Unexpected Enhancement of Molecular n-Doping Efficiency in Polymer Thin Films by a Degradation Product

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.