The reason for the junction temperature of LED is that the electric energy added is not completely converted into light energy, but a part of it is transformed into thermal energy. The optical efficiency of LED is only 100lm/W at present, and its electro-optical conversion efficiency is only about 20 - 30%. That is to say, about 70% of the electricity is turned into thermal energy.
The junction temperature of LED is caused by two factors.
1. the internal generated photons can not be completely ejected to the outside of the chip and finally converted to heat, which is the main part, because at present, this is called the external quantum efficiency of only about 30%, most of which are converted into heat.
2. the internal quantum efficiency is not high, that is, when the electrons and holes are combined, 100% can not produce photons, which is usually called the "current leakage" which reduces the recombination rate of the carrier in the PN region. The leakage current multiplied by the voltage is the power of this part, which is converted to thermal energy, but this part is not the main component, because the internal photon efficiency is now close to 90%..