Tuning of Carrier Balance by Introducing Dipolar Layers at Organic Heterointerfaces in Organic Light‐Emitting Diodes

Tuning the hole–electron carrier balance is essential for improving the performance of organic light‐emitting diodes (OLEDs). This challenge can be addressed through innovative interface engineering techniques. In this study, a thin organic layer exhibiting spontaneous orientation polarization (SOP) is inserted at an organic heterointerface with a large hole‐injection barrier. The introduced negative SOP layer provides a steep vacuum level shift at the organic heterointerface owing to the spontaneous orientation of the permanent dipole moment of the polar molecules. This improves hole injection at the interface and lowers the OLED driving voltage. The effect of voltage reduction is maximized at a film thickness of up to 1.5 nm, and increasing the thickness gradually leads to higher voltages. Furthermore, owing to the good carrier balance at the optimal thickness of 1–3 nm, the introduction of the SOP interlayer improves the current efficiency by up to 35% from that of the OLED without the SOP interlayer. The OLED performance depends on the thickness of the SOP interlayer, indicating that inserting an SOP interlayer at the organic heterointerface can tune the carrier balance in OLEDs. This technique enables precise optimization of the OLED performances.