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For single‐layer OLEDs to achieve efficiencies approaching those of multilayer devices, balanced charge transport is a prerequisite. We demonstrate a single‐layer blue‐emitting OLED with unity internal quantum efficiency and an external quantum efficiency of 27.7% with minor roll‐off at high brightness. These findings lay the foundations for efficient solution‐processed OLEDs.

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. We demonstrate that tuning the energy levels of the organic semiconductor can prevent charge trapping, which is the main cause of unbalanced charge transport. By additionally using a novel strategy to enhance charge injection, efficient single‐layer organic light‐emitting diodes are presented based on a trap‐free organic semiconductor exhibiting thermally activated delayed fluorescence.