Hyun Myung Jang

Exciton Bohr radius (a_B) and exciton binding energy (E_b) of metal halide perovskites are two prime quantities in their applications to both light-emitting diode displays and photovoltaic devices. We develop a reliable theoretical method of simultaneously finding a_B and ε_r^c (dielectric constant) based on the net exciton energy above the bulk band gap. It is estimated that a_B under the dielectric confinement is substantially smaller than a_B in the absence of dielectric confinement: 4.36 nm vs. 5.61 nm in the case of CH3NH3PbBr3. We attribute the enhanced a_B to variations of ε_r^c and the electron-hole correlation energy. We also develop a simple method of finding E_b based on the same net exciton energy. Using this, we attribute the well-known difference in E_b between organic bromide perovskites and iodide counterparts to ε_r^c and explain that iodide perovskites are more suited than bromide counterparts in photovoltaic applications, which require smaller E_b for efficient charge-carriers transport.

The linewidth broadening caused by various physicochemical effects does limit the well-known advantage of ultrahigh color purity of metal halide perovskites (MHPs) for use in next-generation light-emitting diodes (LEDs). We have theoretically examined the quantum- and dielectric-confinement effects of a quantum dot (QD) on the degree of photoluminescence linewidth broadening. It is predicted that the linewidth (Δ λ _QC ) is mainly contributed by the two opposing effects: (i) the linewidth broadening due to the repulsive kinetic energy of confined excitons ( Δ λ Q C K E ) and (ii) the overall linewidth narrowing caused by the attractive Coulomb interaction ( Δ λ Q C C o u l ). It is shown that the relative contribution essentially remains at a constant value and is evaluated as Δ λ Q C C o u l / Δ λ Q C K E = 0.42 , which is independent of the QD size and the chemical nature of semiconducting emitter. We have computed Δ λ _QC for various QD sizes of the prototypical MHP emitter, MAPbBr ₃ , where MA denotes a methylammonium (CH ₃ NH ₃ ) organic cation. The calculated results show that the linewidth broadening due to the quantum confinement (Δ λ _QC ) increases rapidly beginning at the QD radius approximately equal to 6.5 nm but Δ λ _QC is less than 2 nm even at R = 1.5 nm. Thus, Δ λ _QC is much narrower than the linewidth caused by the exciton-LO phonon Fröhlich coupling (∼23.4 nm) which is known as the predominant mechanism of linewidth broadening in hybrid MHPs. Thus, the linewidth broadening due to the quantum confinement (Δ λ _QC ) is not a risk factor in the realization of MHP-based ultrahigh-quality next-generation LEDs.

Perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites suffer from ion migration, which causes overshoot of luminance over time during operation and reduces its operational lifetime. Here, we demonstrate 3D/2D hybrid PeLEDs with extremely reduced luminance overshoot and 21 times longer operational lifetime than 3D PeLEDs. The luminance overshoot ratio of 3D/2D hybrid PeLED is only 7.4% which is greatly lower than that of 3D PeLED (150.4%). The 3D/2D hybrid perovskite is obtained by adding a small amount of neutral benzylamine to methylammonium lead bromide, which induces a proton transfer from methylammonium to benzylamine and enables crystallization of 2D perovskite without destroying the 3D phase. Benzylammonium in the perovskite lattice suppresses formation of deep-trap states and ion migration, thereby enhances both operating stability and luminous efficiency based on its retardation effect in reorientation.

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.

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.

Nucleation of nanoparticles using exsolution phenomenon is a promising pathway to design durable and active materials for catalysis and renewable energy. Here, we focus on the impact of surface orientation of the host lattice on the nucleation dynamics to resolve questions with regards to “preferential nucleation sites”. For this, we carried out a systematic model study on three differently oriented perovskite thin films. Remarkably, in contrast to the previous bulk powder-based study suggesting that the (110)-surface is a preferred plane for exsolution, we identify that other planes such as (001) and (111)-facets also reveal vigorous exsolution. Moreover, particle size and surface coverage vary significantly depending on the surface orientation. Exsolution of (111)-oriented film shows the largest number of particles, the smallest particle size, the deepest embedment, and the smallest and most uniform interparticle distance among the oriented films. Based on classic nucleation theory, we elucidate that the differences in interfacial energies as a function of substrate orientation play a crucial role in controlling the distinct morphology and nucleation behavior of exsolved nanoparticles. Our finding suggests new design principles for tunable solid-state catalyst or nano-scale metal decoration.

Nanoparticles formed on oxide surfaces are of key importance in many fields such as catalysis and renewable energy. Here, we control B-site exsolution via lattice strain to achieve a high degree of exsolution of nanoparticles in perovskite thin films: more than 1100 particles μm⁻² with a particle size as small as ~5 nm can be achieved via strain control. Compressive-strained films show a larger number of exsolved particles as compared with tensile-strained films. Moreover, the strain-enhanced in situ growth of nanoparticles offers high thermal stability and coking resistance, a low reduction temperature (550 °C), rapid release of particles, and wide tunability. The mechanism of lattice strain-enhanced exsolution is illuminated by thermodynamic and kinetic aspects, emphasizing the unique role of the misfit-strain relaxation energy. This study provides critical insights not only into the design of new forms of nanostructures but also to applications ranging from catalysis, energy conversion/storage, nano-composites, nano-magnetism, to nano-optics.