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Inorganic halide double perovskites with optoelectronic properties modulated by sublattice mixing
摘要： All-inorganic halide double perovskites have emerged as a promising class of materials that are potentially more stable and less toxic than lead-containing hybrid organic-inorganic perovskite optoelectronic materials. In this work, 311 cesium chloride double perovskites (Cs2BB’Cl6) were selected from a set of 903 compounds as likely being stable based on a statistically learned tolerance factor (t) for perovskite stability. First-principles calculations on these 311 double perovskites were then performed to assess their stability and identify candidates with band gaps appropriate for optoelectronic applications. We predict that 261 of the 311 Cs2BB’Cl6 compounds are likely synthesizable based on a thermodynamic analysis of their decomposition to competing compounds (decomposition enthalpy < 0.05 eV/atom). Of these 261 likely synthesizable compounds, 47 contain no toxic elements and have direct or nearly direct (within 100 meV) band gaps between 1 and 3 eV as computed with hybrid density functional theory (HSE06). Within this set, we identify the triple alkali perovskites Cs2[Alk]+[TM]3+Cl6, where Alk is a group 1 alkali cation and TM is a transition metal cation, as a class of Cs2BB’Cl6 double perovskites with remarkable optical properties, including large and tunable exciton binding energies as computed by the GW-Bethe Salpeter Equation (GW-BSE) method. We attribute the unusual electronic structure of these compounds to the mixing of the Alk-Cl and TM-Cl sublattices, leading to materials with small band gaps, large exciton binding energies, and absorption spectra that are strongly influenced by the identity of the transition metal. The role of the double perovskite structure in enabling these unique properties is probed through analysis of the electronic structures and chemical bonding of these compounds as compared with other transition metal and alkali transition metal halides.
关键词： stability,sublattice mixing,band gaps,inorganic halide double perovskites,optoelectronic properties,excitons
更新于2025-09-23 15:19:57
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Reduced Nonradiative Voltage Loss in Terpolymer Solar Cells
摘要： The dissociation of hybrid local-charge transfer excitons (LE-CT) in efficient bulk-heterojunction non-fullerene solar cells contributes to reduced non-radiative photovoltage loss, a mechanism that still remains unclear. Herein, we studied the energetic and entropic contribution in the hybrid LE-CT exciton dissociation in devices based on a conjugated terpolymer. Compared to reference devices based on ternary blends, the terpolymer devices demonstrated a significant reduction of the non-radiative photovoltage loss, regardless of the acceptor molecule, be it fullerene or non-fullerene. Fourier transform photocurrent spectroscopy revealed a significant LE-CT character in the terpolymer-based non-fullerene solar cells. Temperature-dependent hole mobility and photovoltage confirm that entropic and energetic effects contribute to the efficient LE-CT dissociation. The energetic disorder value measured in the fullerene- or nonfullerene-based terpolymer devices suggested that this entropic contribution came from the terpolymer, a signature of higher disorder in copolymers with multiple aromatic groups. This gives new insight into the fundamental physics of efficient LE-CT exciton dissociation with smaller non-radiative recombination loss.
关键词： non-radiative photovoltage loss,terpolymer solar cells,entropic and energetic effects,hybrid local-charge transfer excitons,Fourier transform photocurrent spectroscopy
更新于2025-09-23 15:19:57
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The role of self-trapped excitons in polaronic recombination processes in lithium niobate
摘要： Transient absorption and photoluminescence are experimentally investigated in the polaronic reference system lithium niobate, LiNbO3 (LN), with the aim to refine the microscopic model of small polaron dynamics in materials with strong electron-phonon coupling. As a unique feature, our study is performed by using two different spectroscopic methods, in crystals with dopants enhancing photorefraction or damage resistance, and over a broad temperature range from 15?400 K. Although being self-consistent for particular experimental conditions, the hitherto used microscopic polaronic models reveal inconsistencies when applied to this larger data set. We show that comprehensive modeling is unlocked by the inclusion of an additional type of polaronic state with the following characteristics: (i) strongly temperature- and dopant-dependent relaxation times, (ii) an absorption feature in the blue-green spectral range, and (iii) a Kohlrausch-Williams-Watts decay shape with a temperature-dependent stretching factor β(T ) showing a behavior contrary to that of small, strong-coupling polarons. The hypothesis of self-trapped excitons (STEs, i.e., bound electron-hole pairs strongly coupled to Nb5+ and O2? within a niobium-oxygen octahedron) and their pinning on defects as the microscopic origin of these characteristics is supported by a spectroscopic linkage of photoluminescence at low (15 K) and elevated (300 K) temperatures and explains the long-lifetime components in transient absorption as due to pinned STEs.
关键词： self-trapped excitons,lithium niobate,photoluminescence,transient absorption,polaronic recombination
更新于2025-09-23 15:19:57
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Strained bubbles in van der Waals heterostructures as local emitters of photoluminescence with adjustable wavelength
摘要： The possibility to tailor photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDCs) using external factors such as strain, doping and external environment is of significant interest for optoelectronic applications. Strain in particular can be exploited as a means to continuously vary the bandgap. Micrometer-scale strain gradients were proposed for creating ‘artificial atoms’ that can utilize the so-called exciton funneling effect and work, for example, as exciton condensers. Here we describe room-temperature PL emitters that naturally occur whenever monolayer TMDC is deposited on an atomically flat substrate. These are hydrocarbon-filled bubbles which provide predictable, localized PL from well-separated submicron areas. Their emission energy is determined by the built-in strain controlled only by the substrate material, such that both the maximum strain and the strain profile are universal for all bubbles on a given substrate, i.e., independent of the bubble size. We show that for bubbles formed by monolayer MoS2, PL can be tuned between 1.72 to 1.81 eV by choosing bulk PtSe2, WS2, MoS2 or graphite as a substrate and its intensity is strongly enhanced by the funneling effect. Strong substrate-dependent quenching of the PL in areas of good contact between MoS2 and the substrate ensures localization of the luminescence to bubbles only; by employing optical reflectivity measurements we identify the mechanisms responsible for the quenching. Given the variety of available monolayer TMDCs and atomically flat substrates and the ease of creating such bubbles, our findings open a venue for making and studying the discussed light-emitting ‘artificial atoms’ that could be used in applications.
关键词： photoluminescence,exciton funneling,monolayer transition metal chalcogenides,excitons,strain engineering
更新于2025-09-19 17:15:36
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Effects of near-field electromagnetic coupling in dimers of nanoparticles with a silver core and a J-aggregate dye shell
摘要： We report a theoretical study of the plasmon – exciton coupling effect on the absorption spectra of pairs of closely spaced double-layer hybrid nanoparticles consisting of a metallic core and a J-aggregate dye shell. The effect of frequency conversion of plasmonic lines due to the near-field interaction between plasmons and Frenkel excitons of the organic shell is demonstrated. The effect leads to the appearance of additional spectral lines in the long-wavelength part of the spectrum of the system of hybrid particles. The shapes and the relative intensities of the additional lines exactly reproduce the specific features of the original spectrum of plasmonic absorption bands in uncoated metallic nanoparticles. The discovered phenomenon can be used to design new types of high-sensitivity nanosensors, based on plasmon – exciton effects and principles of near-field optics.
关键词： near-field electromagnetic coupling,nanophotonics,Frenkel excitons,dimers of metalorganic nanoparticles,plasmon – exciton interaction,molecular J-aggregates,localised plasmons,absorption spectra
更新于2025-09-19 17:15:36
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Valence and core excitons in solids from velocity-gauge real-time TDDFT with range-separated hybrid functionals: An LCAO approach
摘要： An atomic-orbital basis set framework is presented for carrying out velocity-gauge real-time time-dependent density functional theory (TDDFT) simulations in periodic systems employing range-separated hybrid functionals. Linear optical response obtained from real-time propagation of the time-dependent Kohn-Sham equations including nonlocal exchange is considered in prototypical solid-state materials such as bulk Si, LiF and monolayer hexagonal-BN. Additionally core excitations in monolayer hexagonal-BN at the B and N K-edges are investigated and the role of long-range and short-range nonlocal exchange in capturing valence and core excitonic effects is discussed. Results obtained using this time-domain atomic orbital basis set framework are shown to be consistent with equivalent frequency-domain planewave results in the literature. The developments discussed lead to a time-domain generalized Kohn-Sham TDDFT implementation for the treatment of core and valence electron dynamics and light-matter interaction in periodic solid-state systems.
关键词： Core-level spectroscopy,TDDFT,Solid-state,Real-time,Excitons,Range-separated hybrid
更新于2025-09-19 17:15:36
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Excitonic states in spherical layered quantum dots
摘要： The properties of excitons formed in spherical quantum dots are studied using the ?? ? ?? method within the Hartree approximation. The spherical quantum dots considered have a central core and several concentric layers of different semiconductor materials that are modeled as a succession of potential wells and barriers. The ?? ? ?? Hamiltonian and the Coulomb equations for the electron–hole pair are solved using a self-consistent iterative method. The calculation of the spectrum of the empty quantum dot and the electron–hole pair is performed by means of a very accurate numerical approximation. It is found that the exciton binding energy as a function of the core radius of the quantum dot shows a strong non-linear behavior. In particular, for quantum dots with two potential wells, the binding energy presents a large steep change. This last behavior is explained in terms of the polarization charges at the interfaces between different materials and the matching conditions for the eigenfunctions.
关键词： Spherical quantum dots,Hartree approximation,Excitons
更新于2025-09-19 17:13:59
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[IEEE 2019 International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD) - Ottawa, ON, Canada (2019.7.8-2019.7.12)] 2019 International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD) - 2D materials for optoelectronic devices
摘要： There is currently interest in 2D transition metal dichalcogenide (TMDC) materials, MX2 (M=Mo,W, X=S,Se,Te), for optoelectronic devices. These materials, when thinned down to a single layer, are an example of atomically thin truly two dimensional direct gap semiconductors. The reduction of dimensionality is a reason for strongly enhanced electron - electron interactions, which result in optical properties at room temperature dominated by neutral and charged excitons with binding energies orders of magnitude larger than room temperature and those found in standard compound semiconductors, e.g., GaAs quantum wells.
关键词： Bethe-Salpeter equation,optoelectronic devices,excitons,TMDC,2D materials
更新于2025-09-19 17:13:59
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On the absence of triplet exciton loss pathways in non-fullerene acceptor based organic solar cells
摘要： We investigate the viability of highly efficient organic solar cells (OSCs) based on non-fullerene acceptors (NFA) by taking into consideration efficiency loss channels and stability issues caused by triplet excitons (TE) formation. OSCs based on a blend of the conjugated donor polymer PBDB-T and ITIC as acceptor were fabricated and investigated with electrical, optical and spin-sensitive methods. The spin-Hamiltonian parameters of molecular TEs and charge transfer TEs in ITIC e.g., zero-field splitting and charge distribution, were calculated by Density Functional Theory (DFT) modelling. In addition, the energetic model describing the photophysical processes in the donor-acceptor blend was derived. Spin-sensitive photoluminescence measurements prove the formation of charge transfer (CT) states in the blend and the formation of TEs in the pure materials and the blend. However, no molecular TE signal is observed in the completed devices under working conditions by spin-sensitive electrical measurements. The absence of a molecular triplet state population allows to eliminate a charge carrier loss channel and irreversible photooxidation facilitated by long-lived triplet states. These results correlate well with the high power conversion efficiency of the PBDB-T:ITIC-based OSCs and their high stability.
关键词： triplet excitons,Density Functional Theory,organic solar cells,non-fullerene acceptors,photoluminescence detected magnetic resonance,electrically detected magnetic resonance
更新于2025-09-19 17:13:59
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[IEEE 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Huangshan, China (2019.8.5-2019.8.8)] 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Exciton tunneling behaviors in two dimensional halide perovskite
摘要： Two-dimensional (2D) halide perovskite materials have attracted great attention because of their impressive optoelectronic properties in light-emitting diodes (LED), sensors and solar cells, etc. Recently, it has been shown that the low dimensional perovskites with direct white light emission may act as the single component phosphors in next generation white-LEDs. Here, we demonstrate temperature dependent photoluminescence (PL) properties of white light 2D perovskites. Spectroscopic measurements suggest that the white light spectrum consist of free exciton (FE) and self-trapped exciton (STE). The relative intensities between the FE and STE are temperature dependent, and the energy tunneling effect is dominated at low temperature while the thermal quenching is responsible for the higher temperature region. Our results give the physical insight of the energy transfer and thermal behaviours of the excitons in 2D perovskites.
关键词： Temperature dependent PL,Excitons,2D materials
更新于2025-09-16 10:30:52