In disordered organic semiconductors, there is no band transport, as there are no delocalised, just localised charges. Consequently, there is no simple band-band recombination of free carriers, and no Shockley-Read-Hall recombination! Of course, there is still recombination going on, a lot of it;-)
Here I’ll just quote some definitions concerning different types of recombination, and get back with details later.
For a general classification we take a look at Kwan-Chi Kao’s book “Dielectric Phenomena in Solids“.
Looking for monomolecular recombination, we find
The recombination that involves one free carrier at a time, such as indirect revombination through a recombination center (e.g., an electron captures by a recombination center and then recombined with a hole, each process involving only one carrier), is generally referred to as monomolecular recombination.
In organic semiconductors, a recombination centre can for instance be a trapped hole, localised in a deep state; it can induce a monomolecular recombination with a mobile electron. Even knowing this, it still feels like bimolecular recombination, doesn’t it? ;-)
Bimolecular recombination reads as
The recombination that involves two free carriers simultaneously, such as direct band-to-band recombination, is generally referred to as bimolecular recombination.
As mentioned above, direct band-band recombination of free carriers does not exist as such in organic disordered semiconductors. Generally speaking, in a hopping system with localised charges, the two oppositely charged carriers “wishing” to recombine first have to find each other. This is described by the Langevin recombination rate, which is therefore proportional to the carrier mobility of both, and is often applied to low mobility semiconductors.
Finally, trimolecular recombination is defined in the book as
The recomination that involves three free carriers simultaneously, such as a three-body collision in the Auger intrinsic recombination (in which one electron in the conduction band recombines with a hole in the valence band and the energy released is taken up by a third particle-electron), is generally referred to as trimolecular recombination or three-body recombination (or simply as Auger or impact recombination).
Auger recombination seems to be negligible in organic solids. However, recently, there was an article claiming to observe a trimolecular recombination signature in polymer:fullerene devices [Shuttle 2008]. Other researchers also observed recombination proportional to the third order of charge carrier density [Juska and Pivrikas, private communications 2008]. They told me they were still looking for an explanation before publishing it. Shuttle et al. instead published without giving an explanation: The specific nature of trimolecular recombination in the polymer:fullerene system is not yet known;-)
I’ll come back to recombination soon, this post serving just as a starting point to delve deeper into the topic;-)