Langmuir-Type Build-Up of Self-Generated Interlayers in Organic Solar Cells

Charge selective ohmic contacts are crucial for optimizing the performance of organic electronic devices, particularly organic solar cells (OSC). This study investigates the modification of contacts by examining the formation of thin interlayers through metal-induced additive migration. Two types of OSCs, based on fullerene and non-fullerene systems, are utilized along with five concentration series of polyethylene glycol (PEG) oligomer additives that gradually migrate from the active layer to the contact, generating interlayers. By analyzing the trends of Voc (open circuit voltage) versus additive concentration and length, the arrangement of additives at the organic/metal interface and the origin of point-dipoles projected onto the metal surface are determined. The behavior of the additive's dipole-baring moiety in all additives is found to follow a Langmuir-like adsorption process, with a remarkably stable “effective” ∆G (Gibbs free energy) across the different OSC systems and additives. These results imply that the active organic layer can be treated as a semisolid solution where the additive undergoes adsorption/desorption at the Al contact. The diffusivity of the PEG-oligomer additive in this solution is temperature-dependent, so elevated temperatures allow fast adsorption/desorption equilibrium. Grazing-incidence wide angle X-ray scattering (GIWAX) analysis reveals that the additive's impact on device performance is solely interfacial. By establishing scientific correlations between the additive's properties, quantity, and energetic shifts at the interface, this study provides practical guidelines for implementing the spontaneous interlayer formation methodology in large-scale applications of organic electronics.