Solution N-Doping with Benzimidazole Compounds—A New Derivative for Improved Thermoelectric Performances

The emerging market of the internet of things is insistently demanding for energy power solutions which are cost-effective, compact in size and biocompatible for applications in portable and healthcare technologies. In this framework Organic Thermoelectric MicroGenerators (μOTEGs) offer an interesting opportunity to replace or support conventional batteries exploiting non-toxic, flexible and conformable carbon-based materials.<br/>These devices, relying on the Seebeck effect, are able to sustainbly convert temperature gradients (e.g. body-environment temperature difference, waste heat...) into electrical power in the order of a few mW/cm² which is enough to operate simple sensors and transmitters.<br/><br/>To realize μOTEGs both p- and n-type semiconductors are required, the latter constituting the weak link due to poor performances in terms of electrical conductivity and air-stability, greatly limiting the employment of scalable fabrication techniques such as printing technologies. Molecular doping has therefore stemmed as one of the possible solutions to enhance charge transport. Despite the actual interaction mechanism has not yet been fully disclosed, benzimidazoles derivatives have so far emerged has one of the most promising class of organic n-type dopants due to their solution processability and compatibility with both polymers and small molecules.<br/>Here we report on a modification of the widely used and commercially available 4-(2,3-Dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) obtained by inserting an iminostilbene moiety, producing the innovative derivative named IStBI. The change in the chemical structure does not modify the electronic levels of the benzimidazole core, as measured by cyclic voltammetry, but instead acts mainly on the interactions within the semiconductor matrix. The study is conducted using the widely spread n-type polymer P(NDI2OD-T2), ensuring a large platform of comparable results available in the literature. All the processing was carried out in solution and under nitrogen-atmosphere due to the lack of air and moisture stability of the obtained semiconducting blends. Thin films, casted through spin-coating technique, have been extensively characterized in their electrical conductivity and Seebeck coefficient. After optimizing the processing conditions (i.e. solvent, dopant concentration and mixing temperature) and post-deposition thermal treatment, we obtained an electrical conductivity of 1.14 x 10^-2 S cm^-1, which is to the best of our knowledge the highest value ever reported for solution doped P(NDI2OD-T2) blends. This resulted in also an improved power factor of 4 x 10^-3 μWm^-1K^-². From the analysis of the thermoelectric properties it is evident a reduced dopant miscibility in the polymer matrix compared to other previously reported benzimidazole derivatives but at the same time an improved doping capability.<br/>To support these findings structural and morphological analysis have been performed through GIWAXS analysis and AFM imaging techniques respectively.