Self-trapped Excitons Research Articles

Pressure-Induced Changes in the Phase Distribution and Carrier Dynamics of Quasi-Two-Dimensional Ruddlesden-Popper Perovskites.

Quasi-two-dimensional (quasi-2D) perovskites hold significant potential for diverse design strategies due to their tunable structures, exceptional optical properties, and environmental stability. Due to the complexity of the structure and carrier dynamics, characterization methods such as photoluminescence and absorption spectroscopy can observe but cannot precisely distinguish or identify the phase distribution within quasi-2D perovskite films or correlate phases with carrier dynamics. In this study, we used pressure to modulate the intralayer and interlayer structures of (PEA)2Csn-1PbnBr3n+1 quasi-2D perovskite films, investigating charge carrier dynamics. Steady-state spectroscopy revealed phase transitions at 1.62, 3, and 8 GPa, with free excitons transforming into self-trapped excitons after 8 GPa. Transient absorption spectroscopy elucidated the structural evolution, energy transfer, and pressure-induced transition mechanisms. The results demonstrate that combining pressure and spectroscopy enables the precise identification of phase distribution and pressure response ranges and reveals photophysical mechanisms, providing new insights for optimizing optoelectronic materials.

Exciton-harvesting enabled efficient charged particle detection in zero-dimensional halides

Materials for radiation detection are critically important and urgently demanded in diverse fields, starting from fundamental scientific research to medical diagnostics, homeland security, and environmental monitoring. Low-dimensional halides (LDHs) exhibiting efficient self-trapped exciton (STE) emission with high photoluminescence quantum yield (PLQY) have recently shown a great potential as scintillators. However, an overlooked issue of exciton-exciton interaction in LDHs under ionizing radiation hinders the broadening of its radiation detection applications. Here, we demonstrate an exceptional enhancement of exciton-harvesting efficiency in zero-dimensional (0D) Cs3Cu2I5:Tl halide single crystals by forming strongly localized Tl-bound excitons. Because of the suppression of non-radiative exciton-exciton interaction, an excellent α/β pulse-shape-discrimination (PSD) figure-of-merit (FoM) factor of 2.64, a superior rejection ratio of 10−9, and a high scintillation yield of 26 000 photons MeV−1 under 5.49 MeV α-ray are achieved in Cs3Cu2I5:Tl single crystals, outperforming the commercial ZnS:Ag/PVT composites for charged particle detection applications. Furthermore, a radiation detector prototype based on Cs3Cu2I5:Tl single crystal demonstrates the capability of identifying radioactive 220Rn gas for environmental radiation monitoring applications. We believe that the exciton-harvesting strategy proposed here can greatly boost the applications of LDHs materials.

Small Polaron Dynamics in Transition Metal Oxide Photoelectrodes

The transition metal oxides are widely used in many optoelectronic applications, such as working as charge-transporting layers in solar cells and photoelectrodes in light-driven water-splitting. Considering the ionic bonding nature, the lattice of these oxides can be polarized by the charge carriers due to the strong electron-phonon coupling, giving rise to lattice distortion that fosters a local potential well. This lattice distortion together with the trapped carrier is regarded as a quasi-particle, i.e., a polaron. Here, the photocarrier dynamics in transition metal oxides were investigated using the transient absorption and time-resolved terahertz spectroscopies. We identified the spectral features of small polarons, self-trapped excitons, and free carriers and extracted their dynamics independently. We proposed a molecular model that could successfully explain the non-barrier self-trapping and quantitatively describe the temperature-dependent recombination of self-trapped excitons and the dopant effect on the photocarrier dynamics. Our model also suggested that the recombination of self-trapped excitons at room temperature was dominated by nuclear quantum tunneling. This model also predicted the micro-heterogeneity of the self-trapped species, and thus be able to quantitatively describe the Auger annihilation dynamics of self-trapped excitons.

(Invited) Exploring and Manipulating Luminescence Centers in Lead Halide Perovskites through Hydrostatic Pressure

The application of hydrostatic pressure to materials induces changes in lattice constants, vibrational spectra, and electron-phonon interactions, thereby altering the optoelectronic properties of semiconductors. Consequently, hydrostatic pressure has become a widely utilized tool for investigating and enhancing the optical properties of organic/inorganic halide perovskites and their nanostructures. In this presentation, I will discuss the impact of hydrostatic pressure on two distinct types of luminescence centers in perovskite materials. The first type of luminescence centers originates from excitons within perovskite nanocrystals. I will discuss how hydrostatic pressure serves as a valuable tool for elucidating the origin of luminescence in low-dimensional perovskites, including 2D CsPb2Br5, 0D Cs4PbBr6, and 2D Ruddlesden-Popper Hybrid Perovskite Crystals. The second type of luminescence centers exhibit broadband optical emission from self-trapped excitons in low-dimensional perovskites. I will present our findings on the observation of strong exciton-phonon coupling and demonstrate how hydrostatic pressure can be employed to modulate electron-phonon coupling, offering a means to finely tune the broad emission spectrum.

(Invited) From PL-Inactive to PL-Active: Synthesis and Properties of Novel PL-Active 2D Lead-Free Double Perovskites Based on (PA)4AgInBr8

In this work, novel two-dimensional lead-free double perovskites ((R)4MaMbMcX8, a+b+c=2) were synthesized to achieve novel compositional and dimensional tunability. Specifically, manganese (Mn2+) is incorporated into (PA)4AgInBr8 (PA = CH3(CH2)2NH3+) to form (PA)4Ag1-0.5xMnxIn1-0.5xBr8 (0 ≤ x ≤ 1). These samples were tuned to be PL-emissive under room temperature via this manganese alloying strategy. Three emission pathways can be investigated: band edge emission, emission stemming from self-trapped excitons, and the energy transfer processes associated with Mn2+ center energy levels. Temperature-dependent PL elucidates two emissive modes: self-trapped exciton emission as well as Mn-center emission based on analysis of the associated Huang-Rhys factors. Overall, this work informs the development of additional light-emissive 2D lead-free double perovskites.

(Invited) Optical Excitations and Growth in Layered Organic Metal Halide Semiconductors

Hybrid organic metal halides, such as CH3NH3PbI3, have garnered attention because they are earth-abundant, solution-processable materials that can be used to form solar cells with high power conversion efficiency (>20%). There are significant questions about the unusual electronic properties of this class of materials because of the coupling between excitations and the lattice. We will present our work investigating the optoelectronic properties of layered organic metal halide systems and the relationship to structure and growth conditions. We will discuss the nature of optical excitations in layered compounds of the class R2PbX4 where R is an organic cation and X is a halide. These systems show signatures of unusually strong optical frequency magnetic dipole transitions that can be understood through self-trapped exciton formation. We will then discuss how mechanical strain during growth influences broad photoluminescence in a model layered system based on the large cation ethylammonium (EA) with the structure (EA)2(EA)n-1PbnBr3n+1. In this system the emission is strongly impacted by growth-induced strain in thin films quantified using X-ray scattering. Our results suggest that broad emission, relevant for LEDs, can be tuned in thin films. Recent work on how growth impacts the behavior of low dimensional system will also be presented.

A Single‐Component Sb/Ho: Cs2Na0.9Ag0.1(In/Bi)Cl6 White Phosphor with a Record Color Rendering Index of 97.4

AbstractThe study on phosphors‐converted white light‐emitting diodes (pc‐WLEDs) using lead‐free double perovskites (DPs) as single‐component white phosphors is widely concerned. However, the photoluminescence quantum yields (PLQY) of white luminescence and color rendering index (CRI) of WLED are not satisfactory. Herein, a new Sb/Ho: Cs2Na0.9Ag0.1(In/Bi)Cl6 single‐component white phosphor with the highest PLQY of 93% and a record CRI of 97.4 is reported. Experimental data and theoretical calculations evidence that in addition to broadband yellow emission of the self‐trapped exciton (STE) recombination, the material also exhibits blue emission from Sb3+: 3P1→1S0 transition and red one assigned to Ho3+: 5F5→5I8 transition. Steady‐state and time‐resolved PL spectra verify the existence of two energy transfer channels from both Sb3+ and STE to Ho3+ dopants. As a demo, Sb/Ho: Cs2Na0.9Ag0.1(In/Bi) Cl6‐based WLED is constructed, showing excellent comprehensive optical performance with specially improved saturation red index R9 and blue one R12. This work provides a novel single‐component rare‐earth doped DP white phosphor for high CRI full‐spectrum solid‐state lighting.

Blue-Light-Excitable Red-Emitting Organic Antimony Halides as a Reversible Humidity Sensor.

Zero-dimensional organic antimony halides have attracted significant attention recently due to their structural variety, tunable optical properties, and high luminescence efficiency. Here, a new series of antimony bromide hybrid structures with seesaw [SbBr4] and pyramidal [SbBr5] geometries are reported with low band gaps and blue-light excited red emissions. Their luminescence is attributed to self-trapped excitons with a broadband emission of a large Stokes shift. Their photoluminescence signal is sensitive to water molecules, with a reversible positive correlation in a relative humidity range of 30-90%, enabling them as potential materials for real-time, self-consistent humidity sensors.

Pressure‐Tuning Localized Excitons Toward Enhanced Emission, Photocurrent Enhancement and Piezochromism in Unconventional ACI‐Type 2D Hybrid Perovskites

Simultaneous enhancement of free excitons (FEs) emission and self‐trapped excitons (STEs) emission remains greatly challenging because of the radiative pathway competition. Here, a significant fluorescence improvement, associated with the radiative recombination of both FEs and STEs is firstly achieved in an unconventional ACI‐type hybrid perovskite, (ACA)(MA)PbI4 (ACA=acetamidinium) crystals with {PbI6} octahedron units, through hydrostatic pressure processing. Note that (ACA)(MA)PbI4 exhibits a 91.5‐fold emission enhancement and considerable piezochromism from green to red in a mild pressure interval of 1 atm to 2.5 GPa. The substantial distortion of both individual halide octahedron and the Pb‐I‐Pb angles between two halide octahedra under high pressure indeed determines the pressure‐tuning localized excitons behavior. Upon higher pressure, photocurrent enhancement is also observed, which is attributed to the promoted electronic connectivity in (ACA)(MA)PbI4. The anisotropic compaction reduces the distance between neighboring organic molecules and {PbI6} octahedra, leading to the enhancement of hydrogen bonding interactions. This work not only offers a deep understanding of the structure‐optical relationships of ACI‐type perovskites, but also presents insights into breaking the limits of luminescent efficiency by pressure‐suppressed nonradiative recombination.

Pressure-Tuning Localized Excitons Toward Enhanced Emission, Photocurrent Enhancement and Piezochromism in Unconventional ACI-Type 2D Hybrid Perovskites.

Simultaneous enhancement of free excitons (FEs) emission and self-trapped excitons (STEs) emission remains greatly challenging because of the radiative pathway competition. Here, a significant fluorescence improvement, associated with the radiative recombination of both FEs and STEs is firstly achieved in an unconventional ACI-type hybrid perovskite, (ACA)(MA)PbI4 (ACA=acetamidinium) crystals with {PbI6} octahedron units, through hydrostatic pressure processing. Note that (ACA)(MA)PbI4 exhibits a 91.5-fold emission enhancement and considerable piezochromism from green to red in a mild pressure interval of 1 atm to 2.5 GPa. The substantial distortion of both individual halide octahedron and the Pb-I-Pb angles between two halide octahedra under high pressure indeed determines the pressure-tuning localized excitons behavior. Upon higher pressure, photocurrent enhancement is also observed, which is attributed to the promoted electronic connectivity in (ACA)(MA)PbI4. The anisotropic compaction reduces the distance between neighboring organic molecules and {PbI6} octahedra, leading to the enhancement of hydrogen bonding interactions. This work not only offers a deep understanding of the structure-optical relationships of ACI-type perovskites, but also presents insights into breaking the limits of luminescent efficiency by pressure-suppressed nonradiative recombination.

Ultrafast Scintillator Based on Zirconium‐Doped Cesium Zinc Chloride Single Crystals and Their Charge Carrier Dynamics

AbstractScintillators have been widely used for high‐energy radiation imaging. It is in great demand and challenge to develop scintillator materials with fast decay time, high scintillation light yield, and low detection limit for high‐resolution imaging applications such as time‐of‐flight positron emission tomography. However, it is still a challenge to develop the ultrafast carrier dynamics to achieve 100% ultrafast decay time with high scintillation light yield. To meet the demand, a series of component‐tunable Cs2ZnCl4: x%Zr single crystals as promising ultrafast scintillators is successfully developed. With 8% of Zr‐dopant (Cs2ZnCl4: 8%Zr), the single crystal exhibits high light yield (28000 photons MeV−1) and low detection limit (51 nGy s−1) under X‐ray excitation. Photo‐induced transient absorption signals on sub‐nanosecond and nanosecond scales ensure almost 100% fast decay time (≈3 ns) in nanoseconds under γ‐ray excitation. In particular, the different radiative transition processes are achieved under ultraviolet and high‐energy radiation due to the tunable carrier dynamics from the shallow trapped state to the self‐trapped exciton (STE) state.

Yellow light emitting Cs2Zr0.99Te0.01Cl6 with high efficiency emission via modified co-precipitation method synthesis for the light-emitting device

While lead-free perovskite has attracted widespread interest in the optical field by overcoming their toxicity and stability disadvantages compared to conventional lead-based perovskite, the low photoluminescence quantum yield (PLQY) of lead-free perovskite has greatly limited their commercialization path. In this research, a modified co-precipitation method was used to prepare Cs2ZrCl6, which was achieved by the addition of TeCl4 to shift the emission wavelength from 436 to 556 nm with bright yellow emission, the optimal PLQY of 98.2% was achieved by the addition of Te4+ in the amount of 1 % and the optimization of the reaction concentration. First-principles calculations and spectral analysis of Cs2Zr1-xTexCl6 show that the broadband yellow light emission comes from self-trapped excitons (STEs) in Cs2Zr1-xTexCl6. In addition, the luminous intensity of the prepared Cs2Zr1-xTexCl6 was 57.3, 63.3 and 90.1% of the initial intensity after exposure to air for 30 d, cyclic intermittent heating to 200°C and ultraviolet (UV) irradiation for 7 d, respectively. Finally, Cs2Zr1-xTexCl6 and Cs2ZrCl6:10% Bi were used as phosphors and encapsulated with a UV chip to make three different colors of light-emitting diodes (LED), where the white LED (WLED) had CIE coordinates of (0.3061 0.3244), color temperatures, color rendering indices as well as color tolerances of 6921 K, 84 and 6.52 respectively. This research provides a new approach to improve the performance of lead-free perovskite.

Ultrafast synthesis and luminescence properties of rare earth orthoniobates RENbO4 (RE = La, Eu, Gd, Yb, Lu)

Rare earth orthoniobates (RENbO4) are one kind of important functional materials due to its applications in solid-state phosphors, thermal barrier coatings, and microwave dielectric ceramics. The synthesis of rare earth niobates often needs high reaction temperatures (1300 °C–1700 °C) and long processing times (from hours to tens of hours) in solid-state reactions, which can increase the study time of the relationship between structure and properties. In this work, we used ultrafast high-temperature sintering method to synthesize RENbO4 (RE = La, Eu, Gd, Yb, Lu), and found specific structure and properties in these materials obtained with specific synthetic techniques. Based on the electronegativity scale, the charge transfer energy of lanthanide ions in the YNbO4 crystal was calculated. The rapid synthesis of RENbO4 in a vacuum atmosphere generated more oxygen vacancies, and the structures of [REO8] and [NbO6] were distorted. The shortening of the fluorescence lifetime of LaNbO4 and EuNbO4 was related to the formation of self-trapped excitons facilitated by lattice distortion. The emission peak of LuNbO4 at about 530 nm is attributed to the oxygen vacancy in the niobate group. The reported synthetic methods can provide a fast materials screening route for high melting point inorganic materials.

From nonemission to nearly unity quantum yield: Breaking parity-forbidden transitions in rubidium indium chloride through 5s2 lone pair Sb-doping

Halide perovskites doped with ns2 metal ions have stimulated widespread research owing to their efficient and stable self-trapped emissions. Here we observe the strictly forbidden transition 3P0→1S0 is broken and afford nearly unity triplet orange self-trapped excitons (O-STEs) emission, through 5 s2 lone pair Sb cation doping in host Rb2InCl5·H2O single crystal. In particular, additional triplet blue self-trapped excitons (B-STEs) emission centered at 475 nm, stemming from partially allowed 3P1→1S0, is also observed. Spectroscopic characterization along with literature analysis demonstrates that the behavior of forbidden transition breaking originates from the mixing of 3P0 and 3P1 states. First-principles density-functional theory (DFT) calculations as well confirm the involvement of atomic orbitals in bandgap construction, as evidence of 5s5p-5 s2 transition, affording the STEs emission. Combining the efficient, stable broad emission Rb2In96.39 %Cl5·H2O: 3.61 % Sb3+ crystal with Cs3Cu2Cl5 and Cs3Cu2I5 phosphors, a lead-free white light-emitting diode with a high color rendering index of 93.4 has been achieved. Furthermore, the unique sensitive and steep temperature dependence photoluminescence (PL) lifetimes may prompt this type of self-trapped zero-dimensional (0D) perovskites to great potential in the field of thermometry application.

Warm white-light emission from self-trapped excitons in double-chain metal halide hybrid

Low-dimensional metal halide hybrids with efficient warm white-light emission from self-trapped excitons (STEs) have attracted extensive attention for interior lighting because of their fascinating emission features including high colour rendition, excellent colour stability, and low manufacturing cost. Despite great efforts, the mechanism of STEs in one-dimensional (1D) metal halide hybrids is still unclear, which makes the way to regulate white-light emission is still worth exploring. Here, we prepared a double-chain organic-inorganic metal halide hybrid crystal, (C6H18N2)Pb2Br6∙H2O (DEPBH). Strikingly, DEPBH shows a warm white-light emission with an appropriate correlated colour temperature of 3714 K and high colour rendering index up to 89. Especially, further research reveals that STEs springs from the 1D core-shell quantum-wire frame play a crucial role in such warm white-light emission of DEPBH. This work offers a new route to fabricate single-component warm white-light phosphors in low-dimensional materials and will contribute to the further exploration of their relationships between crystal structure and optical properties.

AbInitio Self-Trapped Excitons.

We propose a new formalism and an effective computational framework to study self-trapped excitons (STEs) in insulators and semiconductors from first principles. Using the many-body Bethe-Salpeter equation in combination with perturbation theory, we are able to obtain the mode- and momentum-resolved exciton-phonon coupling matrix element in a perturbative scheme and explicitly solve the real space localization of the electron (hole), as well as the lattice distortion. Further, this method allows us to compute the STE potential energy surface and evaluate the STE formation energy and Stokes shift. We demonstrate our approach using two-dimensional magnetic semiconductor chromium trihalides and a wide-gap insulator BeO, the latter of which features dark excitons, and make predictions of their Stokes shift and coherent phonon generation which we hope will spark future experiments such as photoluminescence and transient absorption studies.

Enhancing blue light excitation in Pb-free double perovskites by decreasing the energy barrier of the self-trapping exciton and prospects for environment-friendly optoelectronic applications

In Pb-free double perovskites (DPs) optoelectronic material systems, the energy barrier ∆Eb suppression between free excitons (FEs) and self-trapped excitons (STEs) makes it difficult to achieve blue-light excitation even as the bandgap narrows, which has become a major challenge in the development of full-colour displays, lighting, optoelectronics and catalytic technologies. Here, Cs2-aKaAg0.6Na0.4In0.8Bi0.2Cl6-x-yBrx(SH)y DPs, prepared by doping with HS- (hydrosulfide ion), Br- and K+, achieve a blue excitation at 468 nm. The octahedral tilt caused by doping with K+, Br- and HS- ions reduces the local crystal dimensions and thus the energy barrier ∆Eb. The aggregation of electrons around the AgX6 octahedron under the action of HS- contributes to the formation of effective STE emission under blue light excitation. Furthermore, the potential impact of blue-light-excited Cs2-aKaAg0.6Na0.4In0.8Bi0.2Cl6-x-yBrx(SH)y DPs on enhancing the power conversion efficiency (PCE) of photovoltaic modules and the photodegradation efficiency of Rhodamine B (RhB) is explored. The study concludes by highlighting the wide-ranging applications in displays, photovoltaics, and photocatalytic degradation that could be realized through the utilization of blue-light-excited Pb-free DPs.

Efficient deep-blue electroluminescence from Ce-based metal halide

Rare earth ions with d-f transitions (Ce3+, Eu2+) have emerged as promising candidates for electroluminescence applications due to their abundant emission spectra, high light conversion efficiency, and excellent stability. However, directly injecting charge into 4f orbitals remains a significant challenge, resulting in unsatisfied external quantum efficiency and high operating voltage in rare earth light-emitting diodes. Herein, we propose a scheme to solve the difficulty by utilizing the energy transfer process. X-ray photoelectron spectroscopy and transient absorption spectra suggest that the Cs3CeI6 luminescence process is primarily driven by the energy transfer from the I2-based self-trapped exciton to the Ce-based Frenkel exciton. Furthermore, energy transfer efficiency is largely improved by enhancing the spectra overlap between the self-trapped exciton emission and the Ce-based Frenkel exciton excitation. When implemented as an active layer in light-emitting diodes, they show the maximum brightness and external quantum efficiency of 1073 cd m−2 and 7.9%, respectively.

Synthesis and soft crystal structure-induced broad emission of (NH3C6H12NH3)InBr5·2H2O.

We report a simple synthesis of a new lead-free zero-dimensional (0D) hybrid halide compound, (5P1)InBr5·2H2O [(5P1) = NH3C6H12NH3], which hosts isolated and distorted octahedra of [InBr5(H2O)]2-, surrounded by bulky asymmetric organic cations [(5P1)2+] and H2O molecules. The hybrid crystals exhibit broad self trapped excitonic (STE) emission due to strong anharmonic soft structure.

Low-dimensional hybrid copper(I) halides single crystals: synthesis, structures, and tunable photoluminescence

Low-dimensional organic–inorganic hybrid copper(I) compounds have attracted much attention due to their high luminescence efficiency and non-toxicity. In this study, we synthesized three novel centimeter-scale Cu(I)-based hybrid [1,2-PDA]CuX3 (X=Cl, Br, I) single crystals using the solvent evaporation method. [1,2-PDA]CuCl3 and [1,2-PDA]CuBr3 exhibit zero-dimensional crystal structures composed of separated binary edge-sharing [CuX4]3- (X=Cl, Br) tetrahedra, while [1,2-PDA]CuI3 exhibits a one-dimensional crystal structure composed of chains of corner-sharing [CuI4]3- tetrahedra. The photoluminescence emission of [1,2-PDA]CuX3 (X=I, Cl, Br) ranges from cyan to purple light at room temperature, showing a tunable pattern with the halogen changes. The photoluminescence quantum yield of the [1,2-PDA]CuI3 single crystal was measured to be 33.5 %. Density functional theory calculations demonstrate that all three single crystals have direct bandgaps. The emission mechanisms of [1,2-PDA]CuI3 single crystals were investigated by temperature-dependent photoluminescence spectroscopy, and the broadband emissions were found to be typical of self-trapped exciton emission. It is worth mentioning that the discovery of low toxic, stable, and inexpensive copper(I) compounds paves the way for considering low-cost and environmentally friendly copper halides for practical applications.