Enhanced Bimolecular Recombination of Charge Carriers in Amorphous Organic Semiconductors: Overcoming the Langevin Limit

We consider the bimolecular charge carrier recombination in amorphous organic semiconductors having a special kind of energetic disorder where energy levels for electrons and holes at a given transport site move in the same direction with the variation of some disorder-governing parameter (the parallel disorder). This particular kind of disorder could be found in materials where the dominant part of the energetic disorder is provided by the conformational disorder. Contrary to the recently studied case of electrostatic disorder, the conformational disorder, if spatially correlated, leads to the increase of the recombination rate constant, which becomes greater than the corresponding Langevin rate constant. Probably, organic semiconductors with dominating conformational disorder represent the first class of amorphous organic semiconductors where the recombination rate constant could overcome the Langevin limit.