Dec 5, 2017: An electrically injected hybrid organic/inorganic III-nitride white LED has been fabricated by using a two-dimensional (2D) microhole array structure. The hybrid LED geometry significantly enhances proximity between the inorganic active-region and the down-converting yellow organic light-emitting polymers (OLEPs), enabling the near-field nonradiative Förster resonance energy transfer (FRET) process with high efficiency while retaining excellent electrical characteristics of an unpatterned planar LED.
With the aim to remove the inefficiencies and technical limitations of phosphor-based white lights, researchers from the University of Sheffield have developed a unique type of white LED by integrating an inorganic InGaN/GaN blue LED with organic light-emitting polymers (OLEPs). Nonradiative Förster resonance energy transfer (FRET) can be transferred with radiation-less energy from inorganic active-regions (donors) to OLEPs (acceptors) through dipole−dipole Coulombic interactions. As a result, it is necessary to minimize the separation between donor and acceptor dipoles so that they can interact faster.
In a paper titled “Electrically Injected Hybrid Organic/Inorganic III-Nitride White LightEmitting Diodes with Nonradiative Förster Resonance Energy Transfer” published in ACS Photonics, the researchers explained that they hybridized to leverage the OLEPs’ faster response time together with a highly efficient non-radiative FRET between the inorganic active region and the yellow-emitting organic polymer. The white-light emitted by this hybridized concept achieved typical CIE color coordinates at (0.29, 0.32), which can be used in lighting.
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.
Usually, the common approach to create white light is to cap blue- or ultraviolet-emitting LEDs with down-conversion yellow phosphor materials, such white lights usually suffer from several inefficiencies like phosphor inefficiency, stability and a relatively slow response time which limits functionability and hence their applicability to Li-Fi at bandwidths under 1MHz when unfiltered. In fact, the use of blue filters to bypass the slow yellow response of the phosphors impacts the usable signal integrity.
But the researchers used a blue InGaN/GaN LED wafer grown on c-plane sapphire, with a 160nm thick InGaN/GaN MQW active region. 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 closeness between the inorganic active-region and the down-converting yellow OLEPs.
As 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.