Surface Doping of Rubrene Single Crystals by Molecular Electron Donors and Acceptors

Organic semiconductors have shown great promise for sustainable (opto)electronic applications within the past decade. Organic light-emitting diodes are already commercially available, and organic photovoltaics (OPVs) and field-effect transistors (OFETs) are also rapidly advancing, with power conversion efficiency of OPV reaching over 18% and carrier mobility in OFETs higher than 1 cm²/Vs, respectively. However, achieving further improved device performance requires continued optimization of certain aspects of interfacial electronic energy level alignment, such as improving charge injection by minimizing the energetic barriers and creating charge accumulation layers at the interface. Accordingly, this work seeks to zoom in on the mechanisms of interface engineering by considering the surface doping of rubrene single-crystals by molecular electron donors and acceptors. Rubrene is a benchmark example of a high carrier mobility semiconductor, as its charge transport is mediated through electronic bands and not via hopping. In this regard, to effectively tune the energy levels without compromising the band structure, doping should be nondestructive for the crystalline surface of rubrene. Our angle-resolved photoemission results show that deposition of molecular p-type and n-type dopants, Mo(tfd-CO₂Me)₃and CoCp₂respectively, on rubrene causes a corresponding shift of the valence bands, accompanied by the formation of free charge carriers in rubrene, while the electronic band parameters remain unperturbed.