  In this paper, we report on electrical transport in F4TCNQ doped organic
semiconductor (host) materials. By monitoring the conductance of the sample
in-situ during and after deposition, we show that sequential deposition of
F4TCNQ and host semiconductor results in bulk doping of the semiconductor. In
addition, the doping density (and conductivity) of the host can be easily
controlled by adjusting the thickness of the bottom F4TCNQ layer. This
alternative scheme for doping is simpler than the conventional way of doping
small molecules which involves simultaneous co-evaporation of host and dopant.
In the sequential doping scheme outlined here, bulk doping of the host takes
place due to rapid diffusion of F4TCNQ in the host material. The motion of
F4TCNQ in the host is complex and is not always described by a simple diffusion
process. In situ transport measurements provide a quick and easy way of
measuring dopant diffusion in new hosts. Based on the doping results, we also
outline a possible route to improved transconductance for organic thin film
transistors in a manufacturing environment.
