Excitons. Is this the same as radiative recombination?

[Logio]

Hi,

I'm having difficulty getting my head round the concept of excitons, which have recently been introduced into my Uni course.

Previously, I've always understand the operation of LEDs as follows:
Under forward bias, electron and holes are injected and recombine at the p-n junction (electrons fall from CB and holes rise from the VB, so they meet in the bandgap) emitting a photon of energy equal to the bandgap.

However, I've recently learned about EXCITONS, "bound e-h pairs with not quite enough energy for electrons to escape". In describing the operation of an OLED, we're told that in the organic layer an electron and hole form an exciton, which later radiatively relaxes to emit a photon. Also, when a photon is absorbed, an exciton is formed (e.g. in photodetector).

Therefore, could someone perhaps clarify if excitons are just the "more accurate" way of describing radiative recombination and absorption or if they're fundamentally different please?

Further to this, I wonder then, how do you go from an exciton being formed to it radiatively relaxing and when an exciton is formed by photoabsorption, is the exciton used in conduction?

Any help appreciated!

Thanks,

 

[Cthugha]

Therefore, could someone perhaps clarify if excitons are just the "more accurate" way of describing radiative recombination and absorption or if they're fundamentally different please?

In a rough approximation you can think of excitons being similar to hydrogen. You have one positively charged particle and one which is negatively charged. Those two form a bound complex due to Coulomb interaction. This bound complex of course has lower energy compared to having two free particles. So the energy of an exciton state is a bit below the band gap just like you need energy to break a hydrogen molecule.

Whether excitons are important in recombination prcesses depends a bit on the exciton binding energy in your material and the temperature you are working at. If the average thermal energy in the system is large, then most excitons will be ionized and you will have predominantly free carriers. If you are working at low temperatures or have high binding energies, excitons will dominate recombination processes.

There are quite some differences between free carrier recombination and exciton recombination, but for the beginning the density of states may be the most important one. While free carriers have a continuum of states, the exciton has a well defined binding energy. Of course you may add kinetic energy if the center of mass of the exciton moves around, but if you trap excitons, you may get a discrete density of states (for example for quantum dots).

Further to this, I wonder then, how do you go from an exciton being formed to it radiatively relaxing and when an exciton is formed by photoabsorption, is the exciton used in conduction?

The radiative recombination of an exciton is not too different from free electrons and holes. There are no huge differences. There is of course a difference in terms of conduction as an exciton is electrically neutral in first approximation. It is also bosonic which may make a difference sometimes.