Uncovering S-Shaped IV Curves in Organic Photovoltaics: The Role of DIO-Driven Vertical Segregation

Controlling vertical segregation in organic photovoltaics (OPVs) is critical for achieving optimal solar cell performance and stability, requiring careful consideration of a range of factors such as the surface energy and miscibility of components, solvent and solvent additive identity, and film processing methods. In this work, we use neutron reflectivity to compare vertical segregation in two different OPV systems: a fullerene-based system (PBDB-T:PC₇₁BM) and a non-fullerene-based system (PBDB-T:ITIC), processed with different amounts of the solvent additive, 1,8-diiodooctane (DIO). Both systems exhibit vertical segregation, with enrichment of the acceptor at the film/hole transport layer interface. However, the segregation is considerably more pronounced in ITIC-based systems, where the enriched interface consists of pure ITIC, compared to a maximum of 67% PC₇₁BM by volume in the former system. Prolonging film drying with higher solvent additive content exacerbates segregation in both systems, increasing interfacial acceptor concentration in PC₇₁BM systems and broadening the buried interface in ITIC systems. Simulations confirm that extreme vertical segregation in ITIC-based devices induces nonideal 's-shaped' JV curves when the enriched layer is pure and sufficiently thick, as seen in both fresh and aged devices. Our findings highlight that improper vertical segregation not only leads to poor device performance in fresh devices, but gradual segregation can also contribute to morphological degradation and device instabilities over time.