Extending the p-Doping of Polymers to an Air Stable Lewis Acid-Base Adduct by Increasing the Acidity of the Dopant

Among various molecular p-dopants, some Lewis acids have shown recognizable doping effect by the formation of Lewis acid-base adduct for increased hole concentration in organic semiconductors. However, the doping product is often unstable in air by the interference of water. Here near amorphous conjugated polymer poly(indacenodithiophene-co-benzothiadiazole) (IDT-BT) was doped with four Lewis acid dopants of varying acidities, zinc-, borane-, carbonium- and oxonium-based molecules, respectively. Bearing stronger acidity, the oxonium dopant (C6H15O)+BF4- yielded the most air-stable Lewis acid-base adduct, which was confirmed by direct comparison of electron paramagnetic resonance and UV-vis absorption spectra of the four doping systems measured in air. A single polaron species of radical cation was produced between the IDT-BT host and the oxonium dopant, with a binding stoichiometry of 1:1 according to the model study using a benzothiadiazole-derivatized molecule. Via modulating the dopant quantity, the performance of thin-film transistors based on (C6H15O)+BF4--doped IDT-BT was optimized with enhanced mobility, significantly reduced threshold voltage, higher on/off ratio, and excellent air-stability due to the additional carriers produced by the stable Lewis acid-base adduct. Notably, while the semiconductor layer was processed in air, the (C6H15O)+BF4--doped device exhibited almost no loss of performance as compared with that under inert atmosphere. We demonstrate that the performance and air stability of Lewis acid-doped polymer and transistor devices thereby can be improved by increasing the acidity of the dopant.

Automated summary

This study investigated the doping effect of various Lewis acid dopants on the conjugated polymer IDT-BT, and found that the oxonium-based dopant had the best air-stability, forming a stable Lewis acid-base adduct. By adjusting the quantity of the dopant, the performance of the (C6H15O)+BF4--doped IDT-BT thin-film transistors was optimized, with improved mobility, reduced threshold voltage, higher on/off ratio, and excellent air-stability due to the additional carriers produced by the stable Lewis acid-base adduct. Notably, the (C6H15O)+BF4--doped device exhibited almost no loss of performance when processed in air. This study demonstrates that increasing the acidity of the dopant can enhance the performance and air stability of Lewis acid-doped polymer and transistor devices.