What are LEDs for Solar Lighting?

An LED is a polar device that emits light when applied with an appropriate amount of direct current. Generally, LEDs are made of epitaxial wafers with p-n junctions, which are also called solid-state light sources according to their natural properties. LEDs can come in different colors, shapes and sizes.

Under appropriate voltage conditions, electrons and holes in the semiconductor p-n junction recombine to generate photons and emit light. However, at room temperature without applied voltage, thermal excitation is insufficient to produce light emission because the carrier concentration is too low. Therefore, to make the carrier concentration meet the requirements of light emission, a voltage or current must be added to the junction for excitation.

LED light can have two emission modes, namely surface emission and edge emission. In surface emission, the emitted light is perpendicular to the p-n junction and can exit in both directions along the surface or the substrate. When exiting in the direction of the substrate, whether photons are reflected or absorbed depends on the properties of the substrate.

What are LEDs for Solar Lighting?
Figure 1 LED surface emission
What are LEDs for Solar Lighting?
Figure 2 LED edge emission

LEDs are composed of many thin layers of semiconductor material; one is n-doped with excess electrons, and the corresponding layer is p-doped hole regions.

When an appropriate voltage is applied, electrons in the n-region of the diode recombine with holes in the p-region. Recombination produces photons. If electrons and holes recombine to produce photons, it is called radiative recombination, and it is accompanied by light emission; otherwise, it is called non-radiative recombination, and heat is generated. The more electron-hole pairs recombine, the more photons are produced, and therefore the more intense the light emission. We call this phenomenon injection electroluminescence. The luminous intensity of the LED is determined by the current as described above, and the luminous color is determined by the band gap width of the semiconductor material.

The ratio of the number of photons emitted inside a semiconductor (npin) to the number of injected electrons ne, recombined with holes, is defined as the internal quantum efficiency. The internal quantum efficiency depends on the epitaxial growth structure of the material and can be as high as 90%.

The ratio of the number of photons radiated to the outside of the semiconductor (nphe) to the number of photons generated inside the semiconductor is called the light extraction efficiency, and the light extraction efficiency can be as low as 2%.

The light extraction efficiency is limited by the internal reflectivity of the semiconductor material and the encapsulation material and the different refractive indices between them.

The ratio of the number of photons emitted outside the semiconductor to the number of injected electrons is the external quantum efficiency.

External quantum efficiency can also be estimated by the ratio of the light output power of the LED to the electrical power (current multiplied by voltage) dissipated by the LED.

There are many reasons why LEDs have attracted widespread attention, such as brilliant colors, high efficiency, and long service life. LEDs are made of special semiconductor materials, gallium arsenide (GaAs), gallium phosphide (GaP), gallium arsenide phosphorus (GaAsP), zinc sulfide (ZnS) or carbide Silicon (SiC) and the like are all commonly used substrate materials. The color (wavelength) of its emission depends on the selected semiconductor material and doping material (usually Zn or N).

In order to achieve good electrical connection of LED devices, photolithography techniques are usually used to fabricate electrodes with gold or silver. The LED is then encapsulated in a transparent plastic. The material of the encapsulating plastic can affect its light output. The encapsulating plastic can be colored, with optional scattering or transparent geometry to determine the shape of the light beam.

In device fabrication, the purity and uniformity of the epitaxial wafer must be improved, because this will affect the brightness, efficiency and service life of the LED, that is, how bright the LED is, how high its luminous efficiency is, and how long it can be used. As the quality of LEDs improves, it can be used in increasingly demanding applications such as lighting. At present, the service life of LED can reach 100,000h, which is in sharp contrast with the average life of ordinary incandescent lamps of 1,000h. This also makes LEDs more suitable for applications in environments where lamps and lanterns are difficult to replace, such as the seabed and outer space.

Note: The LED lifetime stated here is obtained under laboratory conditions and is not accurate under other application conditions. The lifetime of an LED is heavily dependent on its usage conditions, especially temperature and humidity. In practical lighting applications, the life expectancy of an LED is about 50,000h.

What are LEDs for Solar Lighting?
Figure 3 Structure of LED and its light emission
What are LEDs for Solar Lighting?
Figure 4 Structure of LED

Usually, the following indicators can be seen on the product label of LED:
1) Dominant wavelength (color).
2) Peak wavelength: the wavelength at the highest peak of the spectrum. It does not necessarily determine the color of the LED’s light.
Two LEDs with the same peak wavelength can have different emission colors.
3) Luminous intensity.
4) Spatial distribution of light (viewable angle or beam angle)