Charge Carrier Trapping in Organic Semiconductors

The efficiency and stability of single-layer polymer light-emitting diodes is compromised by unbalanced charge transport, absence of triplet-exciton harvesting and low photoluminescence quantum efficiencies. Especially charge traps have a triple negative effect: they give rise to non-radiative trap-assisted recombination, they dissociate singlet excitons leading to a reduced photoluminescence quantum yield and due to confinement of the emission zone at an electrode they lower the optical outcoupling efficiency. A strategy is developed that precludes trapping in large band gap organic semiconductors. We demonstrate that using thermally activated delayed fluorescence emitters in combination with trap-free transport high external quantum efficiencies for single-layer blue OLEDs can be obtained. Our results show that OLEDs with a simplified single-layer structure can rival the efficiencies of complex multilayer stacks.