Flexible White Light Emitting Diodes Based on Nitride Nanowires and Nanophosphors

Abstract

We report the first demonstration of flexible white phosphor-converted light emitting diodes (LEDs) based on p–n junction core/shell nitride nanowires. GaN nanowires containing seven radial In0.2Ga0.8N/GaN quantum wells were grown by metal–organic chemical vapor deposition on a sapphire substrate by a catalyst-free approach. To fabricate the flexible LED, the nanowires are embedded into a phosphor-doped polymer matrix, peeled off from the growth substrate, and contacted using a flexible and transparent silver nanowire mesh. The electroluminescence of a flexible device presents a cool-white color with a spectral distribution covering a broad spectral range from 400 to 700 nm. Mechanical bending stress down to a curvature radius of 5 mm does not yield any degradation of the LED performance. The maximal measured external quantum efficiency of the white LED is 9.3%, and the wall plug efficiency is 2.4%.

III‐Nitride Nanowires as Building Blocks for Advanced Light Emitting Diodes

“…Another approach is to use layers of silver nanowires, as in ref. . Transparent oxides films, such as indium tin oxide (ITO), deposition is also an option, but in that case one has to prevent ITO from going into the space between the nanopillars and shorting the diode.…”

“…The solution described in the literature is the deposition of polymer in between the nanopillars and planarizing the structure exposing the tops of the nanopillars before deposition of ITO. As a suitable polymer polydimethylsiloxane (PDMS) has become quite popular . Since it is essential that incorporation of LSP nanoparticles does not lead to shorting of the nanopillar p‐n junction LSP nanoparticles with the metal core surrounded by a dielectric shell, such as Ag/SiO 2 nanoparticles used in ref.…”

“…Moreover, the space between the NWs can be usefully employed for embedding nanoparticles (NPs) downconverting the energy of photons emitted from NWs or enhancing the efficiency of radiative luminescence, such as quantum dots nanoparticles (QDNPs) with high quantum yield or NPs producing localized surface plasmons (LSPs), thus increasing the functionality of the devices. Moreover, one can fill the space between the nanopillars with polymer film that can then be used as a handle for detaching the active layer from the substrate and transferring it to another substrate, something seriously considered recently as a means of fabricating LEDs on flexible substrates …”

“…However, the approach is extremely technologically challenging in obtaining the necessary high density of nanopillars and the uniformity of their height and radius as can be seen from Figure taken from ref. that represents the typical current state‐of‐the art. Moreover, the growth conditions that are conducive to stable high‐quality core‐shell QW NW growth in molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) are such that make it quite difficult to obtain high electroluminescence efficiency, the QWs’ thickness and In composition changes along the growth direction causing variations in the EL spectra along the nanopillars, the leakage current of the grown LED structures is a serious problem .…”

“…c) Transversal cross‐sectional STEM‐HAADF image taken along the c‐zone axis showing the shell structure with the MQWs and p‐GaN. Reproduced with permission . Copyright 2016, ACS Publications.…”