Design Dependence of the Interface Structure and Crystalline Order of Organic Semiconductor/Dopant Heterojunctions: Pentacene/C₆₀F₄₈

Planar and mixed heterojunctions are the common architectures respectively used for contact and bulk doping in organic electronics. Beyond the success of empirical methods, knowing how the incorporation of dopant molecules affects the structural properties of the organic semiconductor film is essential for the design optimization of devices with efficient electrical properties. With this aim, we exploit out-of-plane and grazing incidence X-ray diffraction (GIXD) to characterize engineered heterojunctions fabricated on native SiO₂ substrates with pentacene as an organic semiconductor and C₆₀F₄₈ as a dopant. A strong interaction between the two species is inferred, which dramatically influences the crystalline order in the different architectures. While C₆₀F₄₈ crystalline domains orient randomly on the SiO₂ surface, the ulterior deposition of pentacene is demonstrated to induce the orientational order and reorganization of the underlying C₆₀F₄₈. In the inverse architecture, the highly [001]-oriented pentacene film on SiO₂ has a template effect on C₆₀F₄₈, inducing the formation of a face-centered cubic [111]-oriented film and leading to an ordered planar heterostructure. Real-time GIXD reveals the presence of a strained C₆₀F₄₈ layer at the interface with a gradual relaxation and transition to 3D crystallite formation with increasing coverage. When mixed in the bulk by co-evaporation, the dopant adopts an amorphous state, embedded in a polycrystalline pentacene matrix. The interfacial effects reported here would have relevant repercussion on the efficiency of molecular doping, which makes it possible to foresee the establishment of guidelines for device performance improvement through interface engineering.