Sie Young Choi

Fabrication and Characteristics of mc-Si Solar Cells with RIE-Textured Surface

Reactive ion etching (RIE) texturing is well-known as an effective method to form the surface structure on a multi-crystalline (mc-Si) wafer that has grains with randomly oriented crystallites. The saw damage removal (SDR) process using HF/HNO3/D.I (HND) solution was employed in this work, since the etching rate of RIE dry etching was lower than that of wet etching. The surface morphology on mc-Si surface was formed by RIE using a gas flow ratio of SF6:O2 = 1:1.22. The control of RF power and working pressure could etch the mc-Si surface of the 15.6 × 15.6 cm2 area uniformly during RIE texturing process. The surface morphologies textured for 5 and 10 min were needle-like structures and sharp grass-like structures, respectively. Solar cells with the needle-like structure had higher values for open circuit voltage (Voc), short circuit current (Isc), fill factor (FF), and efficiency, despite higher reflectance compared to those with the sharp grass-like structure. The cell textured for 10 min was expected to have non-homogeneous emitter layer as the dark I-V curves of the cells textured for 5 and 10 min were compared.

Vacuum-controlled wafer-level packaging for micromechanical devices

A vacuum-controlled wafer-level packaging process for micromechanical devices was developed. The process includes a thick titanium deposition process and a vacuum anodic bonding process which is performed in argon ambient unlike conventional process which is carried out in nitrogen ambient. Because the thick getter film absorbs most of oxygen molecules released during the bonding, but does not absorb argon at all, the vacuum level in the package is controlled by only adjusting argon pressure. To estimate the vacuum level, a quality factor is obtained from the frequency response of the packaged device. When the devices are packaged in argon ambient at a pressure of 5 × 10−2 Torr, the average value of the quality factors is about 5000. This shows that the vacuum levels of the packaged devices are almost same as the initial argon setting pressure, considering the correlation between the quality factor and the vacuum level which is investigated by a preliminary experiment. The vacuum-level controllability of the proposed vacuum packaging process was verified repeatedly under other ambient pressure conditions. Durability testing was done more than 1000 h, but no significant variation in the quality factor was observed.

Effect of Valeric Acid as a Co-adsorbate on the Performance of Dye-Sensitized Solar Cells

We investigated effects of valeric acid (VA) as a co-adsorbate on the photovoltaic performance of dye-sensitized solar cells (DSSCs). When the introduction of VA onto TiO2surface was performed after dye (N719) adsorption, the DSSC with VA showed an increase in short-circuit current (Jsc ), open-circuit voltage (Voc ) and fill factor, resulting in a power conversion efficiency of 7.27%, compared to that (6.20%) of reference device without VA. Incorporation of VA on free TiO2surface induced longer lifetime of electrons injected from excited dyes to conduction band of TiO2, leading to the increase of electron collection efficiency and the suppression of charge recombination between injected electrons and I3 −ions, and thus larger Jsc and Voc , respectively.

Fabrication, and Characteristics of Pin-Type a-SiGe:H Thin-Film Solar Cells with a-Si:H Buffer and Graded Absorption Layer

We have investigated the characteristics on hydrogenated amorphous silicon (a-Si:H) based solar cells of the various structures between the doped layers. Through basic experiments about p-i-n solar cells of different structures (i-a-Si:H layer, constant-gap i-a-SiGe:H layer, graded-gap i-a-SiGe:H layer and a-Si:H buffer layer at p/i interface), found that each cell has different advantages. Based on these results, we proposed the structure of a-Si:H buffer/graded absorption layer between the doped layers to improve the performance of hydrogenated amorphous silicon-germanium (a-SiGe:H) based p-i-n solar cell. The proposed structure has advantages to reduce the absorption losses for longer wavelengths and dopant penetration to i-layer. In the proposed structure, we achieved a higher open-circuit voltage (Voc: 485 mV) and fill factor (FF: 0.57) than general a-SiGe:H solar cells.

Characteristics of Amorphous Silicon Thin-Film Solar Cells of a-Si:H/a-SiGe:H Superlattices in Different Thickness for Barrier and Well Layers

Amorphous silicon (a-Si:H)/amorphous silicon-germanium (a-SiGe:H) superlattices were deposited by 13.56 MHz plasma enhanced chemical vapor deposition (PECVD) method using a mixture of SiH4, GeH4and H2. The superlattice materials consist of alternating layers of a-Si:H and a-SiGe:H. The a-Si:H layers were used as barrier material and a-SiGe:H layers were used as well material. It was found that the optical bandgap was controlled by changing the thickness of the barrier layer of a-Si:H (2–10 nm). Based on these results, fabricated the pin-type a-Si:H based solar cells of superlattice structures in different thickness for barrier and well layers. The various values of open-circuit voltage (Voc), short-circuit current density (Jsc), and conversion efficiency were measured under 100 mW/cm2(AM 1.5) solar simulator irradiation. In the fabricated structure, we achieved a higher conversion efficiency (η = 4.52%) than general a-Si:H solar cell (η = 2.10%) and a-SiGe:H solar cell (η = 3.06%).

Study of the p-i-n Layer to Enhance a-Si:H Solar Cell Efficiency Based on Single Junction

We investigated the optimum thickness of the p-, i-, and n-layers to improve transmittance of the p-layer, the absorbance of the i-layer and the recombination rate of the n-layer. a-Si:H-based solar cells with different thicknesses for each layer were fabricated using plasma enhanced chemical vapor deposition (PECVD) and their performance parameters were compared. The optical properties of the intrinsic layer (i-layer) and the optimum hydrogen content in the i-layer were investigated and analyzed using UV/Vis/NIR and FT-IR, respectively. The optimum thicknesses of the p-, the i-, and the n-layer represented about 300, 2000, and 600 Å, respectively. The maximum absorbance and the minimum transmittance were measured at an i-layer thickness of 2000 Å in the wavelength range between 400 and 800 nm. The FT-IR measurements showed the optimum hydrogen content in the i-layer was about 10.1 at.%. We verified that the thickness of each layer and the hydrogen content had greatly influenced the electrical properties of the fabricated cells.

Wafer-Level Packaged MEMS Resonators with a Highly Vacuum-Sensitive Quality Factor

Mechanical stress and the vacuum level are the two main factors dominating the quality factor of a resonator operated in the vacuum range 1 mTorr to 10 Torr. This means that if the quality factor of a resonator is very insensitive to the mechanical stress in the vacuum range, it is sensitive to mainly the ambient vacuum level. In this paper, a wafer-level packaged MEMS resonator with a highly vacuumsensitive quality factor is presented. The proposed device is characterized by a package with out-of-plane symmetry and a suspending structure with only a single anchor. Out-of-plane symmetry helps prevent deformation of the packaged device due to thermal mismatch, and a single-clamped structure facilitates constraint-free displacement. As a result, the proposed device is very insensitive to mechanical stress and is sensitive to mainly the ambient vacuum level. The average quality factors of the devices packaged under pressures of 50, 100, and 200 mTorr were 4987, 3415, and 2127, respectively. The results demonstrated the high controllability of the quality factor by vacuum adjustment. The mechanical robustness of the quality factor was confirmed by comparing the quality factors before and after hightemperature storage. Furthermore, through more than 50 days of monitoring, the stability of the quality factor was also certified.

Effect of a-SiN:H Thin Film Deposited by PE/RACVD on a-Si:H Thin Film Transistor

Electron mobility of a hydrogenated amorphous silicon thin film transistor (a-Si:H TFT) a is important factor for a active matrix display device. The electron mobility of a-Si:H TFT is about 0.5 cm2/V·sec in current technology. If the electron mobility the of a-Si:H TFT increases to 1 ∼ 2 cm2/V·sec, TFT-LCD and OLED display can have drive IC within the panel. High-resolution display also can be made. Hydrogenated amorphous silicon nitride (a-SiN:H) thin films, as dielectric layer of hydrogenated amorphous silicon thin film transistor (a-Si:H TFT), were deposited by radical assisted chemical vapor deposition (RACVD) and plasma enhanced chemical vapor deposition (PECVD). Interface roughness between a-SiN:H and a-Si:H is important to improve electron mobility in a-Si:H TFT. We compared surface roughness of a-SiN:H thin films deposited by RACVD and PECVD and investigated field effect mobility of a-Si:H TFTs using a-SiN:H thin film deposited by RACVD and PECVD.

Copper Metalization Using Electroplating Process

Recently, electroplating method which deposits copper films by using cupric sulfate solution, has been proposed. The electroplating method has the advantages of simplicity, safety, low cost, low deposition temperature, high purity, low resistivity, and high capability of gap filling. In this method, an electrical contact is made to the seed layer and a current is passed such that the reaction ⌊Cu2++2e → Cu(0)⌋ occurs at the wafer surface (cathode). At the anode, an oxidation reaction occurs which balances the current flow at the cathode, thus maintaining electrical neutrality in the cupric sulfate solution. The seed layer used in this experiment was a copper thin film deposited using the sputtering method on p-type (100) silicon wafer using Ta thin film for barrier material. The thickness of seed and barrier layer were 400 and 600 A respectively. The electroplating process was performed for 5min with a cathode current of 2A and a electrode distance of 4 cm. The intensity of magnetic field was about 200 – ...

Effect of phosphorus doping on the performance of pin-type a-Si:H thin-film solar cells

ABSTRACT This article shows the characteristics of pin-type a-Si:H thin-film solar cells with various PH3 concentrations in the i-layer. A series of phosphorus doped a-Si:H films were fabricated by mixing PH3 and SiH4 with H2 during the i-layer deposition. The concentration of PH3 was varied in the range of 0–3650 ppm. By phosphorus doping of the i-layer, the properties of the i-layer were changed light doped n layer (n-), and we were able to improve all these electrical parameters (Voc, Jsc, and FF). Consequently, we achieved a higher conversion efficiency η than in conventional pin-type a-Si:H solar cells with undoped i-layer.