Chemical Defects and Electronics States in Organic Semiconductors
Final rept. 31 May 2007-31 May 2008
PALO ALTO RESEARCH CENTER INC CA
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Density functional calculations were performed for polymers including PBTTT, P3HT, and PQT. To understand the origin of the high mobility and molecular ordering in PBTTT, the atomic and electronic structure was calculated. The calculations predicted that the conjugated planes exhibit a rotation tilt around the polymer axis. The degree of tilting depends on the alkyl-side-chain length. Tilting reduces the inter-chain molecular overlap, and this lowers the hole mobility. Calculations for PQT and P3HT show that tilting occurs in these materials also. This discovery led to a natural interpretation of NEXAFS data, which confirms the predicted tilt. It was shown that backbone tilt is a consequence of electrostatic energy reduction. A simple model of scattering indicates the mobility could be reduced by 60 percent by the tilt. Ozone was investigated as an impurity in PQT. Calculations show that ozone molecules that are weakly attached to the conjugated chains give rise to acceptor levels in the gap. This shows that ozone can dope the material p-type, and must be prevented from interacting with organic semiconductor devices. An expt.theoretical study of O2 in pentacene indicated that a positive gate voltage can cause dissociative interaction of O2 with pentacene.
- Polymer Chemistry
- Solid State Physics