Led Research Paper
Generating white light from electricity with maximum efficacy has been a long quest since the first incandescent lamp was invented by Edison at the end of the 19th century. Nowadays, semiconductors are making reality the holy grail of converting electrons into photons with 100% efficiency and with colours that can be mixed for white light illumination. The revolution in solid-state lighting (SSL) dates to 1994 when Nakamura reported the first high-brightness blue LED based on GaN semiconductors. Then, white light was produced by simply combining a blue dye with a yellow phosphor. After more than a decade of intensive research the performance of white LEDs is quite impressive, beating by far the luminous efficacy of compact fluorescent lamps. We are likely close to replacing our current lighting devices by SSL lamps. However, there are still technological and fabrication cost issues that could delay large market penetration of white LEDs. Interestingly, SSL may create novel ways of using light that could potentially limit electricity saving. Whatever the impact of SSL, it will be significant on our daily life.
The purpose of this special cluster issue is to produce a snapshot of the current situation of SSL from different viewing angles. In an introductory paper, Tsao and co-workers from Sandia National Laboratories, present an energy-economics perspective of SSL considering societal changes and SSL technology evolution. In a second article, Narukawa et al working at Nichia Corporation—the pioneer and still the leading company in SSL—describe the state of the art of current research products. They demonstrate record performance with white LEDs exhibiting luminous efficacy of 183 lm W -1 at high-current injection. Then, a series of topical papers discuss in detail various aspects of the physics and technology of white LEDs
Carrier localization in InGaN quantum wells has been considered the key to white LEDs' success despite the huge density of defects. A comprehensive review of the different localization mechanisms and their implication for internal quantum efficiency (IQE) is proposed by Oliver and co-workers from Cambridge University. When discussing IQE in InGaN-based LEDs, the efficiency droop at high-current injection always emerges, which is a major concern for the future of SSL technology. Here, a collaborative work between Samsung and the Gwangju Institute of Science and Technology (Korea) proves that a specific design of the active region can limit this detrimental effect.
Once the issue of the IQE is solved, one still has to let the photons out of the chip. Matioli and Weisbuch from the University of California at Santa Barbara introduce the use of photonic crystals (PhCs) to improve light extraction efficiency. They describe different approaches to overcoming the main limitation of LEDs when implementing surface PhCs.
The technology of SSL, and in particular of colour rendering, is tackled by Zukauskas et al who studied in detail different white light sources. They show that extreme colour-fidelity indices need to cover the entire spectrum, with a broad-band at 530–610 nm and a component beyond 610 nm. Then, the reliability of GaN-based LEDs is discussed in the paper of Meneghesso and co-workers. The authors consider the most important physical mechanisms that are (i) the degradation of the active layer of LEDs, (ii) the degradation of the package/phosphor system, (iii) the failure of GaN-based LEDs against electrostatic discharge.
Finally, GaN LEDs on silicon developed in the group of Egawa at the Nagoya Institute of Technology are presented. This technology could allow a significant decrease in the fabrication cost of white LEDs.
A Light Emitting Diode (LED) is a solid state light source that emits light by the electroluminescence effect. LEDs utilize the radiative recombination process of electrons and holes to generate light through photon emission. Electrons and holes are pumped into the space charge region in multiple quantum wells (QWs) under forward bias and they recombine to emit light.
The increasing demand for light emitting diodes (LEDs) has been driven by a number of application categories, including display backlighting, communications, medical services, signage, and general illumination. One barrier to the acceptance of LEDs in these applications is the relatively sparse information available on their reliability. There are many areas in need of improvement and study regarding LEDs, including the internal quantum efficiency of the active region, light-extraction technology, current-flow design, the minimization of resistive losses, electrostatic discharge stability, increased luminous flux per LED package, and purchase cost.
The construction of LEDs is somewhat similar to microelectronics, but there are functional requirements, materials, and interfaces in LEDs that make their failure modes and mechanisms unique. This means that comprehensive industry and academic research are required on LED failure mechanisms and reliability to help LED developers and end-product manufacturers focus resources in an effective manner. The reliability information provided by the LED manufacturers is not at a mature enough stage to be useful to most consumers and end-product manufacturers.
CALCE LED research group is providing the groundwork for an understanding of the reliability issues of LEDs across failure causes and their associated failure mechanisms, issues in thermal management, and critical areas of investigation and development in LED technology and reliability.
The goals of the CALCE LED research group are to improve the reliability and the qualification of LEDs for the LED manufacturers and LED lighting companies to better understand the LED failure mechanisms and useful lifecycle dynamics.
- Knowledge-based qualification methods to develop data-driven, physics of failure (PoF) based, and fusion prognostic techniques for LEDs
- Development of reliability improvement methods for LEDs utilizing prognostics and health management techniques:
- To facilitate faster product development
- To identify reliability risks under application conditions and mitigate them
- To improve integration of PHM in LED lighting systems.