Sub-Turn-on Exciton Quenching Modulated by Spontaneous Orientation Polarization in Organic Light-Emitting Devices

Many common materials used in organic light-emitting devices (OLEDs) show preferred molecular orientation in thin films. In molecules with a permanent dipole moment, this can lead to spontaneous orientation polarization (SOP) of the film and can generate a large interface charge. This polarization charge is compensated in a device by the accumulation of charge carriers, typically holes, in the adjacent layer. While it has been speculated that these accumulated charges can quench excitons and accelerate degradation, no direct demonstration of these effects has been made. Here, we show that electron transport layers exhibiting SOP induce substantial exciton quenching prior to device turn-on, reducing the peak external quantum efficiency by ~20% relative to layers without SOP. In addition, we find that the accumulated charges accelerate degradation of emitter photoluminescence efficiency under optical pumping. We further show that SOP can be eliminated by substrate heating during deposition, thereby improving device efficiency and stability. These findings confirm that SOP is an important material parameter that must be considered when optimizing efficiency and lifetime of OLEDs.