|Speaker||Prof. Hyojung Kim (Pusan National University)|
|Date||Thu. 23 May. 2019|
|Venue||#331, Asan Hall, College of Science|
Molecular Engineering for Organic Electronics and Synchrotron X-rays
We present molecular engineering for Organic Photovoltaics (OPVs) in conjugated polymer systems with fs-laser, molecular templating growth (MTG) with graphene, and solvent engineering. In case of poly(3-hexylthiophene)(P3HT) films, face-on favorable nanostructure formed by fs-laser irradiation. The dipole moment of P3HT could align along the electric field of irradiated laser, which induced face-on stacking of P3HT polymers in the films. The vertical charge current increased in the fs-laser irradiated region due the enhancement of face-on stacking. [1-4]
For the face-on favorable stacking of P3HT polymers, we adopted dry transferred graphene as a templating layer. In contrast to conventional PMMA-assisted wet transferred wet transferred graphene, dry transferred graphene is found in the current work to be quite suitable for use in the roll-to-roll process due to its lack of PMMA residue, whose removal would require high-temperature annealing. When we produce a P3HT film with a thickness of 30 nm, the face-on populations of P3HT were prominent on both the dry and wet transferred graphene layers. As the film thickness is increased to 50 nm, the face-on population decreased on the wet transferred graphene surface, but retained high levels on the dry transferred graphene.
Finally, we tried to control crystallinity of P3HT layer by solvent to form ideal BHJ (bulk heterojunction) structure in Sequential (Sq) process for OPVs. In the Sq process, a bi-layered structure is formed by the sequential deposition of the acceptor and donor layers, and the final BHJ nanostructure is formed at the interface between the layers by carrying out an additional procedure involving thermal annealing and swelling. We investigated the inter-diffusion of acceptor material at the interface in different crystalline structured layers as a function of gradual thermal annealing. We found that the P3HT films made using a low-boiling-point solvent had fewer crystalline regions and lots of amorphous regions, which enabled fullerene molecules to readily penetrate into P3HT layer within a short period of time.
We took Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) and NEXAFS (Near Edge X-ray Absorption Fine structure) based on synchrotron x-rays for detail structural studies.
 S. Chae, K. H. Jo, S. W. Lee, H.-S. Geum, H. J. Kim, J. Choi, H. H. Lee, Macromol. Chem. Phys. 217, 537-543 (2016).
 S. Chae, A. Yi, C. M. Park, W. S. Chang, H. H. Lee, J. Choi, H. J. Kim, ACS Appl. Mater. Interfaces 9, 24422-24427 (2017).
 S. Chae, K. Jo, S. J. Won, A. Yi, J. Choi, H. H. Lee, J. H. Kim, H. J. Kim, Adv. Mater. Interfaces 4, 1701099 (2017).
 S. Chae, A. Yi, H. H. Lee, J. Choi, H. J. Kim, Journal of Materials Chemistry C (2018, in printing).
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