| AVS 55th International Symposium & Exhibition | |
| Electronic Materials and Processing | Thursday Sessions |
| Session EM-ThA |
| Session: | Organic Electronics |
| Presenter: | L.J. Richter, National Institute of Science and Technology |
| Authors: | L.J. Richter, National Institute of Science and Technology A.J. Moad, National Institute of Science and Technology D.M. DeLongchamp, National Institute of Science and Technology R.J. Kline, National Institute of Science and Technology D.A. Fischer, National Institute of Science and Technology D.J. Gundlach, National Institute of Science and Technology B.H. Hamadani, National Institute of Science and Technology M. Heeney, Imperial College London, UK I. McCulloch, Imperial College London, UK |
| Correspondent: | Click to Email |
Polymer semiconductors are inexpensive solution processable alternatives to amorphous silicon for applications in flexible large area electronics. Recently, thin films of spun-cast poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) (pBTTT) have been demonstrated to exhibit exceptional hole mobilities in thin film transistors (TFTs) after heating into a low temperature (~ 150 °C) mesophase. This mild thermal cycle results in high level of crystalline order in the annealed film with a morphology exhibiting single molecular layer terraces. Recently we have discovered that heating into a second, higher temperature (~240°C) phase results in a distinct ribbon morphology. The ribbon phase can be oriented over large (cm2) areas by a simple flow coating process. We have applied polarized optical spectroscopies: spectroscopic ellipsometry and FTIR, along with x-ray diffraction and NEXAFS to the in-situ study of the structural evolution in the two high temperature phases. The first mesophase is a well ordered liquid crystal characterized by melted side chains, but excellent vertical lamella order. The second, high temperature phase appears to be an isotropic melt. The behavior of pBTTT is contrasted with that of poly(dialkylthieno[3,2-b]thiophene-2,5-bithiophene) (pTTBT), a newly synthesized isomer of pBTTT with side chains attached to the thienothiophene rather than the bithiophene unit. This subtle structural change results in distinct thermal behavior. The structural transitions of the isomers are generally similar; however, the side chain melting transition Tm occurs about 50°C lower in pTTBT than in pBTTT. The significant drop in Tm appears to correlate with a subtle decrease in main chain packing interactions. Both materials exhibit high hole mobility, even in their respective liquid crystal mesophases. The slight overall higher order in pBTTT is reflected in the annealed device performance.