Kyoohee Woo

Highly Transparent Low Resistance ZnO/Ag Nanowire/ZnO Composite Electrode for Thin Film Solar Cells

We present an indium-free transparent conducting composite electrode composed of silver nanowires (AgNWs) and ZnO bilayers. The AgNWs form a random percolating network embedded between the ZnO layers. The unique structural features of our ZnO/AgNW/ZnO multilayered composite allow for a novel transparent conducting electrode with unprecedented excellent thermal stability (∼375 °C), adhesiveness, and flexibility as well as high electrical conductivity (∼8.0 Ω/sq) and good optical transparency (>91% at 550 nm). Cu(In,Ga)(S,Se)₂ (CIGSSe) thin film solar cells incorporating this composite electrode exhibited a 20% increase of the power conversion efficiency compared to a conventional sputtered indium tin oxide-based CIGSSe solar cell. The ZnO/AgNW/ZnO composite structure enables effective light transmission and current collection as well as a reduced leakage current, all of which lead to better cell performance.

A non-toxic, solution-processed, earth abundant absorbing layer for thin-film solar cells

Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) has attracted significant attention in the past few years as a next generation absorber material for the production of thin film solar cells on large scales due to the high natural abundance of all constituents, tunable direct band gap energy ranging from 1.0 to 1.5 eV, and large absorption coefficient. In addition, to address the issue of expensive vacuum-based processes, non-vacuum solution-based approaches are being developed for CZTS absorber layer deposition. Here, we demonstrate the fabrication of a high quality CZTS absorber layer with a thickness of 2.8–3.0 μm and micrometre-scaled grains (1–2.5 μm) using air-stable non-toxic solvent-based inks. Our approach for the fabrication of CZTS absorber, reported here, will be the first step in achieving low-cost and large area solar cells with high efficiency.

Ink-jet printing of cu-ag-based highly conductive tracks on a transparent substrate.

We have developed a Cu-Ag-based mixed metal conductive ink from which highly conductive tracks form on a flexible substrate after annealing at low temperature. Addition of small Ag particles significantly improves the particle packing density by filling the interstices formed between the larger Cu particles, which in turn facilitates better conductivity compared to pure Cu metal film. The particle size and volume ratio of the Ag particles added should be carefully controlled to achieve maximum packing density in the bimodal particle system, which is consistent with the theoretical considerations of the Furnas model. In addition, we demonstrate direct writing of complex patterns that exhibit high conductivity upon annealing at sufficiently low temperature (175-210 degrees C) to not damage the transparent plastic substrate such as polyethersulphone (PES).

High-performance low-temperature solution-processable ZnO thin film transistors by microwave-assisted annealing

Oxide semiconductors afford a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin film transistors (TFTs). We synthesized an aqueous inorganic precursor by a direct dissolution of zinc hydroxide in ammonium hydroxide solution from which a dense and uniform ZnO semiconducting layer is achieved. Solution-processed ZnO-TFTs prepared at 140 °C by microwave irradiation have shown enhanced device characteristics of ∼1.7 cm2 V−1s−1 mobility and a ∼107 on/off current ratio, with good air stability. Spectroscopic analyses confirmed that such a device improvement originates from accelerated dehydroxylation and better crystallization at low temperature by microwave irradiation. Our results suggest that solution-processable oxide semiconductors have potential for low-temperature and high-performance applications in transparent devices.

Bias-Stress-Stable Solution-Processed Oxide Thin Film Transistors

We generated a novel amorphous oxide semiconductor thin film transistor (AOS-TFT) that has exellent bias-stress stability using solution-processed gallium tin zinc oxide (GSZO) layers as the channel. The cause of the resulting stable operation against the gate bias-stress was studied by comparing the TFT characteristics of the GSZO layer with a tin-doped ZnO (ZTO) layer that lacks gallium. By photoluminescence, X-ray photoelectron, and electron paramagnetic resonance spectroscopy, we found that the GSZO layer had a significantly lower oxygen vacancy, which act as trap sites, than did the ZTO film. The successful fabrication of a solution-processable GSZO layer reported here is the first step in realizing all-solution-processed transparent flexible transistors with air-stable, reproducible device characteristics.

Fuzzy system modeling by fuzzy partition and GA hybrid schemes

Abstract This paper presents an approach to building multi-input and single-output fuzzy models. Such a model is composed of fuzzy implications, and its output is inferred by simplified reasoning. The implications are automatically generated by the structure and parameter identification. In structure identification, the optimal or near optimal number of fuzzy implications is determined in view of valid partition of data set. The parameters defining the fuzzy implications are identified by a GA (Genetic Algorithm) hybrid scheme to minimize mean square errors globally. Numerical examples are provided to evaluate the feasibility of the proposed approach. Comparison shows that the suggested approach can produce a fuzzy model with higher accuracy and a smaller number of fuzzy implications than the ones achieved previously in other methods. The proposed approach has also been applied to construct a fuzzy model for the navigation control of a mobile robot. The validity of the resultant model is demonstrated by experimentation.

Band-gap-graded Cu2ZnSn(S1-x,Sex)4 Solar Cells Fabricated by an Ethanol-based, Particulate Precursor Ink Route

Solution processing of earth-abundant Cu2ZnSn(S1-x,Sex)4 (CZTSSe) absorber materials is an attractive research area in the economical and large-scale deployment of photovoltaics. Here, a band-gap-graded CZTSSe thin-film solar cell with 7.1% efficiency was developed using non-toxic solvent-based ink without the involvement of complex particle synthesis, highly toxic solvents, or organic additives. Despite the high series resistance due to the presence of a thick Mo(S,Se)x layer and Zn(S,Se) aggregates, a high short-circuit current density (JSC) was generated. In addition, there was no significant difference in open circuit voltages (VOC) between CZTS (0.517 V) and CZTSSe (0.505–0.479 V) cells, despite a significant band gap change from 1.51 eV to 1.24 eV. The high JSC and less loss of VOC are attributed to the effect of band gap grading induced by Se grading in the CZTSSe absorber layer. Our environmentally benign ink approach will enable the realization of low-cost, large-area, high-efficiency thin-film solar cells.

Effect of Carboxylic Acid on Sintering of Inkjet-Printed Copper Nanoparticulate Films

The reduction effect of various carboxylic acids on inkjet-printed copper film was investigated. Carboxylic acids were exposed to the film by nitrogen gas that was bubbled through the liquid acids during the annealing process. It was observed that in the case of saturated monocarboxylic acid (formic, acetic, propionic, butyric), the acids with shorter hydrocarbon chains perform better in reducing the surface copper oxides in the printed copper conductive film. The printed films exposed to formic acid vapor exhibited the lowest resistivity (3.10 and 2.30 μΩ cm when annealed at 200 and 250 °C, respectively). In addition, the oxalic acid more effectively reduces copper oxide than formic acid and its usage can shorten the annealing time for highly conductive printed copper film. This reductive annealing process allows fabrication of copper patterns with low resistivity, (3.82 μΩ cm annealed at 250 °C) comparable to the resistivity of bulk copper.

Inkjet-printed Cu source/drain electrodes for solution-deposited thin film transistors

We report on the first utility of Cu nanoparticle inks as low-cost, printable electrodes in the fabrication of solution-deposited amorphous oxide semiconductor thin film transistors. The performance of printed Cu electrodes was studied in terms of involvements of surface states in the devices. The surface chemical structures of Cu nanoparticulate electrodes were observed to be modified, dependent on the molecular weight of the polyvinylpyrrolidone capping molecules used in their synthesis. The surface dipoles became weak, and the work function of the printed electrodes decreased with increasing the molecular weight. The work function tailored by introducing the larger capping agents allowed for a better energetic leveling with the metal oxide semiconductor layer, resulting in the improved device performance.

Continuous Patterning of Copper Nanowire-Based Transparent Conducting Electrodes for Use in Flexible Electronic Applications

Simple, low-cost and scalable patterning methods for Cu nanowire (NW)-based flexible transparent conducting electrodes (FTCEs) are essential for the widespread use of Cu NW FTCEs in numerous flexible optoelectronic devices, wearable devices, and electronic skins. In this paper, continuous patterning for Cu NW FTCEs via a combination of selective intense pulsed light (IPL) and roll-to-roll (R2R) wiping process was explored. The development of continuous R2R patterning could be achieved because there was significant difference in adhesion properties between NWs and substrates depending on whether Cu NW coated area was irradiated by IPL or not. Using a custom-built, R2R-based wiping apparatus, it was confirmed that nonirradiated NWs could be clearly removed out without any damage on irradiated NWs strongly adhered to the substrate, resulting in continuous production of low-cost Cu NW FTCE patterns. In addition, the variations in microscale pattern size by varying IPL process parameters/the mask aperture sizes were investigated, and possible factors affecting on developed pattern size were meticulously examined. Finally, the successful implementation of the patterned Cu NW FTCEs into a phosphorescent organic light-emitting diode (PhOLED) and a flexible transparent conductive heater (TCH) were demonstrated, verifying the applicability of the patterned FTCEs. It is believed that our study is the key step toward realizing the practical use of NW FTCEs in various flexible electronic devices.