Bimolecular recombination in few words

Question

I'm making a short seminar about VERY broad topic of fullerenes in photovoltaics, but I'd like it to be educational (not just full of words hard to audience to make them think I'm smart).

In one of the articles, current response from a cell is attributed to bimolecular recombination. What is it? The article admits that bimolecular recombination generates less current than recombination of exciton that reacts with an impurity.

I'd appreciate short, "hand-waving" answers (maybe some pictures?), that would give my audience good notion. However longer explainations afterwards for me to understand better would be also welcome.

Answer 1

A very common approach for modelling the recombination dynamics in semiconductors is,

$$R = An + Bnp$$

This equation assumes,

1. Monomolecular recombination of electrons dominated over that of holes, with a rate $(s^{-1})$ given by the first term.

2. Bimolecular recombination requires and electron and a hole.

Clearly these assumptions will be material specific, and the above equation is a specification of a more general rate equation. Nevertheless it is useful to illustrate the different recombination mechanism in semiconductors.

Monomolecular term

An example of monomolecular recombination is recombination via defects. Here the $A$ coefficient encapsulates details of the specific material: defect density, cross section, thermal velocity of electrons etc. Therefore this term tells us that the rate at which electrons find defects simply scales with the number of electron in the system.

Bimolecular term

The second term gives the rate of bimolecular recombination. An example could be non-radiative recombination via a trap site which accepts both electrons and holes, or direct radiative recombination between an electron and a hole. The key difference is that the recombination rate is dependent on the density of both carrier types. Moreover, radiative recombination cannot occur without the electron finding a hole. Similarly non-radiative recombination cannot occur via recombination centre that pins both types of carries, if only one carrier type is present.

So for a hand waving picture imagine the Brownian motion of two particles. Bimolecular recombination will only occur when the particles find each other.