Engineering Spontaneous Orientation Polarization in Organic Semiconductor Thin Film via Molecular Blending*

Organic light-emitting devices (OLEDs) are widely used in displays for their favorable electronic and optical properties. Previous studies have shown that when thin films of polar organic molecules are processed by physical vapor deposition, their appreciable permanent dipole moments (PDMs) can preferentially align leading to a macroscopic polarization field. This spontaneous orientation polarization (SOP) can lead to considerable charge accumulation in an OLED at low or even zero bias, leading to exciton-polaron quenching and reduced device efficiency. Recent work has shown that the magnitude of SOP may be controlled via blending with a second, non-polar (i.e. zero SOP) molecular species. Here, we systematically examine SOP in blends, including both polar-polar and polar-non-polar mixtures. The results demonstrate that blending non-polar molecules in the polar matrix can frustrate the PDM interaction between polar molecules and enhance the overall PDM alignment as well as the SOP strength of the film. In contrast, polar-polar blends behave as a superposition of neat film PDM alignment behavior, potentially suggesting that dipole-dipole interactions are preserved across different types of polar molecules. This study offers further insight into how dipole-dipole interactions impact the molecular orientation in organic semiconductor thin films and a route to engineer SOP via polar-polar blending.