Structural Evolution in Methylammonium Lead Iodide CH₃NH₃PbI₃

The organic–inorganic hybrid perovskite, in particular, methylammonium lead iodide (MAPbI₃), is currently a subject of intense study due to its desirability in making efficient photovoltaic devices economically. It is known that MAPbI₃ undergoes structural phase transitions from orthorhombic <i>Pnma</i> to tetragonal <i>I</i>4/<i>mcm</i> at ∼170 K and then to cubic <i>Pm</i>3̅<i>m</i> at ∼330 K. A tetragonal <i>P</i>4<i>mm</i> phase is also reported at 400 K considering total cation disorder is not appealing due to its hydrogen-bonding capabilities. Resolving this ambiguity of phase transition necessitates the study of the structural evolution across these phases in our work using ab initio methods. In this work, we show that the structural phase evolves from <i>Pnma</i> to <i>I</i>4/<i>mcm</i> to <i>P</i>4<i>mm</i> to <i>Pm</i>3̅<i>m</i> with increasing volume. The <i>P</i>4<i>mm</i> phase is a quasi-cubic one with slight distortion in one direction from cubic <i>Pm</i>3̅<i>m</i> due to the rotation of MA cations. Biaxial strain on MAPbI₃ reveals that only the <i>Pnma</i> and <i>P</i>4<i>mm</i> phases are energetically stable at <i>a</i> < 9.14 Å and <i>a</i> > 9.14 Å, respectively. The <i>Pnma</i>, <i>I</i>4/<i>mcm</i>, <i>P</i>4<i>mm</i>, and <i>Pm</i>3̅<i>m</i> phases can be stable under various uniaxial strain conditions. Our study provides a clear understanding of the structural phase transitions that occur in MAPbI₃ and provides a guide for the epitaxial growth of specific phases under various strain conditions

High-Efficiency Light-Emitting Diodes of Organometal Halide Perovskite Amorphous Nanoparticles

Organometal halide perovskite has recently emerged as a very promising family of materials with augmented performance in electronic and optoelectronic applications including photovoltaic devices, photodetectors, and light-emitting diodes. Herein, we propose and demonstrate facile solution synthesis of a series of colloidal organometal halide perovskite CH₃NH₃PbX₃ (X = halides) nanoparticles with amorphous structure, which exhibit high quantum yield and tunable emission from ultraviolet to near-infrared. The growth mechanism and photoluminescence properties of the perovskite amorphous nanoparticles were studied in detail. A high-efficiency green-light-emitting diode based on amorphous CH₃NH₃PbBr₃ nanoparticles was demonstrated. The perovskite amorphous nanoparticle-based light-emitting diode shows a maximum luminous efficiency of 11.49 cd/A, a power efficiency of 7.84 lm/W, and an external quantum efficiency of 3.8%, which is 3.5 times higher than that of the best colloidal perovskite quantum-dot-based light-emitting diodes previously reported. Our findings indicate the great potential of colloidal perovskite amorphous nanoparticles in light-emitting devices

Highly Efficient Visible Colloidal Lead-Halide Perovskite Nanocrystal Light-Emitting Diodes

Lead-halide perovskites have been attracting attention for potential use in solid-state lighting. Following the footsteps of solar cells, the field of perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their application prospects in lighting, however, remain still uncertain due to a variety of shortcomings in device performance including their limited levels of luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs based on colloidal perovskite nanocrystals (PeNCs) synthesized at room temperature possessing dominant first-order excitonic radiation (enabling a photoluminescence quantum yield of 71% in solid film), unlike in the case of bulk perovskites with slow electron–hole bimolecular radiative recombination (a second-order process). In these PeLEDs, by reaching charge balance in the recombination zone, we find that the Auger nonradiative recombination, with its significant role in emission quenching, is effectively suppressed in low driving current density range. In consequence, these devices reach a maximum external quantum efficiency of 12.9% and a power efficiency of 30.3 lm W^–1 at luminance levels above 1000 cd m^–2 as required for various applications. These findings suggest that, with feasible levels of device performance, the PeNCs hold great promise for their use in LED lighting and displays