From Sparse Nanoparticles to Graded Interfaces: Beyond Classical Descriptions of LiF Growth on Organic Semiconductors

Small-molecule $\pi$ -conjugated semiconductors provide an ordered platform for probing interfacial mechanisms in organic optoelectronics, where device performance and stability are dominated by interface effects. Lithium fluoride, despite its widespread use as an interlayer, remains paradoxical: essential for high device efficiencies yet governed by poorly understood growth processes that undermine reproducibility. Here, we use diindenoperylene (DIP) as an idealized small-molecule substrate to examine LiF deposition with atomic force microscopy, grazing-incidence x-ray diffraction, and physically constrained stepwise x-ray reflectivity (XRR) modeling. LiF nucleates as sparse nanoparticles distributed across the surface from the earliest measurable stages, roughening the interface and partially perturbing the organic top layer. Film formation proceeds through stochastic densification of nanoscale domains, rather than classical coalescence or wetting-layer growth, ultimately giving rise to a laterally continuous but structurally graded interface. While the DIP lattice remains crystallographically intact, consistent with weak structural coupling, XRR reveals non-monotonic interfacial electron density profiles that are inconsistent with conventional slab or island models. This coupled evolution of nanoparticle morphology and organic density profile provides a structural basis for how spatially inhomogeneous, partial coverage may exert disproportionate electronic effects. These results recast LiF as a sparse nanoscale interlayer whose spatial distribution and site selectivity govern interfacial behavior, establishing a framework for reconstructing graded nanoscale density profiles in organic semiconductor interfaces.