Charge Carrier Recombination in Amorphous Organic Semiconductors

Bimolecular recombination of charge carriers in amorphous organic semiconductors is discussed. A common feature of these materials is their spatial correlation of the random energy landscape in which hopping transport of charge carriers occurs. The recombination rate constant for such materials is calculated, including the case of locally ordered materials. It turns out that the spatial correlation of the random landscape causes violation of the Langevin relation connecting the charge carrier mobilities with the recombination rate constant. For different sources of energetic disorder the true rate constant can be either less or greater than the corresponding Langevin value. Promising classes of organic semiconductors are indicated where the recombination rate constant could exceed the Langevin value, leading to a potential increase in the efficiency of light generation in organic light-emitting diodes. Organic semiconductors with low recombination rate constants are promising for the use in solar cells. Features of two-dimensional bimolecular recombination in materials based on oligo- and polythiophenes, in which two-dimensional lamellae are formed, are considered. The formal recombination rate constant becomes being dependent on the charge carrier concentration. The effect of spatially correlated energetic disorder leads to the development of various rate constant dependences on the charge carrier concentration. Analysis of the current–voltage characteristics of organic devices gives the possibility to distinguish between the two-dimensional and three-dimensional recombination.