Now we will discuss the other side of the coin: recombination. Just as the generation of charge carriers is due to the absorption of incoming photons that create electron-hole pairs, recombination is the reverse process of charge carriers “rejoining” to relax back to their equilibrium states. When light illuminates a material, there is a surplus population of free charge carriers; when this light is switched off, so to speak, recombination sets in and the surplus decays back to its equilibrium concentration of charge carriers. Recombination is therefore a relaxation process that is intimately related with the generation, or original excitation, of free charge carriers. In fact, for every absorption process there is an inverse radiative recombination processes1.
In this section we will go over several different mechanisms of recombination, which will then allow us to more fully grasp the picture of electron-hole pairs and their participation in the processes of solar cell materials.
Above we have an illustration of some of the recombination mechanisms that we will discuss. Depicted are the processes of direct band-to-band recombination, recombination through traps (defects), and auger recombination. Here EC and EV are the conduction and valence bands respectively.
- Green, Martin A. Solar Cells: Operating Principles, Technology, and System Applications. Englewood Cliffs: Prentice-Hall, Inc., 1982. Full book ordering information at www.pv.unsw.edu.au.