Efficient Single-Layer Inverted Organic Light-Emitting Diodes Based on TADF Emitters

Recently, a yellow organic light-emitting diode (OLED) based on a single layer of a neat thermally activated delayed fluorescence (TADF) emitter sandwiched between two ohmic contacts has been demonstrated to achieve similar efficiency and longer lifetime compared to more complex multilayer architectures.^[1]For blue emitters, however, the required energy gap of ~3 eV, will often lead to trap-limited charge transport for either electrons or holes^[2], leading to severe charge imbalance in an OLED. Since for most organic blue emitters the electron affinity is lower than 3.6 eV, electron transport is frequently impaired. As a result, in a single-layer OLED, the emission zone will be situated close to the metallic cathode, leading to severe light-outcoupling losses. Here, this issue is solved by the design of inverted single-layer OLEDs. Despite the severely imbalanced transport of the used emitters, high external quantum efficiencies up to 19% are achieved even for blue TADF emitters. The high efficiencies are the result of the emission zone being shifted away from the metallic top electrode, due to a combination of using an inverted device structure and taking advantage of the superior hole transport through the emitters. This new device strategy bypasses the severe limitations of imbalanced and trap-limited transport inherent to blue emitters, which leads to improved performance in blue OLEDs.<br/><br/><i>1. Nature Photonics, 2019, 13(11): 765-769.</i><br/><i>2. Nature Materials, 2019, 18(11): 1182-1186.</i>