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.
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