What are Light Emitting Diodes?

Light emitting diodes, namely LEDs, refer to a type of solid state lighting device based on various doped semiconductor materials. Invented in early twentieth century, light emitting diodes represented a revolutionary technological progress. This is because conventional components such as light bulbs generates light through incandescence (or heat glow), which radiates electromagnetic wave by a material heated to high temperatures, typically higher than 750 degree Celsius. In contrast, LEDs release photons through electroluminescence, a electrically driven process that occurs at room temperature.

This beautifully engineered technology was pioneered by Professor Holonyak in 1962 and enabled by numerous scientists and engineers with fifty-plus years of active research. Today, it is difficult to get through our daily chores without coming into contact with light emitting diodes, and they have extended their presence from being power indicators on computers and children toys to energy saving solid-state lighting of our daily life.

Applications of Light Emitting Diodes

Lighting: The primary function of light emitting diodes is to provide a source of illumination at a defined wavelength. Today, the development of white-light-generating techniques have enabled a series of lighting applications. For example, a vast majority of modern high-end flashlights now use light emitting diodes. This is because LEDs not only require considerably less power, but also have a lifetime several orders of magnitude longer than that of the traditional flashlights. Other lighting applications of LEDs include traffic lights and architectural lighting.

Displays: Diverse applications of light emitting diodes are achieved through LED arrays controlled by specially programmed IC modules. TVs, computer monitors, cellphone and tablet screens are hence the arenas where LEDs play pivotal roles. With the advancement of LED

devices, the screens on these devices exhibit increasingly improved display performance in terms of resolution, color purity and balance. Most importantly, by implementing novel materials such as organic semiconductors and quantum dots, display screens can be realized on many unconventional places such as clothes, glasses, and even car windows. These futuristic technologies can profoundly enhance the efficiency of information exchange.

Different Types of Light Emitting Diodes

LEDs have various assorting standards such as color, geometry and operation modes. Perhaps the most fundamental assortment is the material, which can exhibit discrete physical mechanisms, thus giving rise to different advantages and limitations.

Inorganic LEDs

LEDs made of inorganic semiconductor materials are the most prevalent products in the market. They enjoy the most mature manufacturing approach and the longest history, from SiC and GaAs in the 1960s to other III-V compounds such as GaN and GaP developed in the recent decades. Despite of the variation in materials, inorganic LEDs generate light in a very similar manner.

An LED device resemble a diode because it represents a chip of semiconducting material doped with impurities to form a p-n junction. When an electron crosses the energy barrier of the p-n junction under forward bias and combine with a hole, it falls into a lower energy level and releases electromagnetic radiation in the form of a photon. Obviously, the wavelength of the emitted photon is associated with energy band gap of the semiconductor material. By selecting proper materials with different dopant concentrations, it is possible to obtain light with desired colors.

Organic LEDs (OLEDs)

While conventional LEDs achieve light emission through inorganic semiconductor materials, OLEDs employ one or multiple layers of organic compounds sandwiched between the anode and cathode for photon generation. Contrary to the belief that organic materials are usually poor conductors, certain organic molecules are electrically conductive as a result of delocalization of pi electrons caused by conjugation over part or all of the molecule. These materials have conductivity levels ranging from insulators to conductors, and are therefore organic semiconductors. Similar to inorganic LEDs, OLEDs emit light via spontaneous recombination of electrons and holes. However, the emission wavelength in OLED is dependent on the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), rather than by the valence and conduction bands in inorganic materials.

Noticeably, OLEDs exhibit several advantages over inorganic LEDs. First, the manufacturing process for OLED devices costs relatively less. This is because the bonding between organic molecules are Van der Waals forces, which is weaker than the bonding in inorganic materials. Hence the energy required for OLED fabrication is theoretically less. Second, OLEDs boast intriguing potentials in their nature of flexibility. Thus, display screens can possibly be realized on curved, bendable and even foldable surfaces. However, as of this writing, OLEDs have not been implemented in large scale, and the most prominent reason is the limited lifetime and reliability due to their vulnerability to water and oxygen.

Quantum Dot LEDs (QD-LEDs)

Quantum dots refer to small semiconductor nano-structures that confines the motion of electrons and holes in all three spatial directions. These artificial particles have atomic like energy levels with spacings inversely proportional to the square of its size. Hence, by changing the quantum dots dimensions one can manipulate the electronic and optical properties of the material.

QD-LEDs have structures very analogous to OLEDs, but replacing organic semiconductors with quantum dot materials in the emissive layer endows the devices with superior properties. For example, both inorganic LEDs and OLEDs radiate photons within limited spectral regions. Especially for inorganic LEDs, the difficulty of generating light in the green region, imposes an obstacle in obtaining desired colors. QD-LEDs however provide a perfect solution to this problem. For instance, CdSe quantum dot light emission have excellent tunability from red (5 nm diameter) to the violet region (1.5 nm dot). Additionally, quantum dots can be inorganic, which means improved lifetimes compared with OLEDs.

Nevertheless, QD-LEDs suffer from their own problems. For example, the manufacturing of CdSe quantum dots involves operations with toxic chemicals. Hence, extra precaution is necessary. Also quantum dots are susceptible to surface defects which can affect the recombination of electrons and holes. Thus, QD-LEDs are not sufficiently mature until related technical issues are fully addressed.

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