Light Emitting Materials

LED - Light Emitting Diode



1920, from an accidentally discovered of Oleg Vladimirovich Losev, a radio technician, the symtom of a diode in radio receiver emitting light when the current pass through became nowadays Light Emitting Diodes or LED.


Image courtesy of Wikipedia

The inner working of LED



Current flow from the p-side/anode(+) to the n-side/cathode(-) in one way only. Electrons from cathode(-) and hole from anode(+) meeting each other at the junction point, falling into a lower state of energy and release energy in the form of a photon.

Typical structure of a LED



Light colors & material

	Color	Wavelength [nm]	Voltage [V]	Semiconductor Material
	Infrared	λ > 760	ΔV <>	Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs)
	Red	610 < λ <>	1.63 < ΔV <>	Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Orange	590 < λ <>	2.03 < ΔV <>	Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Yellow	570 < λ <>	2.10 < ΔV <>	Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)
	Green	500 < λ <>	1.9 < ΔV <>	Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP)
	Blue	450 < λ <>	2.48 < ΔV <>	Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC) as substrate Silicon (Si) as substrate — (under development)
	Violet	400 < λ <>	2.76 < ΔV <>	Indium gallium nitride (InGaN)
	Purple	multiple types	2.48 < ΔV <>	Dual blue/red LEDs, blue with red phosphor, or white with purple plastic
	Ultraviolet	λ <>	3.1 < ΔV <>	diamond (C) Aluminium nitride (AlN) Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN) — (down to 210 nm)
	White	Broad spectrum	ΔV = 3.5	Blue/UV diode with yellow phosphor

Common Types

Organic light-emitting diodes (OLEDs)

LED with an organic compound as the emitting layer. The emitting compound can be a small organic molecule in a crystalline phase or a polymer.

OLEDs are lighter than LED, have a wider range of color and more flexible. It can be applied to low-cost flexible displays, light sources, thin decoration layers or even luminous cloth.

Advantages:

- Efficiency
- Emit light with a designated color
- Compact size
- Fast respond to on/off switch
- Longevity
- Physical force resistance
- No toxic

Disadvantages:

- Color range: the incomplete of the white spectrum of LED can cause some misread in objects color
- Blue pollution: cool-white Leds can emit much more blue light than conventional light sources that may cause light pollute in some area of the urban area.