When a forward voltage is applied to the PN junction of the LED , a current flows through the PN junction. Electrons and holes recombine in the PN junction transition layer to generate photons. However, not every pair of electrons and holes will generate photons. Due to the PN junction of the LED as an impurity semiconductor, there are material quality, dislocation factors, and process. Various defects, such as ionization, excitation scattering and lattice scattering, cause non-radiative transitions when electrons are exchanged from the excited state to the ground state and exchange energy with the lattice atoms or ions, that is, no photons are generated. Converted into light energy and converted into thermal energy loss in the PN junction, so there is a composite carrier conversion efficiency, and is represented by the symbol Nint.
When a forward voltage is applied to the PN junction of the LED, a current flows through the PN junction. Electrons and holes recombine in the PN junction transition layer to generate photons. However, not every pair of electrons and holes will generate photons. Due to the PN junction of the LED as an impurity semiconductor, there are material quality, dislocation factors, and process. Various defects, such as ionization, excitation scattering and lattice scattering, cause non-radiative transitions when electrons are exchanged from the excited state to the ground state and exchange energy with the lattice atoms or ions, that is, no photons are generated. Converted into light energy and converted into thermal energy loss in the PN junction, so there is a composite carrier conversion efficiency, and is represented by the symbol Nint.
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.
To improve the internal quantum efficiency, it is possible to improve the Nint of LED from the manufacturing materials of LED, the epitaxial growth process of PN junction and the light-emitting mode of LED light-emitting layer. This aspect has been significantly improved by the unremitting efforts of the scientific and technological community, from the early stage. A few percent have 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 LEDPN junction can generate 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 0.23 eV (EV is electron volts).
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