Self-trapped Exciton Emission Research Articles

Free and self-trapped exciton emission in perovskite CsPbBr3 microcrystals.

The all-inorganic perovskite CsPbBr3 has been capturing extensive attention due to its high quantum yield in luminescence devices and relatively high stability. Its luminescence is dominated by free exciton (FE) recombination but additional emission peaks were also commonly observed. In this work, a CsPbBr3 microcrystal sample in the orthorhombic phase was prepared by the chemical vapor deposition method. In addition to the FE peak, a broad emission peak was found in this sample and it was attributed to self-trapped excitons (STEs) based on its photophysical properties. The STE emission can only be observed below 70 K. The derived Huang–Rhys factor is ∼12 and the corresponding phonon energy is 15.3 meV. Its lifetime is 123 ns at 10 K, much longer than that of FE emission. The STE emission is thought to be an intrinsic property of CsPbBr3.

Photoluminescence enhancement study in a Bi-doped Cs2AgInCl6 double perovskite by pressure and temperature-dependent self-trapped exciton emission.

Here, we report a halide precursor acid precipitation method to synthesize Cs2AgIn1-xBixCl6 (x = 0, 0.02, 0.04, 0.08, 0.16, 0.32, 0.64, and 1) microcrystals. Cs2AgInCl6 and Bi derivative double perovskites show broadband white light emission via self-trapped excitons (STEs) and have achieved the highest internal quantum efficiency of up to 52.4% at x = 0.08. Synchrotron X-ray diffraction confirmed the linear increase of lattice parameters and cell volume with Bi3+ substitution at In3+ sites. Absorbance, photocurrent excitation, and photoluminescence excitation spectra are used to observe possible transitions from the valence to the conduction band or free exciton (FE) states as well as transitions within local Bi3+ states. The broadband photoluminescence is quenched via a single nonradiative process with an activation energy ΔE = 1490 cm-1 for Cs2AgIn0.92Bi0.08Cl6. Under normal conditions, we observed STE emission, but applying external pressure alters the electronic structure such that at elevated pressure, the only emission via the FE state is observed. We anticipate that structure, temperature and pressure-dependent photoluminescence studies will help the future use of a single-source lead-free double perovskite for white light-emitting diode applications.

Evolution of self-trapped exciton emission tuned by high pressure in 2D all-inorganic cesium lead halide nanosheets

Significantly enhanced broad self-trapped exciton (STE) emission was observed for 2D CsPbCl3 perovskite nanosheets under an applied pressure at room temperature.

Te4+-doped zero-dimensional Cs2ZnCl4 single crystals for broadband yellow light emission

Bright broadband yellow light emission peaking at 570 nm has been successfully obtained via incorporating Te4+ ions into Cs2ZnCl4 single crystals, and this is attributed to self-trapped exciton (STE) emission from the distorted tetrahedral units.

Enhancement of Self-Trapped Exciton and Near-Infrared Emission in Bi3+/Er3+ Co-Doped Cs2ag0.4na0.6incl6 Double Perovskite for Optical Telecommunications

Enhancement of Self-Trapped Exciton and Near-Infrared Emission in Bi3+/Er3+ Co-Doped Cs2ag0.4na0.6incl6 Double Perovskite for Optical Telecommunications

Cu substitution boosts self-trapped exciton emission in zinc-based metal halides for sky-blue light-emitting diodes

The PLQY of Cu:(PEA)2ZnX4 metal halides was greatly boosted after Cu substitution and photophysical mechanisms are revealed. Further, Cu:(PEA)2ZnX4 was used as the active layer and sky-blue light-emitting diodes were prepared for the first time.

Exciton-Phonon Coupling and Low Energy Emission in 2D and Quasi-2D BA2MAn-1PbnI3n+1 Thin Films with Improved Phase Purity.

Phonon scattering with photogenerated excitons and free charges greatly affects optoelectronic properties of metal halide perovskites and governs their emission line width. Benefiting from the improved phase purity, we are able to analyze exciton-phonon coupling in 2D and quasi-2D BA2MAn-1PbnI3n+1 (n = 1-3) thin films using temperature-dependent photoluminescence (PL) spectroscopy. The layer thickness (n value) dependent coupling of free excitons with both acoustic and longitudinal optical (LO) phonons was extracted quantitatively by fitting the temperature-dependent PL line width and band gap. The low energy emissive signatures below free excitons at low temperature might belong to the emission of self-trapped excitons and bounded excitons in structural defects. Our findings provide a systematic picture for the layer thickness (n value) dependent exciton-phonon coupling in 2D and quasi-2D perovskite thin films and could be helpful for improving the optoelectronic performance of devices made by Ruddlesden-Popper perovskite thin films.

Efficient Broadband Yellow-Green Emission of Vacancy Halide Double Perovskites Through the Ion-Exchanged Strategy.

Broadband high-efficiency luminescent materials have become a hot spot in lead-free perovskite research. There are relatively few broadband yellow-green phosphors with both ultraviolet and blue excitation ranges, which make them more suitable for phosphor-converted white LEDs. Through the ion-exchanged strategy, Cs2Hf1-xTexCl6 (CH1-xTxC) vacancy halide double perovskites were successfully prepared at room temperature. Using different excitation ranges of CH1-xTxC, two types of high-quality white LEDs are obtained. By combining density functional theory calculations and experiments, it is proved that this bright broadband yellow-green emission (photoluminescence quantum yield of 83.46%) is not only derived from the ion transitions of Te4+ but also exhibits the inherent characteristics of self-trapped exciton emission. Our results not only broaden the application fields of lead-free halide perovskites but also provide further insights into the luminescence mechanism.

Novel NASICON-type Na3.6Y1.8-x(PO4)3:xDy3+ phosphor: Structure and luminescence

Single-phase Na3.6Y1.8-x(PO4)3:xDy3+ phosphors were synthesized by high-temperature solid-state reaction. Powder X-ray diffraction study reveals that a homogeneous solid solution with a NASICON-type structure is formed at 0 ≤ x ≤ 0.4. Luminescence properties of the phosphors were studied using the methods of UV and VUV spectroscopy. Narrow emission bands originating from intraconfigurational transitions in Dy3+ were detected in the region 450–800 nm and optimal concentration of the dopant determined. The concentration quenching of Dy3+ emission is shown to be due to dipole-interaction. In addition, two broad bands were detected in the UV and blue spectral regions, which were tentatively ascribed to the intrinsic emission of self-trapped excitons and defect-related centers, respectively. Optical bandgap energy of Na3.6Y1.8(PO4)3 was estimated to Eg ∼ 7.1 eV. Influence of temperature on luminescence characteristics was studied.

Bright luminescence of Sb doping in all-inorganic zinc halide perovskite variant

Halide lead perovskites are considered as one of the most promising optoelectronic materials in decades, but the toxicity and bio-accumulation of lead retard the commercialization. Therefore, seeking suitable lead-free perovskites and their variants are of vital importance. In this work, all-inorganic zinc halide perovskite variant Cs2ZnCl4 crystals with Sb3+ doping are synthesized via hydrothermal method. Transparent millimeter-sized crystals exhibit bright red photoluminescence (PL) under room temperature (RT) on account of self-trapped exciton (STE) emission of Sb3+, from 3P1 to 1S0 states. Temperature-dependent PL and time-resolved PL exhibit bimodality and intensity variations, revealing that Jahn-Teller effect of 3P1 state results two energy minima, and thermal activation may contribute to the spectrum change. Further, DFT calculations are carried out and the influences of Sb3+ doping are revealed. A bandgap reduction is achieved via doping, and the band edges are localized at Sb3+. A combination of experimental and theoretical studies demonstrates well that the soft lattice structure and the 5s2 lone pairs of Sb are vital for the STE emission.

Ultra-wideband warm white light emission from self-trapped excitons in CsAgCl2

The materials with self-trapped exciton (STE) emission are considered to have great prospects in application of lighting, due to the large Stokes shift and wide emission band. Here we report a novel STE emission material CsAgCl2 that can emit warm white light with a spectrum covering almost the entire visible range. At room temperature, the peak wavelength of its emission is about 625 nm, and it exhibits a broad half peak width up to 235 nm. A warm white LED was prepared using a 268 nm UV LED as the excitation source. It shows a correlated color temperature (CCT) of 4062 K and a color rendering index (CRI) of 93. The results indicate that CsAgCl2 is an ideal material for high color rendering lighting.

Rapid Synthesis of Bright, Shape‐Controlled, Large Single Crystals of Cs3Cu2X5 for Phase Pure Single (X = Br, Cl) and Mixed Halides (X = Br/Cl) as the Blue and Green Components for Printable White Light‐Emitting Devices

AbstractTernary copper halides have garnered significant interest for their bright, high quantum yield emission stemming from the radiative decay of self‐trapped excitons (STEs). Cesium copper halides have shown promise for use in optoelectronics, including light‐emitting devices (LEDs) for lighting and displays. To date several synthetic procedures for Cs3Cu2X5 (X = Cl, Br, and mixed Br/Cl) have been proposed for making nanocrystals, microcrystals, or polycrystalline thin films. Here, a synthetic method for making large single crystals (SCs) with millimeter dimensions in less than 30 min is presented. Phase pure mixed halide SCs are also produced and in‐depth structural analysis has been performed for the first time, definitively showing the site preferences for mixing chloride into the pure Cs3Cu2Br5 structure. Quantum yields for SCs of X = Cl and Br are 100% and 27% respectively, with long lifetimes and strong evidence of STE emission. This synthesis can be used to produce white light UV‐downconversion LEDs using ternary copper halides as the blue and green components along with the commercial red phosphor K2SiF6:Mn4+. These devices give a Commission Internationale de l'Éclairage (CIE) coordinate of (0.3327, 0.3342) and color rendering index of 90% at a color temperature of ≈5500 K.

Zero-Dimensional Organic-Inorganic Hybrid Copper-Based Halides with Highly Efficient Orange-Red Emission.

Lead-free metal halides are promising luminescent materials with low non-toxicity, tunable emission, and high luminescence quantum yield (PLQY). The development of red light-emitting materials with high PLQY to meet the color coordinate requirements for light-emitting diode (LED) lighting and wide color gamut displays remains a major scientific challenge. Herein, novel lead-free copper-based halide materials are reported. PEA4 Cu4 I4 is an efficient orange-red broad emission with a highest PLQY of 73.6%. All three materials, namely, those of ((R)-(+)-α-PEA)4 Cu4 I4 , ((S)-(-)-α-PEA)4 Cu4 I4 , and (β-PEA)4 Cu4 I4 with zero-dimensional structures have large exciton binding energies and Huang-Rhys factors, thus causing efficient self-trapped exciton emissions. A prototypical orange-red LED device is fabricated successfully that demonstrates the potential of copper-based halide materials in lighting and display applications.

The synthesis of lead-free double perovskite Cs2Ag0.4Na0.6InCl6 phosphor with improved optical properties via ion doping

Lead-free double perovskite (DP) phosphors with nontoxicity and excellent moisture, light, and heat stability are promising alternatives to lead halide perovskites for wide uses in optoelectronic applications. As known, their photoluminescence (PL) quantum yield (PLQY) is low in normal. Here, the optical properties of orange-emitting Cs2Ag0.4Na0.6InCl6 DP via doping Bi3+, Sm3+, or Mn2+ ions in the host lattices are proposed and remarkably enhanced. For the Bi3+-doping, the optical band gap of the Cs2Ag0.4Na0.6In1−xBixCl6 DP could be tuned from 3.82 to 2.74 eV and the optimized sample shows a markedly high PLQY of 97.33%. Sm3+-doped Cs2Ag0.4Na0.6InCl6 microcrystals (MCs) are endowed with broad emission derived from self-trapped excitons (STEs) of host. The orange-red emissions at 570, 608, and 655 nm from the transitions of 4G5/2 to 6HJ (J = 5/2, 7/2, and 9/2) of Sm3+, are demonstrated the first observation of PL from the Sm3+-doped lead-free halide DP. The Mn2+-doped Cs2Ag0.4Na0.6InCl6 MCs show dual emission peaks centered at 538 nm and 618 nm, owing to the STEs emission of Cs2Ag0.4Na0.6InCl6 host and 4T1 → 6A1 transition of Mn2+, respectively. It is interesting that the PLQY of Cs2Ag0.4Na0.6InCl6 MCs is dramatically enhanced from 21% to 41% via doping Mn2+.

Bulk Assembly of Zero-Dimensional Organic Copper Bromide Hybrid with Bright Self-Trapped Exciton Emission and High Antiwater Stability

Bulk Assembly of Zero-Dimensional Organic Copper Bromide Hybrid with Bright Self-Trapped Exciton Emission and High Antiwater Stability

Self-Trapped Exciton Emission in a Zero-Dimensional (TMA)2SbCl5·DMF Single Crystal and Molecular Dynamics Simulation of Structural Stability.

Lead-free lower-dimensional organic-inorganic metal halide materials have recently triggered intense research because of their excellent photophysical properties and chemical stability. Herein, we report a novel zero-dimensional (0D) organic-inorganic hybrid single crystal (TMA)2SbCl5·DMF (TMA = N(CH3)3, DMF= HCON(CH3)2), which exhibits typical self-trapped exciton (STE) emission with an efficient yellow emission at 630 nm and high photoluminescence quantum yield (PLQY) of 67.2%. The dual STE emission is attributed to the singlet and triplet STEs in inorganic [SbCl5]2-, respectively. Further, an ab initio molecular dynamics simulation was performed to estimate the stability of crystal structure at room temperature. The calculated excited-state structure indicates that the deformation parameter (Δd) of the excited-state structure is larger than that of the ground state, illustrating the origin of a large Stokes shift. These results indicate that these new 0D lead-free organic-inorganic hybrid metal halides are promising luminescent materials for optoelectronic applications.

High repetition rate green-pumped supercontinuum generation in calcium fluoride

We compare supercontinuum generation in hbox {CaF}_2 crystal under tight and loose focusing of 150 fs, 515 nm second harmonic pulses from an amplified Yb:KGW laser at a repetition rate of 10 kHz. It is demonstrated that supercontinuum generation geometry applying loose focusing (hbox {NA}=0.004) of the pump beam into a long (25 mm) hbox {CaF}_2 sample is advantageous in terms of supercontinuum spectral extent and durability of damage-free operation of the nonlinear material as compared to a commonly used supercontinuum generation setup which employs tight focusing (hbox {NA}=0.012) into a short (5 mm) sample and to setup which uses tight focusing into a long (25 mm) sample. More specifically, loose focusing into a long sample showed remarkably longer (20 min) damage-free operation of the nonlinear material, which was not translated with respect of the pump beam, while in tight focusing condition the sample is damaged just within 2 min of operation, leading to a complete extinction of the supercontinuum spectrum. The evolution of optical degradation of the nonlinear material in time and its impact to supercontinuum spectrum is studied in terms of filament-induced luminescence due to self-trapped exciton emission and light scattering at the pump wavelength indicating the onset of optical damage. Our findings are supported by the numerical simulations which compare relevant parameters related to filament propagation in tight and loose focusing conditions.

Water-Stable Zero-Dimensional (C4H9)4NCuCl2 Single Crystal with Highly Efficient Broadband Green Emission.

Here, we report (C4H9)4NCuCl2 single crystals with a luminous intensity that remains largely the same after soaking in water for 24 h. (CH9)4NCuCl2 has a new type zero-dimensional framework, in which the isolated [CuCl2]- anions are wrapped by organic (C4H9)4N+ cations. As expected, (C4H9)4NCuCl2 shows a broad emission band at 508 nm with a photoluminescence quantum yield of approximately 82% at room temperature, stemming from self-trapped exciton (STE) emission. Temperature-dependent photoluminescence measurement reveals that there is an energy barrier ΔE (24.0 meV) between the intrinsic state and STE state, which leads to the increase in emission intensity with an increase in temperature (98-278 K), while the emission intensity begins to decrease when the temperature is higher than 278 K due to the effects of both thermal quenching and carrier scattering. Our findings provide a new idea for the design of lead-free anti-water stability metal halide materials.

Light-Emitting 0D Hybrid Metal Halide (C3H12N2)2Sb2Cl10 with Antimony Dimers.

Low-dimensional organic-inorganic metal halides (OIMHs), as emerging light-emitting materials, have aroused widespread attention owing to their unique structural tunability and photoelectric characteristics. OIMHs are also promising materials for optoelectronic equipment, light-emitting diodes, and photodetectors. In this study, (C3H12N2)2Sb2Cl10 (C3H12N22+ is an N-methylethylenediamine cation), a new zero-dimensional OIMH, has been reported, and (C3H12N2)2Sb2Cl10 possesses a P21/n space group. The (C3H12N2)2Sb2Cl10 structure contains [Sb2Cl10]4- dimers (composed of two edge-sharing [SbCl6]3- octahedra) that are surrounded by C3H12N22+ cations. The experimental band gap of (C3H12N2)2Sb2Cl10 is 3.80 eV, and density functional theory calculation demonstrates that (C3H12N2)2Sb2Cl10 possesses a direct band gap, with the edge of the band gap mainly contributed from the inorganic units. (C3H12N2)2Sb2Cl10 exhibits good ambient and thermal stability. Under 395 nm excitation at room temperature, (C3H12N2)2Sb2Cl10 exhibits a broad emission with a full width at half-maximum of ∼114 nm, peaking at 480 nm, and the broad emission was ascribed to self-trapped exciton emission.

Tunable self-trapped excitons in 2D layered rubrene

As emerging efficient emitters, two-dimensional (2D) organic semiconductors offer an intriguing potential to the low-cost and high-performance light emitting devices. However, organic semiconductors usually suffer from serious luminescence quenching owing to the self-trapped exciton formation which prevalently occur in materials with soft lattice and strong exciton-phonon coupling. Therefore, revealing the underlying mechanism that leads to self-trapped excitons is a prerequisite for increasing the photoluminescence (PL) efficiency in organic materials. Here, we grew high-quality layered rubrene films on hexagonal boron nitride through a physical vapor transport method. Combined with time-resolved photoluminescence (TRPL) spectra and laser power dependent TRPL spectra, we confirmed the free exciton (FE) and self-trapped exciton (STE) emission. We found that STEs evolved from FEs from the temperature dependent PL characterization. In addition, we observed tunable STEs in a 2D layered rubrene: reducing the layer number could strongly suppress the exciton transferring efficiency and the rate from a free exciton state to a self-trapped exciton state which result from the decreased self-trapping depth and increased barrier height. Consequently, the average PL intensity is strongly enhanced about seven times, whereas STE emission is quenched. The results provide a method for suppressing the STE formation process and contribute to improving the photoluminescence efficiency in optoelectronic applications.