Youngjun Jeong

Improved Thermal Oxidation Stability of Solution-Processable Silver Nanowire Transparent Electrode by Reduced Graphene Oxide

Solution-processable silver nanowire-reduced graphene oxide (AgNW-rGO) hybrid transparent electrode was prepared in order to replace conventional ITO transparent electrode. AgNW-rGO hybrid transparent electrode exhibited high optical transmittance and low sheet resistance, which is comparable to ITO transparent electrode. In addition, it was found that AgNW-rGO hybrid transparent electrode exhibited highly enhanced thermal oxidation and chemical stabilities due to excellent gas-barrier property of rGO passivation layer onto AgNW film. Furthermore, the organic solar cells with AgNW-rGO hybrid transparent electrode showed good photovoltaic behavior as much as solar cells with AgNW transparent electrode. It is expected that AgNW-rGO hybrid transparent electrode can be used as a key component in various optoelectronic application such as display panels, touch screen panels, and solar cells.

Highly Sensitive, Transparent, and Durable Pressure Sensors Based on Sea‐Urchin Shaped Metal Nanoparticles

Highly sensitive, transparent, and durable pressure sensors are fabricated using sea-urchin-shaped metal nanoparticles and insulating polyurethane elastomer. The pressure sensors exhibit outstanding sensitivity (2.46 kPa-1 ), superior optical transmittance (84.8% at 550 nm), fast response/relaxation time (30 ms), and excellent operational durability. In addition, the pressure sensors successfully detect minute movements of human muscles.

Flexible metal nanowire-parylene C transparent electrodes for next generation optoelectronic devices

We prepared high performance metal nanowire (NW)-parylene C transparent electrodes (TEs) using pyrolytic deposition of a parylene C protection layer onto a silver nanowire (AgNW) or copper nanowire (CuNW) film at room temperature for the first time. The AgNW-parylene C TE showed superior optoelectronic properties such as high optical transmittance (94.7%) and low sheet resistance (41.6 Ω sq−1), comparable to a conventional indium tin oxide (ITO) TE. The AgNW-parylene C TE fabricated on a plastic substrate possessed outstanding flexibility. Moreover, the AgNW-parylene C and CuNW-parylene C TEs exhibited significantly improved oxidation and chemical stability due to the outstanding gas barrier properties of the parylene C protection layer. Furthermore, the potential suitability of the AgNW-parylene C TE was successfully demonstrated by fabricating flexible polymer solar cells. We expect that the flexible metal NW-parylene C TEs can be used as key elements for a variety of next generation optoelectronic devices.

Synthesis and Characterization of Polymers Based on Benzimidazole Segments for Polymer Solar Cells

A series of polymers based on electron-deficient benzimidazole segments and electron-rich benzodithiophene segments for polymer solar cells was designed and synthesized. The polymer solar cell devices with benzimidazole based polymers achieved power conversion efficiency (PCE) close to 1%. This work implies that polymers based on benzimidazole segment can achieve high PCE through structural modification of the polymers.

DC Magnetron Sputtered IZTO Thin Films for Organic Photovoltaic Application

IZTO20 (In0.6Zn0.2Sn0.2O1.5) ceramic target was prepared from oxide mixture of In2O3, ZnO, and SnO2 powders. IZTO20 thin films were then deposited onto glass substrate at 400 °C by DC magnetron sputtering. The average optical transmittance determined by ultraviolet-visible spectroscopy was higher than 85% for all films. The minimum resistivity of the annealed IZTO20 thin film was approximately 6.1×10-4 Ω·cm, which tended to increase with decreasing indium content. Substrate heating and annealing were found to be important parameters affecting the electrical and optical properties. An organic photovoltaic (OPV) cell was fabricated using the IZTO20 film deposited under the optimized condition as an anode electrode and the efficiency of up to 80% compared to that of a similar OPV cell using ITO film was observed. Reduction of surface roughness and electrical resistivity through annealing treatment was found to contribute to the improved efficiency of the OPV cell.

Theory guided systematic molecular design of benzothiadiazole–phenazine based self-assembling electron-acceptors

We report two new self-assembling n-type materials in which the design process was systematically analyzed using theoretical calculations prior to experimental synthesis. Benzothiadiazole (A′) and alkoxyphenazine (A) serve as our basic electron-deficient building blocks to construct two candidate molecules with an acceptor (A) – acceptor (A′) – acceptor (A) configuration. We conducted a computational characterization (B3LYP/6-31G*) of the electronic properties for structural subunits linked together and analyzed in a step-by-step fashion, thereby culminating in the target molecules of interest. We found that ELUMO was controlled primarily by benzothiadiazole as was evident by orbital localization on this subunit. Meanwhile, EHOMO was influenced by the dihedral angle between A and A′. The molecule with A and A′ coupled with a C–C triple bond (BTD-P-T) was found to be planar with a more stabilized ELUMO and a reduced Egap when compared to its C–C single bond counterpart (BTD-P-S). The two molecules were synthesized and characterized to verify the theoretical findings. Optical, electrochemical, and thermal properties were characterized with UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry, and differential scanning calorimetry, respectively. In addition, the molecular packing was examined by X-ray powder diffraction. As predicted by theory, BTD-P-T exhibited a lower Egap based on the systems lower ELUMO in comparison to BTD-P-S. BTD-P-T exhibited a higher Tm and crystallinity due to its planarity. Both BTD-P-S and BTD-P-T exhibited excellent fibrillation ability upon solvent casting. Bulk heterojunction (BHJ) organic solar cell (OSC) devices were fabricated using BTD-P-S or BTD-P-T as an acceptor and P3HT as a donor at different weight ratios. For both P3HT:BTD-P-S and P3HT:BTD-P-T BHJs, the weight ratio of 6 : 1 produced the highest power conversion efficiency (PCE) of 0.17% and 0.44%, respectively. These results are consistent with fluorescence quenching experiments in which 90% of P3HT fluorescence was quenched at that ratio. Nearly two times higher PCE was observed for the BTD-P-T based device compared to that of the BTD-P-S based system, mainly due to the higher Jsc. Presumably, the flat geometry of BTD-P-T allows for more efficient intermolecular π-orbital overlap, enhancing charge transport.