Spontaneous Interfacial Dipole Orientation Effect of Acetic Acid Solubilized PFN

Poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) is a very important interfacial modifier in organic photovoltaic and organic light-emitting diodes to improve device performance, where their molecular dipole has been regarded to play a key role. In this work, we have reported a spontaneous interfacial dipole orientation effect in acetic acid dissolved PFN, which is strongly related to the interfacial dipole and the corresponding device performance. In direct spin-coating, the interfacial dipole is 1.08 Debye with interfacial contact angle 84.8°, whereas after self-assembly of 10 min, the interfacial dipole is balanced at 4.21 Debye, with the interfacial contact angle decreasing to 76.8°. Without strong interaction with the substrate, the energy of upward amine groups is much lower than that of downward ones in theoretical simulation, which would be the driving force of this spontaneous process. The preferred conformations of PFN molecules on hydroxylated substrates have over 99% amine groups outward, and the theoretical average dipole calculated from the weight of these conformations is 4.48 Debye, which is close to the experimental result and indicates a high ratio of upward amine groups in self-assembled films. This effect obviously changes the device performance, such as an obvious positive threshold voltage shift in transistors and a distinct increase of the short-circuit current/open-circuit voltage in organic solar cells. This effect provides a deeper understanding of the PFN interlayer mechanism and has potential application in optoelectronic devices.