Light-emitting Applications Research Articles

Study on structural and photoluminescence property of SrCeO3 and SrCe0.98Sm0.02O3 perovskites

Perovskites with improved structural and photoluminescence properties can be furnished by doping with rare earth elements. Strontium cerate perovskite host matrix plays a prime role in energy devices due to improved functional property, namely electrical conductivity. Electrochemical cells, hydrogen extractors, sensors have been greatly manufactured by innovators in past decade using doped strontium cerate perovskites. But only minimal innovation in the area of luminescence of this particular perovskite was found. In the present study, SrCeO3 and SrCe0.98Sm0.02O3 perovskites are synthesized for investigating the photoluminescence spectra. The perovskites are prepared by gel combustion technique. X-ray diffraction studies were carried out to investigate the evolution of the crystalline phase of the ceramic powder. Absorption bands in the UV region occur due to charge transfer transitions between the 4f orbital of Ce4+ and 2p orbital of oxygen ion in SrCeO3. Under UV excitations, the prepared perovskite nano phosphors show blue emission and orange-red emission respectively and was confirmed using CIE 1931 chromaticity diagram. The vibrational modes were further analysed using Raman spectra. So, these perovskites can be promising candidate as blue phosphor and red phosphor in light-emitting devices and applications.

A comparative analysis on the spectroscopic qualities of modifiers and Sm3+ ions integrated bismuth boro-tellurite glasses for visible orange appliances

Considering the striking features of trivalent samarium ions that can yield novel optoelectronic devices, a set of additive-reliant Sm (III) ions doped bismuth boro-tellurite (BTBiCofXfSm1.0) glasses was synthesized and their physical, structural and lasing features were studied for potential orange emission photonic applications. Structural and elemental investigations are done via XPS and EDX analyses. The spectroscopic paraments as a function of the added additive were analysed to discover the suitable glass. The evaluation of bonding parameters from the absorption studies verified the ionic bonding between the oxygen ligand and the Sm (III) ions in the glasses with respect to the additives. Four luminescence emission bands were observed from the emission spectra and the 4G5/2 → 6H7/2 transition is found to be hypersensitive which denotes intense orange emission. The lasing action determining features like transition probability (Arad), branching ratio (βR) and stimulation emission cross section (σsec) are found to be higher for the Ba oxide containing glass which is a potential medium for red-orange light emission and visible orange photonic applications. The higher quantum efficiency (75.83%) of the Ba containing glass under 476 nm excitation indicates its suitability in high-quality orange light applications. The Judd Ofelt and CIE colorimetric investigations discovered that the studied glass are high-quality media for optoelectronic applications.

Vibrational and Structural Properties of Two-Dimensional Tin Mixed-Halide Perovskites

The emergence of two-dimensional (2D) hybrid metal-halide perovskites has garnered significant attentions for optoelectronic devices and light-emitting applications. Since the toxicity of lead-based perovskites could potentially be harmful to the environment, several works have attempted to change the active metal to tin (Sn). Here, we investigate the characterization of (PEA)2SnBrxI4-x mixed halide perovskites using X-ray fluorescence (XRF), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Qualitative XRF analysis suggests the presence of tin, bromine and iodine emissions under the mid-Z and high-Z ranges. In mid-Z range, Br-Kα peak appeared on 11.96 keV and Br-Kβ was detected on 13.3 keV. Meanwhile Sn-Kα, I-Kα, I-Kβ1, and I-Kβ2 peaks were detected in high-Z range on 25.24 keV, 28.6 keV, 32.35 keV and 33.11 keV, respectively. Thus, the elemental composition of mixed halide components exhibits an indicative control that bromine-rich or iodine-rich can be synthesized via rational chemical design. XRD pattern display a systematic progression at the peak 5.18° (corresponds to (002) plane), which unambiguously demonstrated the feasibility to tune halide composition in tin-based hybrid perovskite. It also confirms that (2D) hybrid metal-halide with tunable halide have identical structure for both bromine-rich and iodine-rich composition. Furthermore, the 2θ peaks slightly shifted to lower angle with increasing bromine composition. The presence of C−I bonding on ~500 cm-1 and C-Br bond on ~600 cm-1 in FTIR spectra highlights the functional group of organic cations. These experimental results promote a foundation to implement compositional engineering on 2D-tin mixed-halide perovskites for optoelectronics and scintillators.

Molecular-Doped Protein-Based Elastomers as a Versatile Platform for Energy-Transfer Studies and Emissive Down-Converting Polymers for Light-Emitting Applications.

Much effort is being employed for designing "green" environmental emissive materials that are capable of color-tuning, i.e., down-converting the emission, and white-light generation (WLG). Here, we introduce a protein-based elastomer that can noncovalently bind a variety of chromophores while preventing their aggregation. Such binding capabilities are unique to the albumin-based materials that we use here in a process we refer to as "molecular doping". In the first part of this study, we explore the energy transfer across five different chromophores within the protein matrix, where the closely packed chromophore organization enables high energy-transfer efficiencies among them. In the second part, we show the easy control of blue, green, and red chromophores within the biopolymer, resulting in tunable emission properties of the film and WLG. The highly affordable chosen protein and the straightforward molecular doping strategy make our protein elastomers an attractive choice for an emissive material, as either a scaffold for investigating energy transfer in proteins or possible integration in light-emitting applications.

Exciton Fine Structure in 2D Perovskites: The Out‐of‐Plane Excitonic State

Abstract2D Ruddlesden‐Popper metal‐halide perovskites feature particularly strong excitonic effects, making them a fascinating playground for studying exciton physics. A complete understanding of the properties of this quasi‐particle is crucial to fully exploit the tremendous potential of 2D perovskites (2DP) in light emission applications. Despite intense investigations, some of the exciton properties remain elusive to date, for example, the energy‐ordering of the exciton states within the so‐called fine structure manifold. Using optical spectroscopy, it demonstrates that in the archetypical 2DP (PEA)2PbI4, in contradiction to theoretical predictions, the energy of the bright out‐of‐plane exciton state is higher than that of two in‐plane states. Having elucidated the order of exciton fine structure, it determines the g‐factor of the dark exciton transition, together with the values of the electron and hole g‐factors in the direction parallel to the c‐axis of the crystal. In this way, it provides for the first time, a complete picture of the exciton fine structure in (PEA)2PbI4 2DP.

Electronic and optical properties of the PtSe2/boron phosphide van der Waals heterostructure

Based on the first-principles calculations, we investigate electronic and optical properties of PtSe2/boron phosphide (BP) van der Waals heterostructure (vdWH). The PtSe2/BP heterostructure displays type-I band alignment with a direct band gap of 0.846 eV. The electric field and biaxial strain allow the vdWH to achieve the band alignments of type-I and type-II. More importantly, the transition from semiconductor to metal is completed in the heterostructure under the applied electric field. Interestingly, the optical absorption of the PtSe2/BP heterostructure occurs to red shift with the positive electric field and blue shift with negative electric field or biaxial strain. In addition, the band alignment has a transition from type-I to type-II for the monolayer BP/few-layer PtSe2 heterostructure. It is meaningful that the monolayer PtSe2/few-layer BP heterostructures maintain type-I band alignment. The results suggest that the PtSe2/BP heterostructure can be widely used in optoelectronic, photocatalytic and light-emitting applications in future research.

The Effect of Halide Composition on the Luminescent Properties of Ternary Cesium–Copper Halide Pseudo‐Perovskite Films

AbstractTernary copper halide pseudo‐perovskites are in the forefront of research as potential active materials in light emission applications. The optoelectronic properties of these compounds can be fine‐tuned by the preparation of mixed‐halide compositions. After irradiation, self‐trapped excitonic states are formed in these materials. However, the emission from these self‐trapped states is not yet fully understood. In this work, mixed‐halide Cs3Cu2X5 films (where X: I and/or Br) are prepared by a simple spray‐coating method. Using ultraviolet photoelectron spectroscopy, the changes in optoelectronic properties are linked to the electronic structure of these materials. It is revealed that the incorporation of bromide into the lattice makes the emission process of these materials more vulnerable to trap states. By combining the different spectroscopic characterization techniques, the exact band structure of these compounds is determined, and the different processes are translated to the absolute energy scale. As an alternative excitation mechanism of self‐trapped states, α‐particles are used to induce radioluminescence response. The Cs3Cu2X5 films exhibit composite decay patterns, most likely attributed to a multitude of different trap state‐mediated recombination processes.

High-Level Reverse Intersystem Crossing and Molecular Rigidity Improve Spin Statistics for Triplet-Triplet Annihilation Upconversion.

The structural factors affecting triplet-triplet annihilation (TTA) at the molecular level are not well-understood. Here, our steady-state photoluminescence and transient absorption results demonstrate that the spin statistical factor, η, decreases from 0.60 to 0.46 and 0.14 going from 9,10-diphenylanthracene (DPA) to the 1,5-DPA and 2,6-DPA isomers, respectively, during photon upconversion with a platinum octaethylporphyrin sensitizer. Density functional theory (DFT) shows that η depends on the energetics of hot triplet states and molecular rigidity. The significantly high conical intersection energy between the S0 and T1 states for 9,10-DPA gives its longer triplet lifetime. Time-dependent DFT calculations show that 9,10-DPA and 1,5-DPA can undergo high-level reverse intersystem crossing from their T2 and T3 states, respectively, to the bright S1 state, increasing the limit of the spin statistical factor. Both factors ultimately serve to enhance the TTA efficiency. This work provides insight into designing molecules for efficient light-emitting and photon upconversion applications.

Enhanced femtosecond nonlinear optical response in Mn-doped Cs2AgInCl6 nanocrystals.

Lead-free halide double perovskite nanocrystals (DPNCs) are emerging materials, recently explored as potential candidates in light-emitting, photovoltaic, and other optoelectronic applications. This Letter reveals unusual photophysical phenomena and nonlinear optical (NLO) properties of Mn-doped Cs2AgInCl6 nanocrystals (NCs) via temperature-dependent photoluminescence (PL) and femtosecond Z-scan measurements. The PL emission measurements suggest that self-trapped excitons (STEs) are present, and more than one STE state is possible for this doped double perovskite. We observed enhanced NLO coefficients, owing to the improved crystallinity arising from the Mn doping. From the closed aperture Z-scan data, we have calculated two fundamental parameters, Kane energy (29 eV) and exciton reduced mass (0.22m0). We further obtained the optical limiting onset (1.84 mJ/cm2) and figure of merit as a proof-of-concept application to demonstrate the potential in optical limiting and optical switching applications. Highlighting the self-trapped excitonic emission and NLO applications, the multifunctionality of this material system is demonstrated. This investigation provides an avenue to design novel photonic and nonlinear optoelectronic devices.

Correlating Photophysical Properties with Stereochemical Expression of 6s2 Lone Pairs in Two-dimensional Lead Halide Perovskites.

Two-dimensional (2D) lead halide perovskites (LHPs) have shown great promises for light-emitting applications and excitonic devices. Fulfilling these promises demands an in-depth understanding on the relationships between the structural dynamics and exciton-phonon interactions that govern the optical properties. Here, we unveil the structural dynamics of 2D lead iodide perovskites with different spacer cations. Loose packing of an undersized spacer cation leads to out-of-plane octahedral tilting, whereas compact packing of an oversized spacer cation stretches Pb-I bond length, resulting in Pb2+ off-center displacement driven by stereochemical expression of the Pb2+ 6s2 lone pair electrons. Density functional theory calculations indicate that the Pb2+ cation is off-center displaced mainly along the direction where the octahedra are stretched the most by the spacer cation. We find dynamic structural distortions associated with either octahedral tilting or Pb2+ off-centering lead to a broad Raman central peak background and phonon softening, which increase the non-radiative recombination loss via exciton-phonon interactions and quench the photoluminescence intensity. The correlations between the structural, phonon, and optical properties are further confirmed by the pressure tuning of the 2D LHPs. Our results demonstrate that minimizing the dynamic structural distortions via a judicious selection of the spacer cations is essential to realize high luminescence properties in 2D LHPs.

Theoretical DFT Investigation of Structure and Electronic Properties of η5-Cyclopentadienyl Half-Sandwich Organochalcogenide Complexes

For the first time, an extensive theoretical comparative study of the electronic structure and spectra of the η5-cyclopentadienyl half-sandwich [(Cp)(EPh3)], E = Se, Te) organochalcogenides was carried out using direct space electronic structure calculations within hybrid, meta, and meta-hybrid DFT GGA functionals coupled with double-ζ polarized 6-31G* and correlation-consistent triple-zeta cc-pVTZ-pp basis sets. The absence of covalent bonding between the cyclopentadienyl (Cp) ligands and Te/Se coordination centers was revealed. It was found that the chalcogens are partially positively charged and Cp ligands are partially negatively charged, which directly indicates a visible ionic contribution to Te/Se-Cp chemical bonding. Simulated UV–Vis absorption spectra show that all complexes have a UV-active nature, with a considerable shift in their visible light absorption due to the addition of methyl groups. The highest occupied molecular orbitals exhibit π-bonding between the Te/Se centers and Cp rings, although the majority of the orbital density is localized inside the Cp π-system. The presence of the chalcogen atoms and the extension of π-bonds across the chalcogen-ligand interface make the species promising for advanced photovoltaic and light-emitting applications.

Synthetic Control over the Surface Chemistry of Blue-Emitting Perovskite Nanocrystals for Photocatalysis

Lead halide perovskite nanocrystals (NCs) are particularly suitable for light-emitting and photocatalysis applications, where their potential can be maximized by controlling the surface composition of their organic shell. In this study, the preparation of CsPbClxBr3–x NCs at room temperature in toluene is described. Three differently structured surfactants are utilized for the synthesis, each with a specific function, namely, the solubilization of the lead precursor (n-Hept4NBr), the surface passivation with halide modification (dimethyldioctadecylammonium chloride), and the protection of the surface-active sites (octanoic acid) for photocatalysis. Under these conditions, nearly monodispersed blue-emitting nanocubes are selectively obtained in a one-pot synthesis by combining specific amounts of the perovskite precursors. As supported by thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy investigations, the organic shell of the obtained NCs is composed of electrostatically bound dimethyldioctadecylammonium ions, granting robustness to the corresponding NCs, and octanoic acid molecules, interacting with the nanoparticle surface through weaker secondary bonds. The obtained NCs exhibit a high photoluminescence quantum yield (PLQY = 72 ± 3%) notwithstanding multiexponential recombination dynamics of the excited state, resulting from the different passivation modes at the NC surface. Moreover, the NCs show a remarkable optical stability after exposure to high temperatures and to water contact due to the high surface density of the multifunctional organic ligands. The introduction of 4-tert-butylphenyl thiol promotes a charge transfer process at the NC/thiol interface formed upon removal of the labile ligands (octanoic acid) at the NC surface. In these conditions, the NCs are prone to the photoinduced conversion of the aromatic thiol into the corresponding disulfide without varying the optical properties of the perovskite photocatalyst upon the substrate conversion. Therefore, the obtained results cast light on the versatility of the surface engineering of lead halide perovskite NCs for efficient blue emission and photocatalysis with improved stability.

Energy transfer controlled color-tunable luminescence of Tm3+/Dy3+ co-doped aluminoborosilicate glass-ceramics

Tm3+/Dy3+-doped glass ceramic (GC) phosphors containing Al4B2O9 nanocrystallites were prepared by isothermal heat-treatment of aluminoborosilicate glass and the structure, optical properties were characterized. The GCs with Tm3+/Dy3+ luminescent center incorporated in the lattice of Al4B2O9 nanocrystals reveal notable energy transfer from Tm3+ to Dy3+ and possess comparable thermal stability. The luminescence emission shift from cool white light to warm white light with increasing concentration ratio of Dy3+/Tm3+ and crystallinity. With extending heat-treatment time, the luminescence emission of GC increase firstly followed by decline in addition that the energy transfer efficiency increases whereas contrarily the lifetime decreases. Crystallization induced Dy3+/Tm3+ penetrating Al4B2O9 is proposed to enhance the ligand field environment at luminescence center and further the energy transfer from Tm3+ to Dy3+. By adjusting concentration ratio of Dy3+/Tm3+ and the heat-treatment time, The doped GC phosphor with suitable chromaticity coordinate and prominent fluorescence potential for white light-emitting applications can be achieved.

A Simple and Fast Access to Phosphine-Substituted Copper(I)-Carbene Complexes via C=Se Bond Cleavage Reaction.

Phosphine coordinated copper(I)-N-heterocyclic carbene complexes have emerged as an efficient material in catalysis and light-emitting applications. In this study, a gentle and sustainable approach to the copper(I)-carbene phosphine complexes is reported through an efficient C=Se activation protocol. The complexes [(Py^NHC)Cu(PPh3 )2 ]X, X=BF4 (1), ClO4 (2), PF6 (3) and OTf (4); Py^NHC=3-isopropyl-1-(pyridin-2-yl)-imidazol-2-ylidene, and [(Py^NHC)Cu(PPh3 )(X)], X=Br (5) and I (6) have been synthesized by treating 1-isopropyl-3-(pyridin-2-yl)-imidazole-2-selone with corresponding copper(I) precursors and triphenylphosphine. In this synthetic strategy, N-heterocyclic carbene gets transferred from N-heterocyclic selone through a C=Se bond cleavage reaction to form copper(I) complexes within five minutes at room temperature. In addition, the mechanism responsible for the C=Se bond cleavage reaction has been fully investigated. These reactions are not sensitive to moisture and oxygen.

Photoluminescence manipulation in two-dimensional transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDCs) have been regarded as an intriguing platform for exploring novel physical phenomena and optoelectronic devices due to their excitonic emission characteristics derived from the atomic thin thickness and reduced dielectric screening effect. Notably, monolayer TMDCs with a direct bandgap exhibiting strong photoluminescence (PL) are promising candidates for the light-emitting devices, while the interlayer excitons in heterostructures hold great potential for the photonic chips and optical communication applications. However, the non-ideal photoluminescent intensity and quality due to the ultrathin thickness and high defect density of experimentally obtained monolayer TMDCs limit the further development for the light-emission applications. Here, we summarize the research progress on the PL manipulation of the excitonic emission in TMDCs, where the PL intensity enhancement and emission wavelength regulation are included. The concept and characteristics of excitons are overviewed firstly, followed by the discussion on the evaluation and characterization of excitonic emission. The state-of-the-art progress on the manipulation of the neutral excitons and interlayer excitons PL are then summarized. Finally, the challenges and prospects are proposed.

The optical characteristics of organic light-emitting diodes with various microlens arrays

It is important to increase the light extraction efficiency (LEE) of devices in which micrometer-sized spheroidal, conical, pyramidal, and truncated microlens arrays (MLAs) are coupled to organic light-emitting diodes (OLEDs). MLAs may be hexagonal or square. To specify the shapes of individual lenses and their arrays in MLAs, the respective variables x (newly defined in this paper) and the fill factor (FF) can be used. MLAs are characterised by their LEE enhancements (LEEEs) and angular intensity distributions (AIDs), and these are the key optical characteristics of MLAs coupled to OLEDs. Here, these characteristics were obtained via computational simulation using a ray-tracing method. The LEEEs of the spheroidal MLAs were the same regardless of whether the arrays were hexagonal or square. The spheroidal MLAs are suitable for light emission applications in forward direction and, for spherical MLAs of FF = 74.55%, the LEEE was 88.3%. For conical hexagonal and pyramidal MLAs, the LEEEs gradually decreased and the difference widened as the extent of truncation increased. For display applications, the largest LEEE for light rays with emission angles < 10° was 56.5% for spheroidal MLA/OLD devices. Considering all emission angles, the largest LEEE was about 120% for pyramidal MLA/OLED devices. For regular pyramidal MLAs, the intensities of emitted light by the azimuth emission angle evidenced four sawtooth shapes precluding applications in displays.

Novel Zn2SiO4: Nb phosphor for light emitting applications

The researchers have focussed on finding new novel phosphors with enhanced emissions under UV/near UV excitation in a suitable host due to the limitation of commercial phosphors. In this study, niobium (Nb) doped zinc silicate (Zn2SiO4) by varying different wt. % of Nb was synthesized through the sol-gel method at 1200 °C under normal atmospheric conditions. The incorporation of Nb in the Zn2SiO4 host lattice significantly modifies their crystallinity, morphology, optical, and photoluminescent behaviours. In the PL study, Zn2SiO4: Nb phosphor shows a broad blue emission peak at 460 nm under 315 nm excitation. The highest PL emission intensity at 460 nm is obtained in 4 wt % of Nb-doped Zn2SiO4 materials among the prepared materials. The highest quantum yield and average lifetime were achieved for 4 wt % Nb-doped Zn2SiO4 materials which enunciates that the 4 wt % Nb-doped Zn2SiO4 is a potential candidate for light-emitting applications.

Exciton Ground State Fine Structure and Excited States Landscape in Layered Halide Perovskites from Combined BSE Simulations and Symmetry Analysis

AbstractLayered halide perovskites are solution‐processed natural heterostructures where quantum and dielectric confinement down to the nanoscale strongly influence the optical properties, leading to stabilization of bound excitons. Detailed understanding of the exciton properties is crucial to boost the exploitation of these materials in energy conversion and light emission applications, with on‐going debate related to the energy order of the four components of the most stable exciton. To provide theoretical feedback and solve among contrasting literature reports, this work performs ab initio solution of the Bethe–Salpeter equation (BSE) for symmetrized reference Cs2PbX4 (X = I and Br) models, with detailed interpretation of the spectroscopic observables based on group‐theory analysis. Simulations predict the following Edark &lt; Ein‐plane &lt; Eout‐of‐plane fine‐structure assignment, consistent with recent magneto‐absorption experiments and obtain similar increase in dark/bright splitting when going from lead‐iodide to a lead‐bromide composition as found experimentally. The authors further suggest that polar distortions may lead to stabilization of the in‐plane component and end‐up in a bright lowest exciton component, discuss exciton landscape over a broad energy range and clarify the exciton spin‐character, when large spin‐orbit coupling is in play, to rationalize the potential of halide perovskites as triplet sensitizers in combination with organic dyes.

Electron structure and exciton energy level of HgI2, CH3NH3HgI3 and (CH3NH3)2HgI4

The electronic structures and exciton properties of HgI2, CH3NH3HgI3 and (CH3NH3)2HgI4 are reported. Compared with HgI2, organic-inorganic hybrids CH3NH3HgI3 and (CH3NH3)2HgI4 have lower dimension and higher band gap, which show significant excitonic properties. The effective masses of electrons and holes of these three materials were predicted by their energy band structures. Their exciton binding energy at room temperature therein was calculated to be 13.77, 166.81, and 256.00 meV, respectively. HgI2 has low exciton binding energy that cannot exist stably, while CH3NH3HgI3 and (CH3NH3)2HgI4 have strong charge localization and high exciton binding energy. In addition, the UV–vis diffuse reflectance spectra reveal the presence of exciton absorption in CH3NH3HgI3 and (CH3NH3)2HgI4, and the exciton energy levels is confirmed by combining the diffuse reflectance spectra with its exciton binding energy. Importantly, the exciton luminescence peaks of CH3NH3HgI3 and (CH3NH3)2HgI4 were observed at 468 nm and 419 nm in their photoluminescence spectra at room temperature, respectively. This indicates that CH3NH3HgI3 and (CH3NH3)2HgI4 have strong exciton properties and are potential candidates for high-light-yield scintillators and light-emitter applications.

Light-Emitting Organic Semiconductor-Incorporated Perovskites: Fundamental Properties and Device Applications.

Recently, organic semiconductor-incorporated perovskites (OSiPs) have emerged as a new subclass of next-generation organic-inorganic hybrid materials. OSiPs combine the advantages of organic semiconductors, such as large design windows and tunable optoelectronic functionalities, with the excellent charge-transport properties of the inorganic metal-halide counterparts. OSiPs provide a new materials platform for the exploitation of charge and lattice dynamics at the organic-inorganic interfaces for various applications. This Perspective reviews recent achievements in OSiPs highlighting the benefits from organic semiconductor incorporation and elucidates the fundamental light-emitting mechanism, energy transfer, as well as band alignment structures at the organic-inorganic interface. Insights on the emission tunability lead toward a discussion of the potential of OSiPs in light-emitting applications, such as perovskite light-emitting diodes or lasing systems.