A Conformationally Driven Mechanism in n-Type Doping of Naphthalene Diimide-Bithiophene Copolymer by 1H-Benzimidazoles

N-doped polymer semiconductors are of great interest in the field of organic thermoelectrics, as high-conductive materials are still highly desired. In this framework, this paper aims to clarify whether the n-doping of naphthalene diimide-bithiophene copolymer, P(NDI2OD-T2), by 1H-benzimidazoles is a thermally activated process. The study interestingly demonstrates that a relevant change in conductivity, with an increase of more than three orders of magnitude with respect to pristine P(NDI2OD-T2), occurs before the annealing process takes place, thus revealing that benzimidazole-derived dopants are already active at room temperature. Moreover, despite the annealing time and temperature affecting the electrical conductivity of the system, their contribution is less relevant, with the increase of electrical conductivity limited to up to three times. The results from the electrical characterization of the samples are supported by infrared spectroscopy investigation and X-ray analysis, revealing the marker bands of polaron and a manifest structural change between the undoped and the just-doped P(NDI2OD-T2) films, accompanied by only minor modifications during the annealing process. Finally, based on the results of density functional theory simulations, the conformational modifications of the 1H-benzimidazole dopant molecules, induced by the interaction with the P(NDI2OD-T2), are proposed as a possible mechanism explaining the effective doping at room temperature.