Hyeonseok Lee

The study of electrical conductivity and diffusion behavior of water-based and ferro/ferricyanide-electrolyte-based alumina nanofluids.

Nanofluids are liquids containing suspensions of solid nanoparticles and have attracted considerable attention because they undergo substantial mass transfer and have many potential applications in energy technologies. Most studies on nanofluids have used low-ionic-strength solutions, such as water and ethanol. However, very few studies have used high-ionic-strength solutions because the aggregation and sedimentation of nanoparticles cause a stability problem. In this study, a stable water-based alumina nanofluid was prepared using stirred bead milling and exhibits a high electrical conductivity of 2420 μS/cm at 23 °C and excellent stability after five severe freezing-melting cycles. We then developed a process for mixing the water-based nanofluid with a high-ionic-strength potassium ferro/ferricyanide electrolyte and sodium dodecyl sulfate by using stirred bead milling and ultrasonication, thus forming a stable electrolyte-based nanofluid. According to the rotating disk electrode study, the electrolyte-based alumina nanofluid exhibits an unusual increase in the limiting current at high angular velocities, resulting from a combination of local percolation behavior and shear-induced diffusion. The electrolyte-based alumina nanofluid was demonstrated in a possible thermogalvanic application, since it is considered to be an alternative electrolyte for thermal energy harvesters because of the increased electrical conductivity and confined value of thermal conductivity.

Phase identification of RF-sputtered SnS thin films using rietveld analysis of X-ray diffraction patterns

Tin monosulfide (SnS) is a promising material for a photovoltaic absorber layer. Significant strides have been taken to better understand its material properties. The X-ray diffraction patterns of radio-frequency sputtered SnS thin films are investigated. Samples were deposited under varying total pressure, target power, substrate-to-target distance, and substrate temperature. Rietveld refinement of samples deposited under varying conditions yielded evidence of multiple phases present in SnS thin films. Refinements were completed with one or more tin sulfide phases, showing a dominant herzenbergite SnS phase (Pbnm). Possible secondary phases include orthrhombic (Cmcm) and cubic (Fm3m) crystal structures. Lattice parameters, cell volume, and unit cell density were investigated as a function of deposition conditions. Results indicate that growth mode is related to deposition rate. Early studies of heated stage depositions showed that SnS thin films have added mobility at the substrate.

Engineered optical and electrical performance of rf–sputtered undoped nickel oxide thin films for inverted perovskite solar cells

Inverted perovskite solar cells incorporating RF sputtered NiO thin films as a hole transport layer and window layer are demonstrated. The electrical and optical properties of the NiO thin films are engineered using varied sputtering conditions. The localized states within bandgap owing to its crystal disorder and nonstoichiometric features affect the transmittance and the optical bandgap of the NiO thin films which in turn influences the Jsc of the perovskite solar cells. In addition, the electrical properties of the NiO thin films can be also varied during sputtering condition affecting the concentration of nickel vacancies and the resulting hole concentration. The conductivity largely originates from the hole concentration relating to the density of states in the NiO thin films which influence the fill factor (FF) of the solar cells. The solar cells fabricated with the NiO thin films made at 4 Pa of deposition pressure show highest performance owing to excellent transmittance and wider bandgap along with moderate conductivity. With further optimization, the perovskite solar cells exhibit ~20 mA/cm2 of Jsc and a 12.4% PCE (11.3% of averaged PCE).

Investigation of RF-sputtered tin sulfide thin films with in situ heating for photovoltaic applications

Tin (II) Monosulfide (SnS) is of increasing interest to researchers due to its near-optimal optoelectronic properties for photovoltaic devices. In this work, we take a new approach using a Tin (IV) Disulfide target to sputter SnS thin films. Sulfur-rich SnS thin films are produced via in situ heating of the substrate. Experimentation with substrate heating has yielded two unique crystal structures, depending on constant or “pulsed” heating. Standardless Electron Dispersive Spectroscopy measurements indicate that the films are nearly 1:1 Sn:S ratio. Films with low resistivity (<;100 Ω-cm) were produced via this method. These results are promising for improving SnS-based photovoltaic device performance.

Investigation of the absorption properties of sputtered tin sulfide thin films for photovoltaic applications

Tin sulfide (SnS) is an absorber with promising optoelectronic properties and low environmental constraints of interest for high efficiency solar cells. Sputtered SnS thin films were deposited at target powers 105-155 W and total pressures of 5 to 60 mTorr in argon. X-ray diffraction patterns confirmed a dominant tin monosulfide phase. The absorption coefficient was determined by spectroscopic ellipsometry and unpolarized spectrophotometry measurements. Both methods show that the films have absorption coefficients above the band gap in the range of 105-106 cm-1.