Understanding the Interplay Between Thermal Activation, Diffusion, and Phase Segregation of Molecular Dopants Blended with Polymeric Semiconductors

Molecular doping of polymeric semiconductors is a key strategy to tune charge transport properties, energy levels alignment and charge injection in printed electronic devices. N-type doping is more challenging than p-type due to the characteristic energy levels of performing polymers requiring the development of oxygen sensitive dopants. Precursor dopants are kinetically stable compounds that cannot directly dope the target semiconductor but can be converted in situ via thermal activation into the real doping species. 1,3-Dimethyl-2-(4-(dimethylamino)phenyl)-2,4-dihydro-1H-benzoimidazole (N-DMBI-H) is the most widely employed example. While in blend with the polymeric semiconductor, the thermal activation of DMBI-H like molecules does not exclusively lead to doping but also causes diffusion and phase segregation of both the dopant itself and the various possible by-products of the doping cascade. All such processes have profound impact on the morphology, microstructure and charge transport properties of the blend. In this paper, we compare different DMBI-H derivatives with comparable thermodynamic doping capability over the benchmark polymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) and we show how the phase segregation process is the main responsible for their different performances and dopant design can help enhance intermolecular interactions and reduce phase segregation.