Polar organic semiconducting molecules can exhibit a preferential alignment of their permanent dipole moments, inducing a giant surface potential via spontaneous orientation polarization (SOP). Previous work has shown how blends of polar and non-polar molecules offer one approach to either engineer SOP and its deleterious effect on OLED efficiency. Here, the emphasis is instead on composite films where both components exhibit SOP. In these systems, the observed SOP follows a linear combination of neat film behaviors. This study expands the avenues for control over SOP via molecular blending that can be further exploited for applications.
An optimization scheme to minimize angular color shifts in OLEDs is developed using a combination of optical simulations and experimental measurements of device performance. This minimization does not compromise other critical device operation parameters, such as efficiency and the angular intensity profile. By considering both bottom- and top-emitting OLEDs, this study utilizes strong feedback between simulation and experiment to identify stack architectures that have a minimum color change with viewing angle while still maintaining high power efficiency.
Preferential alignment of molecular permanent dipole moments, known as spontaneous orientation polarization (SOP), is present in many materials employed in the active layers of organic light-emitting devices (OLEDs). This phenomenon leads to the formation of bound polarization charge, which is compensated by polaron accumulation at voltages below turn-on. While most prior work has focused on polarization in the device electron transport layer (ETL), here we examine the impact of emissive layer SOP by systematically probing polaron accumulation and exciton-polaron quenching in phosphorescent OLEDs. To gain a deeper understanding of polaron accumulation, device capacitance is systematically probed as a function of voltage across samples with polar and nonpolar emissive layers. We find that capacitance measurements can be used to track not only the number of accumulated charges, but also its location within the device active layers. This study provides an analysis framework that allows further insights on the charge accumulation process in OLEDs, thus improving our understanding of SOP in OLEDs
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