Double doping, in
which a single dopant molecule induces two charge
carriers in an organic semiconductor (OSC), was recently experimentally
observed and promises to enhance the efficiency of molecular doping.
Here we present a theoretical investigation of p-type molecular double
doping in a CN6-CP:bithiophene–thienothiophene OSC system.
Our analysis is based on density functional theory (DFT) calculations
for the electronic ground state. In a molecular complex with two OSC
oligomers and one CN6-CP dopant molecule, we explicitly demonstrate
double integer charge transfer and find the formation of two individual
polarons on the OSC molecules and a dianion dopant molecule. We show
that the vibrational modes and related infrared absorption spectrum
of this complex can be traced back to those of the charged dopant
and OSC molecules in their isolated forms. The near-infrared optical
absorption spectrum calculated by time-dependent DFT shows features
of both typical intramolecular polaron excitations and weak intermolecular
charge transfer excitations associated with the doping-induced polaron
states.