By Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106
Light Emitted Diodes
Solid-state lighting (SSL) is an illumination technology that has emerged in the past decade due to the development of white light-emitting diodes (LEDs). Currently, LEDs use a mature technology that can outperform traditional light sources due to their higher efficiencies, longer lifetimes, fast switching, robustness and compact size. The working principle of LEDs is based on electroluminescence, that is, the radiative recombination of injected electron–hole pairs in a material. We expect to see widespread replacement of traditional light sources with LEDs within the next two decades, leading to a considerable reduction in worldwide electricity consumption. To facilitate this transition, we must integrate LEDs into many different applications. To do this, we must be able to accurately and specifically control brightness, color and directionality of light emitted from LEDs. It appears that this control may be achieved using nanostructures
Light-emitting diodes (LEDs) are driving a shift toward energy-efficient illumination. Nonetheless, modifying the emission intensities, colors and directionalities of LEDs in specific ways remains a challenge often tackled by incorporating secondary optical components. Metallic nanostructures supporting plasmonic resonances are an interesting alternative to this approach due to their strong light–matter interaction, which facilitates control over light emission without requiring external secondary optical components. An efficient light source is characterized by the three following aspects:
- a) the generation of photons from electrical power with minimal losses
- b) a maximum of the generated photons illuminates the desired object, which can be for example a secondary optical element, or an open space
- c) the emission spectrum is optimized for the sensitivity of the detector, which in case of general lighting is the human eye.
A lot of progress has been made to improve the electrical LED efficiency mentioned above in a) since the first LED has been introduced. Furthermore, over the past few years a lot of attention has also been paid to point c), the optimization of the white light emission spectrum with respect to the human eye sensitivity curve. However, the latter has to be done with the boundary condition of still representing all the colors visible to the human eye, which is typically characterized by the color rendering index (CRI). Several ways on how to achieve highly efficient systems while maintaining an acceptable CRI have been proposed and realized.
Nanostructures, which have dimensions comparable to the wavelength of light, are especially suited to enhancing light–matter interactions. Metallic surfaces and nanostructures supporting surface plasmon polariton (SPP) resonances are of particular interest in this regard. These resonances have their origin in the coherent oscillation of charge carriers in the metal. The spontaneous emission from sources in the proximity of metals can be modified by SPPs, thereby
influencing the emission rate and directionality. These modifications are analogous to the resonant amplification and directional radiation of antennas. Therefore, metallic nanoparticles supporting SPPs have been referred to as optical antennas or nanoantennas. However, integrating such resonant nanostructures into state-of-the-art lighting applications remains challenging. The vast majority of studies has focused on modification of the emission properties of single and/or low-efficiency emitters, while real applications in SSL require modification of emission over macroscopic areas, typically in the mm2 range, of highly efficient emitters for which the typical photoluminescence quantum yield (QY) exceeds 90%. Until recently, these stringent requirements have limited the use of plasmonic structures for SSL. This situation is quickly changing due to the introduction of cost-effective nanofabrication techniques for use in light extraction, spectral shaping of emissions and strong beaming, without requiring additional external optical components.
Plasmonic nanostructures are known to influence the emission of near-by emitters. They can enhance the absorption and modify the external quantum efficiency of the coupled system. It has been shown that periodic plasmonic metal nanoparticle (NP) arrays can largely enhance the photoluminescence of nearby emitters in particular directions for a certain narrow wavelength range. This enhancement corresponds, on the one hand, to a modification of the angular emission profile, as more light is emitted in a specific direction than in others at a particular wavelength. On the other hand it also corresponds to a modification of the spectrum in a particular direction or small solid angle, as in this angular range more emission occurs at certain wavelengths than at others. Therefore, using periodic plasmonic arrays in combination with LED phosphors can address both points b) and c) mentioned above simultaneously. Using such a plasmonic LED device could therefore show the following advantages over a regular phosphor-converted LED:
- a) Modification of the angular emission profile such that the periodic plasmonic NP array could replace (partly) bulky secondary optical structures for focusing and collimating light in lighting applications.
- b) Shaping of the emission to better fit the eye sensitivity, especially in the red part of the visible spectrum.
- Confinement—which occurs due to the large difference in refractive index between the semiconductor and the ambient media—leads to severe total internal reflection at the interface, lowering the light extraction efficiency (LEE) of III-nitride LEDs. The light trapped inside of the LED device is eventually reabsorbed, thereby decreasing its efficiency. To achieve high light excitation and output performance, LEE enhancement is crucial. By modifying the chip shape or its surface morphology, the LEE can be improved. Several approaches have been proposed for the fabrication of different structures, either inside of the substrate or on its surface (e.g., photonic crystals, nanopyramids, a patterned substrate, surface roughness, and reflectors). The typical scale of these structures ranges from a few hundred nanometers to a few micrometers, depending on the resolution limit of the optical lithographic technology used.
- New methods that enhance the efficiencies of LEDs using nanostructured metals are being investigated. This is an emerging field that incorporates physics, materials science, device technology and industry. To evaluate the possibility of using plasmonics to enhance the light emission of a phosphor-converted LED device and create an efficient directional light source, regular arrays of aluminium nanoparticles covered with a red dye layer are under investigation. In arrays of aluminum nanocylinders with a diameter of ca 140 nm combined with a thin (650 nm) layer of luminescent material, very narrow resonances have been observed, which lead to large enhancement factors of up to 70 and 20 for excitation with a directional blue laser source and a lambertian LED respectively, in a small spectral range for particular angles. These changes in the angular emission profile of the red dye as well as the spectral shape of its emission can help to optimize the efficacy of phosphor-converted LED modules and increase the amount of useable light in a certain angular cone. Using Fourier microscopy, large modifications of the angular emission profile as well as spectral shaping are observed for these plasmonic LED devices if compared to reference samples without plasmonic nanostructures.
- Inorganic blue LEDs that are based on InGaN/GaN multi-quantum-well heterostructures are currently used in advanced architectures to obtain white-light emission. However, light generated in the active region of the multi-quantum-well structure can be reflected at the interfaces and trapped in the layered structure before it reaches the phosphor. To remedy this and maximize light extraction, metallic surfaces and nanostructures have been used. The metallic thin films used with SPPs have been applied directly to LEDs to enhance the spontaneous emission rate of excitons in quantum wells. The process can be explained as follows. Electron–hole pairs are injected in the active region of the LED. When a metal layer is grown at a distance smaller than the evanescent decay length of the SPPs, the electron–hole pairs recombine, giving their energy to the SPPs. Thus, the metal provides additional states for exciton recombination. This enhanced density of states for exciton recombination can significantly increase the recombination rate. Because SPPs are evanescent surface waves, they cannot radiate to free space. The metallic surface can be made rough to efficiently couple SPPs to free space radiation and enhance the emission intensity. Enhancement of the visible light emission originates from a combined higher recombination rate and a higher quantum-well extraction efficiency enabled by the nanometer-sized roughness in the metal layer. Although such random textures result in improved extraction efficiencies, they provide little control over the directionality of the emitted light, which typically displays a Lambertian profile.
- Accurate control over the angular distribution of the emission can be achieved using metallic nanostructures, which are directly fabricated, with predetermined geometries and dimensions, on the emissive semiconductor surface. A periodic designs may also be used to avoid undesirable angular and/or spectral dependencies. Unidirectional beaming of the LED emission has been recently demonstrated using a periodic array of optical antennas with specifically designed geometries. The silver flat film causes a substantial reduction in the intensity of the emitted light for both polarizations because no mechanism is provided to scatter the excited SPPs into radiation. In contrast, in the direction of the maximum intensity for one polarization, the output intensity of the LED with metallic nanostructures is enhanced compared with that of the flat sample. This polarization dependence can be attributed to the asymmetric shape of the nanostructures. Emission enhancements with a preferential light polarization can be beneficial for applications where light impinges upon smooth surfaces at nearly grazing angles, for example, automotive lighting. In these cases, it may be desirable to selectively enhance the emission obtained for one polarization only, because the other polarization may lead to unwanted effects, such as glare from incoming drivers.
- At the LSPR wavelength (~650 nm), the nanopyramid array beams more light toward the bottom of the pyramids. The opposite occurs at the SLR wavelength (~585 nm). These effects are due to the enhanced magnetoelectric response of the nanopyramid array (magnetic dipole moments are excited via the electric field of light), which originates from the pyramidal shape and height of the nanostructures. Future research should further investigate these phenomena in order to increase emission asymmetry and maximize the fraction of the emitted intensity that can be efficiently used in SSL.
LEDs constitute a new technology that is currently driving substantial changes in the way artificial light is generated. Several applications, for example, screen or automotive lighting, require light to be directed in only one direction. For planar structures, such as shallow nanoantenna arrays, light beaming into small angles is enhanced with roughly equal strengths in the forward and backward directions. The light emitted backward must be recycled using secondary optics, resulting in losses. To address this issue, the forward–backward light emission symmetry of planar structures can be broken by integrating an array of nanostructures with a pyramidal shape into the fluorescent layer. The inclusion of metallic nanoparticles minimizes the need for optical components in LEDs, such as parabolic mirrors or condenser lenses that are used for beaming the emission. These optical elements are often bulky, increasing the total size of the LEDs and limiting their integration. Therefore, the performance of metallic nanoparticle arrays in SSL applications must be assessed in terms of overall system efficiency with and without the presence of the metallic nanoparticle arrays. From a device perspective, the enhancement of phosphor-layer emissions enabled by the use of nanoparticles must not only be compared with emissions obtained from the same layer when no nanoparticles are used. We must also compare the results obtained with the same phosphor layer under conditions in which the usual secondary optical elements are in place. An additional advantage of nanoantenna-enhanced emission is that it also reduces the phosphor-layer thickness, which is important with regard to heat dissipation. Heat reduces emission efficiency, limiting the performance of LEDs. So far, it has not been easy to use thin layers in pc LEDs owing to their low blue absorption; the conversion efficiencies of these layers have not been sufficient to generate the desired emission spectrum.
Hurdles to overcome
The texturing process still has several disadvantages, however, including non-uniformity, high cost, material degradation, and limited efficiency enhancement. Due to its high optical transparency and low resistivity, a transparent conductive oxide layer (TCL) can be employed on the LED surface as an effective current-spreading layer and graded refractive index material. Although the physics of strongly coupled plasmon-emitter systems is very rich, and the prospect of strongly interacting emitters is exciting, the potential of these systems for use in light-emitting devices has rarely been discussed. One of the challenges in this regard is related to the poor QY that phosphor layers with high densities of organic molecules display. Although it is required to access the strong coupling regime, a high molecular density degrades the photoluminescence quantum yield QY of the ensemble via an effect known as ‘concentration quenching. Therefore, challenges remain with regard to improving high-QY light-emitting devices via strong emitter–plasmon coupling.