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Reduction of loss processes in polymer light-emitting diodes

PhD ceremony:Mr D. Abbaszadeh
When:May 06, 2016
Supervisor:prof. dr. P.W.M. Blom
Where:Academy building RUG
Faculty:Science and Engineering

Polymer light-emitting diodes (PLEDs) are a promising candidate for replacement of conventional inorganic LEDs but their efficiency and durability need to be improved. The electronic performance of conducting polymers depends on their charge (both electron and hole) transport properties. In devices made of conjugated polymers the hole transport is normally trap-free space-charge-limited, whereas the electron transport is strongly reduced (more than 3 order of magnitude) due to presence of electron trapping states in the forbidden band-gap.

This artifact negatively affects the performance of polymer based electronic devices in two ways. First, due to unbalanced transport, the recombination zone sticks to the metallic cathode and due to dipole-dipole interaction, the exciton can transfer its energy to the metallic cathode. Secondly, the presence of trap states enhances the trap-assisted non-radiative recombination, which is an important loss process in PLEDs. To enhance the efficiency of PLEDs we need to understand the processes that limit the efficiency of PLEDs.

In this thesis, we follow two main strategies to improve the efficiency of PLEDs. First, by making a multi-layer device free carriers and excitons are spatially separated from the electrodes, such that exciton quenching at the electrodes is eliminated. The second method is elimination/dilution of trap states in the polymer, which due to reduced non-radiative recombination also improves PLED efficiency.