Sangmin Chae

Using Femtosecond Laser Irradiation to Enhance the Vertical Electrical Properties and Tailor the Morphology of a Conducting Polymer Blend Film

We report femtosecond infrared laser-induced selective tailoring of carrier transport as well as surface morphology on a conducting polymer blend thin film. Maximal 2.4 times enhancement on vertical current transport in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, was achieved by this irradiation. The laser irradiation induced a photo expansion without deteriorating its molecular structure and the film morphology could be customized in the micron scale by adjusting the laser writing parameters. In the photoexpanded region, the face-on populations were about 2.2 times larger in comparison with the pristine region, which was a major contributor to the enhanced carrier transport.

Syntheses and Properties of Semiconducting Polymers Based on Pyrimidine Series Substituted with Thiazolo-Pyridine

For the utilization in the polymer solar cells, conjugated polymers were synthesized using electron-deficient units, 2-{3-[4,6-bis-(4-hexyl-thiophen-2-yl)-pyrimidin-2-yl]-phenyl}-thiazolo[5,4-b]pyridine (mTP) or 2-{4-[4,6-bis-(4-hexylthiophen-2-yl)-pyrimidin-2-yl]-phenyl}-thiazolo[5,4-b]pyridine (pTP). To increase the electron-deficiency of the pyrimidine unit, thiazolopyridine (TP), strong electron-withdrawing group, was incorporated at the meta-/para-position of the phenylene connector which is attached to the pyrimidine. It was confirmed that the TP unit at para-position has stronger intramolecular charge transfer (ICT) effect to shift the absorption to the longer wavelength region and deepens the highest occupied molecular orbital (HOMO) energy level. The polymers with 4-(1-octylnonyl)-6-(2-thienyl)-4H-thieno[3,2,-b]indole (TTI) having the decent solubility for the morphology show the superior open-circuit voltage (VOC) value of the mTTI/pTTI devices compared to the others. Also, the polymers with TTI show increased short-circuit current (JSC), fill factor (FF), and power conversion efficiency (PCE) due to high face-on populations.

Manipulating the crystal structure of a conjugated polymer for efficient sequentially processed organic solar cells

Recently, the sequential (Sq) process, which forms nanoscale network structures from quasi-solid-state inter-diffusion through swelling and annealing, is considered to be one of the most efficient methods for fabricating organic solar cells and blend films. Here, we examined the effect of the crystallinity and orientation of poly(3-hexylthiophene) (P3HT) molecules on the formation of the nanostructure by carrying out a Sq process using various solvents with different boiling points. We showed that the moderate crystallinity promoted suitable inter-diffusion between the donor (P3HT) and acceptor ([6,6]-pentadeuterophenyl C61 butyric acid methyl ester, PC60BM), and hence was important for achieving high-performance solar cells using Sq processing. Nanostructure formation by inter-diffusion was investigated and visualized by taking a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements. In addition, our Sq-processed solar cell yielded a device efficiency as high as 3.25%, and was also impressive because it was made with an eco-friendly solvent and using a short-duration annealing process, in contrast to the conventional BHJ process. The present findings provided advanced insight into the Sq process, and we anticipate this efficacious sequential process to contribute not only to the development of higher-efficiency organic solar cells but also to the fabrication of functional blend films.

Laser-induced orientation transformation of a conjugated polymer thin film with enhanced vertical charge transport

The development of new techniques that can be used to modify the molecular orientations and structures of organic compounds in organic thin films is in urgent demand to overcome the low levels of charge transport in such films and to ultimately achieve high-efficiency organic opto-electronics. Here, we report structural transformation and electrical characterization of a poly(3-hexylthiophene) (P3HT) semiconductor thin film under femtosecond laser irradiation at a level below the ablation threshold. A combination of grazing incidence X-ray diffraction and near edge X-ray absorption fine structure analyses reveal that the laser procedure effectively switches the P3HT orientation from edge-on to face-on, and the stability of this face-on orientation was also confirmed. Various laser fluence levels ranging up to the ablation threshold were tested, and the P3HT film with a transformed face-on orientation resulting from the optimized fluence level showed a charge current in the vertical direction three times greater than did the pristine film. This enhancement in the charge current was attributed to the vertical π–π stacking of the face-on orientation. These results also confirmed the effectiveness of our novel method for modifying organic thin films, and we expect this method to be attractive for the development of organic electronics and hybrid device applications.

Ultrafast laser materials processing for manufacturing innovation

Ultrafast laser based materials processing has shown remarkable success in various area of industrial applications. A few examples of innovative applications in displays, OSCs, and other consumer electronics fabrication are demonstrated.