Organic Doping at Ultralow Concentrations

Abstract Organic doping is widely used for defining the majority charge carriers of organic thin films, tuning the Fermi level, and improving and stabilizing the performance of organic light‐emitting diodes and organic solar cells. However, in contrast to inorganic semiconductors, the doping concentrations commonly used are quite high (in the wt% range). Such high concentrations not only limit the scope of doping in organic field‐effect transistors (OFETs), but also limit the doping process itself resulting in a low doping efficiency. Here, the mechanism of doping at ultralow doping concentrations is studied. Doped C₆₀ metal‐oxide‐semiconductor (MOS) junctions are used to study doping at the 100 ppm level. With the help of a small‐signal drift‐diffusion model, it is possible to disentangle effects of traps at the gate dielectric/organic semiconductor interface from effects of doping and to determine the doping efficiency and activation energy of the doping process. Doped C₆₀ OFETs with an ultralow operation voltage of 800 mV and an excellent on/off ratio of up to 10⁷ are realized. The devices have low subthreshold swing in the range of 80 mV dec⁻¹ and a large transconductance of up to 8 mS mm⁻¹.