Which part of the LED is affected when it is exposed to liquid nitrogen? I have seen experiments showing LED's brightness increasing and color shifting, when the LED is submerged in liquid nitrogen. I have also seen one where the color went DOWN in frequency, how can you explain this inconsistancy?
The LED is a semiconductor that was designed for a certain voltage at a certain temperature. This certain voltage is called "band gap" and is set by properly doping it during manufacturing. The "band gap" is the voltage (think pressure) required for the electrons to hop the "gap". The larger the band gap, the higher the voltage to cross, the higher the energy of light emitted, and the higher the frequency.
When a semiconductor is cooled its resistance increases as well its band gap. This larger band gap requires a larger corresponding voltage for electrons to hop. This increases the frequency (blue shift) of the light emitted, like the orange to green shift from liquid nitrogen demonstrated in this youtube video. Notice that the measured voltage drop across the LED also increased.
Similarly, solar photo-voltaic cells are also semiconductors and increase output voltage as the temperature gets colder.
Note that the voltage to frequency relationship doesn't always make sense though. I am not in the LED manufacturing industry, but other things are going on physically inside the LED when the opposite occurs. For example, this youtube video shows a yellow LED change to green and green to yellow when submersed in liquid nitrogen. Yellow to green makes sense as we previously discussed. Green to yellow doesn't though.
Perhaps the doping chemical in that specific green led becomes more conductive at low temperatures and actually reduces the band gap. I am not a material scientist, but something similar to that has to occur, because the band gap voltage sets the emitted light frequency.
You can also see color changes by increasing the voltage on an LED beyond its design voltage as shown in this youtube video. The life of the LED is significantly reduced, but its a simple experiment. This color shift is most likely due to over heating the semiconductor inside its plastic housing. At higher temperatures, the band gap decreases and the corresponding frequency decreases (red shift).
Here is a good Digikey article on chromatic shift in LEDs due to temperature. Mid way down the page it better explains the physical nature of the band gap:
As the temperature of the LED rises, the atoms in the lattice vibrate more, which slightly increases the lattice constant. This in turn decreases the band gap which increases the wavelength of the emitted photon.
This is a rather in-depth question but broadly speaking - The part of the LED which is affected by the liquid nitrogen is called the p-n junction. The two halves of the junction are a set distance apart, this is the "band-gap".
Heating and cooling of the LED causes the crystal lattice of the p-n junction materials to either expand or retract. This causes the band-gap to increase and decrease. In this specific case the liquid nitrogen causes the crystal lattice to become more closely packed. This increases the size of the band-gap. The result of which is the decrease in wavelength of the light being emitted.
An LED exposed to liquid nitrogen would last only a few seconds before failing.
Please refer to p-n junctions and band gaps for additional information.