(Organic Photovoltaic Cells) 유기 태양 전지 (Organic Photovoltaic Cells) 김 미 라 부산대학교 플라스틱정보소재연구센터 mrkim2@pusan.ac.kr

Introduction Concept and Principle of Photovoltaic Cell Si Solar Cells Organic Solar Cells Dye-Sensitized Solar Cells Problems of organic solar cells and improvements Future of organic solar cells

Green House Effect

태양광 태양열 바이오매스 Renewable Energy 폐기물 에너지 풍 력 해양 에너지 수 력 지 열

태양전지의 역사 현재 태양전지효율 7∼17%, 수명 20년 이상, 모듈가격 $6/W 내외, 발전단가 $0.25∼0.5/kWh 쿄토 의정서 미국 우주선 보조전원 사용 -5 mW Oil Shock Photovoltaic Effect 발견 -Edmond Becquerel 본격적인 기술개발 추진 대량생상 공정 개발 염료감응형 태양전지 개발 -연간 200MW 돌파 전세계 생산량 -연간 400MW 돌파 전세계 생산량 -연간 500MW 돌파 전세계 생산량 Si 태양 전지 개발 -Bell Lab. 1839 1954 1958 1974 1980 1991 1997 1999 2001 2002 (year)

Center for Plastic Information System

hat Kinds of Solar Cells are? Center for Plastic Information System hat Kinds of Solar Cells are? W Materials for Solar cells Si – monocrystalline, Multi-, amorphous- GaAs, InP, CdTe Aa-Si/CIS, GaAs/Ge Inorganic Solar Cell Organic Solar Cell Dye-Sensitized Solar Cell Conducting polymer – fullerene Conducting polymer – conducting polymer Structures of Solar Cells Organic polymer – Nanoinorganic materials P-n Homojunction Solar Cell P-n Heterojunction Solar Cell Dye-Sensitized Solar Cell

태양전지의 종류 특징 최대 효율(%) 단결정 실리콘 태양전지 높은 순도, 낮은 결정결함밀도가, 고가 28% (도달 한계치 : 35%) 다결정 실리콘 저렴한 공정, 저급재료의 이용 18% (도달 한계치 : 19%) 비정질 실리콘 가장 상업적으로 성공한 최초의 박막형태의 태양전지 12% CuInSe2 (CIS cell) p/n이종 접합의 기본구조를 가짐, 약 1eV의 에너지 밴드를 가짐, 0.5V 이하의 낮은 Voc 11% GaAs 태양전지 태양전지의 재료 중 가장 높은 효율 달성 우주용으로 상용화 성공, 에너지 밴드의 조절이 용이, 높은 광 흡수계수, 고가, As의 유해성 28.7% CdTe 태양전지 높은 광 흡수, 저렴한 생산비용, 물질의 합성이 용이, 높은 접촉저항, 소자제작의 어려움 10% 염료감응 태양전지 (DSSC) 저렴한 제작비용, 제작의 용이성, 환경 친화적, 투명한 태양전지의 제작 가능, 안정성의 문제 유기 태양전지 저렴한 제작비용, 다양한 물질의 적용 가능, 유연한 태양전지의 제작 가능, 낮은 효율, 안정성의 문제 3-4%

일반적인 태양전지의 장,단점

World Best Research-Cell Efficiencies

Applications Building Satellite Cooker Vehicle Military Mobile Remote WIndow

                                                       [그림 1)] 태양전지의 현재 및 미래 활용분야

Product Requirement Triad Durability & Stability Cost Performance Must be minimized Must be maximized Durability & Stability “Life time = 10-30 yr” Must be demonstrated

Concepts and Principles of Solar Cells

W Concept and principle of photovoltaic cell hat Solar Cells are? Solar Cells are solid-state devices that absorb light and convert light energy directly into electricity light Light energy n 형 반도체 p 형 반도체 Electrical energy Solid-state devices 전류

LED devices VS PV devices Center for Plastic Information System LED devices VS PV devices Light Glass ITO Organic material Al, Ca, Mg Input Glass ITO Organic material Al, Ca, Mg Output Light LED mode PV mode

A simple circuit of P-N junction solar cell H ow Solar Cells work? Conducting Band Valence Band ㅡ Light Input + Current Extracted By Contacts Load N-Type P-Type ④ ① ③ ② A simple circuit of P-N junction solar cell An energy-band diagram for a P-N junction solar cell showing the generation and transport of charge carriers.

1. P-N junction with dopants

2. Bandgap Energy Conduction band (empty) Valence band Hole Energy Vacuum zero Electron Majority Carriers C B Work Function Donor levels Vacuum zero Vacuum zero Vacuum zero Ef Conduction band (empty) Electron Affinity C B Work Function Valence band Hole Minority Carriers Work Function Ec Electrons Eg Eg Holes Ef Ef Ev Hole Energy Electron Energy Valence band (Full) Valence band Valence band n-type semiconductor Vacuum zero Electron Affinity Electron Minority Carriers C B insulator conductor semiconductor Ef Acceptor levels Hole Majority Carriers Valence band p-type semiconductor

Dye-sensitized solar cell Valence band n-type p-type Ef Vacuum zero C B Acceptor levels Donor levels Depletion Layer Electron-Hole Pair Back Front Light Electron Energy p-n type junction Dye-sensitized solar cell Dye Molecule Electron conductor Hole conductor

3. 태양광 스펙트럼과 Air Mass Air mass 1.5 ; θ = 48.2 Air mass 2 ; θ = 60 θ Air mass = 1/cosθ

4. Spectral Response

for a solar cell both in the dark and illuminated Center for Plastic Information System The Current-voltage plots for a solar cell both in the dark and illuminated Voltage (V) Current (I) Short-Circuit Current (Isc) Open-Circuit Voltage (Voc) Dark Illumination Vmax Imax Short-Circuit Current ( Isc ) For V = 0, I = Isc Open-Circuit Voltage (Voc ) For I = 0, V = Voc Fill Factor ( FF ) = ( Imax x Vmax ) / ( Isc x Voc ) = Pmax / ( Isc x Voc ) Power Conversion Efficiency ( η ) = Pmax / Pin = ( Imax x Vmax ) / Pin = FF x { ( Isc x Voc ) / Pin } * Pin = 100 [ mW/ cm2 ]

A simple circuit to test a solar cell Voltmeter V Load Ammeter A

Si Solar Cells

Future of Multijunction Solar Cell

Organic Solar Cells

Structure and Energy-Band Diagram of Heterojunction Device-bilayer Photoinduced Charge Transfer (PICT) : 광여기 전하이동 현상 ITO Al LUMO HOMO h+ e- Donor Acceptor (a) Al C60 MEH-PPV ITO Glass (b) C60 (Acceptor) MEH-PPV (Donor) O n h ω e - (c)

hv Photocurrent ITO Al (80 nm) ITO hv Al Glass substrate Wire to ITO Wire to metal electrode Thin film of MEH-PPV in chlorobenzene solution (1:375 at wt%) (100 nm) Thin film of fulleropyrrolidine in chlorobenzene solution (4:375 at wt%) (50nm) ITO hv Al e- Photocurrent

Structure and Energy-Band Diagram of Heterojunction Device hv Vacuum level e- Χ LUMO ΦAl ΦITO h+ IP Eg Electron acceptor의 LUMO 조절 ⇒구조상 전자친화도 조절 HOMO e- 2 LUMO 3 e- ITO LUMO Al 4 1 5 cathode Eg anode Al C60 MEH-PPV ITO Glass C60 (Acceptor)+ MEH-PPV (Donor) h+ 3 HOMO HOMO donor acceptor

hv Photocurrent ITO Al ITO hv Al Glass substrate Wire to metal electrode (80 nm) Wire to ITO Thin film of blend of MEH-PPV and Fullleropyrrolidine in chlorobenzene solution (1:4:375 at wt%) (100-120 nm) ITO hv Al e- Photocurrent

Electron donor로 사용되는 대표적인 반도체 고분자 및 단분자

Electron accepter로 사용되는 대표적인 물질

Interstructure of Semiconductor Polymers and Fullerene h+

Photovoltaic parameters of fullerene-based solar cells

Preparations of ITO substrate 90℃, 30min ITO substrate Resister coating UV irradiation Cutting Washing Resister etching Resister remove Resister washing ITO etching

Preparation of solar cell device and test of device by solar simulator Light source ( 100 mW/cm2, AM 1.5 ) Al ( 80 nm ) vacuum deposition LiF ( 5-10 nm ) vacuum deposition Active layer (100nm ) Spin coating PEDOT ( 30-50 nm) Spin coating ITO Glass substrate Detector Solar simulator Cell holder Vacuum pump

Problems of organic solar cells and improvements Improvement of charge transport Al Improvement of light absorption LiF Active layer Improvement of coating method PEDOT ITO Improvement of recombination in interface Glass substrate Light

Cell output 14 mW ＳＵＮ Resistance loss Fill Factor 0.8 100mW 21 mW No below-bendgap adsorption 31 mW Excess photon energy lost as heat Voltage available 1.1V Current available 44 mA Collection efficiency Incomplete absorption Top-surface reflection Recombination Open-circuit voltage 0.6V Short-circuit current 28 mA Resistance loss Fill Factor 0.8 Cell output 14 mW

Improvements of Solar Cells Isc 의 향상 : 높은 광흡수도(낮은 밴드갭에너지) mobility (charge transport impurities나 defects제거) Light –trapping구조 (photon의 흡수증가)-active layer의 두께 (100nm이하) Voc 의 향상 : electron donor의 HOMO level과 electron acceptor의 LUMO level acceptor의 LUMO level (electron affinity) 조절 FF 의 향상 : 고분자 박막의 morphology, 고분자층과 전극과의 에너지 갭, recombination High Energy Conversion Efficiency !!

Dye-Sensitized Solar Cells

Electron transfer mechanism Center for Plastic Information System Structure and Energy Transfer Mechanism of Dye-Sensitized Solar Cell Device Ti - Counter Electrode Light FTO glass Dye Redox Polymer Electrolyte 3Iㅡ I3ㅡ Load TiO2 nanoparticles FTO glass Dye absorbed TiO2 layer Redox electrolyte Pt layer Electron transfer mechanism The structure of DSSC

Energy Band Diagram of Dye-Sensitized Solar Cell Device CB Ef VB TCO Pt Redox electrolyte Dye n-SC (TiO2) Load E (D+/D) (D+/D*) (I3-/I-) Voc hv 2.5eV 0.8eV 0.2eV - 0.7eV - 0.9eV

Characteristics of Dye-Sensitized Solar Cell Device 다공질의 티타늄 산화물: 염료의 담지량 증가 Carboxylic acid group을 가지는 염료: 전자 주입효율이 높음 광흡수: 염료, 전자 전도: 티타늄 산화물 Improvements of Dye-Sensitized Solar Cell 고체상태의 전해질 개발 바인더를 첨가한 열처리과정의 제거 새로운 염료를 통한 흡착방법의 개발 제 2의 산화물을 이용한 에너지 장벽 형성

Dye-Sensitized Solar Cell Production Calculations Component Quantity Thickness (micron) Vol/area (m3/m2) Density (g/cm3) Percent fill Use rate Kg/yr Reference/Comments Conducting glass substrate (SnO2:F) 3,000 0.003 6.85 100 2,055,000 4 Photo electrode TiO2 (nanocrystalline anatase) 20 0.00002 4.23 50 4,230 1, 5 Dye 0.7 g/m2 70 Calculated from Ru weight and molecular formula Ruthenium – g/m2 0.1 g/m2 10 Personal note from M. Graetzel Dye Depositon solvent (ethanol) 700 g/m2 70,000 Assume 0.1 % by weight dye in ethanol (probably more dilute) Electrolyte Propionitrile 50 ml/m2 3,860 1 Redox – lithium iodide 0.635 g/m2 64 Assume 0.05 moles/L Redox - iodine 3.35 g/m2 335 Assume 0.5 moles/L Counter-electrode Platinum 0.05 5E-0.8 21.45 107.3 7 SnO2 substrate Sealant Thermoplastic – 25 ㎛ x 0.5 mm strip around 10 cm X 10 cm square 0.0000005 1.1 55.0 10, Density used is approximate * Assume Production Rate = 100,000 m2/yr

Structure of N3-TiO2 chemical bonding Center for Plastic Information System Schematic diagram of the device configuration TiO2 particle FTO glass Pt layer Redox electrolyte Dye particle 25nm (powder, Degussa Co.) 12nm (solution, Solaronix Co.) Dye absorbed TiO2 layer Perfect absorption is important FTO glass TiO2 : Titanium dioxide Dye : (Cis-di(thiocyanato)-N,N- bis(2,2’-bypyridil-4,4’- dicarboxylic acid) Ruthenium(II) complex (N3 dye) Pt : Platinum Materials Structure of N3-TiO2 chemical bonding

Fabrication Process Photo-electrode Preparation Transparent conductor + titanium dioxide layer +dye Electrolyte Preparation ·Mix solvent and iodine/iodide redox couple Physical Assembly · Align electrode plates · Install electrical connections · Install electrolyte ports · Seal edges Counter-electrode Preparation Conductive support + Platinum nano-layer Final Assembly · Add electrolyte · Seal ports · Test performance

Fabrication of DSSC device (I) FTO glass TiO2 layer TiO2 layer adsorbed dye Active layer Casting electrolyte Sintering condition

Fabrication and Testing of DSSC device Air mass = 1 Air mass = 1/cosθ θ Air mass 1.5 ; θ = 48.2 Air mass 2 ; θ = 60 Two electrodes Working electrode - anode Counter electrode + cathode Solar Simulator Connect the load

FTO glass TiO2 solution Electrolyte Materials fo DSSC device Fluorine doped tin oxide(SnO2:F) coated electrically conducting glass (Solaronix, TCO22-15) TiO2 solution Ti-Nanoxide HT (Solaronix) Colloidal anatase particles size : 9nm TiO2 layer thickness : 10 μm Liquid Type Solid-State Type 극성고분자, 금속염, 유기용매 이온전도도: 10-8 -10-5 S/cm Electrolyte Quasi-Solid-State Type (Gel Type) 극성고분자, 금속염, 유기용매, 가소제 이온전도도: 10-3 S/cm

Dye ① N3 ② Black dye ③ N719 ④ Os-dye R.Argazzi, J. Photochem. Photobio. A: Chem. 164 (2004) 15-21

Structures of materials of gel polymer electrolyte solutions Polymer matrix Hole conducting Polymers Polyacrylonitrile(PAN) Polyethylene glycol(PEG) Poly[(9,9-dihexyl fluorenyl-2,7-diyl)-co-(bithiophene)] (6P) Polymethylmethacrylate(PMMA) Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N’-(4-butylphenyl-1,1’-biphenylene-4,4’-diamine))] (BE)

B.O’Regan and M.Gratzel, Nature, 335, 737-739, 1991

UV-Vis spectra of TiO2 layer, TiO2 layer adsorbed N3 dye and N3 dye in ethanol solution TiO2 impregnated dye-sensitizer on FTO glass. (solid line) λmax = 375 nm & 527 nm Ru-dye 1mg in ethanol 5ml. (dotted line) λmax = 375 nm & 527 nm TiO2 deposited on FTO glass. (medium dash)

SEM images of TiO2 layer and TiO2 layer adsorbed N3 dye (I) Cross-section of porous nanocrystalline TiO2 layer (particle size : 9nm, wt.20%) Surface of porous nanocrystalline TiO2 layer (particle size : 9nm, wt.20%) C D Cross-section of porous nanocrystalline TiO2 layer adsorbed N3 dyes (particle size : 9nm, wt.20%) Surface of porous nanocrystalline TiO2 layer adsorbed N3 dyes (particle size : 9nm, wt.20%)

SEM images of TiO2 layer and TiO2 layer adsorbed N3 dye (II) Surface of porous nanocrystalline TiO2 layer (particle size : 9nm, wt.11%) Surface of porous nanocrystalline TiO2 layer (particle size : 9nm, wt.14%) C D Surface of porous nanocrystalline TiO2 layer adsorbed N3 dyes (particle size : 9nm, wt.11%) Surface of porous nanocrystalline TiO2 layer adsorbed N3 dyes (particle size : 9nm, wt. 14%)

Preparation of Dye-Sensitized Solar Cell Device (I) FTO glass TiO2 layer TiO2 layer adsorbed dye Casting electrolyte Sintering condition

Preparation of Dye-Sensitized Solar Cell Device (II) Two electrodes Working electrode - anode Counter electrode + cathode Solar simulator Connect the load