• Current Photovoltaic Research
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• v.2 no.3
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• pp.115-119
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• 2014

$Cu(In,Ga)Se_2$ (CIGS) thin films have been used as a light absorbing layer in high-efficiency solar cells. In order to improve the quality of the CIGS thin film, often selenization step is applied. Especially when the thin film was formed by non-vacuum powder process, selenization can help to induce grain growth of powder and densification of the thin film. However, selenization is not trivial. It requires either the use of toxic gas, $H_2Se$, or expensive equipment which raises the overall manufacturing cost. Herein, we would like to deliver a new, simple method for selenization. In this method, instead of using a costly two-zone furnace, use of a regular tube furnace is required and selenium is supplied by a mixture of selenium and ceramic powder such as alumina. By adjusting the ratio of selenium vs. alumina powder, selenium vaporization can be carefully controlled. Under the optimized condition, steady supply of selenium vapor was possible which was evidently shown by large grain growth of CIGS within a thin powder layer.

• 한국신재생에너지학회:학술대회논문집
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• 2009.11a
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• pp.400-400
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• 2009

Despite the success of Cu(In,Ga)$Se_2$ (CIGS) based PV technology now emerging in several industrial initiatives, concerns about the cost of In and Ga are often expressed. It is believed that the cost of those elements will eventually limit the cost reduction of this technology. One candidate to replace CIGS is $Cu_2ZnSnSe_4$ (CZTSe), fabricated by co-evaporation technique. Co-evaporation technique will be one of the best methods to control film composition. This type of absorber derives from the $CuInSe^2$ chalcopyrite structure by substituting half of the indium atoms with zinc and other half with tin. Energy bandgap of this material has been reported to range from 0.8eV for selenide to 1.5eV for the sulfide and large coefficient in the order of $10^{14}cm^{-1}$, which means large possibility of commercial production of the most suitable absorber by using the CZTSe film. In this work, Effects of substrate temperature of $Cu_2ZnSnSe_4$ absorber layer on the performance of thin films solar cells were investigated. We reported on some of the absorber properties and device results.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.97-97
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• 2013

Recently, Cu2ZnSn(S,Se)4 (CZTSS), which is one of the In- and Ga- free absorber materials, has been attracted considerable attention as a new candidate for use as an absorber material in thin film solar cells. The CZTSS-based absorber material has outstanding characteristics such as band gap energy of 1.0 eV to 1.5 eV, high absorption coefficient on the order of 104 cm-1, and high theoretical conversion efficiency of 32.2% in thin film solar cells. Despite these promising characteristics, research into CZTSS based thin film solar cells is still incomprehensive and related reports are quite few compared to those for CIGS thin film solar cells, which show high efficiency of over 20%. I will briefly overview the recent technological development of CZTSS thin film solar cells and then introduce our research results mainly related to sputter based process. CZTSS thin film solar cells are prepared by sulfurization of stacked both metallic and sulfide precursors. Sulfurization process was performed in both furnace annealing system and rapid thermal processing system using S powder as well as 5% diluted H2S gas source at various annealing temperatures ranging from $520^{\circ}C$ to $580^{\circ}C$. Structural, optical, microstructural, and electrical properties of absorber layers were characterized using XRD, SEM, TEM, UV-Vis spectroscopy, Hall-measurement, TRPL, etc. The effects of processing parameters, such as composition ratio, sulfurization pressure, and sulfurization temperature on the properties of CZTSS absorber layers will be discussed in detail. CZTSS thin film solar cell fabricated using metallic precursors shows maximum cell efficiency of 6.9% with Jsc of 25.2 mA/cm2, Voc of 469 mV, and fill factor of 59.1% and CZTS thin film solar cell using sulfide precursors shows that of 4.5% with Jsc of 19.8 mA/cm2, Voc of 492 mV, and fill factor of 46.2%. In addition, other research activities in our lab related to the formation of CZTS absorber layers using solution based processes such as electro-deposition, chemical solution deposition, nano-particle formation will be introduced briefly.

• 한국신재생에너지학회:학술대회논문집
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• 2010.06a
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• pp.62.1-62.1
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• 2010

CIS(CuInSe2)계 화합물 태양전지는 높은 광흡수계수와 열적 안정성 및 조성 조절을 통한 밴드갭 조절이 용이해 고효율 박막 태양전지로 각광 받고 있다. 또한 CIGS 태양전지는 기존의 유리기판 대신 유연한 기판을 사용해 flexible 태양전지 제조가 가능하다. 이러한 유연기판은 보통 stainless steel과 같은 금속 기판이 많이 사용되는데 기존의 soda-lime glass 기판과는 달리 금속기판에는 Na이 첨가되어 있지 않아 별도의 Na첨가를 필요로 한다. Na은 CIGS 흡수층의 조성조절을 용이하게 하여 태양전지의 변환 효율을 향상시키는 역할을 한다. 본 연구에서 기판은 Na이 첨가되어있지 않은 corning glass를 사용 하였으며 NaF를 이용해 Mo가 증착된 기판에 NaF의 두께를 달리하며 증착해 CIGS 흡수층의 grain 사이즈를 비교 하였으며 그 후 태양전지 소자를 제조해 광전특성을 분석하였다. 후면 전극으로 약60nm 두께의 Mo를 DC Sputtering 방법을 이용해 증착 하였다. buffer층으로는 약 50nm의 CdS층을 CBD방법을 이용하여 제조 하였으며 TCO 층으로 약 50nm의 i-ZnO와 약 450nm의 Al-ZnO를 RF Sputtering방법으로 증착 하였다. 마지막으로 앞면 전극으로 약 $1{\mu}m$의 Al을 Thermal Evaporation방법으로 증착하였다. 태양전지 소자의 면적은 $0.49cm^2$로 효율을 비교 분석하였다.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.26 no.9
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• pp.701-706
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• 2013

We have focused on the conversion efficiency of CIGS thin film solar cell prepared by co-evaporation method as well as the optimization of process condition. The total thickness of back electrode was fixed at 1 ${\mu}m$ and the structural, electric and optical properties of CIGS thin film were investigated by varying the thickness of Mo:Na bottom layer from 0 to 500 nm. From the experimental results, the content of Na was appeared as 0.28 atomic percent when the thickness of Mo:Na layer was 300 nm with compactly densified plate-shape surface morphology. From the XRD measurements, (112) plane was the strongest preferential orientation together with secondary (220) and (204) planes affecting to the crystallization. The lowest roughness and resistivity were 2.67 nm and 3.9 ${\Omega}{\cdot}cm$, respectively. In addition, very high carrier density and hole mobility were recorded. From the optimization of Mo:Na layer, we have achieved the conversion efficiency of 9.59 percent.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.459.2-459.2
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• 2014

CdTe계와 CGIS계 태양전지의 광투과층으로 CdS 박막이 많이 사용된다. Cds 박막의 필요한 물성으로는 높은 광투과도와 얇은 두께이다. 광투과층으로 사용되는 CdS 막의 광투과도가 높아야 많은 양의 빛이 손실 없이 투과하여 광흡수층인 CIGS에 도달할 수 있다. 특히, CdS막의 두께가 얇으면 밴드 갭 이상의 에너지를 가지는 파장의 빛도 투과시킬 수 있어 태양전지의 효율의 증가을 얻을 수가 있다. 그러나 CdS 막의 두께가 얇을 경우, pinhole이 생성되는 등 막의 균질성이 문제가 된다. 본 연구에서는 높은 변환 효율을 갖는 CIGS 박막 태양전지 제작에 적합한 chemical bath depostion(츙)법을 이용하여 CdS 박막을 제조하였다. 또한 반응시간, Cd 및 S source 비와 같은 증착 조건에 따른 박막의 특성을 조사하였다.

• Current Photovoltaic Research
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• v.5 no.4
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• pp.135-139
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• 2017

We fabricated two different transparent conducting oxide thin films of ZnO doped with Ga ($Ga_2O_3$ 0.9 wt%) as well as Al ($Al_2O_3$ 2.1 wt%) (GAZO) and ZnO doped only with Al ($Al_2O_3$ 3 wt%) (AZO). It was investigated how it affects the moisture resistance of the transparent electrode. In addition, $Cu(In,Ga)Se_2$ thin film solar cells with two transparent oxides as front electrodes were fabricated, and the correlation between humidity resistance of transparent electrodes and device performance of solar cells was examined. When both transparent electrodes were exposed to high temperature distilled water, they showed a rapid increase in sheet resistance and a decrease in the fill factor of the solar cell. However, AZO showed a drastic decrease in efficiency at the beginning of exposure, while GAZO showed that the deterioration of efficiency occurred over a long period of time and that the long term moisture resistance of GAZO was better.

• Korean Journal of Materials Research
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• v.20 no.8
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• pp.434-438
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• 2010

The selenization process has been a promising method for low-cost and large-scale production of high quality CIGS film. However, there is the problem that most Ga in the CIGS film segregates near the Mo back contact. So the solar cell behaves like a $CuInSe_2$ and lacks the increased open-circuit voltage. In this study we investigated the Ga distribution in CIGS films by using the $Ga_2Se_3$ layer. The $Ga_2Se_3$ layer was applied on the Cu-In-Ga metal layer to increase Ga content at the surface of CIGS films and to restrict Ga diffusion to the CIGS/Mo interface with Ga and Se bonding. The layer made by thermal evaporation was showed to an amorphous $Ga_2Se_3$ layer in the result of AES depth profile, XPS and XRD measurement. As the thickness of $Ga_2Se_3$ layer increased, a small-grained CIGS film was developed and phase seperation was showed using SEM and XRD respectively. Ga distributions in CIGS films were investigated by means of AES depth profile. As a result, the [Ga]/[In+Ga] ratio was 0.2 at the surface and 0.5 near the CIGS/Mo interface when the $Ga_2Se_3$ thickness was 220 nm, suggesting that the $Ga_2Se_3$ layer on the top of metal layer is one of the possible methods for Ga redistribution and open circuit voltage increase.

• 한국신재생에너지학회:학술대회논문집
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• 2009.06a
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• pp.85-87
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• 2009

Despite the success of $Cu(In,Ga)Se_2$ (CIGS) based PV technology now emerging in several industrial initiatives, concerns about the cost of In and Ga are often expressed. It is believed that the cost of those elements will eventually limit the cost reduction of this technology. one candidate to replace CIGS is $Cu_2ZnSnSe_4$ (CZTSe), fabricated by co-evaporation technique. Effects of substrate temperature of $Cu_2ZnSnSe_4$ absorber layer on the performance of thin films solar cells were investigated. As substrate temperature increased, the grain size of $Cu_2ZnSnSe_4$ films increased presumably. At a optimal condition of substrate temperature is $320^{\circ}C$, the solar cell shows a conversion efficiency of 1.79% with $V_{OC}$ of 0.213V, JSC of $16.91mA/cm^2$ and FF of 49.7%.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.389-389
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• 2012

Molybdenum is one of the most important materials used as a back ohmic contact for $Cu(In,Ga)(Se,S)_2$ (CIGS) solar cells because it has good electrical properties as an inert and mechanically durable substrate during the absorber film growth. Sputter deposition is the common deposition process for Mo thin films. Molybdenum thin films were deposited on soda lime glass (SLG) substrates using direct-current planar magnetron sputtering technique. The outdiffusion of Na from the SLG through the Mo film to the CIGS based solar cell, also plays an important role in enhancing the device electrical properties and its performance. The structure, surface morphology and electrical characteristics of Mo thin films are generally dependent on deposition parameters such as DC power, pressure, distance between target and substrate, and deposition temperature. The aim of the present study is to show the resistivity of Mo layers, their crystallinity and morphologies, which are influenced by the substrate temperature. The thickness of Mo films is measured by Tencor-P1 profiler. The crystal structures are analyzed using X-ray diffraction (XRD: X'Pert MPD PRO / Philips). The resistivity of Mo thin films was measured by Hall effect measurement system (HMS-3000/0.55T). The surface morphology and grain shape of the films were examined by field emission scanning electron microscopy (FESEM: Hitachi S-4300). The chemical composition of the films was obtained by the energy dispersive X-ray spectroscopy (EDX). Finally the optimum substrate temperature as well as deposition conditions for Mo thin films will be developed.