Dual role of spontaneous orientation polarization in organic light-emitting diodes: mechanisms, impacts, and control strategies

The advancement of organic light-emitting diode (OLED) technology has been predicated on a fundamental understanding and precise control of molecular orientation within organic thin films. Among these phenomena, spontaneous orientation polarization (SOP), formed by the spontaneous alignment of polar organic molecules during deposition, has emerged as a critical factor influencing device performance. SOP is a macroscopic polarization phenomenon that arises from the statistically dominant orientation of molecules possessing a permanent dipole moment within an amorphous organic thin film. This alignment, which constitutes SOP, results in the formation of fixed polarization charges at the interfaces and induces the accumulation of mobile charge carriers from the adjacent layer. While SOP can facilitate charge injection by modulating interfacial energy barriers, it can also induce charge accumulation at interfaces, leading to exciton-polaron quenching, which is a key cause of reduced device efficiency and lifetime. This dual nature of SOP has made its precise control and optimization, rather than simple elimination, a central challenge in the development of high-performance OLEDs. This review discusses the mechanisms and dual impacts of SOP on OLEDs, recent control strategies via molecular, process, and device engineering, and offers a future outlook.