Electron-hole pair recombination

Each electron-hole recombination typically only releases a tiny amount of energy. The emission of "usable" quantities of energy such as light from LEDs requires a large number of recombinations.

The most efficient method of producing heat from recombination is possibly where the band gap is designed to correspond to emissions in the infra-red portion of the spectrum.

Whether this would be considered to be a reasonable amount of heat is debatable, and as a heat source there are simpler and more direct methods of producing heat than infra-red LEDs.

Carrier recombination (electron-hole pair) can happen through:

1. radiative channel, part of the energy is released by light emission or luminescence

2. non-radiative channel, the excess energy is converted into heat via phonon emission

The main ones are band-to-band recombination, Shockley–Read–Hall (SRH) trap-assisted recombination, Auger recombination and surface recombination.

You are asking about when the energy is released as heat, that is non-radiative emission channels.

Non-radiative recombination is a process in phosphors and semiconductors, whereby charge carriers recombine with releasing phonon instead of photons. Non-radiative recombination in optoelectronics and phosphors is an unwanted process, lowering the light generation efficiency and increasing heat losses. Non-radiative life time is the average time before an electron in the conduction band of a semiconductor recombines with a hole. It is an important parameter in optoelectronics where radiative recombination is required to produce a photon; if the non-radiative life time is shorter than the radiative, a carrier is more likely to recombine non-radiatively. This results in low internal quantum efficiency.

The types of non-radiative emission shannels:

1. Shockley-Read-Hall, the electron passes through the a new energy state created within the band gap by a dopant or defect in the lattice, this is called a trap.

Non-radiative recombination occurs primarily at such sites. The energy is exchanged in the form of lattice vibration, a phonon exchanging thermal energy with the material.

Since traps can absorb differences in momentum between the carriers, SRH is the dominant recombination process in silicon and other indirect bandgap materials. However, trap-assisted recombination can also dominate in direct bandgap materials under conditions of very low carrier densities (very low level injection) or in materials with high density of traps such as Perovskites.

2. Auger, the energy is given to a third carrier which is excited to a higher energy level, and then loses its excess energy to thermal vibrations.

The mechanism causing LED efficiency droop was identified in 2007 as Auger recombination, which met with a mixed reaction.[13] In 2013, an experimental study claimed to have identified Auger recombination as the cause of efficiency droop.[14] However, it remains disputed whether the amount of Auger loss found in this study is sufficient to explain the droop. Other frequently quoted evidence against Auger as the main droop causing mechanism is the low-temperature dependence of this mechanism which is opposite to that found for the drop.

https://en.wikipedia.org/wiki/Carrier_generation_and_recombination