Engineering Spontaneous Orientation Polarization in Blends of Polar Organic Semiconductors

Despite their amorphous character, vapor-deposited organic thin films can exhibit significant anisotropy in molecular orientation. When this anisotropy leads to the preferential alignment of molecular permanent dipoles, spontaneous orientation polarization (SOP) is observed. The resulting large polarization fields can impact film properties and device performance. In organic light-emitting devices (OLEDs), SOP can lead to interfacial polaron accumulation that can limit efficiency and operational lifetime. Solid-state dilution of the SOP-forming material with an additional molecular species has been previously demonstrated as a (often nonlinear) means to tune the magnitude of SOP. Here, emphasis is on the behavior of SOP in blends where both species separately exhibit SOP and can effectively transport charge. Interestingly, it is found that such polar-polar blends can show a linear dependence of the giant surface potential (GSP) slope on composition, allowing for straightforward tuning of SOP. Supporting density functional theory calculations suggest that the observed composition dependence of SOP may reflect the balance of competing intermolecular interactions, which can favor segregation versus more uniform mixing. These findings are applied to engineer SOP-induced quenching in OLEDs, highlighting the range of potential interactions that can be expected for SOP in blends and their usefulness to tune this phenomenon.