Theoretical and experimental investigations were performed for the organic semiconductor poly(3,6-dialkylthieno[3,2-b]thiophene-co-thieno[3,2-b]thiophene); PATT. In PATT the backbone is constructed such that its alkyl side-chain separations are intermediate between those in P3HT and PBTTT. An objective is to test whether this intermediate side-chain separation leads to increased bonding strength between layers, and perhaps promotes greater crystalline order and hole mobility. Calculations indicate that the energy reduction arising from interdigitation is indeed larger in PATT compared to PBTTT. In transistors fabricated with PATT the hole mobility was found to be 0.024 cm2/Vs, somewhat less than values obtained previously with PBTTT. Differential scanning calorimetry indicates differences between PATT and PBTTT in their thermal behavior. Pseudopotential local density functional calculations predict the tilting angle of the planar conjugated backbone in PATT to be 26. The driving force for tilting, a structural feature exhibited by many organic semiconductors, is shown to be electrostatic energy reduction.
Northrup, J. E.; Chabinyc, M.; Hamilton, R.; McCulloch, I.; Heeney, M. Theoretical and experimental investigations of a poly-alkylated-thieno[3,2-b]thiophene semiconductor. Journal of Applied Physics. 2008 October 15; 104 (8): 083705.