Researchers design fast white light from hybridized inorganic/organic LED
The use of blue filters to bypass the slow yellow response of the phosphors severely impacts the usable signal integrity.
Working on removing the inefficiencies and technical limitations of phosphor-based white lights, a team of researchers from the University of Sheffield has designed a novel type of white LED by closely hybridizing an inorganic InGaN/GaN blue LED with pockets of organic light-emitting polymers (OLEPs).
The idea behind this hybridization is to leverage the OLEDs’ much faster response time together with a highly efficient near-field non-radiative Förster resonance energy transfer (FRET) between the inorganic active region and the nearby yellow-emitting organic polymer.
It is understood that nonradiative FRET involves a near-field radiation-less energy transfer from inorganic active-regions (donors) to OLEPs (acceptors) through dipole−dipole Coulombic interactions, hence the need to minimize the separation between donor and acceptor dipoles so they can interact.
The researchers started with a commercial blue InGaN/GaN LED wafer grown on c-plane sapphire, with a 160nm thick InGaN/GaN MQW active region.
In a paper titled “Electrically Injected Hybrid Organic/Inorganic III-Nitride White LightEmitting Diodes with Nonradiative Förster Resonance Energy Transfer” published in ACS Photonics, they detail how they patterned a two-dimensional microhole array structure deep across the MQW active region to drop cast a yellow-emitting polyfluorene copolymer.
The micropockets, 900nm deep, 2.5μm in diameter and distributed at a 3.5μm pitch ensure a close proximity between the inorganic active-region and the down-converting yellow organic light-emitting polymers (OLEPs).
Because the microhole pattern maintains the continuity of the InGaN/GaN diode structure, the overall device keeps inherits from the electrical properties of the unpatterned device.
From their experiments on 350×350μm2 planar-LEDs, the researchers noted a reduction in the recombination lifetime in the InGaN/ GaN blue active region, confirming the nonradiative FRET process occurring between the InGaN/GaN blue active region and the yellow organic polymer.
They observed a typical FRET efficiency of 16.7%, with the FRET interaction area accounting for approximately 0.64% of the remaining blue-emitting inorganic LED, suppressing likely nonradiative recombination processes and therefore enhancing the device’s overall efficiency.
The white-light emitted by this hybridized approach achieved typical CIE color coordinates at (0.29, 0.32), suitable for use in lighting.
University of Sheffield – www.sheffield.ac.uk