Hyung Jun Koo
1050 Engineering bldg 1
Phone: 919-513-4648
Fax: 919-515-3465
Email: hjkoo [at] ncsu [dot] edu

M.S. School of Chemical and Biological Engineering, 2006
Seoul National University, Korea

B.S. School of Chemical and Biological Engineering, 2004
Seoul National University, Korea

CV

Soft Matter Diodes
Our group has reported that a new type of ionic rectifying junction can be formed at the interface of two water-based gels doped with polyelectrolytes of opposite charge (Figure 1). The remarkable feature of these rectifying junctions is that they operate on the basis of water-borne ions. The unidirectional current response at the interface between the cationic and anionic gels originates from anisotropy in the mobile ionic charges in the gels. The current depends on the concentration of polyelectrolyte, the background ionic concentration and the distance traveled by the ions. Even though our devices were first-generation prototypes without any optimization, the gel diodes had comparable or higher current densities and rectification ratios than other organic polymer-based electronic devices (Figure 1). These water gel diodes had good long-term stability in both DC and AC conduction modes. Publication Link

Recently, highly efficient rectifying junction was observed at the interface between SiO₂ and aqueous gel. Figure 2 shows the schematic of the SiO₂/gel interface diode. SiO2 thin film, which has non-linear conductivity, controls the ionic current flows unidirectionally. Since the polymer matrix of the gel has slightly charged backbones, the gel materials increase the ion conductivity by facilitating the ionic transport. Addition of the salt or polyelectrolyte to the gel improves the current density and rectifying performance. This diode structure based on the SiO₂/hydrogel interface exhibits high rectification ratio of more than 30,000, which is comparable to, or even higher than, that of inorganic p-n junction diode (Figure 2). The gels are flexible, biocompatible and environmentally friendly. The materials and the process of preparation of these devices are very simple, inexpensive and scalable. Such device could be used in bioelectronic circuits, liquid sensors, bio-fuel cells and biomimetic photovoltaics.

Hydrogel Solar Cells
My second project is to design and characterize new photovoltaic devices that will be based on aqueous gels (Figure 3). The gel is not only a soft and flexible material but also a good medium for biologically derived molecules. Such biocompatible gel material would be a key component to achieve our overarching goal, fabrication of photovoltaics that mimic an artificial leaf.

The prototypes were tested in terms of voltage output, photocurrent generation under various loads, I-V cycle characteristics with and without illumination, speed of response and physical stability. Finally, we constructed prototypes of flexible and low cost photovoltaic cells based on aqueous gels. The performance of this device is shown in Figure 4.