When a forward voltage is applied to the PN junction of the LED, a current flows through the PN junction, and electrons and holes recombine in the PN junction transition layer to generate photons. However, not every pair of electrons and holes will generate photons. Because of the PN junction as an impurity semiconductor, there are material defects, dislocation factors, and various defects in the process, which may cause problems such as ionization, excitation scattering, and lattice scattering. When the electrons are transitioned from the excited state to the ground state, no radiation transition occurs when the energy is exchanged with the lattice atoms or ions, that is, no photons are generated. This part of energy is not converted into light energy and converted into thermal energy loss in the PN junction, so there is A composite carrier conversion efficiency, expressed as a Nint symbol.
Nint = (number of photons generated by composite carriers / total number of composite carriers) × 100%
Of course, it is difficult to calculate the total number of composite carriers and the total number of photons produced. This efficiency is generally evaluated by measuring the optical power of the LED output. This efficiency, Nint, is called internal quantum efficiency.
Improving the internal quantum efficiency from the LED manufacturing materials, PN junction epitaxial growth process and the light-emitting mode of the LED light-emitting layer can improve the LED Nint, which has been significantly improved by the unremitting efforts of the scientific and technological community, from the early stage The percentage has increased to tens of percent, and there has been considerable progress, LED development in the future, and a lot of room for improving Nint.
Assuming that each composite carrier in the PN junction can produce a photon, can it be said that the LED-to-optical conversion efficiency reaches 100%? The answer is no.
It is known from semiconductor theory that the LEDs produced have different emission wavelengths due to different materials and epitaxial growth processes. It is assumed that the LEDs of these different illuminating wavelengths have an internal quantum efficiency of 100%, but a composite current is generated due to an electron N-type layer moving to the PN junction active layer and a hole moving from the P-type layer to the PN junction active layer. The energy E required for the sub-element is not the same as the energy band position of the LEDs of different wavelengths. The energy E of photons of different wavelengths is also different, and the conversion of electric energy to light energy has a certain loss. The following examples are explained:
For example, a GaInAlP quaternary orange LED with D=630nm is forward biased to VF≈2.2V, which means that the potential energy of one electron and one hole is combined into one carrier is ER=2.2Ev. And the potential energy of a photon entering D=630nm is E=Hc/into D≈1240/630≈1.97eV, so the conversion efficiency of electric energy to light energy is N(EL)=1.97/2.2×100%≈90%, ie There is an energy loss of 0.0.23 eV (EV is electron volts).
If a GaN blue light 470nm LED, then VF ≈ 3.4V, then EB ≈ 3.4EeV, and EB ≈ 1240 / 470 ≈ 2.64eV, then Nb = 2.64 / 3.4 × 100% ≈ 78%, which is assumed in Nint =100%. If Nint=60%, N(EL)=90%×60%=54% for red LEDs and N(EL)B=78%×60%=47s% for blue LEDs. It can be seen that this is the reason why the light-to-electric conversion efficiency of the LED is not very high.
It has been known above that the electrical-to-optical conversion efficiency of the PN junction active layer is not very high, and a considerable part of the electrical energy is not converted into light energy, but is converted into thermal energy loss in the PN junction, which becomes a heat source of the PN junction. The industry is improving this efficiency through the efforts of materials, processes and other mechanisms. If the electrical power applied to the LEDs all becomes photon energy, then ask: Can these photons all escape into the air to "see"? The answer is also negative. Then there is a problem of LED photon escape rate. This can be used to indicate the ratio of photons generated in the LED that escape into the air.
Nout = (number of photons escaping into the air / total number of photons generated by the PN junction) × 100%
The above formula can be the internal quantum efficiency of the LED. For convenience of explanation, we assume that the material of the LED is GaAs, the refractive index of the material is N1=3.9, the interface with the chip is air, and its light refractive index N0=1, which can be known by the law of light refraction of light propagation theory. When the refractive indices of the two different interfaces are different, the reflection function of the light perpendicular to the interface can be expressed by the following formula:
R(L)=[(N1-N0)/(N1+N0)] 2×100%
For GaAs and air, there are,
R(L)=[(3.9-1)/(3.9+1)]2×100%=35.02
That is to say, 35.02% of the photons will be reflected back into the GaAs material, that is, reflected back into the chip, and will not escape into the air, only 64.98% may escape into the air. However, if the illumination of the LED is a point source, the critical half angle Θc of the boundary full emission is related to the refractive index of the two materials of the interface, and is determined by the following formula: Θc=Arcsin(Ndn1)
For GaAs and air: Θc=Arcsin(1/3.9)=14.90°
The critical full-emission angle of the boundary is 29.8°. Exceeding this angle cannot be emitted into the air. Obviously, for a spherical surface, only 8.27% of this angle can be fully emitted. Obviously, the internal quantum efficiency is extremely low.
Of course, for the LED chip, it is a hexahedron, not a point source. When the electrode is blocked, the six faces of the hexahedron can have a full-emission critical angle with a total of 49.6% of the light-emitting area. In fact, because of the reasons for the LED to be led out, fixed on the lead frame, etc., it is also impossible to achieve six-sided light output, that is, less than 49.6% of the total emission area. The quantum efficiency of LED is generally only about 20%. It also has a lot of room for improvement. It is to solve the factors such as LED chip structure, package structure and material refractive index to improve the light extraction efficiency.
In recent years, due to the comprehensive advantages of environmental protection, energy saving and semiconductor, LED has replaced the traditional light source, but it needs a greater breakthrough in the luminous efficiency of LED to achieve wide application. To improve the luminous efficiency, the above quantum efficiency and electro-optic Efficiency is closely related! Technology increases and costs are reduced, and semiconductor lighting can take advantage of technology!
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