I. IQE, LEE and EQE definition
First, the definitions and expressions of the internal quantum efficiency (IQE: Internal Quantum Efficiency), the light extraction efficiency (LEE) and the external quantum efficiency (EQE) are given as follows:
IQE = number of photons emitted by the active layer per unit time / number of electrons injected into the active layer per unit time = (Pint / (hv)) / (I / e) (1.1)
LEE = number of photons emitted into space per unit time / number of photons emitted from the active layer per unit time = (P / (hv)) / = (Pint / (hv)) (1.2)
EQE = number of photons emitted into space per unit time / number of electrons injected into the active layer per unit time = (P / (hv)) / (I / e) = IQE * LEE (1.3)
Among them, Pint is the optical power emitted in the active region, I is the injection current, and P is the optical power emitted into the free space. The internal quantum efficiency characterizes the ability of the LED active region to convert the injected electrical energy into light energy; the light extraction efficiency characterizes the ability of the LED active region to emit light energy; the external quantum efficiency characterizes the LED's ability to convert electrical energy into the outside world. The ability of visible light energy, the higher the external quantum efficiency, the higher the luminous efficiency. For an ideal LED device, all three parameters are 1, which can completely convert the injected electrical energy into visible light energy.
2. Influence IQE, LEE and EQE factors
Defects in the epitaxial layer limit the IQE of the LED. From the working principle of LEDs, we know that LEDs are based on the combined radiation of electrons and holes. However, electrons and holes also have another compounding mechanism - non-radiative recombination, as shown in Figure 1. When electrons and holes are non-radiatively combined, excess energy is transferred to nearby atoms in the form of phonons, increasing the kinetic energy of the atoms. Macroscopically, the LED temperature is raised.
Non-radiative recombination is associated with defects in the epitaxial layer. When a defect exists in the epitaxial layer, a recombination center is formed at the defect, and non-radiative recombination is more likely to occur at the recombination center. The higher the defect density, the more such composite centers are, and more composite carriers will be non-radiatively compounded. Therefore, in the region where the defect is concentrated, the luminous intensity is weak. The LEE limiting factor for LEDs is the refractive index of the material.
figure 1
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