Ohsung Song

Effect of the thickness of the Ru-coating on a counter electrode on the performance of a dye-sensitized solar cell

A ruthenium (Ru) catalytic layer was assessed as the counter electrode (CE) in dye sensitized solar cells (DSSCs) by examining the effect of the Ru thickness on the DSSC performance. Ru films with different thicknesses (34, 46, 69 and 90 nm) were deposited on glass/fluorine-doped tin oxide (FTO) substrates as the CE by atomic layer deposition (ALD) at 250 °C using RuDi as the precursor and O2 as the reaction gas. Finally, a 0.45 cm2 DSSC of glass/FTO/TiO2/dye(N719)/electrolyte(C6DMII, GSCN)/Ru CE structure was prepared. The properties of the DSSCs were examined by field emission scanning electron microscopy (FESEM), four-point-probe, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), current-voltage (I–V), incident photon-to-current conversion efficiency (IPCE), and dark current measurements. FESEM showed that the crystallized Ru films had been deposited quite uniformly and conformally on the glass/FTO surface. The sheet resistance of the Ru film decreased with increasing Ru thickness. CV profiling revealed an increase in catalytic activity with increasing film thickness. The charge transfer resistance at the interface between the Ru-coated CE and electrolyte decreased with increasing Ru thickness. I–V profiling showed that the energy conversion efficiency was increased up to 3.40 % by increasing the Ru thickness. Moreover, the IPCE and dark current results showed the efficiency of the Ru-coated CE was comparable to that of a conventional platinum (Pt) CE.

Properties of an Au/Pt bilayered counter electrode in dye sensitized solar cells

A Ru/Ti bilayer to a flat glass substrate was applied as a counter electrode to improve the energy conversion efficiency of a dye-sensitized solar cell device with the structure of glass/FTO/blocking layer/TiO2/N719(dye)/electrolyte/(50 nm Ru-50 nm Ti)/glass. For comparison, a 100 nm-thick Ru counter electrode on a flat glass substrate was also prepared using the same method. The photovoltaic properties, such as the short circuit current density, open circuit voltage, fill factor, energy conversion efficiency and impedance, were characterized using a solar simulator and potentiostat. The phase of the bilayered films was examined by x-ray diffraction. The measured energy conversion efficiency of the dye-sensitized solar cell device with a Ru/Ti bilayer counter electrode was 2.40%. The efficiency was 1.48 times larger that of the dye-sensitized solar cell with the 100 nm Ru counter electrode. The new phase of RuTi led to a decrease in resistivity and an increase in catalytic activity. The interface resistance at the interface between the counter electrode and electrolyte decreased when Ru/Ti bilayer thin films were applied. This suggests that Ru/Ti bilayer thin films improve the efficiency of dye-sensitized solar cells.

Effect of microstructure on the magnetoresistive properties of NiFe/Co(CoFe)/Al(Ta)–oxide/Co(CoFe) tunnel junctions

The microstructure of the NiFe/Co(CoFe)/Al(Ta)-oxide/Co(CoFe) ferromagnetic tunnel junction was investigated using cross-sectional transmission electron microscopy (TEM). The effect of the insulating layer on the magnetoresistive (MR) properties of the junction was studied. The multilayer junction was formed using magnetron sputtering and the insulating layer was created by plasma oxidation of the deposited metal film. TEM analysis showed that the MR ratio was highly dependent on the insulating layer. For the NiFe/Co/Al-oxide/Co junction, when the Al2O3 layer was 13 A, the oxide layer was flat and the highest MR ratio of 15% was attained. As the Al2O3 thickness increased, the interface roughness rapidly increased, and the MR ratio also markedly dropped. In contrast, NiFe/CoFe/Al-oxide/CoFe junction showed a comparatively flatter interface and recorded a higher MR ratio. The Ta-oxide insulating layer remained flat regardless of the thickness; however, the largest MR ratio of only 9% was obtained within a n...

Formation of ruthenium-dots on counter electrodes for dye sensitized solar cells

A 34 nm-thick ruthenium layer was deposited on a flat glass substrate by atomic layer deposition at low temperatures followed by the formation of self-aligned Ru micro or nano-dots by rapid thermal annealing for 30 seconds at 400°C. The resulting substrates were re-coated with another 34 or 69 nm-thick Ru layers by ALD at 250°C. Finally, the effective area of 0.45 cm2 dye sensitized solar cell with a glass / FTO / TiO2 / dye / electrode / (nano-dots Ru / Ru)/glass structure was fabricated. The microstructure was examined by field emission scanning electron microscopy and atomic force microscopy. The photovoltaic properties, such as the short circuit current density, open circuit voltage, fill factor, and energy conversion efficiency, were characterized using a solar simulator and potentiostat. FE-SEM confirmed that the 34 nm and 69 nm-thick Ru layers re-deposited by the second ALD process showed a 65% and 49% larger surface area, respectively, than the flat glass substrate due to the rearranged nano-dots. The ECE of the final DSSC was 2.62%, which was a 1.45 times larger than that of the 34 nm-Ru flat glass substrate. These results suggest that the efficiency of DSSCs can be improved by increasing the effective surface area of a counter electrode by low temperature ALD.

Effects of film thickness and deposition rate on the diffusion barrier performance of titanium nitride in Cu-through silicon vias

The influence of morphology on the performance of TiN diffusion barriers was studied by investigating the effects of film thickness and deposition rate. Increasing the TiN film thickness was ineffective in preventing Cu migration due to the columnar growth of TiN, which left rapid diffusion paths for Cu. When the thickness of the TiN film was less than 10 nm, slowly deposited TiN films showed better Cu barrier performance than rapidly deposited TiN films due to the formation of an amorphous structure, which is an effective phase for preventing Cu migration.

Properties of the Cu/Pt Bilayered Counter Electrode Employed Dye Sensitized Solar Cells

In order to improve energy-conversion efficiency using a Cu/Pt bilayer on a flat-glass substrate of a counter electrode (CE), a 0.45 cm dye-sensitized solar-cell (DSSC) device (with glass/FTO/blocking layer/ TiO2/N719 (dye)/electrolyte/50 nm-Pt/50 nm-Cu/glass) was prepared. For comparison, 100-nm thick Cu and Pt CEs were also prepared on flat-glass substrates using the same method. The photovoltaic properties of the DSSC device, such as short-circuit current-density (Jsc), open-circuit voltage (Voc), fill factor (FF), energy-conversion efficiency (ECE), and impedance, were checked using a solar simulator and potentiostat. The sheet resistance was examined using a four point probe. The phases of the bi-layered films were examined using X-ray diffraction. The measured energy-conversion efficiency of the DSSC devices with only Pt and Cu/Pt bilayer counter electrodes was 4.60% and 5.72%, respectively. The sheet resistance and interface (CE/electrolyte) resistance of a Cu/Pt bilayer were smaller than those of a Pt-only layer. The phases of the Cu/Pt bi-layered films were identified in pure Cu and Pt without any intermetallic layer. We concluded that the increase in the efficiency of DSSCs employing Cu/Pt, resulted from employing the low-resistive Cu layer. †(Received August 20, 2013)

Effect of inductively coupled plasma oxidation on properties of magnetic tunnel junctions

Magnetic tunnel junctions consisting of Ta(50 A)/NiFe(50 A)/IrMn(150 A)/CoFe(50 A)/Al(13 A) –O/CoFe(40 A)/NiFe(400 A)/Ta(50 A) with a 100×100 μm2 junction area were prepared. The AlOx tunnel barrier was produced by oxidizing the 13 A thick Al metal using inductively coupled plasma (ICP) for 30–360 s and the ensuing junction properties were characterized as a function of oxidation time. It was found that a junction oxidized for 80 sec exhibited the highest magnetoresistance ratio, 30.3%, at room temperature. It was also shown that the junctions with an ICP oxidized tunnel barrier maintained the tunneling magnetoresistance ratio over 15% even when the insulator layer was oxidized for a prolonged period, well beyond the optimal oxidation time. The large processing window for the insulator oxidation was attributed to the dense amorphous AlOx structure formed by the ICP oxidation.

Microstructure and electrical properties of magnetic tunneling junctions: Ta/NiFe/IrMn/M/Al-oxide/M/NiFe (M=Co, NiFe, CoFe)

Abstract The microstructure and electrical properties of the Ta/NiFe/IrMn/M/Al-oxide/M/NiFe (M=Co, NiFe, CoFe) ferromagnetic tunnel junctions with different Al-oxide thickness were investigated. The CoFe junction showed the highest magnetoresistance (MR) ratio of 32% at room temperature with the insulation layer of Al (15 A)-oxide, but also had the highest junction resistance of 34 kΩ (junction area of 200×200 μm) out of the three electrodes. The Co junction had the lowest resistance with reasonably high MR ratio of 24%. Cross-sectional transmission electron microscopy of the junctions showed that the increasing insulation thickness resulted in the rapid increased roughness at the top electrode/Al-oxide interface and the subsequent reduction of the MR ratio for all three electrodes materials. We have demonstrated that the MR effect is not only dictated by the intrinsic properties of the FM electrode materials, but also by the thickness and the microstructure of the oxide layer, which could be utilized to optimize the electrical properties of the ferromagnetic tunneling junction.

Characterization of synthesized and treated gem diamonds

Synthesized diamonds have been widely employed as polishing media for precise machining and noble substrates for microelectronics. The recent development of the split sphere press has led to the commercial HPHT (high pressure high temperature) synthesis of bulk gem diamonds from graphite and to the enhancement of low quality natural diamonds. Synthesized and treated diamonds are sometimes traded deceptively as high quality natural diamonds because it is hard to distinguish among these diamonds with conventional gemological characterization methods. Therefore, we need to develop a new identification method that is non-destructive, fast, and inexpensive. We proposed using new methods of UV fluorescence and X-ray Lang topography for checking the local HPHT stress field as well as using a vibrating sample magnetometer for checking ferromagnetic residue in synthesized diamonds to distinguish these diamonds from natural ones. We observe unique differences in the local stress field images in synthesized and treated diamonds using Lang topography and UV fluorescence characterization. Our result implies that our proposed methods may be appropriate for identification of the synthesized and treated diamonds.

Crystallized Nano-thick TiO2 Films with Low Temperature ALD Process

ZnO thin films were deposited on Si(100) substrates at low temperatures (44°C~210°C) by atomic layer deposition using DEZn (diethyl zinc) and water as precursors. The film thickness was measured by ellipsometry calibrated with cross-sectional TEM. The phase formation, microstructure evolution, UVabsorbance, and chemical composition changes were examined by XRD, SEM, AFM, TEM, UV-VIS-NIR, and AES, respectively. A uniform amorphous ZnO layer was formed even at 44°C while stable crystallized ZnO films were deposited above 90°C. All the samples showed uniform surface roughness below 3 nm. Fully crystallized ZnO layers with a band-gap of 3.37 eV without carbon impurities can be formed at substrate temperatures of less than 90°C. (Received May 26, 2010)