Lately, we have talked about recombination, also discussing instances where trimolecular recombination has been observed experimentally. From the different excited states observed in organic solar cells, it is not obvious which combination could be participating in a trimolecular loss process. By the way, chemists seem to know the occurance of termolecular recombination, though in different circumstances.
One candidate for an excitation involving three species it the so called trion. Coming from inorganic semiconductor physics, and meaning charged exciton, it has been described for organic matter already more than 20 years ago [Pope 1984] as
bound exciton plus hole (excitonic ion)
In this review (including the references therein, in particular [Agranovich 1979]), an attractive interaction between exciton and charge is described.
The respective binding energy is linearly proportional to the change in the molecular polarisability of the excited molecule,
(Note: all in CGS unit system, therefore a pain… for me;-) The binding energy is about 500 cm-1, calculated for a polarisability of 15×10-24cm3, which was – for the matter of obtaining an estimate – taken from the polarizability of the lowest lying singlet exciton state in the rather rigid molecules tetracene or anthracene. Furthermore, epsilon=3 and r=0.5nm where assumed. This corresponds to about 60meV binding energy. For polymers, this estimate looks quite different: Taking also polarisabilities for the lowest lying singlet state for polythiophene [van der Horst 2001], which seems to be 2 orders of magnitude higher (please correct me if I messed up something… 1.6×103Angstrom3 was given in the paper;-) , the binding energy would be on the order of a few eV… which is just an estimate, and hopefully not wrong.
With these rather high binding energies, trions seem to be rather stable entities. Also, they might be the candidate for trimolecular recombination, even though a possible mechanism could also be proportional to singlet density times charge density, i.e., bimolecular again. I am not sure myself, so let’s wait and see … or measure and think;-)