Professor Russell Holmes (CEMS at UMN): The emergence of spontaneous polarization in glassy thin films and its impact on organic light-emitting devices (OLEDs)

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 thin films has been in displays based on organic light-emitting devices (OLEDs). An OLED consists of a vertical thin film stack deposited via a high vacuum, physical vapor deposition process. While these 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.

Central to this talk is how changes in thin film molecular orientation lead to corresponding changes in the associated transition dipole moment (TDM) and permanent dipole moment (PDM) orientations. The TDM orientation plays a role in determining 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 (termed spontaneous orientation polarization, SOP), 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 means to manipulate PDM orientation via choice of thin film processing conditions and molecular blending, as well as showing how device architecture can be used to mitigate the impact of SOP on OLED 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.