Light-emitting diodes based on all-inorganic copper halide perovskite with self-trapped excitons

Light-emitting diodes based on lead halide perovskite have attracted great attention due to their outstanding performance.However,their application is plagued by the toxicity of Pb and the poor stability.Herein novel copper-based all inorganic perovskite CsCu2I3 with much enhanced stability has been reported with a potential photoluminescence quantum yield(PLQY)over 20%and self-trapped excitons(STE).By taking advantage of its extraordinary thermal stability,we successfully fabricate high-quality CsCu2I3 film through direct vacuum-based deposition(VBD)of CsCu2I3 powder.The resulting film shows almost the same PLQY with the synthesized powder,as well as excellent uniformity and stability.The perovskite light-emitting diodes(Pe-LED)based on the evaporated CsCu2I3 emitting layer achieve a luminescence of 10 cd/m2 and an external quantum efficiency(EQE)of 0.02%.To the best of our knowledge,this is the first CsCu2I3 Pe-LED fabricated by VBD with STE property,which offers a new avenue for lead-free Pe-LED.

Efficient energy transfer from self-trapped excitons to Mn^(2+) dopants in CsCdCl_(3):Mn^(2+) perovskite nanocrystals

Mn^(2+)doping has been adopted as an efficient approach to regulating the luminescence properties of halide perovskite nano-crystals(NCs).However,it is still difficult to understand the interplay of Mn^(2+)luminescence and the matrix self-trapped exciton(STE)emission therein.In this study,Mn^(2+)-doped CsCdCl_(3) NCs are prepared by hot injection,in which CsCdCl_(3) is selected because of its unique crystal structure suitable for STE emission.The blue emission at 441 nm of undoped CsCdCl_(3) NCs originates from the defect states in the NCs.Mn^(2+)doping promotes lattice distortion of CsCdCl_(3) and generates bright orange-red light emission at 656 nm.The en-ergy transfer from the STEs of CsCdCl_(3) to the excited levels of the Mn^(2+)ion is confirmed to be a significant factor in achieving efficient luminescence in CsCdCl_(3):Mn^(2+)NCs.This work highlights the crucial role of energy transfer from STEs to Mn^(2+)dopants in Mn^(2+)-doped halide NCs and lays the groundwork for modifying the luminescence of other metal halide perovskite NCs.

Boosting self-trapped exciton emission from Cs_(3)Cu_(2)I_(5)nanocrystals by doping-enhanced exciton-phonon coupling

Self-trapped excitons(STEs)emission from halide perovskites with strong exciton-phonon coupling has attracted considerable attention due to the widespread application in optoelectronic devices.Nevertheless,the in-depth understanding of the relationship between exciton-phonon coupling and luminescence intensity remains incomplete.Herein,a doping-enhanced exciton-phonon coupling effect is observed in Cs_(3)Cu_(2)I_(5)nanocrystals(NCs),which leads to a remarkable increasement of their STEs emission efficiency.Mechanism study shows that the hetero-valent substitution of Cu+with alkaline-earth metal ions(AE^(2+))causes a greater degree of Jahn-Teller distortion between the ground state and excited state structures of[Cu_(2)I_(5)]_(3)-clusters as evidenced by our spectral analysis and first-principles calculations.As a consequence,an X-ray detector based on these Cs_(3)Cu_(2)I_(5):AE NCs delivers an X-ray imaging resolution of up to 10 lp·mm^(-1) and a low detection limit of 0.37μGyair·s^(-1),disclosing the potential of doping-enhanced exciton-phonon coupling effect in improving STEs-emission and practical application for X-ray imaging.

Origin of singlet self-trapped exciton and enhancement of photoluminescence quantum yield of organic–inorganic hybrid antimony(III)chlorides with the[SbCl_(5)]^(2)−units

Sb-based organic–inorganic hybrid metal halides(OIHMHs)with[SbCl5]2−units have been widely reported due to high photoluminescence quantum yield(PLQY)and occasional multiple self-trapped exciton(STE)emission bands mainly out of singlet and triplet states,and their multi-band emission is important in white light-emitting diode(WLED).However,not all these OIHMH compounds can produce both emissions out of singlet STE and triplet STE at room temperature simultaneously.It is crucial to consider how the singlet STE generates and retains to emit light at room temperature for this material’s design and application.Herein,a strategy is proposed that can significantly lift Sb halide PLQY by synthesizing two Sb-based OIHMHs using organic amine cations of different-sized and-quantity,which modulate the distance of neighboring emission centers.Therein,the occurrence of singlet STE emission is found to be closely related to the distance of[SbCl_(5)]^(2)−units and local unit distortion in the lattice.The larger distance can produce smaller local distortions,favoring the formation of the singlet STE emission band at higher energy.This is the first work to reveal the relationship between the local structure and the origin of the singlet STE emission band,providing new insights into the modulation of the Sb-based OIHMH’s emission.

Boosting triplet self-trapped exciton emission in Te(IV)-doped Cs_(2)SnCl_(6) perovskite variants

Perovskite variants have attracted wide interest because of the lead-free nature and strong self-trapped exciton (STE) emission. Divalent Sn(II) in CsSnX3 perovskites is easily oxidized to tetravalent Sn(IV), and the resulted Cs2SnCl6 vacancy-ordered perovskite variant exhibits poor photoluminescence property although it has a direct band gap. Controllable doping is an effective strategy to regulate the optical properties of Cs2SnX6. Herein, combining the first principles calculation and spectral analysis, we attempted to understand the luminescence mechanism of Te4+-doped Cs2SnCl6 lead-free perovskite variants. The chemical potential and defect formation energy are calculated to confirm theoretically the feasible substitutability of tetravalent Te4+ ions in Cs2SnCl6 lattices for the Sn-site. Through analysis of the absorption, emission/excitation, and time-resolved photoluminescence (PL) spectroscopy, the intense green-yellow emission in Te4+:Cs2SnCl6 was considered to originate from the triplet Te(IV) ion 3P1→1S0 STE recombination. Temperature-dependent PL spectra demonstrated the strong electron-phonon coupling that inducing an evident lattice distortion to produce STEs. We further calculated the electronic band structure and molecular orbital levels to reveal the underlying photophysical process. These results will shed light on the doping modulated luminescence properties in stable lead-free Cs2MX6 vacancy-ordered perovskite variants and be helpful to understand the optical properties and physical processes of doped perovskite variants.

Self-trapped excitons in two-dimensional perovskites

With strong electron-phonon coupling,the self-trapped excitons are usually formed in materials,which leads to the local lattice distortion and localized excitons.The self-trapping strongly depends on the dimensionality of the materials.In the three dimensional case,there is a potential barrier for self-trapping,whereas no such barrier is present for quasi-one-dimensional systems.Two-dimensional(2D)systems are marginal cases with a much lower potential barrier or nonex istent potential barrier for the self-trapping,leading to the easier formation of self-trapped states.Self-trapped excitons emission exhibits a broadband emission with a large Stokes shift below the bandgap.2D perovskites are a class of layered structure material with unique optical properties and would find potential promising optoelectronic.In particular,self-trapped excitons are present in 2D per-ovskites and can significantly influence the optical and electrical properties of 2D perovskites due to the soft characteristic and strong electron-phonon interaction.Here,we summarized the luminescence characteristics,origins,and characterizations of self-trapped excitons in 2D perovskites and finally gave an introduction to their applications in optoelectronics.

Identifying self-trapped excitons in 2D perovskites by Raman spectroscopy[Invited]

Two-dimensional(2D)perovskites exhibit broadband emission due to strong exciton–phonon coupling-induced self-trapped excitons and thus would find important applications in the field of white-light emitting devices.However,the available identifying methods for self-trapped excitons are currently rather limited and complex.Here,we identify the existence of self-trapped excitons by Raman spectroscopy in both excited and non-excited states.Under excited states,the shifting of the Raman peak indicates the presence of the lattice distortion,which together with the extra Raman scattering peak reveals the presence of self-trapped excitons.Our work provides an alternative simple method to study self-trapped excitons in 2 D perovskites.

Self-trapped exciton emission in inorganic copper(Ⅰ)metal halides

The broad emission and high photoluminescence quantum yield of self-trapped exciton(STE)radiative recombination emitters make them an ideal solution for single-substrate,white,solid-state lighting sources.Unlike impurities and defects in semiconductors,the formation of STEs requires a lattice distortion,along with strong electron–phonon coupling,in low electrondimensional materials.The photoluminescence of inorganic copper(Ⅰ)metal halides with low electron-dimensionality has been found to be the result of STEs.These materials were of significant interest because of their leadfree,all-inorganic structures,and high luminous efficiencies.In this paper,we summarize the luminescence characteristics of zero-and one-dimensional inorganic copper(I)metal halides with STEs to provide an overview of future research opportunities.

Effect of electron-electron interaction on polarization process of exciton and biexciton in conjugated polymer

By using one-dimensional tight-binding model modified to include electron-electric field interaction and electron-electron interaction,we theoretically explore the polarization process of exciton and biexciton in cis-polyacetylene.The dynamical simulation is performed by adopting the non-adiabatic evolution approach.The results show that under the effect of moderate electric field,when the strength of electron-electron interaction is weak,the singlet exciton is stable but its polarization presents obvious oscillation.With the enhancement of interaction,it is dissociated into polaron pairs,the spin-flip of which can be observed through modulating the interaction strength.For the triplet exciton,the strong electron-electron interaction restrains its normal polarization,but it is still stable.In the case of biexciton,the strong electron-electron interaction not only dissociate it,but also flip its charge distribution.The yield of the possible states formed after the dissociation of exciton and biexciton is also calculated.

Behavior of exciton in direct−indirect band gap Al_(x)Ga_(1−x)As crystal lattice quantum wells

Excitons have significant impacts on the properties of semiconductors.They exhibit significantly different properties when a direct semiconductor turns in to an indirect one by doping.Huybrecht variational method is also found to influence the study of exciton ground state energy and ground state binding energy in Al_(x)Ga_(1−x)As semiconductor spherical quantum dots.The Al_(x)Ga_(1−x)As is considered to be a direct semiconductor at AI concentration below 0.45,and an indirect one at the concentration above 0.45.With regards to the former,the ground state binding energy increases and decreases with AI concentration and eigenfrequency,respectively;however,while the ground state energy increases with AI concentration,it is marginally influenced by eigenfrequency.On the other hand,considering the latter,while the ground state binding energy increases with AI concentration,it decreases with eigenfrequency;nevertheless,the ground state energy increases both with AI concentration and eigenfrequency.Hence,for the better practical performance of the semiconductors,the properties of the excitons are suggested to vary by adjusting AI concentration and eigenfrequency.

Optical limiting of niobic tellurite glass induced by self-trapped exciton absorption of the AgCl nanocrystal dopant

Niobic tellurite glass doped by silver chloride nanocrystal was prepared with the melting-quenching and heat treatment method, and the self-trapped exciton absorption band of the silver chloride nanocrystal was observed at 532 nm in the UV-visible absorption spectrum. The glass structure characteristics were investigated by Raman spectroscopy, and the mechanism of self-trapped exciton was analyzed by Jahn-Teller model. Its optical limiting was measured with 532 nm picosecond laser pulses, and the corresponding nonlinear absorption coefficient was measured with open-aperture Z-scan. The experimental results showed that optical limiting at 532 nm was attributed to free carrier absorption between the self-trapped state and the continuum band.

Doping suppresses lattice distortion of vacant quadruple perovskites to activate self-trapped excitons emission

The vacancy-ordered quadruple perovskite Cs_(4)CdBi_(2)Cl_(12),as a newly-emerging lead-free perovskite system,has attracted great research interest due to its excellent stability and direct band gap.However,the poor luminescence performance limits its application in light-emitting diodes(LEDs)and other fields.Herein,for the first time,an Ag^(+)ion doping strategy was proposed to greatly improve the emission performance of Cs_(4)CdBi_(2)Cl_(12) synthesized by hydrothermal method.Density functional theory calculations combined with experimental results evidence that the weak orange emission from Cs_(4)CdBi_(2)Cl_(12) is attributed to the phonon scattering and energy level crossing due to the large lattice distortion under excited states.Fortunately,Ag^(+)ion doping breaks the intrinsic crystal field environment of Cs_(4)CdBi_(2)Cl_(12),suppresses the crossover between ground and excited states,and reduces the energy loss in the form of nonradiative recombination.At a critical doping amount of 0.8%,the emission intensity of Cs_(4)CdBi_(2)Cl_(12):Ag^(+)reaches the maximum,about eight times that of the pristine sample.Moreover,the doped Cs_(4)CdBi_(2)Cl_(12) still maintains excellent stability against heat,ultraviolet irradiation,and environmental oxygen/moisture.The above advantages make it possible for this material to be used as solid-state phosphors for white LEDs applications,and the Commission International de I’Eclairage color coordinates of(0.31,0.34)and high color rendering index of 90.6 were achieved.More importantly,the white LED demonstrates remarkable operation stability in air ambient,showing almost no emission decay after a long working time for 48 h.We believe that this study puts forward an effective ion-doping strategy for emission enhancement of vacancy-ordered quadruple perovskite Cs_(4)CdBi_(2)Cl_(12),highlighting its great potential as efficient emitter compatible for practical applications.

Localized self-trapping in two-dimensional molecular lattice with interaction between Wannier-Mott excitons and phonon lattice

We investigate the interactions of lattice phonons with Wannier-Mott exciton, the exciton that has a large radius in two-dimensional molecular lattice, by the method of continuum limit approximation, and obtain that the self-trapping can also appear in two-dimensional molecular lattice with a harmonic and nonlinear potential. The exciton effect on molecular lattice does not distort the molecular lattice but only makes it localized and the localization can also react, again through phonon coupling, to trap the energy and prevents its dispersion.

Localized self-trapping in the two-dimensional discrete molecular lattice with the interaction between Frenkel excitons and phonons

We investigate the interactions of lattice phonons with Frenkel exciton,which has a small radius in a twodimensional discrete molecular lattice,by the virtue of the quasi-discreteness approximation and the method of multiplescale,and obtain that the self-trapping can also appear in the two-dimensional discrete molecular lattice with harmonic and nonlinear potential.The excitons' effect on the molecular lattice does not distort it but only causes it to localize which enables it to react again through phonon coupling to trap the energy and prevent its dispersion.

Charge self-trapping in two strand biomolecules:Adiabatic polaron approach

We investigate the properties of the excess charge(electron, hole) introduced into a two-strand biomolecule. We consider the possibility that the stable soliton excitation can be formed due to interaction of excess charge with the phonon subsystem. The influence of overlap of the molecular orbitals between adjacent structure elements of the macromolecular chain on the soliton properties is discussed. Special attention is paid to the influence of the overlapping of the molecular orbitals between structure elements placed on the different chains. Using the literature values of the basic energy parameters of the two-chain biomolecular structures, possible types of soliton solutions are discussed.

Interaction of moiréexcitons with cavity photons in two-dimensional semiconductor hetero-bilayers

Moirématerials,composed of two single-layer two-dimensional semiconductors,are important because they are good platforms for studying strongly correlated physics.Among them,moirématerials based on transition metal dichalcogenides(TMDs)have been intensively studied.The hetero-bilayer can support moiréinterlayer excitons if there is a small twist angle or small lattice constant difference between the TMDs in the hetero-bilayer and form a type-Ⅱ band alignment.The coupling of moiréinterlayer excitons to cavity modes can induce exotic phenomena.Here,we review recent advances in the coupling of moiréinterlayer excitons to cavities,and comment on the current difficulties and possible future research directions in this field.

Thickness-dependent exciton relaxation dynamics of few-layer rhenium diselenide

Rhenium diselenide(ReSe_(2))has gathered much attention due to its low symmetry of lattice structure,which makes it possess in-plane anisotropic optical,electrical as well as excitonic properties and further enables ReSe_(2)have an important application in optoelectronic devices.Here,we report the thickness-dependent exciton relaxation dynamics of mechanically exfoliated few-layer ReSe_(2)flakes by using time-resolved pump–probe transient transmission spectroscopies.The results reveal two thickness-dependent relaxation processes of the excitons.The fast one correlates with the exciton formation(i.e.,the conversion of hot carriers to excitons),while the slow one is attributed to the exciton recombination dominated by defect-assisted exciton trapping besides photon emission channel.The decrease of scattering probability caused by defects leads to the increase of fast lifetime with thickness,and the increase of slow lifetime with thickness is related to the trap-mediated exciton depopulation induced by surface defects.Polarization-dependent transient spectroscopy indicates the isotropic exciton dynamics in the two-dimensional(2D)plane.These results are insightful for better understanding of excitonic dynamics of ReSe_(2)materials and its application in future optoelectronic and electronic devices.

Efficient transfer of metallophosphor excitons via confined polaritons in organic nanocrystals

We investigate the transfer of phosphorescent energy between co-assembled metallophosphors in crystalline nanostructures [Angew. Chem. Int. Ed. 57 7820(2018) and J. Am. Chem. Soc. 140 4269(2018)]. Neither Dexter's nor Forster's mechanism of resonance energy transfer(RET) could account fully for the observed rates, which exceed 85% with significant temperature dependence. But there exists an alternative pathway on RET mediated by intermediate states of resonantly confined exciton–polaritons. Such a mechanism was used to analyze artificial photosynthesis in organic fluorescents [Phys.Rev. Lett. 122 257402(2019)]. For metallophosphors, the confined modes act as extended states lying between the molecular S_(1) and T_(1) states, offering a bridge for the long-lived T_(1) excitons to migrate from donors to acceptors. Population dynamics with parameters taken entirely based on experiments fits the observed lifetimes of phosphorescence across a broad range of doping and temperature.

Melting of electronic/excitonic crystals in 2D semiconductor moirépatterns:A perspective from the Lindemann criterion

Using the Lindemann criterion,we analyzed the quantum and thermal melting of electronic/excitonic crystals recently discovered in two-dimensional(2D)semiconductor moirépatterns.We show that the finite 2D screening of the atomically thin material can suppress(enhance)the inter-site Coulomb(dipolar)interaction strength,thus inhibits(facilitates)the formation of the electronic(excitonic)crystal.Meanwhile,a strong enough moiréconfinement is found to be essential for realizing the crystal phase with a wavelength near 10 nm or shorter.From the calculated Lindemann ratio which quantifies the fluctuation of the site displacement,we estimate that the crystal will melt into a liquid above a critical temperature ranging from several tens Kelvin to above 100 K(depending on the system parameters).

Exciton radiative lifetime in CdSe quantum dots

Colloidal CdSe quantum dots(QDs)are promising materials for solar cells because of their simple preparation pro-cess and compatibility with flexible substrates.The QD radiative recombination lifetime has attracted enormous attention as it affects the probability of photogenerated charges leaving the QDs and being collected at the battery electrodes.However,the scaling law for the exciton radiative lifetime in CdSe QDs is still a puzzle.This article presents a novel explanation that recon-ciles this controversy.Our calculations agree with the experimental measurements of all three divergent trends in a broadened energy window.Further,we proved that the exciton radiative lifetime is a consequence of the thermal average of decays for all thermally accessible exciton states.Each of the contradictory size-dependent patterns reflects this trend in a specific size range.As the optical band gap increases,the radiative lifetime decreases in larger QDs,increases in smaller QDs,and is weakly depend-ent on size in the intermediate energy region.This study addresses the inconsistencies in the scaling law of the exciton life-time and gives a unified interpretation over a widened framework.Moreover,it provides valuable guidance for carrier separa-tion in the thin film solar cell of CdSe QDs.