Formation Mechanism of PFN Dipole Interlayer in Organic Solar Cells

Water-/alcohol-soluble polyelectrolyte poly[(9, 9-bis (3 '-(N,N-dimethylamino) propyl)-2, 7-fluorene)-alt-2, 7-(9, 9-dioctylfluorene)] (PFN) used in organic solar cells (OSCs) reduces the work function of the electrode due to the effect of an interfacial dipole, which is beneficial for the energy-level alignment between the electrode and the active layer. To date, the studies on the working mechanism of PFN are mainly conducted through topographical and electronic research. Herein, a dynamic insight into the formation mechanisms of the PFN interlayer at the molecular structural level is established. The charge transfer between PFN and the substrates is verified for the first time by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) studies, which results in chemisorption dipoles with their direction aligned with the intrinsic dipole of the PFN molecule, thereby reducing the work function of the substrate. The larger adsorption energy in the substrates of the nitrogen-containing side chains of PFN is also identified, which induces the preferential orientation of PFN molecule to reduce the work function of the substrate. By incorporating this interlayer, high efficiency in single-junction OSCs is achieved using commercial materials. The findings are of great significance for understanding and optimizing the polymer dipole interlayers for OSCs.