Recombination Of Self-trapped Excitons Research Articles

Sublattice Distortion Enabled Strong Interplay between Phonon Vibrations, Electron-Phonon Coupling, and Self-Trapped Excitonic Emissions in Cs2Ag1-xNaxBiCl6 Double Perovskites.

Sublattice distortion resulting from alloying compositionally distinct double perovskites is shown to influence photoluminescence emission in Cs2Ag1-xNaxBiCl6 (0 < x < 1). The end members show negligible photoluminescence, whereas interestingly the alloys exhibit broad photoluminescence. These emissions are attributed to self-trapped excitons (STE) resulting from sublattice distortions arising due to the mismatch in [AgCl6]5- and [BiCl6]3- octahedra. Change in sublattice distortions plays significant role in the formation and recombination of STEs. The STE emission intensity and quantum yield greatly depend on x, with highest intensity observed for x = 0.75, consistent with a large change in sublattice found at this x. Variation in photoluminescence properties with composition follows a similar trend as that of bandgap and phonon vibrational changes observed due to sublattice distortion. Temperature-dependent phonon vibrations and photoluminescence studies reveal a giant electron-phonon coupling. A strong synergy between STE emissions, electron-phonon coupling, bandgap, and phonon vibrations in double perovskites with sublattice distortions is demonstrated.

Examining Impacts of Acidic Bath Temperature on Nano-Synthesized Lead Selenide Thin Films for the Application of Solar Cells.

The influence of bath temperature on nano-manufactured PbSe (lead selenide) films was successfully generated by utilizing CBD on the acid solution's metal surface tool. Pb (NO3)2 was employed as a lead ion source as a precursor, while Na2O4Se was used as a selenide ion source. The XRD characterization revealed that the prepared samples are the property of crystalline structure (111), (101), (100), and (110) Miller indices. The scanning electron microscope indicated that the particles have a rock-like shape. There was a decrement of energy bandgap that is from 2.4 eV to 1.2 eV with increasing temperature 20°C–85°C. Thin films prepared at 85°C revealed the best polycrystal structure as well as homogeneously dispersed on the substrate at superior particle scales. The photoluminescence spectrophotometer witnessed that as the temperature of the solution bath increases from 20°C to 85°C, the average strength of PL emission of the film decreases. The maximum photoluminescence strength predominantly exists at high temperatures because of self-trapped exciton recombination, formed from O2 vacancy and particle size what we call defect centres, for the deposited thin films at 45°C and 85°C. Therefore, the finest solution temperature is 85°C.

Water-Molecule-Induced Emission Transformation of Zero-Dimension Antimony-Based Metal Halide.

Low-dimensional organic-inorganic metal halides have recently emerged as a class of promising luminescent materials. However, the intrinsic toxicity of lead would strongly hamper future application. Herein, we synthesized a new type of lead-free zero-dimensional (0D) antimony-based organic-inorganic metal halide single crystals, (PPZ)2SbCl7·5H2O (PPZ = 1-phenylpiperazine), which features a broadband emission at 720 nm. Ultrafast transient absorption and temperature-dependent photoluminescence (PL) spectra are combined to investigate the PL mechanism, revealing that self-trapped exciton recombination was involved. Furthermore, it is interesting that (PPZ)2SbCl7·5H2O material shows reversible PL emission transformation between red light (720 nm) and yellow light (590 nm) as water molecules are inserted or removed from the lattice. Such reversible emission transformation phenomenon renders the (PPZ)2SbCl7·5H2O as a potential low-cost water sensing material.

Three homologous 1D lead halide perovskites with broadband white-light emissions

Three new one-dimensional (1D) face-shared octahedral perovskites with unified APbBr3 composition, namely, [DTHPE]Pb2Br6 (1), [TMPDA]PbBr3 (2) and [MPD]PbBr3 (3) have been synthesized by using different types of organic cations. These 1D perovskites exhibit tunable broadband luminescence from orange to blue light emissions. Experimental and computational studies unveil that these broadband emissions are derived from the radiative recombination of self-trapped excitons due to strong electron–phonon coupling.

A Zero-Dimensional Hybrid Cadmium Perovskite with Highly Efficient Orange-Red Light Emission.

Low-dimensional organic-inorganic hybrid metal halide materials have been extensively studied due to their excellent optoelectronic performances. Herein, by using the facile wet-chemistry method, we designed one new hybrid cadmium bromide of (H3AEP)2CdBr6·2Br based on discrete octahedral [CdBr6]4- units. Remarkably, the bulk crystal of (H3AEP)2CdBr6·2Br exhibits strong broadband orange-red light emission from the radiative recombination of self-trapped excitons (STEs) with a high photoluminescence quantum yield (PLQY) of 9%. Benefiting from the highly efficient luminescent performance, this 0D cadmium perovskite can be utilized as an excellent down-conversion red phosphor to assemble a white light-emitting diode, and a high color rendering index (CRI) of 93 is realized. As far as we know, this is the first orange-red light-emitting hybrid cadmium perovskite which promotes the full-color display in this system.

Low-temperature luminescence in organic-inorganic lead iodide perovskite single crystals

We present a temperature and laser-power dependent photoluminescence (PL) study of methylammonium lead iodide (CH3NH3PbI3) single crystals in the orthorhombic phase. At temperatures below 140 K, we revealed the multi-component PL emission. In addition to a free exciton with an energy of 1.65 eV, we found emission bands with peaks approximately equal to 1.6 eV, 1.52 eV, and 1.48 eV. Analysis of the thermal evolution of the intensities, peak positions, and linewidths of all the PL bands allowed one to determine their origin. We attributed the PL peak with energy of 1.6 eV to a bound exciton, while the free exciton-bound exciton splitting energy is 50–60 meV. The PL emission with an energy of 1.52 eV can be explained by the donor-acceptor pair (DAP) recombination, where donor and acceptor defects have a depth of about 12 meV and 120 meV, respectively. MA (CH3NH3) interstitials (MA) and lead vacancies (V) are the most suitable for the DAP transition to occur in CH3NH3PbI3 crystals. The 1.48 eV PL emission is consistent with the recombination of self-trapped excitons, and interstitial iodine is likely to be an active trap source. We found the variation of the self-trapped depth from 15 meV (at 80 K) to 53 meV (at 80 K) with increasing the temperature. Although the multi-component PL emission in CH3NH3PbI3 single crystals appears at low temperatures, defects and excitonic traps that cause this emission can affect the photophysics of hybrid perovskites at higher temperatures.

Co-Doping of Cerium and Bismuth into Lead-Free Double Perovskite Cs2AgInCl6 Nanocrystals Results in Improved Photoluminescence Efficiency.

Lead-free double cation metal halide perovskites have recently attracted considerable attention, with continuing research efforts focusing on the improvement of their stability and photoluminescence quantum yield (PL QY). In this study, Ce3+ has been co-doped together with Bi3+ into lead-free double perovskite Cs2AgInCl6 nanocrystals (NCs) in order to improve their crystallinity and PL QY. Both uncoordinated chloride ions and silver vacancies could be eliminated using this co-doping strategy, and the resulting Ce3+,Bi3+-co-doped Cs2AgInCl6 NCs showed adjustable PL emission peaks in the range of 589 to 577 nm by varying the doping amount of Ce3+ with a fixed feeding ratio of bismuth precursor set at 1%. Cs2AgInCl6 NCs doped with 1% Bi alone reached a PL QY of 10% for the PL peak centered at 591 nm, while those co-doped with 1% Bi and 2% Ce together achieved the highest PL QY of 26% for the PL peak centered at 580 nm. The use of Ce3+ as a dopant promoted the localization of self-trapped excitons to prevent PL quenching, although the ion's 5d excited state may potentially provide an energetically favorable indirect route for the radiative relaxation process. This also resulted in a blue shift of the PL maximum and increased the exciton binding energy, thus promoting the radiative recombination of self-trapped excitons.

Three-Dimensional Cuprous Iodide Framework with Intrinsic Broadband Red-to-Near-Infrared Light Emission.

Herein, a new organic-inorganic hybrid cuprous iodide of [(Me)2-DABCO]Cu6I8 was prepared and structurally characterized with a novel three-dimensional (3D) [Cu6I8]2- framework. Significantly, this 3D cuprous iodide displays infrequent broadband red-to-near-infrared light emission (600-1000 nm) stemming from the radiative recombination of self-trapped excitons.

Light-Induced Effects in Amorphous Chalcogenide Glasses: Femtoseconds to Seconds

Amorphous chalcogenide glasses are intrinsically metastable, highly photosensitive, and therefore exhibit numerous light-induced effects upon bandgap and sub-bandgap illumination. Depending on the pulse duration of the excitation laser, ChGs exhibit a series of light-induced effects spanning over femtosecond to seconds time domain. For continuous wave (CW) illumination, the effects are dominantly metastable in terms of photodarkening (PD) and photobleaching (PB) that take place via homopolar to heteropolar bond conversion. On the other hand, under nanosecond and ultrafast pulsed illumination, ChGs exhibit transient absorption (TA) that is instigated from the transient bonding rearrangements through self-trapped exciton recombination. In the first part of the review, we pay special attention to continuous wave light-induced PD and PB, while in the second part we will focus on the TA and controlling such effects via internal and external parameters, e.g., chemical composition, temperature, sample history, etc.

Pressure-Induced Emission toward Harvesting Cold White Light from Warm White Light.

The pressure-induced emission (PIE) behavior of halide perovskites has attracted widespread attention and has potential application in pressure sensing. However, high-pressure reversibility largely inhibits practical applications. Here, we describe the emission enhancement and non-doping control of the color temperature in two-dimensional perovskite (C6 H5 CH2 CH2 NH3 )2 PbCl4 ((PEA)2 PbCl4 ) nanocrystals (NCs) through high-pressure processing. A remarkable 5 times PIE was achieved at a mild pressure of 0.4 GPa, which was highly associated with the enhanced radiative recombination of self-trapped excitons. Of particular importance is the retention of the 1.6 times emission of dense (PEA)2 PbCl4 NCs upon the complete release of pressure, accompanied by a color change from "warm" (4403 K) to "cold" white light with 14295 K. The irreversible pressure-induced structural amorphization, which facilitates the remaining local distortion of inorganic Pb-Cl octahedra with respect to the steric hindrance of organic PEA+ cations, should be greatly responsible for the quenched high-efficiency photoluminescence.

Intrinsic polaronic photocarrier dynamics in hematite

Self-trapping of photocarriers due to the strong carrier-phonon interaction is ubiquitous in metal oxides, which exerts detrimental influence on the performance of the oxide-based optoelectronic devices. The self-trapped species could be isolated self-trapped charges (i.e., isolated polarons) or bound self-trapped electron-hole pairs (i.e., polaron-pairs, self-trapped excitons), and they may show distinct dynamic behaviors. Here, we find that the self-trapping in hematite ($\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$) primarily results in self-trapped excitons rather than spatially separated polaronic charges or electron-hole polaron-pairs based on both dynamic and spectral evidence. The formation and recombination of self-trapped excitons are found to be substantially independent of defect concentration varying from monocrystalline to polycrystalline $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ films. The spectral feature attributed to small electron polarons can only be resolved above the noise floor by raising excitation intensity so that the Auger process dominates the recombination. Based on the comparison of monocrystalline and polycrystalline sample data, we suggest that the isolated electron polaron formation is likely facilitated by defects. Our finding invokes reconsideration of the photocurrent generation mechanism in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ photoanodes.

Color Tunable Self-Trapped Emissions from Lead-Free All Inorganic IA-IB Bimetallic Halides Cs-Ag-X (X = Cl, Br, I).

Multi-metallic halides of group IA and IB metals are emerged as a new class of color tunable emitters. While chalcogenides and perovskites are extensively studied, these families of materials are little explored. In comparison, herein, lead and cadmium free bimetallic Cs-Ag-X (X = Cl, Br, I) halides are reported where the larger ion Ag+ helped in incorporating all the halide ions which in turn tune their emission color in spanning from 397nm (violet) to 820nm (near infrared) as a function of their composition. The synthesis method adopted here is the solvent free ball milling of respective halides of Cs and Ag and took the record shortest time and in bulk scale. From decay lifetimes, emissions from these bimetallic halides are found as a result of fast recombination of self-trapped excitons, which exhibited not only reasonably high quantum yield in the range of 17-68% but also excellent stability to air and moisture under ambient conditions. These also show wide Stokes shift with relatively longer decay lifetimes ranging above the exciton and below the surface trap or dopant induced emissions of inorganic semiconductors, indicating a new class of materials having unique identity of their optical behaviors.

Highly luminescent and stable lead-free cesium copper halide perovskite powders for UV-pumped phosphor-converted light-emitting diodes

Lead halide perovskites have drawn extensive attention over recent decades owing to their outstanding photoelectric performances. However, their toxicity and instability are big issues that need to be solved for further commercialization. Herein, we adopt a facile dry ball milling method to synthesize lead-free Cs 3 Cu 2 X 5 ( X = I , Cl) perovskites with photoluminescence (PL) quantum yield up to 60%. The optical features including broad emission spectrum, large Stokes shift, and long PL lifetime can be attributed to self-trapped exciton recombination. The as-synthesized blue emissive Cs 3 Cu 2 I 5 and green emissive Cs 3 Cu 2 Cl 5 lead-free perovskite powders have good thermal stability and photostability. Furthermore, UV-pumped phosphor-converted light-emitting diodes were obtained by using Cs 3 Cu 2 I 5 and Cs 3 Cu 2 Cl 5 as phosphors.

Mercury-indium-sulfide nanocrystals: A new member of the family of ternary in based chalcogenides.

A general synthesis approach of aqueous glutathione-capped ternary Ag-In-S, Cu-In-S, and Hg-In-S nanocrystals (NCs) is introduced, allowing the NC composition to be varied in a broad range. Ternary Hg-In-S (HIS) NCs are reported for the first time and found to have the same tetragonal chalcopyrite motif as Cu-In-S and Ag-In-S NCs, corroborated by phonon spectra, while X-ray photoelectron spectroscopic data indicate mercury to be present as Hg+ in the Hg-In-S NCs. Colloidal HIS and Hg-In-S/ZnS NCs showed little or no variations of the spectral width of the photoluminescence band upon NC size selection, temperature variation in a broad range of 10-350 K, deposition of a ZnS shell, or postsynthesis annealing. All these observations are similar to those reported earlier for Ag-In-S and Ag-In-S/ZnS NCs and allowed us to assume a general photoluminescence mechanism for all three ternary compounds, based on the model of radiative self-trapped exciton recombination.

Insights into different photoluminescence mechanisms of binary and ternary aqueous nanocrystals from the temperature dependence: A case study of CdSe and Ag-In-S

A comparative study of the temperature dependences of the photoluminescence (PL) of aqueous CdSe and silver-indium-sulfide (AIS) nanocrystals (NCs) in a broad range of 10–310 K is reported. Gelatin-incorporated CdSe (and core/shell CdSe/ZnS) NCs reveal both direct (excitonic) and trap-state-mediated PL components with different temperature dependences of the PL band maximum and spectral width. Glutathione-capped AIS (and core/shell AIS/ZnS) NCs emit a single broad PL band resembling the trap-related PL of CdSe NCs but showing nevertheless a temperature PL dependence typical for excitonic PL. In combination with previously reported results, these special features allow the PL emission of ternary AIS (AIS/ZnS) NCs to be explained based on the recombination of self-trapped excitons.

Pressure-Induced Emission (PIE) of One-Dimensional Organic Tin Bromide Perovskites.

Low-dimensional halide perovskites easily suffer from the structural distortion related to significant quantum confinement effects. Organic tin bromide perovskite C4N2H14SnBr4 is a unique one-dimensional (1D) structure in which the edge sharing octahedral tin bromide chains [SnBr42-]∞ are embraced by the organic cations C4N2H142+ to form the bulk assembly of core-shell quantum wires. Some unusual phenomena under high pressure are accordingly expected. Here, an intriguing pressure-induced emission (PIE) in C4N2H14SnBr4 was successfully achieved by means of a diamond anvil cell. The observed PIE is greatly associated with the large distortion of [SnBr6]4- octahedral motifs resulting from a structural phase transition, which can be corroborated by in situ high-pressure photoluminescence, absorption, and angle-dispersive X-ray diffraction spectra. The distorted [SnBr6]4- octahedra would accordingly facilitate the radiative recombination of self-trapped excitons (STEs) by lifting the activation energy of detrapping of self-trapped states. First-principles calculations indicate that the enhanced transition dipole moment and the increased binding energy of STEs are highly responsible for the remarkable PIE. This work will improve the potential applications in the fields of pressure sensors, trademark security, and information storage.

Pushing the limit of Cs incorporation into FAPbBr3 perovskite to enhance solar cells performances

Cation compositional engineering has revealed a powerful design tool to manipulate the perovskite structural and optoelectronic characteristics with a tremendous impact on device performances. Tuning the bandgap by cation and anion compositional mixing, for instance, is paramount to target different optoelectronic segments, from light emitting applications to tandem solar cells. However, structural and photo instabilities, and phase segregation come along, imposing a severe control on the material composition and structure. Here we develop highly uniform alloy of mixed cation FA(1−x)CsxPbBr3 perovskite thin films pushing for the first time the Cs content up to 30%. In contrast to what has been reported so far, this composition leads to a high quality crystalline film, maintaining a single cubic phase arrangement. In addition, a remarkably high robustness against moisture and phase purity is observed. The experimental finding is also supported by density functional theory simulations, demonstrating at the atomistic level Cs segregation starting from Cs concentration around 37.5%. Beyond that, phase segregation happens, leading to formation of an unstable pure Cs-rich region. Low temperature photoluminescence (PL) measurements reveal that the addition of Cs eliminates the non-radiative channel into mid-gap traps, as evident by the lack of the broad emission band, often associated with recombination of self-trapped exciton, present for 0% Cs. This, in turn, reduces the non-radiative recombination losses which manifests as high performance solar cells. Indeed, when embodied in solar devices, Cs incorporation leads to enhanced device performances, with an open circuit voltage beyond 1.33 V.

Organic-Inorganic Layered and Hollow Tin Bromide Perovskite with Tunable Broadband Emission.

Recently, layered perovskites attracted great attention for its excellent stability and light-emitting property. However, most of them rely on the toxic element lead and their emission quantum yields are generally low. Here, a unique hollow two-dimensional perovskite was developed in which the organic hexamethylene diamines (C6H18N22+) strongly coupled with distorted tin bromide anions (SnBr64-). This toxic-free low-dimensional tin perovskite exhibits a broadband emission in the visible region with a high luminescence quantum yield of 86%. First-principles calculation indicate the broadband emission is associated with the recombination of self-trapped excitons. And the emission is related to the geometry of tin bromide anions. An ultraviolet light-pumped white light emitting diode with excellent color-rendering index of 94 was fabricated using it together with a commercially available blue phosphor.

Absorption and emission features of Ho3+ ion in Nb2O5 mixed Lithium zirconium silicate glasses

Li2O–ZrO2–SiO2: Ho3+ glasses mixed with Nb2O5 were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The optical absorption spectra of all glasses recorded at room temperature in the wavelength region 300-2000 nm exhibited several absorption bands all from the ground state 5I8 to 5G5, 5G6, 5F1, 5F3, (5F4+5S2), 5F5, 5I5, 5I6, 5I7. The luminescence spectra of all the glasses recorded at room temperature in the visible and NIR regions exhibited the following prominent emission bands 5F3 → 5I8, 5S2 → 5I8, 5G4 → 5I6, 5K8 → 5I7, 5F5 → 5I8 ,5G5 → 5I6, 5F4 →5I7, 5G5 → 5I5, 5F2 → 5I6, 5F3 → 5I6, 5I5 → 5I8 (Visible region) and ,5I7 → 5I8 (NIR region). The luminescence spectra of Nb2O5 mixed Li2O–ZrO2–SiO2 glasses (free of Ho3+ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Nb5+ ions in the glass network. The Judd–Ofelt theory was successfully applied to characterize Ho3+ spectra of all the glasses. From this theory, various radiative properties like transition probability A, branching ratio βr, the radiative lifetime τr, for 5S2 emission levels in the spectra of these glasses has been evaluated. The radiative life time for 5S2 level of Ho3+ ions has also been measured and quantum efficiencies were estimated.

Luminescence studies on undoped and lanthanide doped Zinc Tin Hydroxide

Undoped and rare earth ions (Eu3+ and Dy3+) doped ZnSn(OH)6 particles were prepared at room temperature in aqueous medium. XRD studies confirmed that unit cell parameters increases with increase in concentration of lanthanide ions in the sample, suggesting its incorporation in the lattice. The particles are having cube like shape with size in the range of 200nm, as revealed by TEM measurements. Luminescence studies confirm that host emission from the samples arises due to self-trapped exciton recombination with an activation energy for thermal quenching ~ 16.7meV. Further it is inferred from the studies that lanthanide ions get incorporated up to 2% in the lattice. Energy transfer from host matrix to lanthanide ions has been observed in these samples.