Molecular orientation anisotropy and spontaneous polarization in vapor-deposited organic semiconductor thin films

Organic semiconductors are conjugated molecular materials with highly tunable electrical and optical functionality. These materials have found broad application in organic light-emitting devices (OLEDs) for a variety of display applications including mobile/wearable devices, televisions, lighting, and recently for augmented/virtual reality devices. While OLEDs consist of amorphous thin films deposited via high vacuum, physical vapor deposition, anisotropy in molecular orientation can play a significant role in both the optical and electrical behavior of the film. A notable example is the practice of engineering emitter molecular structure to realize an in-plane alignment of the molecular transition dipole moment (TDM). In-plane alignment of the TDM increases OLED optical outcoupling efficiency by frustrating the excitation of lossy modes in the device. In this talk, emphasis instead is on understanding how molecular anisotropy impacts the orientation of the permanent dipole moment (PDM). Preferential PDM alignment leads to a phenomenon known as spontaneous orientation polarization (SOP) in which a large polarization field may develop across the film exhibiting molecular anisotropy. SOP can significantly alter charge accumulation and exciton relaxation kinetics in an OLED, leading to an impact on both device efficiency and operational lifetime. This talk will elucidate the link between molecular anisotropy, SOP, and OLED performance. In addition, this talk will demonstrate several approaches to manipulate molecular anisotropy and PDM orientation including via film processing conditions and molecular blending, as well as by thoughtfully engineering device architecture to mitigate the potentially negative impacts of SOP on performance. The talk will conclude with a discussion of how tuning of molecular orientation in organic thin films represents a largely open axis for engineering the behavior of this important materials class.