Host Exciton Research Articles

Suppression of Auger Cross Relaxation in Mn-Doped Core-Shell Perovskite Nanocrystals via Wave Function Engineering.

Mn-doped perovskite nanocrystals (NCs) exhibit great application potential because of their unique optical properties. However, the long-lived nature of excited Mn2+ easily leads to the coexistence of excited Mn2+ and host excitons in a single NC, which inevitably induces an Auger cross relaxation between them, thus significantly limiting the luminescent efficiency of Mn2+ due to its competition with internal energy transfer. Herein, we design and prepare a kind of Mn-doped core-shell CsPbCl3@Cs4PbCl6 perovskite NC with Mn2+ doped only in the shell layer, which is expected to suppress this Auger process by spatially separating the electronic wave functions. By using pump-pump-probe transient absorption spectroscopy, we demonstrate that the core-shell structure effectively suppresses the Auger process with the Auger relaxation time notably extended from 12.1 to 148.3 ps. Our finding offers an effective strategy to suppress this Auger process in Mn-doped NCs, which is greatly significant for improving their luminescent efficiency.

Visible to Near‐Infrared Luminescence and Reversible Photochromism in Mn2+/Nd3+ Co‐Doped Double Perovskite for Versatile Applications

AbstractMultifunctional optical materials with temperature sensing and anti‐counterfeiting properties play an essential role in the commercial applications. Herein, Mn2+/Nd3+ co‐doped Cs2AgInCl6 (CAIC) lead‐free double perovskites (DPs) is synthesized through a hydrothermal method, which exhibits visible to near‐infrared (NIR) luminescence and reversible photochromic properties from yellowish to dark purple. The mechanism investigations on the photoluminescence and photochromic phenomena reveal that the incorporation of Mn2+ ions not only acts as an intermediary in the energy transfer process from the host exciton to Nd3+ ions, but also plays a pivotal role in the reversible photochromic response. The temperature‐dependent luminescence, attributed to the contrasting thermal responses of Mn2+ and Nd3+ ions, enables precise temperature sensing via the fluorescence intensity ratio method. In addition, the integration of visible red emission, NIR luminescence, and photochromic properties in a single CAIC: Mn2+/Nd3+ DPs, offers a multilevel anti‐counterfeiting strategy. The multifunctional CAIC: Mn2+/Nd3+ DPs would open up new avenues for advanced optical applications, particularly in the realms of temperature sensing and security anti‐counterfeiting.

Near-critical dark opalescence in out-of-equilibrium SF6

The first-order phase transition between the liquid and gaseous phases ends at a critical point. Critical opalescence occurs at this singularity. Discovered in 1822, it is known to be driven by diverging fluctuations in the density. During the past two decades, boundaries between the gas-like and liquid-like regimes have been theoretically proposed and experimentally explored. Here, we show that fast cooling of near-critical sulfur hexafluoride (SF6), in presence of Earth’s gravity, favors dark opalescence, where visible photons are not merely scattered, but also absorbed. When the isochore fluid is quenched across the critical point, its optical transmittance drops by more than three orders of magnitude in the whole visible range, a feature which does not occur during slow cooling. We show that transmittance shows a dip at 2eV near the critical point, and the system can host excitons with binding energies ranging from 0.5 to 4 eV. The spinodal decomposition of the liquid-gas mixture, by inducing a periodical modulation of the fluid density, can provide a scenario to explain the emergence of this platform for coupling between light and matter. The possible formation of excitons and polaritons points to the irruption of quantum effects in a quintessentially classical context.

Two-Dimensional Hybrid Perovskite with High-Sensitivity Optical Thermometry Sensors.

Optical thermometry has gained significant attention due to its remarkable sensitivity and noninvasive, rapid response to temperature changes. However, achieving both high absolute and relative temperature sensitivity in two-dimensional perovskites presents a substantial challenge. Here, we propose a novel approach to address this issue by designing and synthesizing a new narrow-band blue light-emitting two-dimensional perovskite named (C8H12NO2)2PbBr4 using a straightforward solution-based method. Under excitation of near-ultraviolet light, (C8H12NO2)2PbBr4 shows an ultranarrow emission band with the full width at half-maximum (FWHM) of only 19 nm. Furthermore, its luminescence property can be efficiently tuned by incorporating energy transfer from host excitons to Mn2+. This energy transfer leads to dual emission, encompassing both blue and orange emissions, with an impressive energy transfer efficiency of 38.3%. Additionally, we investigated the temperature-dependent fluorescence intensity ratio between blue emission of (C8H12NO2)2PbBr4 and orange emission of Mn2+. Remarkably, (C8H12NO2)2PbBr4:Mn2+ exhibited maximum absolute sensitivity and relative sensitivity values of 0.055 K-1 and 3.207% K-1, respectively, within the temperature range of 80-360 K. This work highlights the potential of (C8H12NO2)2PbBr4:Mn2+ as a promising candidate for optical thermometry sensor application. Moreover, our findings provide valuable insights into the design of narrow-band blue light-emitting perovskites, enabling the achievement of single-component dual emission in optical thermometry sensors.

Mn2+/Er3+‐Codoped Cs2AgInCl6 Double Perovskites with Dual Emission and Photochromism Properties for Anti‐Counterfeiting Application

AbstractHighly transparent inorganic photochromic materials have several promising applications, especially for anti‐counterfeiting application. Enriching and enhancing the luminescent properties of perovskite by doping transition metals and rare earth elements into the host is a feasible and attractive route. Codoping of Mn2+ and Er3+ can realize novel anti‐counterfeiting properties with upconversion emission and photochromic ability within a double perovskite structure of Cs2AgInCl6.The Cs2AgInCl6: Mn2+, Er3+ single crystal can be observed in a reversible color variation from yellow to dark violet by implementing under 365, 520 nm illumination or thermal treatment. In addition, the crystal emits red and green light under the excitation of 365 nm ultraviolet and 980 nm laser, respectively. The introduction of Mn2+ endowed Cs2AgInCl6 with photochromic ability, and Mn2+ acted as a bridge for energy transfer from host excitons to Er3+, facilitating emission in the visible region. Er3+ achieve upconversion luminescence in the material through its abundant energy levels and acting synergistically with Mn2+. Herein, the Cs2AgInCl6:Mn2+, Er3+ combines various optical properties and has applications for specific anti‐counterfeiting demands.

Effect of temperature on lanthanide charge transition levels and vacuum referred binding energies

Effect of temperature on lanthanide charge transition levels and vacuum referred binding energies

Narrowband Ultraviolet-B Persistent Luminescence in an Indoor-Lighting Environment through Energy Transfer from Host Excitons to Gd3+ Emitters in ScPO4.

Narrowband ultraviolet-B (NB-UVB) luminescent materials are characterized by high photon energy, narrow spectral width, and visible-blind emission, thus holding great promise for photochemistry and photomedicine. However, most NB-UVB phosphors developed so far are photoluminescent, where continuous external excitation is needed. Herein, we realize NB-UVB persistent luminescence (PersL) in an indoor-lighting environment by exploiting the interaction between self-trapped/defect-trapped excitons and Gd3+ emitters in ScPO4. The phosphor shows a self-luminescing feature with a peak maximum at 313 nm with a time duration of >24 h after ceasing X-ray irradiation, which can be clearly imaged by an UVB camera in a bright environment. Spectroscopic and theoretical approaches reveal that thermo- and photo-stimulations of energies trapped at intrinsic lattice defects followed by energy transfer to Gd3+ emitters account for the emergence of the afterglow. The present results can initiate more exploration of NB-UVB PersL phosphors for emerging applications in secret optical tagging and phototherapy.

Strong light-matter interactions of exciton in bulk WS2 and a toroidal dipole resonance.

Exciton-polaritonic states are generated by strong interactions between photons and excitons in nanocavities. Bulk transition metal dichalcogenides (TMDCs) host excitons with a large binding energy at room temperature, and thus are regarded as an ideal platform for realizing exciton-polaritons. In this work, we investigate the strong coupling properties between high-Q toroidal dipole (TD) resonance and bulk WS2 excitons in a hybrid metasurface, consisting of Si3N4 nanodisk arrays with embedded WS2. Multipole decomposition and near-field distribution confirm that Si3N4 nanodisk arrays support strong TD resonance. The TD resonance wavelength is easily tuned to overlap with the bulk WS2 exciton wavelength, and strong coupling is observed when the bulk WS2 is integrated with the hollow nanodisk and the oscillator strength of the WS2 material is adjusted to be greater than 0.6. The Rabi splitting of the hybrid device is up to 65 meV. In addition, strong coupling is confirmed by the anticrossing of fluorescence enhancement in the hybrid Si3N4-WS2 metastructure. Our findings are expected to be of importance for both fundamental research in TMDC-based light-matter interactions and practical applications in the design of high-performance exciton-polariton devices.

Doping Mn2+ in hybrid Ruddlesden–Popper phase of layered double perovskite (BA)4AgBiBr8

The layered hybrid double perovskites emerged as excellent semiconductor materials owing to their environment compatibility and stability. However, these materials are weakly luminescent, and their photoluminescence (PL) properties can be modulated via doping. While Mn2+ doping in perovskites is well known, but to the best of our knowledge the doping of Mn2+ in layered double perovskites (LDPs) is yet to be explored. Herein, for the first time, we demonstrate the doping of Mn2+ in hybrid inorganic-organic two-dimensional (2D) LDPs, (BA)4AgBiBr8 (BA = n-butyl amine) via a simple solid-state mechanochemical route. The powder x-ray diffraction pattern, and electron paramagnetic resonance analysis confirm the successful incorporation of Mn2+ ions inside (BA)4AgBiBr8 lattice. The Mn2+ doped 2D LDP shows energy transfer from host excitons to d-electrons of Mn2+ ions, which results in red-shifted broad Mn2+ emission band centered at 625 nm, attributed to the spin-forbidden 4T1 to 6A1 internal transition. This work opens up new possibilities to dope metal ions in 2D LDPs to tune the optical as well as magnetic properties.

Halogen‐content‐dependent photoluminescence of Mn2+‐doped CsPbCl3 nanocrystals

AbstractThe thermodynamically viable halogen vacancy defects in CsPbCl3 perovskite nanocrystals (NCs) are considered the main factor in weakening their optical quality. However, their separate impact on the dopant emission in Mn2+‐doped CsPbCl3 perovskite NCs is still under exploration owing to the absence of comparable samples. Herein, a series of Mn2+‐doped CsPbCl3 samples were synthesized with different oleylamine‐Cl (OAm‐Cl) contents based on a halogen‐hot‐injection strategy. It is found that the Mn2+ concentration of as‐prepared samples fixed at 0.65%, and the Mn2+ photoluminescence (PL) also exhibited an OAm‐Cl content‐independent lifetime of ∼ 1.78 ms, implying the efficient and identical luminescence of isolated Mn2+ ion in all samples. In contrast, the excitonic and Mn2+ PL intensities of the NCs are strong related to the amount of OAm‐Cl, which are attributed to the coordination effect of the suppressed excitonic nonradiative recombination owing to the passivation of chloride vacancies and the enhanced energy transfer from the host exciton to Mn2+ ions in the samples with sufficient OAm‐Cl contents. The temperature‐dependent of Mn2+ PL disclosed an initial increase and was followed by a decrease with increasing temperature for the samples with relatively higher OAm‐Cl contents, while it monotonously decreases for the sample with lower OAm‐Cl content of 0.4 mL. The different PL intensity change trend results from the effective passivation of chloride vacancies by extra OAm‐Cl. Above results present here will help clarify the underlying influence mechanism of halogen vacancy on the PL properties of Mn2+‐doped perovskite NCs, which is essential for targeted modulation of their optical properties.

Host-to-Guest Energy Transfer and Its Role in the Lower Stability of Solution-Coated versus Vacuum-Deposited Phosphorescent OLEDs

Electroluminescence (EL) degradation mechanisms in solution-coated (SOL) host:guest (H:G) systems commonly used in phosphorescent organic light-emitting diodes (OLEDs) are investigated and compared to their vacuum-deposited (VAC) counterparts. Changes in the EL, photoluminescence (PL), and time-resolved PL (TRPL) characteristics of devices comprising SOL or VAC H:G light-emitting layers (EMLs) made of the same materials and in the same device architectures during prolonged electrical driving are compared and analyzed. Hole-only devices are also utilized to study the effects of charges and excitons, separately and combined. Moreover, devices with double EMLs comprising SOL and VAC components are tested to glean additional insights into the role of host excitons in device degradation. The results indicate that the faster degradation of SOL EML devices relative to their VAC EML counterparts under electrical stress is due–at least in part–to the less efficient host-to-guest (H → G) energy transfer in these systems, which accelerates molecular aggregation in the EML. Interactions between excitons and polarons in the EMLs induce this aggregation phenomenon which occurs more strongly in the case of SOL EMLs compared to their VAC counterparts because of the higher host exciton concentration in the former as a result of the less efficient H → G energy transfer. The findings shed light on one of the root causes of the limited stability of SOL OLEDs.

In situ preparation of Mn-doped perovskite nanocrystalline films and application to white light emitting devices

Mn-doped perovskite nanocrystals have promised new optoelectronic applications due to their unique material properties. In the present study, Mn-doped perovskite nanocrystalline films were prepared in situ in a polymer matrix. The Mn-doped perovskite nanocrystals (PNCs) had good crystallinity and uniform size/spatial distributions in the polymer film. Bright dual-color emission and the long lifetime of the excited state of the dopant were observed from the host exciton and the Mn2+ dopant, respectively. Furthermore, magnetism was observed in the optimal Mn2+ concentration, implying that magnetic coupling was achieved in the Mn-doped perovskite lattice. The Mn-doped perovskite films also showed superior stability against moisture. To demonstrate the practicality of this composite film, a white light emitting device was fabricated by combining a single composite film with a blue light emitting diode; the device showed a high-quality white light emission, and the Commission Internationale De L'Eclairage (CIE) chromaticity coordinate of the white light emitting diode (WLED) (0.361, 0.326) was close to the optimal white color index. In this single-layer WLED, self-absorption among the luminous multilayers in traditional white light emitting diodes can be avoided. The study findings revealed that Mn-doped perovskite nanocrystalline films have many exciting properties, which bodes well for the fundamental study and design of high-performance optoelectronic devices.

Organic Light‐Emitting Diodes: Modeling Electron‐Transfer Degradation of Organic Light‐Emitting Devices (Adv. Mater. 12/2021)

A chemical model capable of predicting the operational stability for organic light-emitting devices is established, as reported by Dongho Kim, Jun Yeob Lee, Youngmin You, and co-workers in article number 2003832. This model involves Langevin recombination, followed by electron transfer from a dopant to a host exciton, as the key degradation step. The research disentangles the chemical processes in the degradation and provides a useful foundation for enhancing the operational stability of electroluminescence devices.

Modeling Electron-Transfer Degradation of Organic Light-Emitting Devices.

The operational lifetime of organic light-emitting devices (OLEDs) is governed primarily by the intrinsic degradation of the materials. Therefore, a chemical model capable of predicting the operational stability is highly important. Here, a degradation model for OLEDs that exhibit thermally activated delayed fluorescence (TADF) is constructed and validated. The degradation model involves Langevin recombination of charge carriers on hosts, followed by the generation of a polaron pair through reductive electron transfer from a dopant to a host exciton as the initiation steps. The polarons undergo spontaneous decomposition, which competes with ultrafast recovery of the intact materials through charge recombination. Electrical and spectroscopic investigations provide information about the kinetics of each step in the operation and degradation of the devices, thereby enabling the building of mass balances for the key species in the emitting layers. Numerical solutions enable predictions of temporal decreases of the dopant concentration in various TADF emitting layers. The simulation results are in good agreement with experimental operational stabilities. This research disentangles the chemical processes in intrinsic electron-transfer degradation, and provides a useful foundation for improving the longevity of OLEDs.

Efficient External Energy Transfer from Mn-Doped Perovskite Nanocrystals.

Doping with a transition metal is an effective way to tune the optical properties of semiconductor nanocrystals (NCs). The excitation of transition-metal dopants in NCs is through an internal energy transfer from a host exciton, by which the short-lived exciton energy can be "stored" at the dopant for a significantly longer lifetime. Herein, using Mn-doped CsPbCl3 perovskite NCs as an example, we report that the long-lived excited state at Mn dopants can be efficiently extracted from the NCs through an external energy transfer (EET) to rhodamine B (RhB) molecules adsorbed on the NC surface. The EET process leads to a delayed RhB emission. The EET rate is found to increase from 0.16 to 1.42 ms-1 as the number of RhB molecules adsorbed per NC increases from 1 to 8.9, leading to energy extraction efficiency up to 71%. This work suggests the potential of Mn-doped perovskite NCs for applications in photon energy conversion and biological imaging.

Multidimensional Perovskite Cesium Lead Halide Nanocrystal for Light Conversion Applications

Zero-dimensional (0D) inorganic perovskites have recently emerged as a new class of material for optoelectronics owing to their outstanding excitonic properties, strong photoluminescence (PL), and high exciton binding energy. These materials have a unique quantum-confined structure, which originates from the presence of fully isolated octahedra exhibiting single-molecule behavior. In this work, we probed the optical behavior of single molecule-like isolated octahedra in 0D Cs4PbX6 (X = Cl, Br/Cl) perovskite through isovalent (Mn2+) doping at Pb sites. The incorporation of Mn2+ stabilizes the Cs4PbX6 phase by lowering the symmetry of PbX6 via enhanced octahedral distortion (preventing the high-symmetry cubic CsPbX3 impurity) and controlling the compositional variation of Cs-Pb salts. This strategy enabled the synthesis of CsPbX3 free Cs4PbX6 nanocrystals. A high PL quantum yield (QY) of Mn2+ emission was obtained in the colloidal (29%) and solid (21%, powder) forms. These performances can be attributed to structure-induced confinement effects, which enhance the energy transfer from localized host exciton states to Mn2+ dopant within the isolated octahedra of 0D perovskite. The present findings could lead to designing of low-dimensional perovskites as efficient light emitters with photo and chemical stability for high-performance optoelectronic devices.

62‐1: Invited Paper: Understanding Degradation Processes of Organic Light‐Emitting Devices

The short operation lifetime of organic light‐emitting devices originates from irreversible degradation of organic materials, but the mechanism by which degradation initiates has been insufficiently understood. We discovered that degradation is triggered by electron‐transfer interactions between a dopant and a host exciton. A numerical model predicting operational stability has been developed.

Rigid Oxygen‐Bridged Boron‐Based Blue Thermally Activated Delayed Fluorescence Emitter for Organic Light‐Emitting Diode: Approach towards Satisfying High Efficiency and Long Lifetime Together

AbstractThermally activated delayed fluorescence (TADF) materials have emerged as an efficient emitter for achieving high efficiency of blue organic light emitting diodes (OLEDs). However, it is challenging to satisfy both high device efficiency and long operational lifetime together. Here, highly efficient and electrochemically stable blue TADF emitter, 5‐(5,9‐dioxa‐13b‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐10,15‐diphenyl‐10,15‐dihydro‐5H‐diindolo[3,2‐a:3′,2′‐c]carbazole (DBA‐DI) is designed and synthesized for high efficiency and long lifetime OLED. This emitter exhibits high photoluminescence quantum yield of 95.3%, small single‐triplet energy gap of 0.03 eV, short delayed exciton lifetime of 1.25 µs, and high bond dissociation energy (BDE). Also, phosphine oxide free high triplet energy host systems (single and mixed) and exciton blocking layer materials are analyzed using molecular and optical simulations to find an efficient host system with high BDE and suitable emission zone for high efficiency and stable OLEDs. The fabricated OLED with DBA‐DI and high triplet host exhibited a maximum external quantum efficiency (EQE) of 28.1% with blue CIE color coordinates of (0.16, 0.39) and long operational lifetime (LT50) of 329 h at the initial luminance of 1000 cd m−2. Furthermore, the mixed host‐based TADF device showed a slightly lower EQE of 26.4% and almost two times longer lifetime (LT50: 540 h) than the single host device.

Investigating the electronic structure of confined multiexcitons with nonlinear spectroscopies.

Strong confinement in semiconductor quantum dots enables them to host multiple electron-hole pairs or excitons. The excitons in these materials are forced to interact, resulting in quantum-confined multiexcitons (MXs). The MXs are integral to the physics of the electronic properties of these materials and impact their key properties for applications such as gain and light emission. Despite their importance, the electronic structure of MX has yet to be fully characterized. MXs have a complex electronic structure arising from quantum many-body effects, which is challenging for both experiments and theory. Here, we report on the investigation of the electronic structure of MX in colloidal CdSe QDs using time-resolved photoluminescence, state-resolved pump-probe, and two-dimensional spectroscopies. The use of varying excitation energy and intensities enables the observation of many signals from biexcitons and triexcitons. The experiments enable the study of MX structures and dynamics on time scales spanning 6 orders of magnitude and directly reveal dynamics in the biexciton manifold. These results outline the limits of the simple concept of binding energy. The methods of investigations should be applicable to reveal complex many-body physics in other nanomaterials and low-dimensional materials of interest.

Ultrafast Dopant-Induced Exciton Auger-like Recombination in Mn-Doped Perovskite Nanocrystals

Manganese(II)-doped perovskite nanocrystals (NCs), because of their unique dual-color (from host excitons and Mn-dopants) emission property, are promising materials for various optical applications...