New Polythiophenes with Large Domains and High Mobilities

 

R. Joseph Kline

Eric K. Lin

Polymers Division

Materials Science and Engineering Laboratory

Building 224, Room A325

Mail stop 8541

Office- 301 975-4356

Fax- 301 975-3928

joe.kline@nist.gov

Not a sigma-xi member

Category: Materials

 

Increasing the charge carrier mobility of conjugated polymers has been an intense focus of the development of polymer-based thin-film transistors for several years. Currently used polymers such as regioregular poly(3-hexylthiophene) (P3HT) have a mobility of about 0.1 cm^2/Vs. These mobilities are sufficient for low-end applications, but a higher mobility would greatly increase the impact of this technology. We report the results of a study of the recently developed liquid crystalline polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes), having mobilities of 0.2 0.6 cm^2/Vs. We use atomic force microscopy and x-ray diffraction rocking curves to study the domain structure and orientation to determine why this polymer is better than P3HT. Annealing in the liquid crystalline phase produces terraced domains greater than 200 nm in diameter, substantially larger than any previously observed in conjugated polymers. Rocking curves show that these domains grow from the substrate and extend through the entire thickness of the film. Furthermore, the domains are highly oriented (within 0.03 degrees of the substrate normal). The intensity of the rocking curve peaks is at least an order of magnitude greater than that observed for the best P3HT films, indicating a significant increase in crystallinity. We will show how the combination increased order and domain size result in the high mobilities.