Structure Property Relationships for Developing Plastic Electronics

 

R. Joseph Kline, Dean M. DeLongchamp, Lee J. Richter, Eric K. Lin

 

The charge carrier mobility of semiconducting polymers is the key materials parameter for determining the performance of thin-film transistors. The liquid crystalline polymer poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (PBTTT) has been recently shown to have mobilities of 0.2 0.7 cm^2/Vs. This represents a substantial improvement over regioregular poly(3-hexylthiophene) (P3HT). We have used a combination of atomic force microscopy (AFM), x-ray diffraction (XRD), near-edge x-ray absorption spectroscopy (NEXAFS), spectral ellipsometry (SE), and polarized infrared (IR) spectroscopy to solve details of the packing arrangement of the polymer crystals not possible to solve by a single technique. AFM reveals molecular terraces that are the first visualization of the lamellar packing motif inferred from XRD. The combination of the lamellar spacing measured by XRD and the side chain tilt angles measured by IR and NEXAFS allows the direct determination of the degree of side chain interdigitation. We find that P3HT does not interdigitate while pBTTT and PQT interdigitated substantially. We find that this difference in interdigitation is due to the side chain attachment density. The lack of interdigitation in P3HT substantially reduces the three-dimensional ordering, resulting in a lower charge carrier mobility. Side chain attachment density is thus a critical parameter for designing high performance molecules.

 

 

Author Information

 

R. Joseph Kline

Mentor: Dean DeLongchamp

Polymers Division, 854

MSEL

Building 224/ Room A325

Stop 8541

301-975-4356 / Fax 301-975-3928

joe.kline@nist.gov

Sigma Xi Member: No

Category: Materials