Jaehyeong Lee

Growth and properties of the Cd1−xZnxS thin films for solar cell applications

Cd1−xZnxS (0≤x≤1) thin films have been prepared by the co-evaporation of CdS and ZnS. When the ZnS mole ratio was less than 0.85, the crystal structure of Cd1−xZnxS films was hexagonal with the c axis aligned perpendicular to the substrate. For x>0.85, however, the Cd1−xZnxS films were grown with cubic zincblende structure. As the ZnS mole ratio increased, the lattice constant of Cd1−xZnxS films decreased. The optical band gap of Cd1−xZnxS films varies from 2.41 eV for CdS to 3.48 eV for ZnS with x. The open circuit voltage of Cd1−xZnxS/CdTe solar cells increased with x due to reducing of the electron affinity difference between Cd1−xZnxS and CdTe films, having approximately 830 mV of the maximum value at x=0.35.

Electrical and optical properties of CdTe films prepared by vacuum evaporation with close spacing between source and substrate

CdTe films for solar cell application were prepared at various growth temperatures by a vacuum evaporation system having close spacing between the source and substrate. The CdTe thin films had a cubic structure highly oriented with the (111) direction perpendicular to the substrate surface, regardless of the growth temperature. The crystallites are of random shape and reach up to about 2 μm in size with increasing growth temperature. The higher growth temperature contributed to the reduction of dark resistivity from 6 × 10 7 Ωcm at room temperature to 5.4 × 10 6 Ω cm at 300°C. The photovoltaic properties of the CdS/CdTe solar cell were considerably improved with the increase in the growth temperature, which was caused by the increase of (111) texture and grain size in CdTe films.

Characteristics of the CdZnS thin film doped by indium diffusion

Abstract Effects of the thickness of indium films and the annealing temperature on structural, optical and electrical properties of chemically deposited CdZnS films were investigated. The diffusion process of evaporated indium was carried out by heating the sample in air at 150–550 °C for 1 h. X-Ray diffraction patterns of CdZnS films indicate that the minimum thickness and annealing temperature for the formation of an In 2 O 3 layer, which acts as a barrier preventing the out-diffusion of indium and gives a high optical transmittance, are 20 nm and 350 °C, respectively. As the thickness of indium film and the annealing temperature increase, the conductivity of CdZnS films improves and the lowest resistivity of 0.3 Ω-cm is attained for CdZnS films with a 40 nm indium coating and annealed at 450 °C.

Influence of SnO2:F/ZnO:Al bi-layer as a front electrode on the properties of p-i-n amorphous silicon based thin film solar cells

The effect of aluminum doped zinc oxide film used between a fluorine doped tin oxide layer and a hydrogenated amorphous silicon carbide layer to improve the open circuit voltage (Voc) and fill factor (FF) for high efficiency thin film solar cells. The efficiency enhancement was accomplished by the insertion of high work-function layers engineered in the interfaces to raise FF as well as Voc. Therefore, we were able to obtain the conversion efficiency of 10.34% at 16.14 mA/cm2 of the current density (Jsc) and 70.37% of FF.

Effects of the Growth Temperature on the Properties of CdTe Thin Films for Solar Cell Applications

CdTe films for solar cell application were prepared at various growth temperatures by a vacuum evaporation system having close spacing between the source and substrate. The CdTe thin films had a cubic structure highly oriented with the (111) direction perpendicular to the substrate surface, regardless of the growth temperature. The crystallites are of random shape and reach up to about 2 µm in size with increasing growth temperature. The higher growth temperature contributed to the reduction of dark resistivity from 6 ×107 Ωcm at room temperature to 45.4 ×106 Ωcm at 300°C. The photovoltaic properties of the CdS/CdTe solar cell were considerably improved with the increase in the growth temperature, which was caused by the increase of (111) texture and grain size in CdTe films.

Influence of Surface Texturing on the Electrical and Optical Properties of Aluminum Doped Zinc Oxide Thin Films

An aluminum doped zinc oxide (AZO) film for front contacts of thin film solar cells, in this work, were deposited by r.f. magnetron sputtering, and then etched in diluted hydrochloric acid solution for different times. Effects of surface texturing on the electro-optical properties of AZO films were investigated. Also, to clarify the light trapping of textured AZO film, amorphous silicon thin film solar cells were fabricated on the textured AZO/glass substrate and the performance of solar cells were studied. After texturing, the spectral haze at the visible range of 400 ~750 nm increased substantially with the etching time, without a change in the resistivity. The conversion efficiency of amorphous Si solar cells with textured AZO film as a front electrode was improved by the increase of short-circuit current density (J sc ), compared to cell with flat AZO films.

Influence of Sputter Power on the Structural and Optical Properties of CdS Films for Photovoltaic Applications

CdS films have been prepared on polycarbonate, polyethylene terephthalate, and Coming 7059 substrates by r.f magnetron sputtering technique at room temperature. A comparison of the properties of the films deposited on polymer and glass substrates was performed. In addition, the influence of the sputter power on the structural and optical properties of these films was evaluated. The XRD measurements revealed that CdS films were polycrystalline and retained the mixed structure of hexagonal wurtzite and cubic phase, regardless of substrate types. As the sputter power was increased from 75 to 150 Watt, the structure of CdS films was converted from the mixed of hexagonal and cubic phase to hexagonal phase. The morphology of CdS films is found to be continuous and dense. Also, the grain of CdS films is larger with increasing the sputter power. The average transmittance exceeded 80 % in the visible spectrum for all films and decreases slightly with the sputter power.

Incorporation of Si and Zr into Pure HfO2 and Its Effects on Dielectric Integrity

Hafnium-silicate (HfSiO4, (HfO2)x(SiO2)1-x) and hafnium-zirconate (HfZrO4, (HfO2)x(ZrO2)1-x) films were employed as a gate dielectric to enhance the electrical properties of pure HfO2. (HfO2)x(SiO2)1-x and (HfO2)x(ZrO2)1-x films were formed onto p-Si substrates with varying degrees of Hf content x (x = 1, 0.9, 0.7, and 0.5) via solution processing. With regard to (HfO2)x(SiO2)1-x, the leakage current decreased from 1.94 × 10-8 to 4.29 × 10-9 A/cm2 at a gate voltage of VG = -1 V when the HfO2 content was reduced. These resulted from the reduction of leakage paths through the interface between HfSiO4 and Si substrate. Additionally, (HfO2)x(ZrO2)1-x exhibited the lowest interfacial trap density of 3.4 × 1011 cm-2 eV-1 for x = 0.5 due to a reduction in root mean square (RMS) roughness of the film from 6.0 to 4.2 nm. From the results, it was found that (HfO2)0.5(SiO2)0.5 demonstrated excellent oxide integrity in contact with Si substrates, whereas (HfO2)0.5(ZrO2)0.5 demonstrated an enhanced film morphology and maintained a high dielectric constant value. Finally, the HfZrO4/HfSiO4/Si structure revealed a gate oxide with enhanced integrity compared to pure HfO2-based devices.

Characteristics of Sputtered Cr Thin Films and Application as a Working Electrode in Transparent Conductive Oxide-Less Dye-Sensitized Solar Cells

Cr metal electrode was suggested as the working electrode material to fabricate DSSCs without the TCO, and thin films were fabricated by an unbalanced magnetron sputtering system. The surface morphologies show uniform and smooth surfaces regardless of various film thicknesses, and the small crystallites of various sizes were showed with the vertical direction on the surface of Cr thin films with the increase of film thickness. And also, the root mean square (RMS) surface roughness value of Cr thin films increased, and the sheet resistance is decreased with the increase of film thickness. The maximum cell efficiency of the TCO-less DSSC was observed when a Cr working electrode with a thickness of 80 nm was applied to the TCO-less DSSC. Consequently, these results are related to the result of the optimization of conduction characteristics, transmission properties and surface properties of Cr thin films.

Tribological and Electrical Properties of Diamond-Like Carbon Films Deposited by Filtered Vacuum Arc Method for Medical Guidewire Application

A good medical guidewires are used to introduce stents, catheters, and other medical devices inside the human body. In this study, diamond-like carbon (DLC) film was proposed to solve the poor adhesion problem of guidewire and to improve the tribological performance of guidewire. DLC films were fabricated on Si substrate by using FVA (Filtered Vacuum Arc) method. In this work, the tribological, structural, and electrical properties of the fabricated DLC films with various arc currents were experimentally investigated. All DLC films showed smooth and uniform surface with increasing applied arc current. The rms surface roughness was increased and the value of contact angle on the film surface was decreased with increasing arc current. The hardness and elastic modulus of DLC films were improved, and the resistivity value of DLC films were decreased with increasing arc current. These results are associated with ion bombardment effects by the applied arc current and bias voltage.