Charge Transport in Organic Semiconductors: The Interplay of Crystalline Morphology and MoO₃ Doping in NPB Thin Films

Abstract

Organic semiconductors offer significant advantages for electronic device fabrication, including low cost, enhanced accessibility, simplified processing, and mechanical flexibility. Although disordered amorphous films are known to limit charge carrier mobility and exciton diffusion length, they remain widely used in devices and extensively studied, particularly in the context of improving charge transport via molecular doping. In contrast, crystalline films can mitigate these limitations, yet they have received less attention in relation to the potential of doping. This study investigates the interplay between crystalline morphology and molecular doping in the organic semiconductor N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) doped with molybdenum trioxide (MoO3). Undoped and doped thin films, prepared via co-deposition and layered doping methods, were fabricated using physical vapor deposition. Successful formation of large-area single-crystal grains (platelets) is observed at significant doping concentrations for both methods, with improved crystalline domain preservation in the layered films. Moreover, a clear trend of enhanced conductivity in crystalline films compared to the amorphous ones is observed. The conductivity trends across varying MoO3 concentrations further demonstrate the enhancement even at low doping levels. However, more complex doping effects are also observed, especially in the layered systems, including higher conductivities compared to their co-deposited counterparts and an unexpected gradual decrease at the highest concentrations. The findings of this study provide key insights into the integration of large-area platelet films with improved conductivity into next-generation electronic device architectures, and motivate further investigation of crystalline doped organic semiconductors.