posted on 2022-01-27, 19:04authored byFrancesca Pallini, Sara Mattiello, Marco Cassinelli, Pietro Rossi, Sara Mecca, Wen Liang Tan, Mauro Sassi, Guglielmo Lanzani, Christopher R. McNeill, Mario Caironi, Luca Beverina
Molecular doping
of conjugated polymers is extremely desirable
to control charge density gradients and shape the electric field across
polymer electronic devices, including highly efficient organic solar
cells. It is also a fundamental requirement for organic thermoelectrics
and a powerful strategy to boost charge injection and transport properties
in transistors. Yet, currently available doping approaches are far
from offering a suitable level of control, particularly in the case
of n-type doping. We here reveal that part of this limitation lies
in the lack of understanding of dominant factors in doping efficiency.
In particular, we highlight the key role played by very small amounts
of a specific decomposition product formed during processing of the
widely used molecular dopant 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (DMBI-H) in influencing
the n-type conductivity in polymer blends. We show that such an overlooked
decomposition product acts as a nucleating agent for a new crystalline
phase of DMBI-H, with the overall effect of boosting the electrical
conductivity of the final doped polymer films. Such results, confirmed
by control experiments performed with a different nucleating agent,
focus on the crucial role played by the solid-state microstructure
in molecular doped semiconductors and offer ground for a significant
change in design guidelines for molecular doping strategies.