Carrier Decay Research Articles

Photoluminescence decay of mobile carriers influenced by imperfect quenching at particle surfaces with subdiffusive spread.

We recently presented a quantitative model to explain the particle-size dependence of photoluminescence (PL) quantum yields and revealed that exciton quenching is not diffusion controlled, but limited by surface reactions. However, the exciton decay kinetics has not been analyzed yet using our theoretical model. Here, we study kinetic aspects of the model and show that it should be extended to take into account subdiffusion rather than normal diffusion to maintain consistency with the observed complex decay kinetics; we also show that the PL decay kinetics is nonexponential even when the PL quenching is limited by surface reactions under subdiffusion. Our theoretical analysis of the PL quantum yield and the PL decay kinetics provides a comprehensive picture of mobile charge carriers, immobile polarons, and self-trapped excitons.

Exploring the Feasibility of Copper Incorporation in Halide Perovskites: Impact on CO2 Photoreduction Performance

AbstractDouble perovskite have attracted substantial attention as prospective materials for applications in optoelectronics and photocatalysis. Significant efforts are devoted to modulating the properties of double perovskites to improve their performance. One promising approach involves substituting silver (Ag) with copper (Cu), which offers favorable electronic characteristics. Despite promising theoretical predictions, the experimental synthesis of copper‐based double perovskites has presented notable challenges. Here, the challenges of Cu incorporation in double perovskites and the subsequent – impact of Cu on the photocatalytic activity of halide perovskites toward CO2 reduction are explored. Combining detailed computational thermodynamic studies, it is found that Cu does not form the traditional double perovskite structure that is Cs2CuBiCl6; it stays as an interstitial dopant in its pristine Cs3Bi2Cl9 structure. The Cu‐Cs3Bi2Cl9 are found to exhibit enhanced CO2 photoreduction activity than the pristine Cs3Bi2Cl9. Further, transient absorption results show that Cu dopants enhance the carrier generation because of introduced sub‐bandgap states, and it is found that carrier decay lifetime is elevated in Cu‐Cs3Bi2Cl9, which can enhance the participation of carriers in CO2 photoreduction. The study explores the challenges and opportunities of copper doping in halide perovskites, offering the potential for developing efficient CO2 reduction photocatalysts.

Band Gap Renormalization at Different Symmetry Points in Perovskites.

Using ultrafast broad-band transient absorption (TA) spectroscopy of photoexcited MAPbBr3 thin films with probe continua in the visible and the mid- to deep-ultraviolet (UV) ranges, we capture the ultrafast renormalization at the fundamental gap at the R symmetry point of the Brillouin zone (BZ) and a higher energy gap at the M symmetry point. Advanced global lifetime analysis and lifetime density distribution analysis are applied to extract quantitative information. Our work confirms the similarity of the response at both high-symmetry points, which indicates a band edge renormalization that rises within the instrument response function (IRF, ∼250 fs) and decays in ca. 400-600 fs, undergoing an energy red shift of 90-150 meV. The reported time scale corresponds to the decay of free carriers into neutral excitons. The ability to monitor different high-symmetry points in photoexcited perovskites opens exciting prospects for the characterization of a large class of materials and for photonic applications.

Magnetic-order-mediated carrier and phonon dynamics in MnBi2Te4

We investigate the quasiparticle dynamics in MnBi2Te4 single crystals using ultrafast optical spectroscopy. Our results show that there exist anomalous dynamical optical responses below the antiferromagnetic (AFM) ordering temperature TN. In specific, we reveal that both the initial carrier decay and recombination processes can be modulated via introducing the AFM order in subpicosecond and picosecond timescales, respectively. We also discover a long relaxation process emerging below TN with a timescale approaching the nanosecond regime, and can be attributed to the T-dependent spin-lattice interaction. There also emerges an unusual phonon energy renormalization below TN, which is found to arise from its coupling to the spin degree via the exchange interaction and magnetic anisotropy. Our findings provide key information for understanding the dynamical properties of nonequilibrium carrier, spin, and lattice in MnBi2Te4. Published by the American Physical Society 2024

Invariant Charge Carrier Dynamics Using a Non-Planar Non-Fullerene Acceptor across Multiple Processing Solvents.

Conventional non-fullerene acceptors (NFAs) typically have planar structures that can enable improved electron mobility and produce more efficient organic photovoltaic devices. A relatively simple A-D-A'-D-A type NFA specifically designed to match with poly(3-hexylthiophene-2,5-diyl) (P3HT) for green-absorbing agrivoltaic applications has been examined using a variety of techniques: microsecond transient absorption spectroscopy, atomic force microscopy, and photoluminescence. Relatively invariant charge carrier decay dynamics are observed in the blend films across a variety of processing solvents. Raman spectroscopy in conjunction with computational studies reveals that this NFA is non-planar and that multiple conformations are present in films, while preserving the crystalline nature of P3HT. The non-planarity of the NFA therefore creates a dispersive acceptor environment, irrespective of processing solvent, and this leads to the observed relative invariance in charge carrier decay dynamics and high tolerance to morphological variation. The findings presented in this work highlight the potential of non-planar materials as acceptors in organic photovoltaic devices.

Influence of Halogen-Solvent Hydrogen Bonding on Gold Nanocluster Photoluminescence.

The influence of gold nanocluster-solvent interactions on nanostructure optical properties was determined. Using [Au11(BINAP)4X2]+, where X = Cl or Br, as a model system, the dramatic influence of halogen-solvent hydrogen bonding on nanocluster optical properties was resolved. The creation of a nanocluster-solvent hydrogen-bond network yielded intense photoluminescence (PL) and an accompanying 2-fold reduction in vibration-mediated nonradiative decay rates. PL was quenched for systems that did not support hydrogen bonding. As reflected by absorption line widths, Raman scattering, and transient absorption spectroscopy measurements, the hydrogen-bond network increased nanocluster structural rigidity and reduced nonradiative carrier decay rates. The results highlight the significant role of the nanocluster-solvent interface in determining the properties of structurally precise materials.

Band dispersion, scattering rate, and carrier mobility using the poles of Green’s function for dilute nitride alloys

The band-anticrossing (BAC) model provides the basis for the self-consistent Green’s function method that we have previously developed to calculate the density of states of GaNxAs1−x dilute nitride alloys. In this paper, we extend this Green’s function method to include the complex energy states and to find the poles of the Green’s function, thereby allowing one to calculate the dispersion relation, group velocity, and the carrier decay rate in disordered dilute nitride alloys. Two different models of the N states have been studied to investigate the band structure of these materials: (1) the conventional two-band BAC model, which assumes that all N states are located at the same energy, and (2) a model which includes N states distributed over a range of energies, as expected in actual dilute nitride samples. Our results for the second model show a much shorter carrier mean-free path, and lower carrier mobility for GaNxAs1−x, with the magnitude of the calculated mobility in good agreement with the experimental data.

Reversible Ultrafast Chiroptical Responses in Planar Plasmonic Nano-Oligomer.

Ultracompact chiral plasmonic nanostructures with unique chiral light-matter interactions are vital for future photonic technologies. However, previous studies are limited to reporting their steady-state performance, presenting a fundamental obstacle to the development of high-speed optical devices with polarization sensitivity. Here, a comprehensive analysis of ultrafast chiroptical response of chiral gold nano-oligomers using time-resolved polarimetric measurements is provided. Significant differences are observed in terms of the absorption intensity, thus hot electron generation, and hot carrier decay time upon polarized photopumping, which are explained by a phenomenological model of the helicity-resolved optical transitions. Moreover, the chiroptical signal is switchable by reversing the direction of the pump pulse, demonstrating the versatile modulation of polarization selection in a single device. The results offer fundamental insights into the helicity-resolved optical transitions in photoexcited chiral plasmonics and can facilitate the development of high-speed polarization-sensitive flat optics with potential applications in nanophotonics and quantum optics.

Carrier Decay Lifetimes in the Narrow-gap Hg1–xCdxTe at the Interband and Intraband Excitations

The lifetimes of photoconductive decay carriers under interband and intraband excitations are studied in epitaxial layers of narrow-gap Hg1−xCdxTe (x ∼0.2). Samples with large distances (>3 mm) between small-area electrical contacts and small distances (∼10 μm) with largearea contacts (THz antennas) are studied. The lifetimes of decay carriers for intraband and interband excitations are measured and compared. It has been established that, in samples with n-type conductivity, the lifetimes are comparable (in the interval of 40 ns) for both methods of excitation. At the same time, in samples with a small distance between contacts and a large area (bow-tie antennas), contacts make the main contribution to recombination. The elimination of recombination at the contacts leads to a lifetime of ∼10−6 s.

Improved electrical carrier dynamics and UV detection performance of Zn1-xWxO nanostructured thin films

Current work reports development of Zn1-xWxO nanostructured thin films with Tungsten (W) doping level of 0.0, 1.0, 2.5 and 5.0 wt.% onto glass substrates via successive ionic layer adsorption and reaction (SILAR) method. Films demonstrates improved carrier dynamics and UV photodetection in correlation with W doping level and their suitability in advanced optoelectronics devices. The specimen morphological and structural properties is examined via field emission scanning electron spectroscopy (FESEM) and X-ray diffraction (XRD). In order to understand the electronic structure and carrier dynamics, Raman, UV–Vis, photoluminescence (PL) and time resolve photoluminescence (TRPL) Spectroscopy were employed. Films growth reveals crystallization in hexagonal wurtzite structure and preferred orientation along (002) plane (c-axis). In addition, no phase was observed in diffractograms due to W doping in ZnO matrix. An important effect of W incorporation in ZnO matrix was observed on the grain size and morphology as can be seen in FESEM images. Films optical parameters such as band gap, absorption index and refractive index of films are obtained from absorbance, transmittance and reflectance spectra in UV–Vis-NIR region. The PL spectra of films reveals quenching effect after adding W doping level. The TRPL spectra of films shows that the carrier decay time have been improved with increase in W doping concentrations. On the other hand, films UV detection performance are examined, which reveals significant improvement in response-recovery time, responsivity, external quantum efficiency and detectivity.

Origin of Enhanced Overall Water Splitting Efficiency in Aluminum‐Doped SrTiO3 Photocatalyst

AbstractWith near unity quantum efficiency and operational stability surpassing 250 days in outdoor conditions, aluminum‐doped SrTiO3 (Al:SrTiO3) with tailored cocatalysts is one of the promising photocatalysts for scalable solar H2 production. Nevertheless, mechanistic insights behind Al‐doping and Rh cocatalyst‐induced enhanced overall water splitting (OWS) efficiency are not well elucidated. Herein, detailed charge carrier dynamics from sub‐picosecond to milliseconds are unveiled for Al:SrTiO3 by transient (optical and microwave probe) spectroscopy measurements. The obtained transients are rationalized using a theoretical model considering bimolecular recombination, trapping and detrapping processes. Due to a decrease in an n‐type doping density, Al doping of SrTiO3 significantly prolongs bulk carrier lifetime from 50 ns to 12.5 µs (consistent with the previous report). The crucial electron extraction process by the Rh cocatalyst located on the surface from Al:SrTiO3 occurs well before the decay of charge carriers. In contrast, µs‐long electron extraction time observed in SrTiO3 is significantly slower than tens of ns bulk carrier lifetime, thus reducing the photocatalytic OWS reaction. Complementary analysis in conjunction with in situ charge carrier dynamics in water interface addresses the mechanistic insight into Al‐doping‐induced enhancement of OWS activity. Correlating material properties, carrier dynamics and photocatalytic activity is expected to help design next‐generation photocatalysts via dopant and/or defects engineering for efficient solar‐fuel production.

Unraveling the impact of Cu-doping in lead free halide perovskites for markedly enhancing photocatalytic CO2 reduction performance

Excessive CO2 emissions from the consumption of fossil fuels contribute to both global warming and the energy crisis. To address these pressing issues, it is essential to capture and convert CO2 into chemical feedstock using renewable energy such as solar energy. Solar-powered photocatalytic fuel production is widely acknowledged as a viable solution for a sustainable energy future. However, the development of high-performance photocatalysts remains a significant challenge to overcome in this endeavor. Recently, lead-free perovskites have emerged as a promising category of materials for CO2 photoreduction, thanks to their remarkable optoelectronic properties, cost-effective solution processing, and non-toxic nature. Nonetheless, their inherent photocatalytic CO2 reduction activity remains low due to the insufficient separation of photogenerated charges and the lack of active sites. Here, we demonstrate that Cu doping in lead-free double perovskites can lead to markedly enhanced CO2 reduction performance. The interstitial Cu dopants modulate the fermi level and introduce the active energy sites near the conduction band, which significantly enhances light absorption and separation of charge carriers. The Cu-Cs2AgBiCl6 shows the activity of 88 and 48 µmol/gcat for CH4 and CO products which are both higher than those of the pristine Cs2AgBiCl6 (27 and 10 µmol/gcat). The total yield of Cu-Cs2AgBiCl6 perovskite for CO2 photoreduction is superior to those reported under similar conditions. Transient absorption results show that hot carrier relaxation is slower and carrier decay lifetime is extended in Cu-Cs2AgBiCl6, which can enhance the charge migration efficiency and boost the CO2 reduction performance. These findings provide valuable insight into the mechanism of CO2 reduction in Cu-doped Cs2AgBiCl6 perovskite crystals and suggest that Cu-doping is a promising strategy for developing highly efficient and selective CO2 reduction photocatalysts.

Helical Nanofiber Photoelectric Synaptic Devices for an Artificial Vision Nervous System.

Inspired by the helical structure and the resultant exquisite functions of biomolecules, helical polymers have received increasing attention. Here, a series of poly(3-hexylthiophene)-block-poly(phenyl isocyanide) (P3HT-b-PPI) copolymers were prepared using a simple one-pot living polymerization method. Interestingly, the P3HT80-b-PPI30 films were found to have a helical nanofiber structure. The corresponding device has superior optoelectronic properties, such as a broadened spectral response range from the visible band to the deep ultraviolet (DUV) and an approximately 5-fold longer carrier decay time after DUV light stimulation. An energy consumption of 1.44 fJ per synaptic event was obtained, which is the lowest energy consumption achieved so far with DUV light stimulation. The encryption and decryption of images are implemented using an array of devices. Finally, a photoreceptor neural pathway was constructed to achieve early warning for the recognition of the display of harmful light. This research provides an effective strategy for the development of a novel optoelectronic synaptic device.

Ultra-Short Lifetime of Intersubband Electrons in Resonance to GaN-Based LO-Phonons at 92 meV

In this study, we report on the ultra-short lifetime of excited intersubband electrons in a 38 Å wide AlGaN/GaN-based quantum well. The rapid decay of these charge carriers occurs due to a resonance between the relevant intersubband transition energy and the size of the GaN-based LO-phonon at 92 meV. Based on the experimentally observed Lorentz-shaped intersubband emission peak with a spectral width of roughly 6 meV (48 cm−1) respecting the Fourier transform limit, a very short lifetime, namely 111 fs, could be calculated. By comparing this lifetime to the existing literature data, our value confirms the potential high-speed capability of III-nitride-based optoelectronics.

Bulk passivation of perovskite films utilizing halide anion ionic liquids

Chemical additives to perovskite precursors constitute a predominant route to passivate defects in perovskite films and improve carrier charge dynamics. In this work, ionic liquids (ILs) with chloride and iodide anions are used as additives for bulk passivation of perovskite films. The ionic liquids in the form of 1‑butyl‑4-methylpyradinium iodide (IL-I) and 1-ethylimidazolium chloride (IL-Cl) are mixed individually with a precursor of Cesium Formamidinium Lead Triiodide and Methylammonium Chloride (CsFAPbI3−MACl). The decay of charge carriers within the perovskite film before and after passivation with ILs was measured by Time Resolved Photoluminescence Spectroscopy (TRPL). The effect of added ILs on the recombination rate of the photocarriers was investigated by analyzing the decay of the TRPL signal in all samples. The average lifetimes obtained within one day of preparations were 79 ns, 73 ns and 275 ns for pristine perovskite, IL-I passivated perovskite and IL-Cl passivated perovskite films, respectively. These values show that the presence of the IL with chloride anion greatly improves the average lifetime of the charge carrier by around 250%, while IL with iodide anion decreased the lifetime by 32% as compared with the pristine perovskite sample. This finding renders the IL-Cl passivated perovskite films as a valuable resource in applications ranging from photoelectric to hydrogen generation devices.

Boosting Carrier-Phonon Scattering in Cu2-xS Nanochains for 90.5% Photothermal Conversion Efficiency in Near-Infrared II Region.

For plasmonic systems, pursuit of higher photothermal conversion efficiency (PCE) in the near-infrared II region and a relevant photophysical mechanism is helpful for practical applications. Here, we measure the femtosecond transient absorption spectra of Cu2-xS nanochains (PAA-chains-8.9 and PSS-chains-7.3) and nanoparticles (PSS-particles-8.2) to track the excited carrier decay dynamics. The ultrafast carrier-phonon scattering (∼0.33 ps) in PAA-chains-8.9 depletes a vast majority of the excited-state population (>90%). Moreover, the particles have longer decay time for phonon-phonon scattering than the chains. The Fermi level of nanochains is higher than that of nanoparitcles, which affects the attenuation dynamic process of the excited carriers. The PSS-chains-7.3 exhibit higher PCE (88.0%) than PSS-particles-8.2 (82.1%) with a slower phonon-phonon scattering. A remarkable PCE of 90.5% is achieved in PAA-chains-8.9, which represents the highest value in plasmonic photothermal agents. This research indicates strong carrier-phonon scattering and short phonon-phonon scattering processes have great contribution in boosting the PCE.

Quantum Shell in a Shell: Engineering Colloidal Nanocrystals for a High-Intensity Excitation Regime.

Many optoelectronic processes in colloidal semiconductor nanocrystals (NCs) suffer an efficiency decline under high-intensity excitation. This issue is caused by Auger recombination of multiple excitons, which converts the NC energy into excess heat, reducing the efficiency and life span of NC-based devices, including photodetectors, X-ray scintillators, lasers, and high-brightness light-emitting diodes (LEDs). Recently, semiconductor quantum shells (QSs) have emerged as a promising NC geometry for the suppression of Auger decay; however, their optoelectronic performance has been hindered by surface-related carrier losses. Here, we address this issue by introducing quantum shells with a CdS-CdSe-CdS-ZnS core-shell-shell-shell multilayer structure. The ZnS barrier inhibits the surface carrier decay, which increases the photoluminescence (PL) quantum yield (QY) to 90% while retaining a high biexciton emission QY of 79%. The improved QS morphology allows demonstrating one of the longest Auger lifetimes reported for colloidal NCs to date. The reduction of nonradiative losses in QSs also leads to suppressed blinking in single nanoparticles and low-threshold amplified spontaneous emission. We expect that ZnS-encapsulated quantum shells will benefit many applications exploiting high-power optical or electrical excitation regimes.

Liquid Crystalline Mixtures with Induced Polymorphic Smectic Phases Targeted for Energy Investigations

In this study, 4-Biphenyle-4′-alkyloxybenzenamines were synthesized as a homologous series of liquid crystals based on the biphenyl moiety. Their mesomorphic and optical properties were examined for both pure and mixed components. Elemental analysis, NMR, and FT-IR spectroscopy were used to determine the molecular structures of the developed materials. Using differential scanning calorimetry (DSC), the mesophase transitions were studied, and polarized optical microscopy was used to identify the textures of the mesophases (POM). The obtained results showed that all compounds are dimorphic and only have smectic B (SmB) and smectic A (SmA) phases for all terminal side chains, which are enantiotropic. With variably proportionated terminal side chains and a focus on the mesomorphic temperature range, binary phase diagrams were constructed and an induced smectic C phase was achieved (SmC). It was found that terminal chain length affects both conformation and steric impact in the mixed states. The absorption and fluorescence emission spectra of pure as well as their binary mixtures liquid crystalline films were recorded to investigate the optical and photophysical properties. It was noted that, with the increase in alkyl chain length, the energy bandgap increases from 3.24 eV (for C6H13) to 3.37 eV (for C16H33) and charge carrier lifetime decreases, ascribing to the increase in stearic hindrance causing, consequently, the faster decay of charge carriers.

Growth modification via indium surfactant for InGaN/GaN green LED

In this work, indium (In) was introduced as a surfactant during growth of high temperature GaN quantum barriers (QBs) and GaN interlayer of InGaN/GaN green LEDs. A reference LED grown without In-surfactant was also included for comparison. Results suggested that the LED growth was improved by introducing the In-surfactant, especially during the growth of the GaN interlayer. The In-surfactant improved the morphology of the interlayer, hence allowed it to serve as a good surface growth for the LED. Moreover, the LED showed the lowest full width at half maximum of each x-ray diffraction satellite peak when the In-surfactant was introduced in the GaN interlayer, suggesting an effective way to improve the multi-quantum wells. The introduction of the In-surfactant in the GaN interlayer and GaN QBs growths shifted the emission wavelength of the corresponding LEDs towards red (λ emission = 534 nm) with respect to the reference LED where λ emission = 526 nm. Furthermore, the In-surfactant introduction reduced the forward voltage, V f of the corresponding LEDs down to 4.56 V, compared to the reference LED with V f of 5.33 V. It also allowed the LEDs to show faster carrier decay lifetime, and hence higher radiative recombination, particularly when it was introduced in the GaN interlayer growth.

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.