Self-trapped Carriers Research Articles

Pressure-Induced Distinct Self-Trapped Exciton Emission in Sb3+-Doped Cs2NaInCl6 Double Perovskite

Abstract The Cs2NaInCl6 double perovskite is one of the most promising lead-free perovskites due to its exceptional stability and straightforward synthesis. However, it faces challenges related to inefficient photoluminescence. Doping and high pressure are employed to tailor the optical properties of Cs2NaInCl6. Herein, Sb3+ doped Cs2NaInCl6 (Sb3+:Cs2NaInCl6) was synthesized and it exhibits blue emission with a photoluminescence quantum yield of up to 37.3%. Further, by employing pressure tuning, a blue stable emission under a very wide range from 2.7 GPa to 9.8 GPa is realized in Sb3+:Cs2NaInCl6. Subsequently, the emission intensity of Sb3+:Cs2NaInCl6 experiences a significant increase (3.3 times) at 19.0 GPa. It is revealed that the pressure-induced distinct emissions can be attributed to the carrier self-trapping and detrapping between Cs2NaInCl6 and Sb3+. Notably, the lattice compression in the cubic phase inevitably modifies the band gap of Sb3+:Cs2NaInCl6. Our findings provide valuable insights into effects of the high pressure in further boosting unique emission characteristics but also offer promising opportunities for development of doped double perovskites with enhanced optical functionalities.

The influence of solvent refractive index on the photoluminescence decay of thick-shell gradient-alloyed colloidal quantum dots investigated in a wide range of delay times.

Colloidal semiconductor quantum dots have many potential optical applications, including quantum dot light-emitting diodes, single-photon sources, or biological luminescent markers. The optical properties of colloidal quantum dots can be affected by their dielectric environment. This study investigated the photoluminescence (PL) decay of thick-shell gradient-alloyed colloidal semiconductor quantum dots as a function of solvent refractive index. These measurements were conducted in a wide range of delay times to account for both the initial spontaneous decay of excitons and the delayed emission of excitons that has the form of a power law. It is shown that whereas the initial spontaneous PL decay is very sensitive to the refractive index of the solvent, the power-law delayed emission of excitons is not. Our results seem to exclude the possibility of carrier self-trapping in the considered solvents and suggest the existence of trap states inside the quantum dots. Finally, our data show that the average exciton lifetime significantly decreases as a function of the solvent refractive index. The change in exciton lifetime is qualitatively modeled and discussed.

Coherent phonon signal amplification technique based on a numerical differentiation method

Coherent optical phonon (COP) and coherent acoustic phonon (CAP) play vital roles in probing ultrafast dynamics across various materials and systems. Their applications range from metamaterial diagnosis to manipulating ferroelectric polarization and self-trapping of carriers in perovskites. However, the inherently small signals associated with COP and CAP pose challenges in extracting valuable information. In this paper, we address this challenge by presenting a numerical differential method for signal amplification and deduction of phonon dephasing time. In addition, application of this method has been demonstrated in a superlattice system, establishing the validity and effectiveness of this technique.

Role of Self-Trapped Excitons in the Broadband Emission of Lead-Free Perovskite-Inspired Cu2AgBiI6.

The perovskite-inspired Cu2AgBiI6 (CABI) absorber shows promise for low-toxicity indoor photovoltaics. However, the carrier self-trapping in this material limits its photovoltaic performance. Herein, we examine the self-trapping mechanism in CABI by analyzing the excited-state dynamics of its absorption band at 425 nm, which is responsible for the self-trapped exciton emission, using a combination of photoluminescence and ultrafast transient absorption spectroscopies. Photoexcitation in CABI rapidly generates charge carriers in the silver iodide lattice sites, which localize into the self-trapped states and luminesce. Furthermore, a Cu-Ag-I-rich phase that exhibits similar spectral responses as CABI is synthesized, and a comprehensive structural and photophysical study of this phase provides insights into the nature of the excited states of CABI. Overall, this work explains the origin of self-trapping in CABI. This understanding will play a crucial role in optimizing its optoelectronic properties. It also encourages compositional engineering as the key to suppressing self-trapping in CABI.

(Invited) Small Polarons and Surface Defects in Metal Oxide Photocatalysts Studied Using Xuv Reflection-Absorption Spectroscopy

In photocatalytic transition metal oxides, the surface electronic structure and carrier kinetics are complicated by charge trapping at defects as well as by carrier self-trapping as small polarons. These processes are unique to the material interface, where surface carrier dynamics deviate from bulk behavior due to the presence of surface defect states, undercoordination, or interface bonding. This talk will highlight the ability to directly visualize these processes with surface sensitivity, chemical state resolution, and ultrafast time resolution using extreme ultraviolet (XUV) reflection-absorption spectroscopy. This method measures element-specific, core-to-valence spectra analogous to x-ray absorption, but with the added benefits of surface sensitivity and ultrafast time resolution. Using this method, we investigate surface trapping and recombination in α-Fe2O3 and TiO2 as two prototypical metal oxide photocatalysts. In α-Fe2O3, we observe ultrafast polaron formation with a rate that is three times slower than observed in bulk. Kinetic analysis reveals that this is a result of increased lattice relaxation energy for polaron formation at a surface, and we show that these rates can be rationally tuned by surface molecular functionalization. In contrast, in photoexcited TiO2 we observe that hot holes trap as small polarons at oxygen vacancy defects in approximately 50 fs, and hot electrons couple to polar optical phonons leading to vibrational coherence and large polaron formation in approximately 1 ps. These findings provide examples of the unique opportunities enabled by XUV spectroscopy for understanding surface electron dynamics in photocatalytic systems.

Coupled Electronic and Anharmonic Structural Dynamics for Carrier Self-Trapping in Photovoltaic Antimony Chalcogenides.

V–VI antimony chalcogenide semiconductors have shown exciting potentials for thin film photovoltaic applications. However, their solar cell efficiencies are strongly hampered by anomalously large voltage loss (>0.6 V), whose origin remains controversial so far. Herein, by combining ultrafast pump–probe spectroscopy and density functional theory (DFT) calculation, the coupled electronic and structural dynamics leading to excited state self‐trapping in antimony chalcogenides with atomic level characterizations is reported. The electronic dynamics in Sb2Se3 indicates a ≈20 ps barrierless intrinsic self‐trapping, with electron localization and accompanied lattice distortion given by DFT calculations. Furthermore, impulsive vibrational coherences unveil key Sb—Se vibrational modes and their real‐time interplay that drive initial excited state relaxation and energy dissipation toward stabilized small polaron through electron–phonon and subsequent phonon–phonon coupling. This study's findings provide conclusive evidence of carrier self‐trapping arising from intrinsic lattice anharmonicity and polaronic effect in antimony chalcogenides and a new understanding on the coupled electronic and structural dynamics for redefining excited state properties in soft semiconductor materials.

Bandgap and electrochemical engineering for disordered LaFeO3

Being a charge transfer Mott insulator with low metal-insulator temperature, disordered LaFeO3 (as an air electrode) is a potential material for electrochemical engineering. High electronic conductivity and inexpensive catalytic activity are the prime requisites for electrode materials. Here, we systematically substituted hole (Sr) at La-site and electron (Ti) at Fe-site. Our objective is here twofold, first to get material with improved catalytic behavior and to understand the role of bandgap engineering. All the investigated samples are oxygen-rich and show orthorhombic(Pbnm) phase at 300 K. The prominent cation vacancies are closely associated with the self-trapping of carriers. These trapping centers may be detrimental if they shift toward conduction band minima. However, this energy shift can be controlled with proper selection of substitution but require intricate understanding. At 300 K, we observe higher conductivity with band shrinkage for hole substitution, i.e., La0.5Sr0.5FeO3 (LSF). A field dependent electrical study reveals trap free conduction for the hole substituted sample while undoped and compensated show ohmic conduction. Our results suggest that there is a correlation between band tuning and corrosion resistance. Dynamic numerical simulation, where interfacial electrochemical behavior up to 4000 s is studied, suggests that LSF has the least surface charge degradation. Experimentally, LSF proves to be a robust and efficient electrode due to chronopotentiometry stability with a potential of 2 V (vs Ag/AgCl) at a higher current of 150 mA/cm2 in neutral media. Further, this study provides concomitant charge dynamics for these samples and the foster mechanism to get better air electrodes.

Ultrafast Excited-State Localization in Cs2AgBiBr6 Double Perovskite.

Cs2AgBiBr6 is a promising metal halide double perovskite offering the possibility of efficient photovoltaic devices based on lead-free materials. Here, we report on the evolution of photoexcited charge carriers in Cs2AgBiBr6 using a combination of temperature-dependent photoluminescence, absorption and optical pump–terahertz probe spectroscopy. We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm2 V–1 s–1 at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge whose band gap and associated exciton binding energy shrink with increasing temperature in a correlated manner. Overall, our observations suggest that strong electron–phonon coupling in this material induces rapid charge-carrier localization.

Large (bi)Polarons for Novel Energy Conversion and Superconductivity

Materials containing high densities of exceptionally displaceable ions (e.g. perovskites) have extremely large ratios of their static to high-frequency dielectric constants, > 2. Large polarons form in such materials as their electronic charge carriers self-trap by displacing surrounding ions. Large polarons are very heavy-massed slow-moving quasi-particles that are very weakly scattered by ambient phonons. Large-polaron mobilities, e.g. 1 cm2/V-sec at 300 K, are much smaller than the minimum possible for conventional electronic charge carriers. The minimum mobility for an itinerant charge carrier of effective mass m, eh/mkT, occurs when its mean-free-path falls to its de Broglie wavelength, e.g. 300 cm2/V-sec at room temperature for m equaling the free-electron mass. Distinctively, large-polarons frequency-dependent conductivities consist of two contributions that separate as the temperature is reduced. Large polarons Drude-like contributions are relegated to frequencies below those of characteristic phonons. Contributions from excitations of large polarons self-trapped electronic carriers occur above those of characteristic phonons. Oppositely charged large polarons repel one another at short range. The resulting suppressed recombination facilitates exceptionally efficient solar cells. Large polarons of the same charge attract one another at short range to enable their real-space pairing into singlet bipolarons. Additional attractions between large bipolarons facilitates their condensation into liquids that can exhibit superconductivity.

Controlling polaron formation at hematite surfaces by molecular functionalization probed by XUV reflection-absorption spectroscopy.

Small polaron formation is known to limit the photocatalytic charge transport efficiency of hematite via ultrafast carrier self-trapping. While small polaron formation is known to occur in bulk hematite, a complete description of surface polaron formation in this material is not fully understood. Theoretical predictions indicate that the kinetics and thermodynamics of surface polaron formation are different than those in bulk. However, to test these predictions requires the ability to experimentally differentiate polaron formation dynamics at the surface. Near grazing angle extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy is surface sensitive and provides element and oxidation state specific information on a femtosecond time scale. Using XUV-RA, we provide a systematic comparison between surface and bulk polaron formation kinetics and energetics in photoexcited hematite. We find that the rate of surface polaron formation (250 ± 40 fs) is about three times slower than bulk polaron formation (90 ± 5 fs) in photoexcited hematite. Additionally, we show that the surface polaron formation rate can be systematically tuned by surface molecular functionalization. Within the framework of a Marcus type model, the kinetics and energetics of polaron formation are discussed. The slower polaron formation rate observed at the surface is found to result from a greater lattice reorganization relative to bulk hematite, while surface functionalization is shown to tune both the lattice reorganization as well as the polaron stabilization energies. The ability to tune the kinetics and energetics of polaron formation and hopping by molecular functionalization provides the opportunity to synthetically control electron transport in hematite.

Exploring Polaronic, Excitonic Structures and Luminescence in Cs4PbBr6/CsPbBr3.

Among the important family of halide perovskites, one particular case of all-inorganic, 0-D Cs4PbBr6 and 3-D CsPbBr3-based nanostructures and thin films is witnessing intense activity due to ultrafast luminescence with high quantum yield. To understand their emissive behavior, we use hybrid density functional calculations to first compare the ground-state electronic structure of the two prospective compounds. The dispersive band edges of CsPbBr3 do not support self-trapped carriers, which agrees with reports of weak exciton binding energy and high photocurrent. The larger gap 0-D material Cs4PbBr6, however, reveals polaronic and excitonic features. We show that those lattice-coupled carriers are likely responsible for observed ultraviolet emission around ∼375 nm, reported in bulk Cs4PbBr6 and Cs4PbBr6/CsPbBr3 composites. Ionization potential calculations and estimates of type-I band alignment support the notion of quantum confinement leading to fast, green emission from CsPbBr3 nanostructures embedded in Cs4PbBr6.

Intrinsic Broadband White-Light Emission from Ultrastable, Cationic Lead Halide Layered Materials.

We report a family of cationic lead halide layered materials, formulated as [Pb2 X2 ]2+ [- O2 C(CH)2 CO2- ] (X=F, Cl, Br), exhibiting pronounced broadband white-light emission in bulk form. These well-defined PbX-based structures achieve an external quantum efficiency as high as 11.8 %, which is comparable to the highest reported value (ca.9 %) for broadband phosphors based on layered organolead halide perovskites. More importantly, our cationic materials are ultrastable lead halide materials, which overcome the air/moisture-sensitivity problems of lead perovskites. In contrast to the perovskites and other bulk emitters, the white-light emission intensity of our materials remains undiminished after continuous UV irradiation for 30 days under atmospheric conditions (ca.60 % relative humidity). Our mechanistic studies confirm that the broadband emission is ascribed to short-range electron-phonon coupling in the strongly deformable lattice and generated self-trapped carriers.

Anderson metal–insulator transition in doped polar cuprates

In this paper, we investigate the possibility of the Anderson transition for polarons, different dopants, and impurities in doped cuprates. We have developed the continuum theory of carrier self-trapping in a deformable lattice and near different dopants (impurities), the appropriate variational methods and tight-binding model for studying the formation of the localized hole states and the narrow energy bands of large polarons and dopants (impurities) in hole-doped cuprates $$\mathrm{La}_{2-x}\mathrm{Sr}_{x}\mathrm{CuO}_{4}$$ (LSCO) and $$\mathrm{YBa}_{2}\mathrm{Cu}_{3}\mathrm{O}_{7-\delta }$$ (YBCO). To develop the quantitative theory of the Anderson localization and derive the adequate quantitative criteria for the Anderson metal–insulator transition (MIT) in doped materials, we have studied localized states of non-interacting hole carriers in crystalline solids and metal–insulator transitions (MITs) caused by disorder in distribution of large polarons, dopants (or impurities). It is suggested that the large polarons, dopants, and impurities from different superlattices with different degrees of disorder. We have considered a crystalline array of potential wells with the depth $$U = U_{0}$$ in the absence of disorder and developed the approximate methods for the quantitative estimation of the possible changes (or fluctuations) of the depth U of these potential wells between the limits $$U_{0}-V_{0}/2<U<U_{0}+V_{0}/2$$ in the presence of disorder, where $$V_{0}$$ is a random potential which is determined as the amplitude of the change of U due to random distribution of large polarons and dopants (impurities). Using the uncertainty principle, we have obtained the new and more adequate criteria for the Anderson transition in doped polar materials and analyzed the applicability of these criteria to the hole-doped cuprates both in the absence and in the presence of polaronic effects. We have shown that our theoretical results on Anderson MIT obtained by taking into account polaronic effects are in quantitative agreement with the well-established experimental results on MITs in hole-doped cuprates LSCO and YBCO.

Atomic insight to lattice distortions caused by carrier self-trapping in oxide materials.

We gain hitherto missing access to the spatio-temporal evolution of lattice distortions caused by carrier self-trapping in the class of oxide materials - and beyond. The joint experimental/theoretical tool introduced combines femtosecond mid-infrared probe spectroscopy with potential landscape modeling and is based on the original approach that the vibration mode of a biatomic molecule is capable to probe strongly localized, short-lived lattice distortions in its neighborhood. Optically generated, small, strong-coupling polarons in lithium niobate, mediated by OH− ions present as ubiquitous impurities, serve as a prominent example. Polaron trapping is found to result in an experimentally determined redshift of the OH− stretching mode amounting to Δνvib = −3 cm−1, that is successfully modeled by a static Morse potential modified by Coulomb potential changes due to the displacements of the surrounding ions and the trapped charge carrier. The evolution of the trapping process can also be highlighted by monitoring the dynamics of the vibrational shift making the method an important tool for studying various systems and applications.

Direct Time-Resolved Observation of Carrier Trapping and Polaron Conductivity in BiVO4

BiVO4 is a promising photoanode candidate for water splitting applications, but its microscopic charge carrier transport properties are not yet fully understood. We investigated the photoinduced carrier mobility for undoped and 1% tungsten-doped BiVO4 thin films in an early time window from 1 ps to 1 ns using THz spectroscopy. The combined electron–hole effective mobility gradually decreases with time by 1 order of magnitude starting at an upper limit of ∼0.4 cm2 V–1 s–1. The loss is attributed to carrier localization. We provide for the first time direct time-resolved evidence of hole polaron formation accompanied by the temporal buildup of a polaron population in parallel to initial carrier trapping. A mobility of 0.02 cm2 V–1 s–1 is found for the self-trapped carriers, which leads to a thermal hopping activation energy of ∼90 meV.

Probing the Metal-Insulator Transition in BaTiO_{3} by Electrostatic Doping.

The metal-to-insulator transition in BaTiO_{3} is investigated using electrostatic doping, which avoids effects from disorder and strain that would accompany chemical doping. SmTiO_{3}/BaTiO_{3}/SrTiO_{3} heterostructures are doped with a constant sheet carrier density of 3×10^{14} cm^{-2} that is introduced via the polar SmTiO_{3}/BaTiO_{3} interface. Below a critical BaTiO_{3} thickness, the structures exhibit metallic behavior with high carrier mobilities at low temperatures, similar to SmTiO_{3}/SrTiO_{3} interfaces. Above this thickness, data indicate that the BaTiO_{3} layer becomes ferroelectric. The BaTiO_{3} lattice parameters increase to a value consistent with a strained, tetragonal unit cell, the structures are insulating below ∼125 K, and the mobility drops by more than an order of magnitude, indicating self-trapping of carriers. The results shed light on the interplay between charge carriers and ferroelectricity.

Unravelling Small-Polaron Transport in Metal Oxide Photoelectrodes.

Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10(-4) and 10(-2) cm(2) V(-1) s(-1), respectively. Future directions to build a full picture of charge transport in this family of materials are discussed.

Low temperature luminescence and charge carrier trapping in a cryogenic scintillator Li2MoO4

The luminescence and optical properties of promising cryogenic scintillator Li2MoO4 were studied in the temperature region of 2–300K. The data on luminescence spectra and decay characteristics, excitation spectra, thermostimulated luminescence curves and spectra as well as transmission and reflectivity spectra are presented for the single crystals grown by two different procedures, the conventional Czochralski method and the low-temperature gradient Czochralski technique. The bandgap of Li2MoO4 is estimated from the analysis of transmission, luminescence excitation and reflectivity spectra. Up to three luminescence bands with the maxima at 1.98, 2.08 and 2.25eV are detected in the emission spectra of crystals and their origin is discussed. In the thermoluminescence curves of both studied crystals, two high-intensity peaks were observed at 22 and 42K, which are ascribed to the thermal release of self-trapped charge carriers. The coexistence of self-trapped electrons and holes allows one to explain the poor scintillation light yield of Li2MoO4 at low temperatures.

In-plane conductivity of a layered large-bipolaron liquid

Distinctive normal-state properties of cuprate superconductors follow from their charge carriers forming a large-bipolaron liquid. The very weak scattering of the liquid’s slow-moving heavy-massed excitations by acoustic phonons yields a scattering rate that is less than the Debye frequency. The liquid’s moderate mobility, >1 cm2/V-sec at 300 K, results from its weak scattering compensating for its large mass. In resolution of a long-standing dilemma, the dc resistivity resulting from scattering by long-wavelength phonons remains nearly proportional to temperature to well below the Debye temperature. Above the Debye frequency, the frequency-dependent conductivity is dominated by excitation and photo-ionization of the liquid’s self-trapped electronic carriers. Below the Debye frequency, the frequency-dependent conductivity is dominated by the liquid’s Drude-like collective motion. The ‘gap’ between these two domains sharpens with decreasing temperature as phonon scattering of the liquid’s collective excitations diminishes. The high-frequency electronic excitations survive in the superconducting state.

Semiconducting transition-metal oxides based ond5cations: Theory for MnO and Fe2O3

Transition-metal oxides with partially filled $d$ shells are typically Mott or charge-transfer insulators with notoriously poor transport properties due to large effective electron/hole masses or due to carrier self-trapping. Employing band-structure calculations and ab initio small-polaron theory for MnO and Fe${}_{2}$O${}_{3}$, we explore the potential of ${d}^{5}$ oxides for achieving desirable semiconducting properties, e.g., in solar energy applications. The quantification of self-trapping energies and the trends with the coordination symmetry suggest strategies to overcome the main bottlenecks, i.e., the tendency for self-trapping of holes due to Mn(II) and of electrons due to Fe(III).