Durable n-type carbon nanotubes double-doped with 1,8-diazabicyclo[5.4.0]undec-7-ene and polyamidoamine dendrimers

Abstract

Owing to their well-known instability in air, organic n-type semiconductors must be doped to protect them from moisture and oxygen. Such doping is crucial for producing soft flexible devices that can convert large amounts of low-temperature waste heat into electrical energy. Herein, we report the preparation and thermoelectric properties of drop-cast carbon nanotube (CNT) films double-doped with the 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) superbase and electron-donating polyamidoamine dendrimers (PAMAM). Our method not only enhances the chemical stabilities of the electron carriers but also is favorable for large-scale applications. Despite being prepared in aqueous solvents, the p-type CNTs were found to exhibit n-type behavior, irrespective of the pKa of the electron-donating organic molecule. In particular, the addition of DBU resulted in the largest thermoelectric conversion output factor (Seebeck coefficient and power factor of −32.2 μV K−1 and 261 μW m−1 K−2, respectively). The inclusion of PAMAM as a dispersant and secondary dopant significantly prolonged the stability of the n-type behavior of DBU-doped CNTs in air at 80 °C from approximately 15 days for an undoped system to more than 30 days. Surprisingly, after 30 days, the thermoelectric conversion power factor of the doped system was found to be approximately 20% higher than that of the undoped system, confirming the excellent performance of hybrid thermoelectric material with a PAMAM shell/DBU-doped CNT core structure. In addition, its n-type doping process does not require organic solvents. These results create new avenues for the development of atmospherically stable drop-cast n-type CNT films.