Myunghun Shin

Material characteristics and equivalent circuit models of stacked graphene oxide for capacitive humidity sensors

The oxidation properties of graphene oxide (GO) are systematically correlated with their chemical sensing properties. Based on an impedance analysis, the equivalent circuit models of the capacitive sensors are established, and it is demonstrated that capacitive operations are related to the degree of oxidation. This is also confirmed by X-ray diffraction and Raman analysis. Finally, highly sensitive stacked GO sensors are shown to detect humidity in capacitive mode, which can be useful in various applications requiring low power consumption.

Fabrication of honeycomb textured glass substrate and nanotexturing of zinc oxide front electrode for its application in high efficiency thin film amorphous silicon solar cell

Abstract. A significant part of broad band sunlight remains unabsorbed in a simple structured amorphous silicon solar cell. This absorption can be enhanced by adopting a light-trapping scheme with the help of a textured front surface and back reflector. For this purpose, honeycomb-type texture was fabricated on a glass surface by chemical etching. A 3  μm×3  μm honeycomb patterned optical-mask was used to create an image-pattern of an etch-mask on the glass surfaces. We used 0.5% hydrofluoric acid (HF) as an etchant solution with an optimized etching time ranging between 25 and 28 min. This single textured glass shows an enhancement in diffused transmission of light. In solar cell application, a 630-nm-thick Al-doped ZnO (AZO) layer was deposited over the textured glass surface. An additional random etching was carried out on the AZO with 1% HF acid solution for 10 s. This results to a double textured AZO superstrate on which solar cells were fabricated. The solar cells show higher short circuit current density to 17.2  mA/cm2. Finally, we obtained photovoltaic conversion efficiency of an optimized solar cell as 10.75% (with the corresponding Jsc as 17.03  mA/cm2).

Impedance Spectroscopy Analysis and Equivalent Circuit Modeling of Graphene Oxide Solutions

The optical and electrical characteristics of a graphene oxide solution (GS) with different graphene oxide (GO) concentrations in de-ionized water are investigated via the electrochemical impedance spectroscopy (EIS) method. The measurement results produced by the EIS for the GS are represented with both Bode and Nyquist plots in a frequency range from 1 kHz to 10 MHz. Using these results, we develop an equivalent circuit model as a function of the GO concentration, representing the GS as a mixed circuit of two-dimensional (2D) GO dispersed in parallel in de-ionized (DI) water. The underlying physics of the current-flowing behavior in the GS are explained and interpreted using empirical circuit models; the circuit model also shows that highly resistive GO becomes conductive in GS form in the DI water. The findings in this work should draw new attention toward GSes and related applications, including functional composite materials, catalysts, and filter membranes.

Two-dimensional diode equivalent circuit model for large-area Z-type transparent DSC module

A diode equivalent circuit and extended two-dimensional network circuit models were developed for a unit dye-sensitized solar cell (DSC) and modules. Based on a P-spice simulation using the developed models, we designed and fabricated 300 × 300 mm Z-type transparent DSC modules for building integrated photovoltaic windows. The developed models demonstrated that the optimized cell designs and performance can depend heavily on solar-irradiation conditions. The developed circuit models exhibited good agreement with the operational behaviors of the fabricated modules under complex irradiation conditions, which will be very useful in developing high-performance transparent DSC modules.