3.KH (242~248).hwp

Size: px

Start display at page:

Download "3.KH (242~248).hwp"

하나 서
5 years ago
Views:

1 Appl. Chem. Eng., Vol. 25, No. 3, June 2014, Review 정문현 김세열 유도혁 김중현 연세대학교화공 생명공학과 (2014 년 2 월 19 일접수, 2014 년 3 월 17 일심사, 2014 년 4 월 16 일채택 ) Materials and Characteristics of Emerging Transparent Electrodes Moon Hyun Chung, Seyul Kim, Dohyuk Yoo, and Jung Hyun Kim Department of Chemical & Biomolecular Engineering, Yonsei University, Seoul , Korea (Received February 19, 2014; Revised March 17, 2014; Accepted April 16, 2014) 정보 통신분야의발전에따라기존의전자기기들은평면성을벗어나투명 유연하고깨지지않는특성이요구되고있다. 이러한부가적인특성을갖춘기기들의제조를위해서는전극의투명성과유연성을동시에갖고있어야하지만, 현재가장대표적으로이용되는투명전극인 ITO (Indium Tin Oxide) 는유연하지못하다는단점과자원적인한계를갖고있다. 이에따라 ITO 의한계를극복하기위해다양한물질들을이용한대체재료개발이활발히연구되고있으며대체물질들의복합화를통해더향상된물성을발현시키기위한연구가진행되고있다. 본총설에서는 ITO 의한계를극복하고투명전극으로서의응용가능한대체물질들에대한연구현황을정리하였다. Flexibility of a transparent device has been required in accordance with miniaturization and mobilization needs in recent industry. The most representative material used as a transparent electrode is indium tin oxide (ITO). However, a couple of disadvantages of ITO are the exhaustion of natural resource of indium and its inflexibility due to inorganic substance. To overcome the limit of ITO, a variety of alternative materials have been researched on development of transparent electrodes and its properties through composite materials. In this review, we classify some of emerged materials with their general studies. Keywords: Transparent electrode, Transparent conducting oxide, Carbon materials, Conducting polymer, Metal nanowire 1) 1. 서론 현재통신및전자기기들은과학기술이급속도로발달해가면서점점소형화, 경량화가되어가고있으며, 유연성그외에도다양한기능을갖춘기기들이요구되고있다. 플렉시블디스플레이, 트랜지스터, 터치패널, 태양전지등과같은유연한전자기기를제작하기위해서는투명하면서유연성을갖는전극을사용해야하며이러한전극은 PET (polyethylene terephthalate), PES (polyethersulfone) 등과같은유연성을가진기판위에박막을형성하여높은전도도와가시광영역에서우수한투과도를나타내야한다. 또한, 면저항의값에따라안테나, 광학필터, 정전기방지막등으로활용이가능해응용분야가매우다양하다. 현재투명전극으로사용가능한차세대소재로투명전도성산화물 (transparent conducting oxide, TCO), 은나노와이어 (silver nanowire), 탄소나노튜브 (carbon nanotube, CNT), 그래핀 (graphene), 전도성고분자 (conducting polymer) 가연구되고있으며, 현재가장보편적으로사 Corresponding Author: Yonsei University Department of Chemical & Biomolecular Engineering, 50 Yonsei-ro, Seodaemun-gu, Seoul , Korea Tel: jayhkim@yonsei.ac.kr pissn: eissn: 2014 The Korean Society of Industrial and Engineering Chemistry. All rights reserved. 용되는투명전극으로는인듐주석산화물 (indium tin oxide, ITO) 박막이있다. 하지만, ITO박막은미래의전자기기에서유연한소재로의사용에있어서여러단점들을갖고있어사용되기는어려운실정이다 [1]. 이에 ITO박막의단점을극복하고대체하기위해앞서언급한차세대투명전극에대한연구가미래디스플레이산업의성장과함께경쟁적으로연구가진행되고있다. 또한, 제조공정의간소화를위해전극박막을형성하는방법을기존고온과진공조건이아닌잉크젯프린팅, 스프레이, 그라비아, 슬롯다이코팅등과같은상온, 상압조건에서롤투롤공정을적용하여투명전극을제작하는연구가활발히진행되고있다 [2]. 본총설에서는 ITO를대체할차세대투명전극으로써의응용가능한소재의종류와그의연구동향에대해집중적으로알아보고자한다. 2. 본론 2.1. Transparent conducting oxide (TCO) 투명전도성산화물인 TCO는가시광영역대에서높은투과율과전기전도성을동시에만족하기때문에태양전지나디스플레이등다양한소자의투명전극재료로사용되고인듐산화물 (In 2 O 3 ) 에주석산화물 (SnO 2 ) 10 wt% 가도핑된 ITO박막이현재가장보편적으로사용되고있다 [3]. 일반적으로 ITO박막은밴드갭이 ev 정도인 n-type의반도체 242

243 (c) (d) Figure 1. SEM images of Ag NW film. The sheet resistance depends on the different density of the Ag NW[8].

PET, (d) Measured the change in sheet resistance of the bending cycle[12]. 이며전기전도도가우수하고높은투과도를가지며동시에전기신호를공급하고 [4] 빛을투과시킬수있으며디스플레이할수있는모든기기들에서투명전극재료로이용되고있다.

그러나, 제조공정에서스퍼터링또는펄스레이저증착시발생하는기계적인자극에의한결함이 ITO 박막의저항을급격히높아지게되고, 세라믹특성상 2 3% 의낮은변형률 (strain) 을갖기때문에 [5,6] 밴딩에대해쉽게부서져전도성이저하되는치명적인단점이있다 [7]. 또한, 인듐의제한된공급과이로인한가격상승, 인체에해로운독성역시해결해야할문제점으로지적되고있다.

2 243 (c) (d) Figure 1. SEM images of Ag NW film. The sheet resistance depends on the different density of the Ag NW[8]. Compare the different concentrations of Ag NWs lead to different transmittance and ITO coated on PET[9], (c) Measured change in sheet resistance of the bending radius between ITO / PET and Ag NW / PET, (d) Measured the change in sheet resistance of the bending cycle[12]. 이며전기전도도가우수하고높은투과도를가지며동시에전기신호를공급하고 [4] 빛을투과시킬수있으며디스플레이할수있는모든기기들에서투명전극재료로이용되고있다. ITO박막은고진공의스퍼터공정으로진행되며 In의자리를 Sn이치환하게되는데이때결정도를유도하기위해기판의온도를 300 의고온으로만들어 Sn이 In의자리를치환하기쉽게하는공정을이용하여저항이낮은 ITO를제작한다. 그러나, 제조공정에서스퍼터링또는펄스레이저증착시발생하는기계적인자극에의한결함이 ITO 박막의저항을급격히높아지게되고, 세라믹특성상 2 3% 의낮은변형률 (strain) 을갖기때문에 [5,6] 밴딩에대해쉽게부서져전도성이저하되는치명적인단점이있다 [7]. 또한, 인듐의제한된공급과이로인한가격상승, 인체에해로운독성역시해결해야할문제점으로지적되고있다. 이로인해 ITO는플렉시블소자의생산을위한롤투롤방식에의한공정이불가능하여이에따라 In을함유하지않는다른무기재료를이용하는연구가진행되었으며, 그대안물질로 ITO보다상대적으로가격이싸고매장량이많은 ZnO (Zinc Oxide), SnO 2 (Tin Dioxide) 계를중심으로상용화를위한연구개발이진행중에있다. 이외에도 TiO 2 (Titanium Dioxide), GZO (Ga-doped ZnO), AZO (Al-doped ZnO), 산화물 -금속- 산화물구조의다층박막등이활발히연구되고있다 [5]. ZnO는도핑물질에따라전기적, 광학적성질의조절이가능하며 ZnO박막의경우저비용으로제작이가능하고 ITO만큼의높은투과율과전기전도성을갖고있어투명전극재료로상용화가기대되며 광전소자로사용하기위한물질로써많은장점을가지고있다. SnO 2 의경우 ITO에비해값이저렴하고기판에화학적으로증착되기때문에접착강도가매우뛰어나 ITO와 ZnO를비교하였을때내산, 내염기성, 기계적성질이우수하다. 하지만, 내마모성으로인해에칭이어려워아연과염화수소의반응을통해생성된수소에의해서만패터닝공정을할수있고, 결정화온도가다른 TCO 박막들에비해높기때문에플라스틱기판에증착하는것이굉장히어렵다는것이단점이다 Metallic nanowire 소재제작시랜덤네트워크형태를이룰수있는이루어진금속나노와이어는최근들어투명전극의대체재로서활발히연구중에있다. 금속나노와이어는전극필름의제조를저렴한비용으로진행할수있으며, 용액을이용하여롤투롤공정을통해박막필름을제조할수있다는것이가장큰장점으로부각되고있다. 특히 silver nanowires (Ag NWs) 는우수한광학적특성, 유연성과높은전도성을가지고있어투명전도체로의사용에대한잠재력이매우큰물질로각광받고있다. Ag NWs의탁도 (haze) 와표면의거칠기는 Ag NW의지름에영향을받을수있으며보통직경이 nm, 길이가 1 20 µm이다 [5,8]. Ag NWs의밀도를조절하여다양한면저항 ( Ω/ ) 을구현할수있지만면저항이낮아질수록 Ag NWs의밀도가증가하여투과도가낮아지고탁도가높아지는단점을갖고있다 [8,9](Figure 1, ). 또 Appl. Chem. Eng., Vol. 25, No. 3, 2014

244 정문현 김세열 유도혁 김중현 Figure 3. The transmittance of the different sheet resistance of SWCNT[19]. Figure 2.

이프를이용하여흑연단일층을떼어내는방법으로 Geim 연구진이개발하였고 [14] 구조적으로우수하며큰결정을제공한다는장점이있으나대량생산에있어서최종수율이굉장히좋지못해실용성이떨어진다. 따라서 graphene의우수한성질을이용한소자를만들기위해서는 graphene의특성을유지하면서대면적의필름을생산할수있는방법의연구가필요하다.

이방법을통해제작한 graphene 투명전극필름은 280 Ω/, 투과율 80% 를보고하였다 [15]. 또한, 2010년에는롤투롤공정을통해 30 inch의 graphene 투명전극필름을제조하였는데 125 Ω/, 97% 의투과율을보고하였다. 롤투롤공정으로그래핀의대면적생산의가능성을열었다 [16,17](Figure 2).

3 244 정문현 김세열 유도혁 김중현 Figure 3. The transmittance of the different sheet resistance of SWCNT[19]. Figure 2. Schematic of graphene transfer process[14], Graphene transparent electrode film of the 30 inch size which is manufactured using a roll-to-roll process[15]. 이프를이용하여흑연단일층을떼어내는방법으로 Geim 연구진이개발하였고 [14] 구조적으로우수하며큰결정을제공한다는장점이있으나대량생산에있어서최종수율이굉장히좋지못해실용성이떨어진다. 따라서 graphene의우수한성질을이용한소자를만들기위해서는 graphene의특성을유지하면서대면적의필름을생산할수있는방법의연구가필요하다. 2009년삼성종합기술원과성균관대에서 Nature 지를통해대면적 graphene을제조할수있는방법을게재하였다 (Figure 2). 이방법은 graphene 성장에필요한촉매인 Ni의두께와성장시간을조절하여촉매 Ni 층을만들고, CVD (Chemical Vapor Deposition) 를이용하여 graphene을형성한후전사하는방식을통해제작을한다. 이방법을통해제작한 graphene 투명전극필름은 280 Ω/, 투과율 80% 를보고하였다 [15]. 또한, 2010년에는롤투롤공정을통해 30 inch의 graphene 투명전극필름을제조하였는데 125 Ω/, 97% 의투과율을보고하였다. 롤투롤공정으로그래핀의대면적생산의가능성을열었다 [16,17](Figure 2). 한, 나노와이어들은표면이거칠어낮은병렬저항과높은암전류를나타내광전자 (optoelectronics) 소자제작에있어서낮은효율을보이고기판과의접착력이좋지않은단점이있다 [10]. 따라서낮은면저항을유지하면서기판과의접착력을높이기위해서는 Ag NWs 의코팅후에높은온도 (150 ) 에서열처리나가압등의처리가필요하며, 기판에 O 2 플라즈마와같은표면처리를해야안정적인 Ag NWs 필름을얻을수있다. 이러한방법들을통하여제조된 Ag NW 필름은은의금속적인성질을갖고있어유연한기판에적용이가능하며밴딩테스트에서도면저항의변화가거의발생하지않아앞으로의플렉시블전자소자에많이응용될것으로기대된다 [12](Figure 1(c), (d)) Carbon material Graphene Graphene은 2차원으로이루어진탄소원자가벌집모양으로배치를이루고있는탄소구조체이며이를둥글게모으면플러렌, CNT (Carbon Nanotube) 형태가되며, graphene을다시층층이쌓으면흑연이된다. 상온에서 quantum hall effect가나타나며이동도가 200,000 cm 2 /V s 의매우높은전도도를가지고있고 [13] 기계적물성, 열전도도, 두께등 ITO 박막보다우수한성질을갖고있어기존의투명전극재료를대체할수있는잠재력이매우높은재료이다. 가장널리알려진 graphene 제조방법중하나인접착테이프법은접착테 Carbon nanotubes Carbon Nanotubes (CNT) 는우수한기계적강도, 강성, 열과전기전도도, 낮은밀도를가지고있으며고분자, 탄소섬유그리고금속의장점들을갖고있는이상적인재료이다. CNT의종류에는 Single Wall Carbon Nanotubes (SWCNT), Double Wall Carbon Nanotubes (DWCNT), Multi Wall Carbon Nanotubes (MWCNT) 가있으며, 이들의전기적특성이정제공정에따라크게영향을받기때문에연구그룹에따라서로상이한면저항과투과도를보고하고있다 [18]. 특히, CNT의경우면저항을낮추기위해선두껍게코팅해야하고투과도를높이기위해선얇게코팅해야하기때문에최적두께조절은필수적인요소이다. CNT는각각의소자에맞는특성을적용하기위해나노튜브를각각분리해내는 CNT분산기술이필요한데 CNT의크기가굉장히작아서로뒤엉키면서뭉쳐있는형태로존재하기때문에쉽게분리하기가굉장히어렵다. 분산기술은기계적분산, 물리적분산, 화학적분산으로구분할수있으며화학적분산은강산을이용하여분산을시켜야하기때문에공정의환경에좋지않으며, CNT를부식시키기때문에 CNT의결정성을낮춰소재의성능구현에한계가있다. 결정성을유지하면서분산시킬수있는물리적 / 기계적분산방법은 CNT의엉킴, 뭉침현상을피할수없으며이를해결할수있는획기적인방법이아직까지는개발되고있지않은실정이다 [5]. 현재양산되는 CNT 필름전극대부분은약 Ω/ 수준의 공업화학, 제 25 권제 3 호, 2014

4 245 Figure 5. Variation of the concentration of EMIM TCB conductivity ( ), thickness( ), transmittance changes measured( )[23]. Figure 4. Compare the transmittance of ITO and PEDOT : PSS film, Comparison of transmittance and sheet resistance corresponding to the thickness of the PEDOT : PSS film[22]. 면저항과 80% 의투과도를나타내고있으나 [19](Figure 3), 미국 Eikos 사에서는가시영역대에서투과율 90% 에면저항 200 Ω/ 정도의 SWNT 를개발해 invisicon 이라는이름으로투명전극을개발하여출시하였다. CNT가갖고있는특성을잘활용하여대량생산할수있는공정, 분산될수있는기술이연구개발을통해실행될수있다면투명전극으로써 CNT 소재가광범위하게적용가능할것으로예상된다 Conducting polymer 전도성고분자는주사슬이불포화탄화수소의구조인이중결합과포화탄화수소구조인단일결합이교차하여이들형태가반복되는구조를갖고있으며, π-공액고분자라고도한다. π-공액형태가고분자골격에존재하면도핑에의해전자밀도가비편재화되며이것으로부터전기전도가일어나게되어전기가흐를수있게된다. Polyacetylene에할로겐원소들을도핑시키는실험 [5] 이후금속의전도도에가까운높은전도도가가능할수있음을보이며전도도를가질수있는고분자물질에대한연구가시작되었다. polyaniline, polypyrrole, polythiophene이전도성고분자로서다양한분야에응용이이루어졌지만 Figure 6. Difference in the H 2 SO 4 content of added PEDOT : PSS change the conductivity and Drying time according to the change of conductivity after H 2 SO 4 100% treatments (inner square)[24]. 중합체내의독성이존재하고불융, 불용성의특징을갖고있는것이단점이다 [20]. 위의단점을극복하고자현재가장활발하게연구되고있는전도성고분자는 thiophene 계열의고분자인 PEDOT (Poly 3,4-ethylenedioxythiophene) : PSS (Polystyrene Sulfonate) 로서중합된소수성인 PEDOT과친수성인 PSS가 coulombic attraction에의해목걸이구조를가지며이를이용하여용액상의전도성고분자를만들수있다 [21]. 유기전자소자에서주로사용되었던 [5] PEDOT : PSS는낮은전도도를나타내었지만, PEDOT : PSS의단독사용이아닌이차적인유기용매첨가에의한전도도의상승효과가발견되면서 S/cm 정도의전도도를구현할수있다. 가장대표적인이차도펀트로유기용매인 DMSO (Dimethyl Sulfoxide) 가사용되고있으며이외에도 NMP (N-methylpyrrolidone), EG (Ethylene Glycol), MeOH (Methanol), EtOH (Ethanol), IPA (Isopropyl Alcohol) 등이사용되고있다 [22]. 이는 PEDOT : PSS의 crystallinity를개선시켜 배의전도도의향상을시켰고 [5] 이후고분자투명전극을이용하여플렉시블전자소자에응용이이뤄지고있다. 현재시중에시판되고있는상품으로는 Heraeus사의 Clevios제품군이가장대표적이며 Clevios PH 1000은 1000 S/cm의높은전도도를구현할수있다. 또한, 2012년프랑스국립과학연구센터 (National Center for Scientific Research) 의 Chantal Badre 연구진은 ionic liquid의한종류인 1-Ethyl 3 - methylimidazolium Tetracyanoborate (EMIM TCB) 을 DMSO대신 Appl. Chem. Eng., Vol. 25, No. 3, 2014

246 정문현 김세열 유도혁 김중현 (c) (d) (e) (f) Figure 7.

side is coated Ag NW - PEDOT : PSS hybrid, right side is coated only Ag NW (SEM image of Ag NWs joined by PEDOT : PSS (red colored) through nanosoldering.

, (e) Compare the deviation of Ag NW and Ag NW - PEDOT : PSS hybrid film according to concentration of Ag NW, (f) Compare the transmittance of Ag NW - PEDOT : PSS film and ITO film.

고분자투명전극은 PET와같은유연한기판에박막을형성하였을때밴딩에대한저항변화가거의없으며용액공정, 롤투롤공정이가능하기때문에주목을받고있다. 최근에는많은합성법및박막제조공정기술의개발로비정질 ITO 전극의광학적, 전기적특성에버금가는고분자투명전극이개발되고있는추세이다.

5 246 정문현 김세열 유도혁 김중현 (c) (d) (e) (f) Figure 7. Compare the transmittance of ITO, AZO, AZO Ag - AZO(inner picture is AZO Ag - AZO film)[29], Photograph of graphene - Ag NW coated on PET substrate and SEM image (Scale bar is 5 µm)[31], (c) Left side is coated Ag NW - PEDOT : PSS hybrid, right side is coated only Ag NW (SEM image of Ag NWs joined by PEDOT : PSS (red colored) through nanosoldering. Scale bar is 4 µm)[32], (d) Compare the sheet resistance (- -) and transmittance (- -) of Ag NW and Ag NW PEDOT : PSS hybrid film according to the concentration Ag NW., (e) Compare the deviation of Ag NW and Ag NW - PEDOT : PSS hybrid film according to concentration of Ag NW, (f) Compare the transmittance of Ag NW - PEDOT : PSS film and ITO film. 첨가하여 2084 S/cm의전도도를보였으며 550 nm 파장영역대에서 95% 이상의투과율을나타냈다고보고했다 [23](Figure 5). 2013년광주과학기술원의이광희교수연구그룹에서 H 2 SO 4 의 post treatment방법을이용하여 4380 S/cm의높은전도도를보였고 90% 의투과도를가시광선영역대에서나타냈다 [24](Figure 6). 고분자투명전극은 PET와같은유연한기판에박막을형성하였을때밴딩에대한저항변화가거의없으며용액공정, 롤투롤공정이가능하기때문에주목을받고있다. 최근에는많은합성법및박막제조공정기술의개발로비정질 ITO 전극의광학적, 전기적특성에버금가는고분자투명전극이개발되고있는추세이다. 고분자투명전극을제작하는데있어서전도성고분자는에너지밴드갭이 3 ev 이하로가시광영역대의파장에서빛을흡수하기때문에특유의색을띄며 [26], 대기중에서는산화속도가굉장히빠르기때문에대기중에서의안정성에대한확보가매우중요하다 [27]. 전도성고분자를당장응용가능한투명전극으로이용하기에는아직어려움이있지만앞서말한단점을극복하고전기적, 광학적특성을 ITO 투명전극의수준까지끌어올릴수있다면향후 ITO 투명전극을대체할수있는유연하고인쇄방법이접목가능한투명전극물질로이용될것이분명하다 Composite materials 앞서말한재료들을이용하여투명전극의상업화가가능할것이라예상되겠지만, ITO 투명전극을당장대체하기는아직해결해야할 문제점이많다. 따라서각단일재료의장점은유지시키면서단점들을보완하기위해두가지이상의재료를복합화하여기존의 ITO를대체할수있는복합소재연구가많이진행되고있다. TCO를이용한복합소재연구로는 AZO/ITO 의구조로인듐의사용을최소화하면서 ITO투명전극이갖지못한유연성을부여함과동시에낮은면저항과높은투과율을유지할수있다. AZO/ITO는비저항이 Ω cm와투과율 79% 를나타냈다 [28]. 또한, TCO 복합소재에유연함을부여하기위해 AZO/metal/AZO와같은다층산화물구조로제조함으로써값싼제조비용을가지며원소의이용에제약이없다. AZO/Mo/AZO의경우 Ω/, 80% 의투과율의투명전극을제작하였으며 [29] AZO/Ag/AZO는 8.3 Ω/ 와투과율약 88% 를나타냈다 [30](Figure 7). Carbon material기반의복합소재를살펴보면 CVD graphene 위에 Ag NWs를코팅하는방식으로 graphene과 Ag NWs를복합화하여 33 Ω/, 94% 의투과율을갖는투명전극을보고하였다 [31](Figure 7). Silver nanowire는단독코팅시갖고있는문제점들을해결하기위해 PEDOT : PSS를사용해복합화를하여기판과의접착력이향상시키고 Ag NW-Ag NW 사이의접합을강화하였다. 또한, NW-NW 사이의빈부분들이 PEDOT : PSS로채워지면서캐리어들의 pathway가증가되어저항값을낮추고, 균일한필름의형태로제조됨으로써저항편차를감소시켰다. 이렇게제작한투명전극은 25 Ω/, 85% 의투과율을보여준다 [32](Figure 7(c)-(f)). 최근 PEDOT : PSS, polyanilne 등의전도성고분자와 CNT, gra- 공업화학, 제 25 권제 3 호, 2014

substrate (right bottom). Compare the transmittance of PEDOT : PSS and PEDOT : PSS - graphene composite film[33]. phene의탄소구조체를 in-situ 및 blending 방법을통해복합화를진행하는연구도활발히진행중이다.

지금까지 ITO를대체하기위해 TCO, silver nanowire, CNT, graphene, conducting polymer와같은물질들이발견되면서연구가진행되고있지만아직까지는 ITO를대체하여상업화를할수있을만큼의물리적성질과면저항, 투과도가나오지는않고있다.

결과및고찰 현재까지의투명전극은딱딱한유리기판위에형성하여응용분야에접목되어왔지만인듐자원의제한과향후디스플레이, 일렉트로닉스, 에너지분야등에서필요로하는기술들을실현시키기위해서유연한투명전극에대한연구의필요성이증가하고있다.

Graphene은저렴한비용의흑연을이용하여생산할수있으며 CNT보다표면의거칠기가우수한장점을가지고있지만높은결정성의대면적 graphene 필름을만드는데있어서보다많은연구가필요하다 [5]. Silver nanowire는다른재료들에비해표면거칠기가좋지못하고헤이즈가높아디스플레이에응용하기에는쉽지않지만낮은면저항과높은투과도를요구하는태양전지의투명전극으로적용할수있을것이라예상한다.

6 247 Figure 8. Picture of PEDOT : PSS (left top), PEDOT : PSS - graphene 3 wt% (right top), PEDOT : PSS - graphene 5 wt% (left bottom) is coated on glass substrate and PEDOT : PSS - graphene 3 wt% is coated on PET substrate (right bottom). Compare the transmittance of PEDOT : PSS and PEDOT : PSS - graphene composite film[33]. phene의탄소구조체를 in-situ 및 blending 방법을통해복합화를진행하는연구도활발히진행중이다. 이는두물질모두방향성 (aromaticity) 을갖고있으며, 방향성의 π-π 전자간 stacking interaction을통해전자이동의다리역할을갖게하여시너지효과를구현한다 [33](Figure 8). 이외에도 CNT-graphene[34], CNT-PEDOT : PSS[35], metal gridgraphene[36] 등을이용하여복합소재가연구되고있다. 지금까지 ITO를대체하기위해 TCO, silver nanowire, CNT, graphene, conducting polymer와같은물질들이발견되면서연구가진행되고있지만아직까지는 ITO를대체하여상업화를할수있을만큼의물리적성질과면저항, 투과도가나오지는않고있다. 하지만, 앞으로의시장흐름을볼때분명유연한투명전극을이용한플렉시블전자제품이실생활에응용될것으로예측되기때문에우리나라에서도끊임없는연구개발을통해원천기술을확보하기위한노력이필요하다. 3. 결과및고찰 현재까지의투명전극은딱딱한유리기판위에형성하여응용분야에접목되어왔지만인듐자원의제한과향후디스플레이, 일렉트로닉스, 에너지분야등에서필요로하는기술들을실현시키기위해서유연한투명전극에대한연구의필요성이증가하고있다. TCO는 ZnO, SnO 2 에다른물질들을도핑시켜 ITO를대체하기위해연구가진행되고있지만, 습한환경에서의안정성이취약하며부족한유연성의개선과다른재료들에비해비싼제조비용의절감이필요하다. CNT는상당한연구가진행되어있으며상업적으로다양한분야에응용될가능성이가장큰재료이다. 그러나 CNT의도핑, 정제와합성의개선방안이앞으로도꾸준히연구가되어야한다. Graphene은저렴한비용의흑연을이용하여생산할수있으며 CNT보다표면의거칠기가우수한장점을가지고있지만높은결정성의대면적 graphene 필름을만드는데있어서보다많은연구가필요하다 [5]. Silver nanowire는다른재료들에비해표면거칠기가좋지못하고헤이즈가높아디스플레이에응용하기에는쉽지않지만낮은면저항과높은투과도를요구하는태양전지의투명전극으로적용할수있을것이라예상한다. Conducting Polymer는지난 20년동안꾸준히투명전극으로사용하기위한연구가상당부분진행되어 TCO가가지고있는문제들중일부를해결할수있는재료이지만기본적으로박막이특유의색을갖고있으며대기안정성이부족하다는단점이있다. References 1. J. K. Wassei and R. B. Kaner, Graphene a promising transparent conductor, Materialstoday, 13, (2010). 2. K.-H. Lee, S.-M. Kim, J. Jeong, Y. Pak, H. Song, J. Park, K.-H. Lim, J.-H. Kim, Y. S. Kim, H. C. Ko, I. K. Kwon, and G.-Y. Jung, All-solution-processed transparent thin film transistor and its application to liquid crystals driving, Adv. Mater., 25, (2013). 3. K. Nakashima and Y. Kumahara, Effect of tin oxide dispersion on nodule formation in ITO Sputtering, Vacuum, 66, (2002). 4. N. Manavizadeh, F. A. Boroumand, E. A. Soleimani, F. Raissi, S. Bagherzadeh, A. Khodayari, and M. A. Rasouil, Influence of substrates on the structural and morphological properties of RF sputtered ITO thin films for photovoltaic application, Thin Solid Films, 517, (2009). 5. D. S. Hecht, L. Hu, and G. Irvin, Emerging transparent electrodes based on thin films of carbon nanotubes, graphene, and metallic nanostructures, Adv. Mater., 23, (2011). 6. K. A. Sierros, N. J. Morris, K. Ramji, and D. R. Cairns, Stress corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices, Thin Solid Films, 517, (2009). 7. G. A. Potoczny, T. S. Bejitual, J. S. Abell, K. A. Sierros, D. R. Cairns, and S. N. Kukureka, Flexibility and electrical stability of polyester- based device electrodes under monotonic and cyclic buckling conditions, Thin Solid Films, 528, (2013). Appl. Chem. Eng., Vol. 25, No. 3, 2014

7 248 정문현 김세열 유도혁 김중현 8. L. Hu, H. S. Kim, J. Y. Lee, P. Peumans, and Y. Cui, Scalable coating and properties of transparent, flexible, silver nanowire electrodes, ACS Nano, 4, (2010). 9. A. B. V. K. Kumar, C. W. Bae, L. Piao, and S.-H. Kim, Silver nanowire based flexible electrodes with improved properties: high conductivity, transparency, adhesion and low haze, Materials Research Bulletin, 48, (2013). 10. D. Y. Choi, H. W. Kang, H. J. Sung, and S. S. Kim, Annealing-free, flexible silver nanowire-polymer composite electrodes via a continuous two-step spray-coating method, Nanoscale, 5, (2013). 11. R. Zhu, C.-H. Chung, K. C. Cha, W. Yang, Y. B. Zheng, H. Zhou, T.-B. Song, C.-C Chen, P. S. Weiss, G. Li, and Y. Yang, Fused silver nanowires with metal oxide nanoparticles and organic polymers for highly transparent conductors, ACS Nano, 5, (2011). 12. M. K. Song, D. S. You, K. Lim, S. Park, S. Jung, C. S. Kim, D. -H. Kim, D.-G. Kim, J.-K. Kim, J. Park, Y.-C. Kang, J. Heo, S. -H. Jin, J. H. Park, and J.-W. Kang, Highly efficient and bendable organic solar cells with solution-processed silver nanowire electrodes, Adv. Funct. Mater., 23, (2013). 13. K. I. Bolotin, K. J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, and H. L. Stormer, Ultrahigh electron mobility in suspended graphene, Solid State Communications, 146, (2008). 14. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Electric field effect in atomically thin carbon films, Science, 306, (2004). 15. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi, and B. H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature, 457, (2009). 16. S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y.-J. Kim, K. S. Kim, B. O. zyilmaz, J.-H. Ahn, B. H. Hong, and S. Iijima, Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nature nanotechnology, 5, (2010). 17. S. Pang, Y. Hernandez, X. Feng, and K. Müllen, Graphene as transparent electrode material for organic electronics, Adv. Mater., 23, (2011). 18. D. S. Hecht, A. M. Heintz, R. S. Lee, L. Hu, B. Moore, C. Cucksey, and S. Risser, High conductivity transparent carbon nanotube films deposited from superacid, Nanotechnology, 22, (2011). 19. J. Li, L. Hu, L. Wang, Y. Zhou, G. Gruner, and T. J. Marks, Organic light-emitting diodes having carbon nanotube anodes, Nano Lett., 6, (2006). 20. L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J. R. Reynolds, Poly(3,4-ethylenedioxythiophene) and its derivatives: past, present, and future, Adv. Mater., 12, (2000). 21. J. Ouyang, Secondary doping methods to significantly enhance the conductivity of PEDOT : PSS for its application as transparent electrode of optoelectronic devices, Displays, 34, (2013). 22. D. Alemu, H.-Y. Wei, K.-C. Ho, and C.-W. Chu, Highly conductive PEDOT:PSS electrode by simple film treatment with methanol for ITO-free polymer solar cells, Energy Environ. Sci., 5, (2012). 23. C. Badre, L. Marquant, A. M. Alsayed, and L. A. Hough, Highly conductive Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) films using 1-Ethyl-3-methylimidazolium tetracyanoborate ionic liquid, Adv. Funct. Mater., 22, (2012). 24. N. Kim, S. Kee, S. H. Lee, B. H. Lee, Y. H. Kahng, Y.-R. Jo, B.-J. Kim, and K. Lee, Highly conductive PEDOT:PSS nanofibrils induced by solution-processed crystallization, Adv. Mater., 26, (2014). 25. Y. Xia, K. Sun, and J. Ouyang, Solution-processed metallic conducting polymer films as transparent electrode of optoelectronic devices, Adv. Mater., 24, (2012). 26. Q. Pei, G. Zuccarello, M. Ahlskog, and O. Inganas, Electrochromic and highly stable poly(3,4-ethylenedioxythiophene) switches between opaque blue-black and transparent sky blue, Polymer, 35, (1994). 27. A. M. Nardes, M. Kemerink, M. M. de Kok, E. Vinken, K. Maturova, and R. A. J. Janssen, Conductivity, work function, and environmental stability of PEDOT:PSS thin films treated with sorbitol, Organic electronics, 9, (2008). 28. J.-H. Yun and J. Kim, Double transparent conducting oxide films for photoelectric devices, Materials Letters, 70, 4-6 (2012). 29. H.-W. Wu and C.-H. Chu, Structural and optoelectronic properties of AZO/Mo/AZO thin films prepared by rf magnetron sputtering, Materials Letters, 105, (2013). 30. F. Li, Y. Zhang, C. Wu, Z. Lin, B. Zhang, and T. Guo, Improving efficiency of organic light-emitting diodes fabricated utilizing AZO/Ag/AZO multilayer electrode Vacuum, 86, (2012). 31. M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.-Y. Lee, S. W. Nam, and J.-U. Park, High-performance, transparent, and stretchable electrodes using graphene-metal nanowire hybrid structures, Nano Lett., 13, (2013). 32. J. Lee, P. Lee, H. B. Lee, S. Hong, I. Lee, J. Yeo, S. S. Lee, T.-S. Kim, D. Lee, and S. H. Ko, Room-temperature nanosoldering of a very long metal nanowire network by conducting-polymer-sssisted joining for a flexible touch-panel application, Adv. Funct. Mater., 23, (2013). 33. D. Yoo, J. Kim, and J. H. Kim, Direct synthesis of highly conductive PEDOT:PSS/graphene composites and their applications in energy harvesting systems, Nano Res., DOI: /s z. 34. Y.-K. Kim and D.-H. Min, Durable large-area thin films of graphene/carbon nanotube double layers as a transparent electrode, Langmuir, 25, (2009). 35. S. De, P. E. Lyons, S. Sorel, E. M. Doherty, P. J. King, W. J. Blau, P. N. Nirmalraj, J. J. Boland, V. Scardaci, J. Joimel, and J. N. Coleman, Transparent, flexible, and highly conductive thin films based on polymer-nanotube composites, ACS Nano, 3, (2009). 36. X. Ho, H. Lu, W. Liu, J. N. Tey, C. K. Cheng, E. Kok, and J. Wei, Electrical and optical properties of hybrid transparent electrodes that use metal grids and graphene films, J. Mater. Res., 28, (2013). 공업화학, 제 25 권제 3 호, 2014

jaeryomading review.pdf

jaeryomading review.pdf 4 5 6 7 8 9 10 11 12 13 1. S. Kim, H. Y. Jeong, S. K. Kim, S. Y. Choi and K. J. Lee, Nano Lett. 11, 5438 (2011). 2. E. Menard, K. J. Lee, D. Y. Khang, R. G. Nuzzo and J. A. Rogers, Appl. Phys. Lett. 84,