Low Turn‐On‐Voltage Single‐Layer Organic Light‐Emitting Diodes With Interlayer‐Free Ohmic Contacts

ABSTRACT Organic light‐emitting diodes (OLEDs) with single‐layer architecture offer distinct advantages over conventional multi‐layer devices, but high‐performance single‐layer OLEDs have long been hindered by unbalanced carrier transport and inefficient charge injection, exhibiting relatively high driving voltages and severe efficiency roll‐off. Different from previous ways in constructing Ohmic contact by introducing additional organic interlayers or complex doping schemes to eliminate the energy barrier at the interfaces, we report a simplified method that establishes a direct Ohmic hole contact between molybdenum oxide (MoO 3 ) and a carbazole‐based thermally activated delayed fluorescence (TADF) emitter of 1,2,3,5‐tetrakis(carbazol‐9‐yl)‐4,6‐dicyanobenzene (4CzIPN). The Ohmic contact was confirmed by space‐charge‐limited current measurements and after carefully optimizing the layer thicknesses, we successfully achieve barrier‐free hole injection with significantly improved carrier balance, resulting in a remarkably low turn‐on voltage of 2.22 V and efficiency roll‐off of only 1.5% at 1000 cd m −2 of green OLEDs. Furthermore, applying this strategy to Förster resonance energy transfer (FRET)‑sensitized systems with additional guest components enables color‑tunable emissions from green to red, with turn‑on voltages below 3 V and low roll‑off. These results provide a clean and low‑cost route to high‑performance interlayer‑free single‑layer TADF OLEDs and offer new insights into designing Ohmic contacts for barrier‑free hole injection.