Transport and Recombination in Polymer LEDs

The processes of charge carrier transport and recombination are crucial to the operation of polymer LEDs and, in the case of good injection, can directly determine the electrical characteristics. However, both these processes are difficult to study at a fundamental level and remain poorly understood. 

Transport properties are usually described by the effective mobilities, /reff, of electrons and holes, such that the current density, /, due to a single carrier type is given by 

The recombination of carriers in polymer LEDs is clearly critical to their operation. The recombination rate, R, is related to the density of electrons and holes (n and p) by 

The theory for recombination in a system where the mean free path of a carrier is small was developed by Langevin75 who showed that the recombination rate 

This formalism has been applied to polymer LEDs to estimate the position of the recombination zone. Particularly useful experimental data here come from studies of multiple-layer LEDs, where the emission spectmm can be used to determine the amount of recombination occurring in each layer.76 The simple theory has been extended by Gartstein et al. to take into account the discrete nature of the hopping process and the distribution in site energies.77 

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