Ph.D. Oral Examination

Joe Kline rjkline at stanford.edu
Mon May 2 13:38:24 PDT 2005


Stanford University Ph.D. Oral Examination

"A Fundamental Study of Morphology and Charge Transport in Regioregular
Polythiophene"

R. Joseph Kline
Materials Science and Engineering

Building 550 - 550A
Friday, May 6, 2005
1:00 pm
(refreshments at 12:45 pm)

Conjugated polymers include some of the most promising candidates for the active
layer of low-cost thin-film transistors (TFTs) and bulk heterojunction
photovoltaic (PV) cells. The charge carrier mobility of these conjugated
polymers is the key materials property limiting the performance of both of
these devices. This thesis outlines a fundamental investigation of the charge
transport and morphology of the first high mobility conjugated polymer,
regioregular poly(3-hexylthiophene) (P3HT). The charge carrier mobility in TFTs
was found to increase by four orders-of-magnitude as the molecular weight (MW)
of P3HT is increased from 3000 g/mole to 36,000 g/mole. P3HT films with
different MWs provided an ideal system for correlating morphological changes in
conjugated polymers to resulting changes in charge transport. Atomic force
microscopy, x-ray diffraction and grazing incidence x-ray scattering (GIXS)
were used to measure changes in the crystallinity and crystal orientation
associated with varying the spin-casting solvent, annealing conditions,
substrate surface treatment, and drop-casting at a constant MW. The GIXS
results showed that at a constant MW in both low- and high-MW films, the
mobility correlated to the strength of in-plane pi-stacking. When comparing
different MWs, however, this correlation broke down. Rocking curves on samples
with a chemically modified surface showed highly oriented crystals that were
nucleated from the substrate and correlated with variations in charge
transport. Switching to low-MW P3HT improves the overall crystallinity, the
intensity of in-plane pi-stacking, and the concentration of highly oriented
crystals, but the mobility is more than a factor of 100 lower than high-MW
P3HT. These counterintuitive results clearly show that the charge carrier
mobility of conjugated polymers is coupled to several different aspects of the
morphology. In the case of the low-MW films, the strong driving force for
ordering creates grain boundaries that isolate the ordered regions from their
neighbors. Whereas in high-MW films, the long chains connect the small ordered
regions and provide a clear pathway for charges to move through the film. These
results were used to develop a model for relating charge transport and structure
that can be used as a guide for the development of new, improved chemical
structures.



More information about the labmembers mailing list