  The polymer (PBDTTT-c) p-doped with the molecular dopant (Mo(tfd-COCF3)3)
exhibits a decline in transport properties at high doping concentrations, which
limits the performance attainable through organic semiconductor doping.
Scanning Electron Microscopy is used to correlate the evolution of hole
conductivity and hopping transport activation energy with the formation of
aggregates in the layer. Transmission Electron Microscopy with
energy-dispersive X-ray analysis along with liquid-state Nuclear Magnetic
Resonance experiments are carried out to determine the composition of the
aggregates. This study offers an explanation to the limited efficiency of
doping at high dopant concentrations and reinforces the need to increase doping
efficiency in order to be able to reduce the dopant concentration and not
negatively affect conductivity.
