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Abstract Colloidal nanocrystals (NCs) can be used to prepare high-color-purity metal halide perovskites (MHPs) for light-emitting displays. However, the NCs have a finite particle-size distribution, which broadens the linewidth of photoluminescence spectra under strong quantum confinement, and thereby degrades the color purity of the MHPs. This paper presents a simple method to quantify this externally-introduced broadening of linewidth $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{SD}}}}}}}$$ Γ SD by combining experimental size-distribution histogram with size-dependent photoluminescence-wavelength (PL- $${{{{{\boldsymbol{\lambda }}}}}}$$ λ ) curve. We develop a semi-empirical method to estimate the other three contributions to the experimentally-measured full-width at half-maximum, $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{EXP}}}}}}}$$ Γ EXP . Namely: the intrinsic linewidth $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{LO}}}}}}}$$ Γ LO caused by exciton-longitudinal optical (LO) phonon Fröhlich coupling; the inhomogeneous linewidth $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{o}}}}}}}$$ Γ o caused by imperfections-related scattering, and the broadening $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{QC}}}}}}}$$ Γ QC due to the quantum-confinement effect. We show that $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{LO}}}}}}}$$ Γ LO of a nanocrystal decreases together with the particle size, disappearing at 1.6 nm radius. Finally, we show that $${{{{{{\boldsymbol{\Gamma }}}}}}}_{{{{{{\bf{LO}}}}}}}$$ Γ LO is correlated with the degree of Fröhlich-polaron formation, hence proportional to the long-range LO-phonon-electron coupling.

Ion migration can induce overshoot of luminance in normal 3D perovskite light-emitting diode devices and results in reduced lifetime. Here Kim et al. show that the ion migration and overshoot can be suppressed in 3D/2D hybrid perovskites, leading to 21 times longer operational lifetime.

Dispersion of metallic nanoparticles is promising for energy conversion and storage, but gaining control of size and distribution is not trivial. Here the authors use lattice mismatch to manipulate exsolution of nanoparticles, achieving a high population of small nanoparticles in perovskite thin films.

Metal halide perovskites (MHPs) show superior optoelectronic properties, which give them the great potential for use in next generation light-emitting diodes (LEDs). In particular, their narrow emission linewidths can achieve ultrahigh color purity. However, the reported luminescence efficiency (LE) values are not high enough to be commercialized in displays and solid-state lightings. Moreover, the operational stability of LEDs associated with the overshooting of luminance and the high relative standard deviation of reported external quantum efficiencies are still problematic. In this perspective, we review photophysical factors that limit the photoluminescence quantum efficiency of perovskite-based LEDs. These factors are categorized into (i) weak exciton binding, (ii) nonradiative recombinations, (iii) slow cooling of long-lived hot carriers, (iv) deep-level defects, and (v) interband transition rates. We then present various physicochemical methods to effectively overcome these luminescence-limiting factors. We finally suggest some useful research directions to further improve the LE of MHP emitters as core components in displays and solid-state lightings.

Abstract We report direct evidence that oxygen vacancies affect the structural and electrical parameters in tensile-strained NdNiO3−δ epitaxial thin films by elaborately adjusting the amount of oxygen deficiency (δ) with changing growth temperature T D. The modulation in tensile strain and T D tended to increase oxygen deficiency (δ) in NdNiO3−δ thin films; this process relieves tensile strain of the thin film by oxygen vacancy incorporation. The oxygen deficiency is directly correlated with unit-cell volume and the metal-insulator transition temperature (T MI), i.e., resulting in the increase of both unit-cell volume and metal-insulator transition temperature as oxygen vacancies are incorporated. Our study suggests that the intrinsic defect sensitively influences both structural and electronic properties, and provides useful knobs for tailoring correlation-induced properties in complex oxides.

The original version of this Article contained an error in the Data Availability section, which incorrectly read ‘The data that support the findings of this study are available from the corresponding authors upon request.’ The correct version replaces this sentence with ‘The research data underpinning this publication can be accessed at https://doi.org/10.17630/21d12144-58ef-4f82-acd0-ba3c9a44ed72’. This has been corrected in both the PDF and HTML versions of the Article.

Despite their potential to exceed the theoretical Shockley-Queisser limit, ferroelectric photovoltaics (FPVs) have performed inefficiently due to their extremely low photocurrents. Incorporating Bi2FeCrO6 (BFCO) as the light absorber in FPVs has recently led to impressively high and record photocurrents [Nechache R, et al. (2015) Nat Photonics 9:61-67], which has revived the FPV field. However, our understanding of this remarkable phenomenon is far from satisfactory. Here, we use first-principles calculations to determine that such excellent performance mainly lies in the efficient separation of electron-hole (e-h) pairs. We show that photoexcited electrons and holes in BFCO are spatially separated on the Fe and Cr sites, respectively. This separation is much more pronounced in disordered BFCO phases, which adequately explains the observed exceptional PV responses. We further establish a design strategy to discover next-generation FPV materials. By exploring 44 additional Bi-based double-perovskite oxides, we suggest five active-layer materials that offer a combination of strong e-h separations and visible-light absorptions for FPV applications. Our work indicates that charge separation is the most important issue to be addressed for FPVs to compete with conventional devices. [ABSTRACT FROM AUTHOR]

Orthorhombic TbMn2O5 (o-TMO) is a well-known multiferroic manganite with the remarkable property of polarization switching at 3 K under a bias magnetic (H) field along the a axis of Pb21m. To theoretically account for this outstanding observation, we have proposed a modulated spin structure under the saturated bias H field by considering the relative strength of the three relevant exchange parameters in o-TMO. The proposed modulated structure based on density-functional theory (DFT) calculations is described in terms of the spin angle ø between the neighboring Mn4+-Mn3+ spin moments on the a-b plane. We have shown that the computed DFT polarization plotted as a function of ø satisfactorily accounts for the observed H-field-induced polarization switching. We have further theoretically shown that the square of the critical field strength (Hc) needed for the polarization switching is inversely proportional to the degree of the extrinsic magnetoelectric coupling. The computed partial charge density demonstrates that the H-field-induced polarization switching also accompanies with the switching in the sign of the excess valence-electron density. [ABSTRACT FROM AUTHOR]

Size-tunable mesoporous spherical TiO2(MS TiO2) with a surface area of ∼110 m2g−1have been prepared through combination of “dilute mixing”-driven hydrolysis of titanium(iv) tetraethoxide and solvothermal treatment. The hierarchically structured MS TiO2are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen sorption analysis. Using three different MS TiO2(587, 757, and 1554 nm in diameter) as a scattering overlayer on a transparent nanocrystalline TiO2film, bi-layered dye-sensitized solar cells (DSCs) have been fabricated. Since the MS TiO2particles are comprised of ∼10 nm nanocrystallites that cluster together to form large secondary spheres, they can function as light scatterers without sacrificing the surface area for dye-uptake. As a result, the present MS TiO2-based cells perform a noticeable improvement in the overall efficiency: maximum 9.37% versus6.80% for the reference cell made of a TiO2nanocrystalline film. This extraordinary result is attributed to the dual effects of enhanced dye loading and light scattering. [ABSTRACT FROM AUTHOR]

We propose an asymmetric bilayer structure in which the magnetic remanence (MR) is controlled by the in-plane strain of the top NiFe2O4 (NFO) layer epitaxially constrained by the bottom Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) substrate. In this asymmetric structure, an electric-field-induced giant piezoelectric strain from the bottom PMN-PT layer is effectively transferred to the top NFO layer. We have shown that the room-temperature magnetic remanence (MR) of the 100-nm-thick NFO layer is enhanced by 46% when an electric-field-induced in-plane compressive strain is about -0.1%. Synchrotron x-ray absorption near edge structure study supports a scenario of the cation-charge redistribution between Ni2+ and Fe3+ ions under the condition of an electric-field-induced in-plane compressive strain. [ABSTRACT FROM AUTHOR]

Orthorhombic YMO3 (o-YMO) is currently being intensively investigated since a single-crystalline thin film of o-YMO recently showed a pronounced degree of magnetoelectric coupling. According to first-principles calculations, the ferroelectric ground state is represented by two degenerate E-type collinear spin configurations with the computed polarization P of ∼1.4 μC/cm2. The bc-cycloidal spin phase is identified as the spin configuration that corresponds to the transition state in the +P↔−P polarization switching. The remarkable coexistence of the E-type and cycloidal spin phases below ∼40 K is attributed to a small value of the Kohn-Sham energy difference between these two phases, 2.2 meV per formula unit. A modulated spin structure, which is characterized by the tilting of the Mn-spin vectors to the a-axis direction of Pbnm setting, is proposed to account for the observed strong magnetic-field-dependent polarization in o-YMO. [ABSTRACT FROM AUTHOR]

HoMnO3 (HMO) is one of the most extensively studied hexagonal manganites owing to its strong tendency of the magnetoelectric coupling. In spite of extensive studies on its multiferroic properties, however, little progress has been made in our understanding on the electronic origin of the hexagonal ferroelectricity in HMO. Considering this, we have examined the bonding mechanism that gives rise to an off-centering ferroelectric distortion along the c axis of the hexagonal P63cm unit cell by exploiting density-functional theory calculations. We have shown that the asymmetric Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed hybridization is primarily responsible for the manifestation of this structural ferroelectricity in HMO. [ABSTRACT FROM AUTHOR]

We propose an asymmetric bilayer structure in which the magnetic remanence (MR) is controlled by the in-plane strain of the top NiFe2O4 (NFO) layer epitaxially constrained by the bottom Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) substrate. In this asymmetric structure, an electric-field-induced giant piezoelectric strain from the bottom PMN-PT layer is effectively transferred to the top NFO layer. We have shown that the room-temperature magnetic remanence (MR) of the 100-nm-thick NFO layer is enhanced by 46% when an electric-field-induced in-plane compressive strain is about -0.1%. Synchrotron x-ray absorption near edge structure study supports a scenario of the cation-charge redistribution between Ni2+ and Fe3+ ions under the condition of an electric-field-induced in-plane compressive strain. [ABSTRACT FROM AUTHOR]