Controlling Spontaneous Orientation Polarization in Organic Semiconductors─The Case of Phosphine Oxides

Upon film growth by physical vapor deposition, the preferential orientation of polar organic molecules can result in a nonzero permanent dipole moment (PDM) alignment, causing a macroscopic film polarization. This effect, known as spontaneous orientation polarization (SOP), was studied in the case of different phosphine oxides (POs). We investigate the control of SOP by molecular design and film-growth conditions. Our results show that using less polar POs with just one phosphor-oxygen bond yields an exceptionally high degree of SOP with the so-called giant surface potential (slope), reaching more than 150 mV nm^-1 in a neat bis-4-(N-carbazol(yl)phenyl)phenyl phosphine oxide (BCPO) film grown at room temperature. Additionally, by altering the evaporation rate and substrate temperature, we are able to control the SOP magnitude over a broad range from 0 to almost 300 mV nm^-1. Diluting BCPO in a nonpolar host enhances the PDM alignment only marginally, but combining temperature control with dipolar doping can result in highly aligned molecules with more than 80% of their PDMs standing upright on the substrate on average.