intrinsic Self-Trapped Excitons in Graphitic Carbon Nitride.

Graphitic carbon nitrides (g-C3N4) as low-cost, chemically stable, and ecofriendly layered semiconductors have attracted rapidly growing interest in optoelectronics and photocatalysis. However, the nature of photoexcited carriers in g-C3N4 is still controversial, and an independent charge-carrier picture based on the band theory is commonly adopted. Here, by performing transient spectroscopy studies, we show characteristics of self-trapped excitons (STEs) in g-C3N4 nanosheets including broad trapped exciton-induced absorption, picosecond exciton trapping without saturation at high photoexcitation density, and transient STE-induced stimulated emissions. These features, together with the ultrafast exciton trapping polarization memory, strongly suggest that STEs intrinsically define the nature of the photoexcited states in g-C3N4. These observations provide new insights into the fundamental photophysics of carbon nitrides, which may enlighten novel designs to boost energy conversion efficiency.

Modulating Carrier Dynamics in PdSe2: The Role of Pressure in Electronic and Phononic Interactions.

PdSe2 is a puckered transition metal dichalcogenide that has been reported to undergo a two-dimensional to three-dimensional structural transition under pressure. Here, we investigated the electronic and phononic evolution of PdSe2 under high pressure using pump-probe spectroscopy. We observed the electronic intraband and interband transitions occurring in the d orbitals of Pd, revealing the disappearance of the Jahn-Teller effect under high pressure. Furthermore, we found that the decay rates of interband recombination and intraband relaxation lifetimes change at 3 and 7 GPa, respectively. First-principles calculations suggest that the bandgap closure slows the decay rate of interband recombination after 3 GPa, while the saturation of phonon-phonon scattering is the main reason for the relatively constant intraband relaxation lifetime. Our work provides a novel perspective for understanding the evolution of the electron and modulation of the carrier dynamics by phonons under pressure.

Modulating the Structure and Optoelectronic Properties of Solution-Processed Bismuth-Based Nanocrystals.

Bismuth-based chalcogenides have emerged as promising candidates for next-generation, solution-processable semiconductors, mainly benefiting from their facile fabrication, low cost, excellent stability, and tunable optoelectronic properties. Particularly, the recently developed AgBiS2 solar cells have shown striking power conversion efficiencies. High performance bismuth-based photodetectors have also been extensively studied in the past few years. However, the fundamental properties of these Bi-based semiconductors have not been sufficiently investigated, which is crucial for further improving the device performance. Here, we introduce multiple time-resolved and steady-state techniques to fully characterize the charge carrier dynamics and charge transport of solution-processed Bi-based nanocrystals. It was found that the Ag-Bi ratio plays a critical role in charge transport. For Ag-deficient samples, silver bismuth sulfide thin films behave as localized state induced hopping charge transport, and the Ag-excess samples present band-like charge transport. This finding is crucial for developing more efficient Bi-based semiconductors and optoelectronic devices.

Time-Dependent Ultrafast Quadratic Nonlinearity in an Epsilon-Near-Zero Platform.

Ultrafast nonlinearity, which results in modulation of the linear optical response, is a basis for the development of time-varying media, in particular those operating in the epsilon-near-zero (ENZ) regime. Here, we demonstrate that the intraband excitation of hot electrons in the ENZ film results in a second-harmonic resonance shift of ∼10 THz (40 nm) and second-harmonic generation (SHG) intensity changes of >100% with only minor (<1%) changes in linear transmission. The modulation is 10-fold enhanced by a plasmonic metasurface coupled to a film, allowing for ultrafast modulation of circularly polarized SHG. The effect is described by the plasma frequency renormalization in the ENZ material and the modification of the electron damping, with a possible influence of the hot-electron dynamics on the quadratic susceptibility. The results elucidate the nature of the second-order nonlinearity in ENZ materials and pave the way to the rational engineering of active nonlinear metamaterials and metasurfaces for time-varying applications.

Interlayer Carrier Dynamics in Two-Dimensional Perovskites Determined by the Length of Conjugated Organic Cations.

Unlocking the restricted interlayer carrier transfer in a two-dimensional perovskite is a crucial means to achieve the harmonization of efficiency and stability in perovskite solar cells. In this work, the effects of conjugated organic molecules on the interlayer carrier dynamics of 2D perovskites were investigated through nonadiabatic molecular dynamics simulations. We found that elongated conjugated organic cations contributed significantly to the accelerated interlayer carrier dynamics, originating from lowered transport barrier and boosted π-p coupling between organic and inorganic layers. Utilizing conjugated molecules of moderate length as spacer cations can yield both superior efficiency and exceptional stability simultaneously. However, conjugated chains that are too long lead to structural instability and stronger carrier recombination. The potential of conjugated chain-like molecules as spacer cations in 2D perovskites has been demonstrated in our work, offering valuable insights for the development of high-performance perovskite solar cells.

Defect Compensation and Lattice Stabilization Enables High Voltage Output in Tin Halide Perovskite Solar Cells.

Tin halide perovskite solar cells (PSCs) are regarded as the most promising lead-free alternatives for photovoltaic applications. However, they still suffer from uncompetitive photovoltaic performance because of the facile Sn2+ oxidation and Sn-related defects. Herein, a defect and carrier management strategy by using diaminopyridine (DP) and 4-bromo-2,6-diaminopyridine (4BrDP) as multifunctional additives for tin halide perovskites is reported. Both DP and 4BrDP induced strong interaction with tin perovskites by coordinate bonding and N─H···I hydrogen bonding, which greatly suppresses the micro-strain and Urbach energy of tin halide perovskite films. The strong hydrogen bonding inhibits the formation of I3 - and related defect density. Meanwhile, the electron-donor species of halogen bond in 4BrDP provides higher reactivity of 2 and 6 sites, which indicates stronger passivation ability with tin halide perovskites. These advances enable a champion power conversion efficiency (PCE) of 13.40% in 4BrDP-processed devices with remarkable improvement in both open-circuit voltage (Voc) of 881 mV and fill factor (FF) of 71.26%. The 4BrDP devices retain 91% and 82% of the pristine PCE after 2000 h storage in N2 atmosphere and 1000 h under 85 °C, respectively. Therefore, this work provides new insight into molecular design for high-performance and stable lead-free optoelectronics.

Advanced Interface Engineering in Gradient Core/Shell Quantum Dots Enables Efficient Photoelectrochemical Hydrogen Evolution.

Semiconductor core/shell quantum dots (QDs) are considered promising building blocks to fabricate photoelectrochemical (PEC) cells for the direct conversion of solar energy into hydrogen (H2). However, the lattice mismatch between core and shell in such QDs results in undesirable defects and severe carrier recombination, limiting photo-induced carrier separation/transfer and solar-to-fuel conversion efficiency. Here, an interface engineering approach is explored to minimize the core-shell lattice mismatch in CdS/CdSexS1-x (x = 0.09-1) core/shell QDs (g-CSG). As a proof-of-concept, PEC cells based on g-CSG QDs yield a remarkable photocurrent density of 13.1 mA cm-2 under AM 1.5 G one-sun illumination (100 mW cm-2), which is ≈54.1% and ≈33.7% higher compared to that in CdS/CdSe0.5S0.5 (g-CSA) and CdS/CdSe QDs (g-CS), respectively. Theoretical calculations and carrier dynamics confirm more efficient carrier separation and charge transfer rate in g-CSG QDs with respect to g-CSA and g-CS QDs. These results are attributed to the minimization of the core-shell lattice mismatch by the cascade gradient shell in g-CSG QDs, which modifies carrier confinement potential and reduces interfacial defects. This work provides fundamental insights into the interface engineering of core/shell QDs and may open up new avenues to boost the performance of PEC cells for H2 evolution and other QDs-based optoelectronic devices.

Ammonium Sulfate to Modulate Crystallization for High-Performance Rigid and Flexible Perovskite Solar Cells.

Perovskite solar cells (PSCs) are attracting widespread research and attention as highly promising candidates in the field of electronic photovoltaics owing to their exceptional power conversion efficiency (PCE). However, rigid or flexible PSCs still face challenges in preparing full-coverage and low-defect perovskite films, as well as achieving highly reproducible and highly stable devices. Herein, a multifunctional additive 2-aminoethyl hydrogen sulfate (AES) is designed to regulate the film crystallization and thereby form flat and pinhole-free perovskite films. It is found that the introduction of AES can effectively passivate defects, restrain charge carrier recombination, and then achieve a higher fill factor. As seen with grazing incidence wide-angle X-ray scattering (GIWAXS), this approach does not affect the crystal orientation distribution. It is observed that AES addition shows a universality across different perovskite components since the PCE is improved up to 20.7% for FA0.97MA0.03Pb(I0.97Br0.03)3-AES, 22.85% for Cs0.05FA0.95PbI3-AES, 22.23% for FAPbI2.7Br0.3-AES, and 23.32% for FAPI-AES rigid devices. Remarkably, the non-encapsulated flexible Cs0.05 (FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 device with AES additive delivers a PCE of 20.1% and maintains over 97% of its initial efficiency under ambient conditions (25 ± 5% relative humidity) over 2280 h of aging.

Managing Interfacial Defects and Charge-Carriers Dynamics by a Cesium-Doped SnO2 for Air Stable Perovskite Solar Cells.

A high-quality nanostructured tin oxide (SnO2) has garnered massive attention as an electron transport layer (ETL) for efficient perovskite solar cells (PSCs). SnO2 is considered the most effective alternative to titanium oxide (TiO2) as ETL because of its low-temperature processing and promising optical and electrical characteristics. However, some essential modifications are still required to further improve the intrinsic characteristics of SnO2, such as mismatch band alignments, charge extraction, transportation, conductivity, and interfacial recombination losses. Herein, an inorganic-based cesium (Cs) dopant is used to modify the SnO2 ETL and to investigate the impact of Cs-dopant in curing interfacial defects, charge-carrier dynamics, and improving the optoelectronic characteristics of PSCs. The incorporation of Cs contents efficiently improves the perovskite film quality by enhancing the transparency, crystallinity, grain size, and light absorption and reduces the defect states and trap densities, resulting in an improved power conversion efficiency (PCE) of ≈22.1% with Cs:SnO2 ETL, in-contrast to pristine SnO2-based PSCs (20.23%). Moreover, the Cs-modified SnO2-based PSCs exhibit remarkable environmental stability in a relatively higher relative humidity environment (>65%) and without encapsulation. Therefore, this work suggests that Cs-doped SnO2 is a highly favorable electron extraction material for preparing highly efficient and air-stable planar PSCs.

Tailoring Broadband Nonlinear Optical Characteristics and Ultrafast Photocarrier Dynamics of Bi2O2S Nanosheets by Defect Engineering.

Low-dimensional bismuth oxychalcogenides have shown promising potential in optoelectronics due to their high stability, photoresponse, and carrier mobility. However, the relevant studies on deep understanding for Bi2O2S is quite limited. Here, comprehensive experimental and computational investigations are conducted in the regulated band structure, nonlinear optical (NLO) characteristics, and carrier dynamics of Bi2O2S nanosheets via defect engineering, taking O vacancy (OV) and substitutional Se doping as examples. As the OV continuously increased to ≈35%, the optical bandgaps progressively narrow from ≈1.21 to ≈0.81 eV and NLO wavelengths are extended to near-infrared regions with enhanced saturable absorption. Simultaneously, the relaxation processes are effectively accelerated from tens of picoseconds to several picoseconds, as the generated defect energy levels can serve as both additional absorption cross-sections and fast relaxation channels supported by theoretical calculations. Furthermore, substitutional Se doping in Bi2O2S nanosheets also modulate their optical properties with the similar trends. As a proof-of-concept, passively mode-locked pulsed lasers in the ≈1.0 µm based on the defect-rich samples (≈35% OV and ≈50% Se-doping) exhibit excellent performance. This work deepens the insight of defect functions on optical properties of Bi2O2S nanosheets and provides new avenues for designing advanced photonic devices based on low-dimensional bismuth oxychalcogenides.

Electron Transport Layer Engineering Induced Carrier Dynamics Optimization for Efficient Cd-Free Sb2 Se3 Thin-Film Solar Cells.

Antimony selenide (Sb2 Se3 ) is a highly promising photovoltaic material thanks to its outstanding optoelectronic properties, as well as its cost-effective and eco-friendly merits. However, toxic CdS is widely used as an electron transport layer (ETL) in efficient Sb2 Se3 solar cells, which largely limit their development toward market commercialization. Herein, an effective green Cd-free ETL of SnOx is introduced and deposited by atomic layer deposition method. Additionally, an important post-annealing treatment is designed to further optimize the functional layers and the heterojunction interface properties. Such engineering strategy can optimize SnOx ETL with higher nano-crystallinity, higher carrier density, and less defect groups, modify Sb2 Se3 /SnOx heterojunction with better interface performance and much desirable "spike-like" band alignment, and also improve the Sb2 Se3 light absorber layer quality with passivated bulk defects and prolonged carrier lifetime, and therefore to enhance carrier separation and transport while suppressing non-radiative recombination. Finally, the as-fabricated Cd-free Mo/Sb2 Se3 /SnOx /ITO/Ag thin-film solar cell exhibits a stimulating efficiency of 7.39%, contributing a record value for Cd-free substrate structured Sb2 Se3 solar cells reported to date. This work provides a viable strategy for developing and broadening practical applications of environmental-friendly Sb2 Se3 photovoltaic devices.

A New Metal-Free Molecular Perovskite-Related Single Crystal with Quantum Wire Structure for High-Performance X-Ray Detection.

The family of metal-free molecular perovskites, an emerging novel class of eco-friendly semiconductor, welcomes a new member with a unique 1D hexagonal perovskite structure. Lowering dimensionality at molecular level is a facile strategy for crystal structure conversion, optoelectronic property regulation, and device performance optimization. Herein, the study reports the design, synthesis, packing structure, and photophysical properties of the 1D metal-free molecular perovskite-related single crystal, rac-3APD-NH4I3(rac-3APD= racemic-3-Aminopiperidinium), that features a quantum wire structure formed by infinite chains of face-sharing NH4I6 octahedra, enabling strong quantum confinement with strongly self-trapped excited (STE) states to give efficient warm orange emission with a photoluminescence quantum yield (PLQY) as high as ≈41.6%. The study accordingly unveils its photoexcited carrier dynamics: rac-3APD-NH4I3 relaxes to STE state with a short lifetime of 10 ps but decays to ground state by emitting photons with a relatively longer lifetime of 560 ps. Additionally, strong quantum confinement effect is conducive to charge transport along the octahedral channels that enables the co-planar single-crystal X-ray detectors to achieve a sensitivity as high as 1556 µC Gyair -1 cm-2. This work demonstrates the first case of photoluminescence mechanism and photophysical dynamics of 1D metal-free perovskite-related semiconductor, as well as the promise for high-performance X-ray detector.

Novel Benzothiadiazole-based Donor-Acceptor Systems: Synthesis, Ultrafast Charge Transfer and Separation Dynamics.

Near-infrared (NIR) absorbing electron donor-acceptor (D-A) chromophores have been at the forefront of current energy research owing to their facile charge transfer (CT) characteristics, which are primitive for photovoltaic applications. Herein, we have designed and developed a new set of benzothiadiazole (BTD)-based tetracyanobutadiene (TCBD)/dicyanoquinodimethane (DCNQ)-embedded multimodular D-A systems (BTD1-BTD6) and investigated their inherent photo-electro-chemical responses for the first time having identical and mixed terminal donors of variable donor-ability. Apart from poor luminescence, the appearance of broad low-lying optical transitions extendable even in the NIR region (> 1000 nm), particularly in the presence of the auxiliary acceptors, are indicative of underlying nonradiative excited state processes leading to strong intramolecular CT and subsequent charge separation (CS) processes in these D-A constructs. The spectral and temporal responses of different photoproducts are obtained from  transient studies. All the systems are found to be susceptible to ultrafast (~ps) CT and CS before carrier recombination to the ground state, which is, however, significantly facilitated after incorporation of the secondary TCBD/DCNQ acceptors, leading to faster and thus efficient CT processes. These findings are likely to expand the horizons of BTD-based multimodular CT systems to revolutionize the realm of solar energy conversion and associated photonic applications.

Ultrafast Optical Nanoscopy of Carrier Dynamics in Silicon Nanowires.

Carrier distribution and dynamics in semiconductor materials often govern their physical properties that are critical to functionalities and performance in industrial applications. The continued miniaturization of electronic and photonic devices calls for tools to probe carrier behavior in semiconductors simultaneously at the picosecond time and nanometer length scales. Here, we report pump-probe optical nanoscopy in the visible-near-infrared spectral region to characterize the carrier dynamics in silicon nanostructures. By coupling experiments with the point-dipole model, we resolve the size-dependent photoexcited carrier lifetime in individual silicon nanowires. We further demonstrate local carrier decay time mapping in silicon nanostructures with a sub-50 nm spatial resolution. Our study enables the nanoimaging of ultrafast carrier kinetics, which will find promising applications in the future design of a broad range of electronic, photonic, and optoelectronic devices.

Electric-Field-Enhanced Electroluminescence Color Tuning of Colloidal Type-II Tetrapods.

Color-tunable electroluminescence (EL) from a single emitting material can be used to develop single-pixel multicolor displays. However, finding materials capable of broad EL color tuning remains challenging. Herein, we report the observation of broad voltage-tunable EL in colloidal type-II InP/ZnS quantum-dot-seeded CdS tetrapod (TP) LEDs. The EL color can be tuned from red to bluish white by varying the red and blue emission intensities from type-II interfaces and arms, respectively. The capacitor device proves that an external electric field can enhance the color tuning in type-II TPs. COMSOL simulations, numerical calculations, and transient absorption measurements are performed to understand the underlying photophysical mechanism. Our results indicate that the reduced hole relaxation rate from the arm to the quantum dot core can enhance the emission from the CdS arms, which is favorable for EL color tuning. This study provides a novel method to realize voltage-tunable EL colors with potential in display and micro-optoelectronic applications.

Design, Synthesis, and Ultrafast Carrier Dynamics of Core-Substituted Naphthalene Diimide-Based Covalent Organic Frameworks.

A series of two-dimensional polyimide covalent organic frameworks (2D COF) based on core-substituted naphthalene diimides (cNDIs) were designed and synthesized with the characteristic of tunable bandgap without global structural changes. Cyclic voltammetry (CV) and DFT calculations indicated that COFcNDI-OEt and COFcNDI-SEt possess higher HOMO/LUMO levels and narrower bandgaps than COFNDI-H. Further investigation indicated that the COF bandgaps are not only related to the electron-donating substituents but also varied with respect to the interlayer distances. Moreover, the femtosecond transient absorption (TA) spectra manifested that the electron donor substituents are beneficial to the charge delocalization in the π-columnar unit, resulting in a longer lifetime of charge recombination, which is one of the pivotal prerequisites for high-performance solar cells and photocatalysis.

Auger Recombination and Carrier-Lattice Thermalization in Semiconductor Quantum Dots under Intense Excitation.

A thorough understanding of the photocarrier relaxation dynamics in semiconductor quantum dots (QDs) is essential to optimize their device performance. However, resolving hot carrier kinetics under high excitation conditions with multiple excitons per dot is challenging because it convolutes several ultrafast processes, including Auger recombination, carrier-phonon scattering, and phonon thermalization. Here, we report a systematic study of the lattice dynamics induced by intense photoexcitation in PbSe QDs. By probing the dynamics from the lattice perspective using ultrafast electron diffraction together with modeling the correlated processes collectively, we can differentiate their roles in photocarrier relaxation. The results reveal that the observed lattice heating time scale is longer than that of carrier intraband relaxation obtained previously using transient optical spectroscopy. Moreover, we find that Auger recombination efficiently annihilates excitons and speeds up lattice heating. This work can be readily extended to other semiconductor QDs systems with varying dot sizes.