Junyoung Choi

Properties of cadmium sulfide thin films deposited by chemical bath deposition with ultrasonication

Ultrasonic agitation was applied during the chemical bath deposition of CdS thin films. Ultrasonication resulted in a dramatic change in the surface morphology, growth rate, and optical properties of CdS films. There were virtually no colloidal particles adsorbed on the surface. The surface roughness measured by atomic force microscopy was reduced by a factor of two. Band gap energy increased to 2.39 eV from 2.37 eV. X-ray patterns showed that the preferred orientation changed from hexagonal (002)/cubic (111) to hexagonal (101). Optical transmission improved in the wavelength range larger than 520nm. The chemical reaction for CdS formation started at a lower temperature under ultrasonication, and dense films were obtained even when the chemical composition of the aqueous solution deviated far from optimum conditions.

17.6% Conversion Efficiency Multicrystalline Silicon Solar Cells Using the Reactive Ion Etching with the Damage Removal Etching

For lower reflectance, we applied a maskless plasma texturing technique using reactive ion etching (RIE) on acidic-textured multicrystalline silicon (mc-Si) wafer. RIE texturing had a deep and narrow textured surface and showed excellent low reflectance. Due to plasma-induced damage, unless the RIE-textured surfaces have the proper damage removal etching (DRE), they have a drop in and FF. RIE texturing with a proper DRE had sufficiently higher short circuit current than acidic-textured samples without a drop in open circuit voltage . And in order to improve efficiency of mc-Si solar cell, we applied RIE texturing with optimized DRE condition to selective emitter structure. In comparison with the acidic-textured solar cells, RIE-textured solar cells have above 200 mA absolute gain in Isc. And optimized RIE samples with a DRE by HNO3/HF mixture showed 17.6% conversion efficiency, which were made using an industrial screen printing process with selective emitter structure.

The potential efficiency of laser doped solar cells using photoluminescence imaging

In this work we evaluate the implied open circuit voltages (iVoc) at various stages of production of laser doped selective emitter (LDSE) solar cells using photoluminescence (PL) imaging The change of the iVoc values after the laser doping (LD) has shown that this process has minimal effect on cell performance. It is also found that the order of processes such as edge junction isolation and silicon nitride is of critical importance prior to light induced plating. If these processes are not performed in the correct order, localised shunts may form during the light induced plating (LIP) process which then inhibits plating near the edge of the cells. In this work, an impressive efficiency of 18.3% has been achieved, even though the fill factor was only 74.2% and the cell suffered from additional shading losses due to overplating. With the optimization of the LD and metallization processes cells have the potential to reach efficiencies of more than 19.2% on full size commercial substrates.

Improved front side metallization for laser doped solar cells

A simple but effective way to create a selective emitter is by using laser doping (LD) to selectively remove the anti-reflection coating (ARC) layer and simultaneously melt the silicon underneath it while incorporating dopants into the melted region, creating a heavily doped layer. In conjunction with LD, a plating technique is often used to selectively plate self-aligned metal contacts onto the laser doped lines. For light induced plating (LIP), the significant advantage compared to ordinary electroplating processes is that no electrical contacts are required to be connected to the front side metal grid as the illuminated solar cell itself generates the required current serving as both the cathode and anode of the reaction. This is also the reason why this plating technique has the potential to provide a very uniform layer provided a reasonably uniform light source is used to illuminate the cell surface. In this paper, we have improved the front side metallization for laser doped solar cell via a pre-treatment prior to Ni plating. The cells with homogenous Ni film have higher FF, so we have obtained over 19% conversion efficiency.

An efficiency of 18.88% in selective emitter solar cells by metal pattern optimization

A high efficiency (η∼18.88 %) mono-crystalline Si solar cell (Area=15.6 × 15.6 cm2) with a selective emitter structure used in the conventional diffusion process (POCl3) and the screen printed contact is presented in this paper. In particular, we focus on the improvement of fill factor (FF) by the front metal Ag portion and aspect ratio optimization. As a result, the front metal portion of 5.15% (printing thrice) represents the best in the FF of 77.68%. The fabricated solar cell with a selective emitter has the portion of high doping concentration (30 ohm/sq.) to just below the metal contact and the shallow emitter (80 ∼ 90 ohm/sq.) of the rest of contacts. As a result, our selective emitter solar cell is accomplished by the Eff 18.88% when the front metal portion of 5.15% is applied during its printing thrice.

Improvements of Voc by selective emitter pattern optimization in screen printed crystalline Si solar cells

We present a high efficiency (η∼18.5%) mono-crystalline Si solar cells (A=15.6×15.6cm2) with a selective emitter structure using a conventional diffusion process (POCl3) and screen printed contact. Especially, in this paper, we focus on the improvement of Voc by selective emitter pattern optimization. As a result, we improve about 4mV Voc as a function of heavy emitter area, investigate the cause of the Voc increase by internal quantum efficiency and life time. The fabricated solar cell with selective emitter has accomplished Voc 635.78 mV, Jsc 37.03 mA/cm2, FF 78.35%, Eff 18.45%. The result shows improvement of Voc 8 mV, Jsc 0.57 A/cm2, Eff 0.5 % in comparison with the reference cell(homogeneous emitter structure) and improvement of Voc 3.7 mV, Eff 0.1 % as a function of heavy emitter size.

Fabrication of screen-printed multi-crystalline silicon solar cells exceeding 16% efficiency using double layer anti-reflective coatings

Aside from having very good optical properties, the good ARC needs passivation effect (Si dangling bonds reduced) and thermal stability (during the firing of screen-printed metal contact). SiNx and SiO2 are the most popular films as ARC and have been applied on the silicon solar cell for a long time. In order to prevent disadvantage like thermal degradation and combine the benefits (low reflectance, and good passivation) of the SiNx and SiO2, the ARC was modified to incorporate a thermally grown thin SiO2 layer beneath the PECVD SiNx. The SiO2/SiNx double layer ARC has a better output performance in particular Voc and FF than SiNx single layer. The Si/SiO2 interface is grown into the silicon crystal at a relatively high temperature, and the subsequent very effective hydrogenation of dangling bond interface states during the SiNx deposition. The gain of FF is predicted due to fewer hole defects in the SiO2/SiNx double ARC layer. The surface passivation by thermally grown SiO2 and PECVD SiNx double ARC is more effective than the SiNx single layer. Adding only a short oxidation process, we can have a +0.4% improved conversion efficiency of screen-printed mc-Si solar cell.

Influence of the ultrasonic agitation on chemical bath deposition of cadmium sulfide thin films

Ultrasonic agitation was applied during the chemical bath deposition of CdS thin films. Ultrasonication resulted in a large difference in surface morphology, growth rate, and optical properties of CdS films. Virtually there were no colloidal particles adsorbed on the surface. The surface roughness measured by atomic force microscopy was reduced by a factor of two. Band gap energy increased to 2.39eV from 2.37eV. X-ray patterns !showed that the preferred orientation changed from hexagona1(002)/cubic(111) to hexagonal(lO1). Optical transmission improved in the longer wavelength range larger than 520nm. The chemical reaction for CdS formation started at a lower temperature under ultrasonication, and dense films were obtained even when the chemical composition of the aqueous :solution was far deviated from the optimum conditions.