Kilwon Cho

Control of the electrode work function and active layer morphology via surface modification of indium tin oxide for high efficiency organic photovoltaics

Indium tin oxide (ITO) substrates modified with self-assembled monolayers (SAMs) were used to control the anode work function and active layer morphology of organic solar cells based on poly(3-hexylthiophene)/[6:6]-phenyl-C61 butyric acid methyl ester heterojunctions. By using SAMs with the terminal groups –NH2, –CH3, and –CF3, the authors were able to control the hole injection barrier of the ITO closer to the highest occupied molecular orbital level of active layer and surface energy of the ITO substrate. A solar cell device with CF3 SAM treated ITO was found to exhibit high efficiency performance, about 3.15%.

Surface-directed molecular assembly of pentacene on monolayer graphene for high-performance organic transistors.

Organic electronic devices that use graphene electrodes have received considerable attention because graphene is regarded as an ideal candidate electrode material. Transfer and lithographic processes during fabrication of patterned graphene electrodes typically leave polymer residues on the graphene surfaces. However, the impact of these residues on the organic semiconductor growth mechanism on graphene surface has not been reported yet. Here, we demonstrate that polymer residues remaining on graphene surfaces induce a stand-up orientation of pentacene, thereby controlling pentacene growth such that the molecular assembly is optimal for charge transport. Thus, pentacene field-effect transistors (FETs) using source/drain monolayer graphene electrodes with polymer residues show a high field-effect mobility of 1.2 cm(2)/V s. In contrast, epitaxial growth of pentacene having molecular assembly of lying-down structure is facilitated by π-π interaction between pentacene and the clean graphene electrode without polymer residues, which adversely affects lateral charge transport at the interface between electrode and channel. Our studies provide that the obtained high field-effect mobility in pentacene FETs using monolayer graphene electrodes arises from the extrinsic effects of polymer residues as well as the intrinsic characteristics of the highly conductive, ultrathin two-dimensional monolayer graphene electrodes.

Recent Advances in Organic Transistor Printing Processes

Recent progress in organic field-effect transistor (OFET) printing processes is reviewed, and a perspective on the future of the field is discussed. The principles underlying the OFET printing techniques are introduced according to two categories: direct write printing and transfer printing. A comprehensive overview of the use of printing techniques in OFET production processes is also provided. Considerations for improving OFET device performance using printing processes are explored. Prior to OFET commercialization, the OFET printing techniques must satisfy several requirements, as discussed here.

Interface engineering in organic transistors

Recent technological advances in organic field-effect transistors (OFETs) have triggered intensive research into the molecular and mesoscale structures of organic semiconductor films that determine their charge-transport characteristics. Since the molecular structure and morphology of an organic semiconductor are largely determined by the properties of the interface between the organic film and the insulator, a great deal of research has focused on interface engineering. We review recent progress in interface engineering for the fabrication of high-performance OFETs and, in particular, engineering of the interfaces between semiconductors and insulators. The effects of interfacial characteristics on the molecular and mesoscale structures of π-conjugated molecules and the performance of OFET devices are discussed.

Work-Function Engineering of Graphene Electrodes by Self-Assembled Monolayers for High-Performance Organic Field-Effect Transistors

We have devised a method to optimize the performance of organic field-effect transistors (OFETs) by controlling the work functions of graphene electrodes by functionalizing the surface of SiO2 substrates with self-assembled monolayers (SAMs). The electron-donating NH2-terminated SAMs induce strong n-doping in graphene, whereas the CH3-terminated SAMs neutralize the p-doping induced by SiO2 substrates, resulting in considerable changes in the work functions of graphene electrodes. This approach was successfully utilized to optimize electrical properties of graphene field-effect transistors and organic electronic devices using graphene electrodes. Considering the patternability and robustness of SAMs, this method would find numerous applications in graphene-based organic electronics and optoelectronic devices such as organic light-emitting diodes and organic photovoltaic devices.

Effect of the phase states of self-assembled monolayers on pentacene growth and thin-film transistor characteristics.

To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90 degrees C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.

Single‐Gate Bandgap Opening of Bilayer Graphene by Dual Molecular Doping

Dual doping-driven perpendicular electric field with opposite directions remarkably increase the on/off current ratio of bilayer graphene field-effect transistors. This unambiguously proves that it is possible to open a bandgap with two molecular dopants (F4-TCNQ and NH2 -functionalized self-assembled monolayers (SAMs)) even in a single-gate device structure.

Transparent flexible organic transistors based on monolayer graphene electrodes on plastic.

There has been much interest in graphene-based electronic devices because graphene provides excellent electrical, optical, and mechanical properties. [ 1 ] In this sense, organic electronic devices using graphene electrodes have attracted considerable attention, and several reports have described the use of graphene source/drain electrodes in organic fi eld-effect transistors (OFETs). [ 2 ] One of the ultimate goals in the fabrication of OFETs using graphene electrodes lies in the fabrication of fl exible and transparent organic transistors, assembled on plastics substrates, that maintain their high performance under ambient conditions. However, no reports have described the fabrication of organic transistors assembled on plastic substrates because the synthesis of either graphene or reduced graphene oxide requires high-temperature fabrication processes. Another important goal in the context of fabricating organic electronic devices with graphene electrodes lies in the fabrication of highly transparent graphene electrodes that cover large areas. Graphene transmittance decreases linearly as the number of layers increases in n-layer graphene. [ 3 ] Thus, the use of monolayer graphene is necessary to achieve high transparency in graphene electrodes, provided that the conductivity of the graphene is suffi cient for device electrode applications. Another merit of monolayer graphene is its extremely low thickness (3–4 Å). Source/drain electrodes in staggered bottomcontact FET structures should be thin to ensure step coverage of the active layer during sequential transistor fabrication. [ 4 ] For this reason, one-atom-thick monolayer graphene provides ideal source/drain electrodes for effi cient charge injection. Recently, several groups succeeded in fabricating high-quality/largearea graphene with preferential monolayer thickness using a

High‐Efficiency Organic Solar Cells Based on End‐Functional‐Group‐Modified Poly(3‐hexylthiophene)

[*] Prof. K. Cho, Prof. J. K. Kim, Y. Lee, J. H. Park Department of Chemical Engineering Polymer Research Institute Pohang University of Science and Engineering Pohang, 790-784 (Korea) E-mail: kwcho@postech.ac.kr; jkkim@postech.ac.kr Dr. J. S. Kim, Dr. J. H. Lee School of Environmental Science and Engineering Polymer Research Institute Pohang University of Science and Engineering Pohang, 790-784 (Korea)

Switchable Transparency and Wetting of Elastomeric Smart Windows

Smart windows with switchable light transmittance properties have recently attracted signifi cant attention because of their many applications, such as architectural or vehicle windows, skylights, and internal partitions. To date, reversible switching has been achieved either by a change in molecular arrange-ments or by the oxidation–reduction reaction of chromogenic materials activated by light, electrical voltage, or tempera-ture.