(409b) Engineering Molecular Orientation in Vapor-Processed Organic Semiconductors and Its Role in Optoelectronics

Organic semiconductors are conjugated molecular materials with highly tunable electrical and optical functionality. These materials have found wide interest as thin film components in optoelectronic and photovoltaic devices, where attractive physical properties are combined with high throughput processing on mechanically flexible substrates enabling novel device form factors. To date, the most successful application of organic semiconductor has been in displays based on organic light-emitting devices (OLEDs). OLED displays have been commercialized in mobile/wearable devices, televisions, lighting, and more recently AR/VR. OLEDs generally consist of a vertical thin film stack deposited via high vacuum, physical vapor deposition. While the resulting layers are typically amorphous and glassy, ongoing work has revealed the complexity and tunability in property and performance that can come with active engineering of molecular orientation.

This talk will focus on the connection between thin film molecular orientation and crucial properties that impact OLED efficiency and stability. Of particular interest is how preferential molecular orientation can lead to anisotropy in the associated transition dipole moment (TDM) and permanent dipole moment (PDM) orientations. The TDM orientation determines key thin film phenomena including birefringence and OLED efficiency, and manipulation of this parameter is already an active part of display design. This talk will focus more heavily on the less widely investigated case of preferential PDM orientation, its impact on charge and exciton behavior in OLEDs, and ultimately its impact on efficiency and operational lifetime. Emphasis will also be placed on discussing the means to manipulate PDM orientation via thin film processing conditions and the inclusion of additional dopant molecules, as well as showing how device architecture can be used to mitigate the impact 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.