Selecting Semiconducting Single-Walled Carbon Nanotubes with Narrow Bandgap Naphthalene Diimide-Based Polymers

Noncovalent functionalization of carbon nanotubes by wrapping them using π-conjugated polymers is one of the most promising techniques to sort, separate, and purify semiconducting nanotube species for applications in optoelectronic devices. However, wide energy bandgap polymers commonly used in this technique reduce charge transport through the nanotube network. To avoid the formation of insulating barriers between the tubes, challenging procedures for the removal of the polymer from the nanotube walls are necessary. Here, the use of two narrow bandgap polymers based on naphthalene-bis(dicarboximide) (NDI), namely, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)}, (P(NDI2OD-T2 or ActivInk N2200) and its molecular cousin, poly{[(N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)]-co-[(N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl)-alt-5,5′-(2,2′-(4,4′-dimethoxybithiophene))]}(ActivInk PE-N-73), for selecting semiconducting single-walled carbon nanotubes (s-SWNTs) is demonstrated. The influence of the chemical structure of these polymers on the nanotube selectivity, as well as the effect of residual excess polymer and their band-gaps, are investigated through optical spectroscopy and charge transport measurements. While the electron transport of the devices fabricated with PE-N-73 and N2200 wrapped SWNTs is comparable, a substantial difference is observed in the hole transport. The better alignment of the HOMO level of PE-N-73 with that of the nanotubes allows achieving improved p-type characteristics even with a large amount of residual polymer in the network.