hujiao

Publication date: December 2018

Source: Nano Energy, Volume 54

Author(s): Kyungeun Jung, Jae-Ho Lee, Kwonwoo Oh, Chan Im, Junghwan Do, Joosun Kim, Weon-Sik Chae, Man-Jong Lee

Abstract

Graphical abstract

The light to be trapped inside light-emitting diodes (LEDs) greatly affects the luminous efficiency and device lifetime. Abrupt difference in refractive index between the sapphire substrate and GaN-based LEDs causes light trapping by total internal reflection, however, its optical loss has been taken for granted. In this study, we demonstrate that nanoporous GaN can be used as a refractive-index-matching layer to enhance the light transmittance at the sapphire–GaN interface in InGaN/GaN flip-chip light-emitting diodes (FCLEDs). The porosity and the refractive index of the nanoporous GaN layer are controlled by electrochemical etching of n -type GaN layer. The optical output power of FCLEDs with the nanoporous GaN layer grown on flat and patterned sapphire substrates is increased by 355% and 65% at an injection current of 20 mA, respectively, compared with that of an FCLED without the nanoporous GaN layer. The remarkable enhancement of optical output is mostly attributed to...

Nanostructured luminescent materials based on perovskite nanocrystals (p-NCs) are attractive since their optical properties can be tuned in a wide spectral range with high luminescence quantum yields and lifetimes, however, they lack stability. In this work, the optical properties of highly luminescent colloidal p-NCs (CsPbX 3 , where X = Cl/Br, Br, I) embedded in porous opal matrices are presented. It is shown that the photoluminescence of the p-NCs embedded into opal matrices possess increased longtime stability of its spectral and kinetic parameters under ambient conditions. LEDs based on the developed materials show pure color p-NC emission with stability of its parameters. The results of this work may expand the knowledge of interactions between luminescent nanoparticles within multicomponent nanostructured materials for further photonic applications.

Under pressure: Cs₃Sb₂I₉ undergoes large‐scale band‐gap narrowing, reversible amorphization, and piezochromism under high pressure. The narrow band gap of Cs₃Sb₂I₉ at the highest pressure satisfies the optimal band‐gap requirement of the Shockley–Queisser limit. The sample exhibits metallic conductor properties at 44.3 GPa, revealing a wholly new electronic landscape and transport properties.

Abstract

Among photovoltaic materials, the antimony‐based, perovskite‐like structure Cs₃Sb₂I₉ stands out owing to its low toxicity and air stability. Here, changes in the optoelectronic properties and crystal structure of the lead‐free perovskite derivative Cs₃Sb₂I₉ are reported, caused by pressure‐induced lattice compression. At 20.0 GPa, Cs₃Sb₂I₉ with a wide band gap (2.34 eV) successfully broke through the Shockley–Queisser limit (1.34 eV), accompanied by clear piezochromism from orange–yellow to opaque black. Additionally, Cs₃Sb₂I₉ experienced completely reversible amorphization at 20.0 GPa. These optical changes could be attributed to atomic‐orbital overlap enhancement caused by contraction of the Sb−I bond length and diminution of the Sb−I bond angle. In addition, Cs₃Sb₂I₉ underwent a transition from semiconductor to conductor upon compression and obtained metallic properties at 44.3 GPa, indicating new electronic properties. The obtained results may further broaden the research prospects of halide perovskite materials in the field of photovoltaics.