Organic Light Emitting Diode (OLEDs)
OLED? A light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. OLED Structure or HBL
OLED-Display 출처 : Combivap
OLED applications-display LG LG SAMSUNG SAMSUNG
OLED-Display Paradigm shift of displays Cathode-Ray Tube (CRT) Plasma Display Panel (PDP) Liquid-Crystal Display (LCD) Organic Light-Emitting Diode (OLED) Large displays Thinner, lighter and more Simple fabrication process Good color rendering Lightweight construction flexible than other display Simple driving system Thin panels Thin panels Much less power Low price No dependence on Low energy consumption Large fields of viewing viewing angle Transparent or Flexible Displays The 1 st generation Display The 2 nd generation Display The next generation Display
OLED vs LCD LCD OLED Structure 1~5 ms 5000:1 120 (left/right) 110 (up/down) Response time Contrast ratio Viewing angle Below 0.001ms Infinity Free Below 100% Color reproduction 108% 출처 : LG Display
Advantages of OLED Thin and Light Self Emitting Wide View Angle High Resolution?? Flexible Fast Response Time
Other OLED applications-oled lighting Phillips LG KONICA MINOLTA R-Display & Lighting
OLED operating mechanism
Organic compound? Compounds containing the carbon atom Classification of Organic Compound Small molecules -Low molecular weight materials ( below Mw~1000) Polymer -High molecular weight materials (More Mw~15000) TPD Polymerization F8BT Poly-TPD Alq 3 TPBi PFO-DBT
OLED Structure 및발광원리 (Energy level) Vacuum Level Conduction Band Work Function (Anode) Electron Affinity Band gap Energy LUMO Ionization Potential ΔE Cathode Work Function (Cathode) Anode HOMO Valence Band Work Function : 일함수, 전자및정공하나가탈출하기위해필요한에너지 ΔE : 정공혹은전자의이동에필요한에너지장벽 LUMO : 반도체의 conduction band와같은전자의이동경로, Lowest Unoccupied Molecular Orbitals HOMO : 반도체의 valence band와같은정공의이동경로, Highest Occupied Molecular Orbitals
Semiconductor A material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. Conduction Band Energy Band gap (<3~4eV) Valence Band Metal Semi conductor Insulator
Hybrid orbital Electronic configuration of Carbon 1s 2s 2px 2py 2pz < Ground state of carbon atom > 1s 2s 2px 2py 2pz < Excited state of carbon atom > sp hybrid orbital sp hybrid obital Alkyne: C n H 2n-2 sp hybrid obital (σ bond) unhybrid obital (π bond) 1s 2s 2px 2py 2pz H C C H < Excited state of carbon atom > < Structural formula of acetylene >
Hybrid orbital sp 2 hybrid orbital sp 2 hybrid obital 1s 2s 2px 2py 2pz sp 3 hybrid orbital sp 3 hybrid obital 1s 2s 2px 2py 2pz Alkene: C n H 2n H C C H H H < Structural formula of ethylene > H H C H sp 2 hybrid obital (σ bond) unhybrid obital (π bond) H < Structural formula of methane > sp 3 hybrid obital (σ bond)
Chemical bond Sigma bond and pi bond (ex. Ethylene) A double bond (sigma bond + pi bond) Each single bond (sigma bond) s * p * p s Ground state Sigma bonds in ethylene Sigma bond & pi bond on carbon in ethylene 출처 : http://slideplayer.com/slide/273513/
Chemical bond Sigma bond (σ bond) -The covalent bond formed by the overlap of atomic orbitals along the internuclear axis -The overlapping orbitals are oriented along the internuclear axis -The bond is rotationally symmetrical around the internuclear axis -Stronger than a pi bond Pi bond (π bond) -The covalent bond formed by the lateral overlap of two p orbitals which are mutually parallel but oriented perpendicular to the internuclear axis -The overlapping orbitals are oriented perpendicular to the internuclear axis -The bond is not rotationally symmetrical around the internuclear axis -Only p orbitals can form pi bond -Weaker than a sigma bond
Chemical bond Anti-bond Empty 2p z 2p z Bond Full Molecular Orbital: a mathematical function describing the wave-like behavior of an electron in a molecule. Anti-bond E Bond
Conjugated organic compound Organic compound which is composed of repeated single and double bond. Butadiene C 4 H 6 Polyacetylene Pentacene Fullerene
Conjugated organic compound Butadiene Node 4 P orbitals 0 bonding 3 anti-bonding 3 node 1 bonding 2 anti-bonding 2 node 2 bonding 1 anti-bonding 1 node 3 bonding 0 anti-bonding 0 node Delocalized ground state of π orbital Node: place of zero electron density between the atoms 출처 : https://www.chem.wisc.edu/
Orbital Energy Conjugated organic compound Benzene Resonance Polyacetylene Antibonding orbitals π* Conducting band (Empty) p z Band gap π Bonding orbitals pi electron density Chain Length Infinity Valence band (Filled) Blue emission in polymer?
Energy band gap The difference energy gap between electron orbitals in which the electrons are not free to move (the valence band or HOMO) and orbitals in which they are relative free and will carry a current (the conduction band or LUMO). HOMO : highest occupied molecular orbital, LUMO : lowest unoccupied molecular orbital Band gap in small molecule -Each single level formation for HOMO and LUMO level LUMO HOMO Band gap in polymer Gap Width Band width was increased Band gap was decresed poly acetylene Box type
Energy band gap V ac (Vacuum level) Ionization potential (I p ) V ac Electron affinity (E a ) Unoccupied Energy band Gap LUMO Occupied HOMO Ionization potential (I p ) : The energy usually required to remove an electron from an atom, molecule, or radical, usually measured in electron volts. Electron affinity (E a ) : The amount of energy released when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion. Energy gap = E HOMO -E LUMO I p -E a I p -E a + Polarization potential
P-conjugated polymers: Energy bandgap 4 E g opt ex Eg 1.4 Chain length n 1 Energy gap (HOMO-LUMO) decreases with increasing p-conjugation
1-D system
Peierls Instability : Energy Bandgap high electron-phonon coupling : in organic, doping or adding molecular functional group can change the lattice (molecular structure) compared with metal, elastic energy & electric energy
Electron-phonon coupling : Polaron
Soliton
Polaron : energy gap
Stokes shift
OLED operating mechanism Energy barrier at Organic semiconductor/metal electrode interface Carrier transport Carrier injection Exciton Electron-hole recombination
Energy barrier at organic semiconductor/metal electrode interface Energy barrier at Organic semiconductor/metal electrode interface
Energy barrier at organic semiconductor/metal electrode interface Energy barrier is determined by energy gap between metal work function and energy level of organic material 출처 : OLED 기초공정이해와실습
Carrier injection Carrier injection
Carrier injection Injection Dominated Mechanisms -Thermionic emission -Tunneling (Fowler-Nordheim Tunneling) -Carrier injection by impurity level Barrier gap ~1nm Thermionic emission Fowler-Nordheim Tunneling Carrier injection by impurity level
Carrier injection Injection Dominated Mechanisms: Thermionic emission : The thermally induced flow of charge carriers from a surface or over a potential-energy barrier - Richardson constant (=120(m*/m)A/cm 2 /K 2 ) m* : Effective mass of hole, electron, T: Temperature, H: Planck s constant, kb : Boltzman constant, q: Quantity of charge, Φ : Barrier gap, V: Voltage
Carrier injection Injection Dominated Mechanisms: Fowler-Nordheim Tunneling : Carrier tunneling Injection by locally formed high electric field (10 6 ~10 7 V/cm) at organic film/metal electrode interface Where
Carrier injection Bulk Dominated Mechanisms -Ohmic Conduction -Space Charge Limited Current Conduction -Space Charge Limited Current Conduction with Traps
Carrier injection Bulk Dominated Mechanisms: Ohmic Conduction -Injected carrier concentration was lower than free charge concentration at low bias voltage condition Current depends linearly on the bias voltage n o : Carrier concentration, μ: Mobility of hole or electron, e: Quantity of charge, Voltage bias, d: Film thickness 출처 : http://www.slideshare.net/richardyang13/richard-yang-phd-defense-talk
Carrier injection Bulk Dominated Mechanisms: Space Charge Limited Current Conduction -Injected carrier concentration was higher than free charge concentration at high bias voltage condition (No traps) Carrier concentration Metal Organic material Carrier concentration is non-uniform in organic semiconductor because low mobility of organic materials (Transition time > dielectric relaxation time) 출처 : http://www.stallinga.org/
Carrier injection Low mobility of Organic materials (Organic semiconductors) -holes: μ ~10-4 -10-8 cm 2 /Vs due to poor wave function overlap (large hopping distance) -electrons: μ ~10-6 -10-10 cm 2 /Vs due to increased disorder and trapping at defect sites caused by impurities such as O 2. Build-up of space-charge density (bulk limited current)
Current Carrier injection Bulk Dominated Mechanisms: Space Charge Limited Current Conduction with traps J~V 2 (Trap free SCLC) J~V (Ohm s law) E c : Conduction band E t : Trap level Ohmic conduction (Low bias voltage) V th Voltage Similar current-voltage curve of trap free SCLC (Because trap site was fully filled by injected electrons Similar organic film without trap sites) Current vertically increased by bias voltage Filled shallow trap by injected
Carrier transport in organic materials Carrier transport
Carrier transport in organic materials Hopping model Poole-Frenkel model Small polaron model Multiple trapping & release model Etc (Exciton model, Soliton model, Ishiguro model )
Carrier transport in organic materials Hopping model -Strong π-orbital overlap -Mobility decreases as T -Band transport -Week π-orbital overlap -Mobility increases as T -Hopping transport -Delocalized states propagating Fi : intermolecular interaction force Fv : thermal vibration force -Localized states hopping Fi > Fv Fi ~ Fv 출처 : OLED 기초공정이해와실습
Carrier transport in organic materials Poole-Frenkel model - Field dependent mobility at high field (E > 10 5 V/cm) - Coulomb potential near localized states is modified by the applied field. μ(0): Zero filed mobility, β=(e/πεε 0 ) 1/2 : Pool-Frenkel factor, F: Magnitude of electric field Electric field Electron energy Electron energy Conduction band edge at F=0 Conduction band edge at F
Carrier transport in organic materials Small polaron model (Small polaron hopping) - In a conjugated molecules, a charge is self-trapped by the lattice deformation. creation of the localized mid gap states. The localized mid gap states Multiple trapping & release model - Narrow delocalized band + localized trap level (highly concentrated) Trap level Process 1) Carriers arrive at a trap and are trapped 2) Thermally activated and released
Rate of Charge Transfer the rate of charge transfer is limited by the reorganization of the molecules intrachain transport of soliton (m~1000) >> interchain transport (m ~ 10-2 ~10-3 ) due to weak intermolecular interaction ( < 10 kcal/mol) ex. Si : 76 kcal/mol Carrier mobility
Electron-hole recombination Exciton Electron-hole recombination
Recombination-Excitons -A bound state of an electron and an hole which are attracted to each other by the electrostatic Coulomb force. (bound electron-hole pairs) : Treated as chargeless particles (because of recombination) : Capable of diffusion (singlet ~ 10 nm, triplet ~ 140 nm : Exciton diffusion length) : position of exciton formation is important : Excited state of molecule -Classification of excitons Wannier exciton Charge transfer excitons Frenkel exciton 출처 : http://image.slidesharecdn.com/
Recombination -The process in which an electron, which has been excited from the valence band to the conduction band of a semiconductor material, falls back into an empty state in the valence band, which is known as a hole. -Process to generate exciton in organic materials -It depends coulomb energy -Coulomb capture radius : The distance at which the hole s coulomb binding energy equals kt Coulomb capture radius (r c ) > Distance between electron and hole Recombination and exciton generation Coulomb capture radius (r c ) < Distance between electron and hole Generated electron and hole thickness of EML > r c (14~18 nm) Coulomb potential
Recombination-Excitons Wannier exciton Frenkel exciton Delocalized (free e & h carrier) localized -Typical of inorganic semiconductors -Binding energy ~10 mev -Radius ~100 A -Difficult existence at RT condition Thermal energy at RT ~ 25 mev -Typical of organic semiconductors -Binding energy ~1 ev -Radius ~10 A -Existence at RT condition 출처 : Electronic Processes in Organic Crystals and Polymers by M.Pope and C. E. Swenberg
Recombination 1. Unbounded electron-hole pair on neighboring chains 2. Electron-hole pair is captured to form a weakly bound charge transfer exciton 3. Inter-conversion to a strongly bound exciton 4. Exciton decays radiatively 출처 : OLED 기초공정이해와실습
Emission mechanism -Exciton formation and emission mechanism by electron-hole recombination Electron-hole Jablonski Diagram Singlet Exciton Triplet Exciton Intersystem 25% crossing 75% hv Fluorescence Thermal deactivation hv Phosphorescence Thermal deactivation Ground state
Emission mechanism-singlet & triplet 출처 : Nature Physics 7, 307 310 (2011) - 3 TE states (S = 1): ( ) (S z = +1), ( + )/ 2 (S z = 0), ( ) (S z = -1). - 1 SE state (S = 0): ( - )/ 2 (S = 0) - In fluorescent organic materials, only the SE decays radiatively, to yield the photoluminescence (PL) or electroluminescence (EL) TE: Triplet exciton, SE: singlet exciton
Emission mechanism -Fluorescence : Spin anti-symmetric (25%) : Symmetry conserved fast process ~10-9 s -Phosphorescence : Spin symmetric (75%) : Triplet to ground state transition is not permitted slow process ~1s 출처 : LG 전자 & http://www.shsu.edu/
Emission mechanism -Fluorescence & Phosphorescence emitter Carrier injection & moving Exciton Coupling of Spin-orbital angular momentum increases with Z 4 Fluorescence materials Singlet exciton formation Max.η s =25% Phosphorescence materials Singlet/triplet exciton formation Max.(η s +η t )=25%+75%=100% Ideal internal quantum efficiency 25% Ideal internal quantum efficiency 100% Phosphorescence materials efficiency is four time better than fluorescence materials efficiency Doped Phosphorescence materials (guest) in fluorescence materials (host) was suggested (phosphorescence guest : organics with heavy metals ( Pt, Ir, Eu, ) ref: LG 전자
Energy transfer (Doping case) Föster energy transfer Dexter energy transfer -Energy transfer by dipole-dipole interaction -Very fast <10-9 s -Long range ~30-100 A -Commonly, singlet to singlet energy -Energy transfer by exchange electrons (Diffusion of excitons from donor to accepter) -Short range ~6-20 A -Singlet to singlet & triplet to triplet transfer host dopant D: donor A: Accepter *: Excited state
Exciton Energy Transfer
Dexter Energy Transfer doping : color tuning, luminance efficiency, device stability, life time, operation voltage
OLED materials
Key performances of OLED materials Energy level(ev) : relation with efficiency and driving voltage - HOMO(highest occupied molecular orbital) - LUMO(lowest unoccupied molecular orbital) Mobility(cm 2 /v s) : relation with efficiency and driving voltage - Hole and electron mobility Emitting wavelength(nm) : relation with color gamut - Photoluminescence PL quantum efficiency(%) : relation with luminance efficiency - Absolute PL efficiency Thermal stability : relation with life time - Glass transition temperature - Melting temperature - Decomposition temperature
OLED material requirements LUMO EML > LUMO ETL LUMO EIL HOMO EML > HOMO ETL ETL(E T ) > EML(E T ) LUMO : Lowest Unoccupied Molecular Orbital HOMO : Highest Occupied Molecular Orbital HOMO HIL > HOMO HTL > HOMO EML LUMO HTL > LUMO EML HTL(E T ) > EML(E T ) High triplet energy transfer host (Host(E T ) > Dopant(E T )) Bipolar charge transport properties Charge balance
OLED material requirements Cathode EIL ETL EML HTL HIL Anode Substrate Light Material requirement - Purity : Removal of other impurity - Amorphous : Non-crystalline - Thermal stability : High T g* and T * d - Appropriate HOMO and LUMO level - Interfacial property : Good adhesion - Simple preparation and purification - transparency depending on emission direction (color purity) *T g :glass transition temperature *T d : degradation temperature
OLED electrodes(cathode, Anode) Anode: - Requirement of transparency Indium-tin-oxide (ITO): 4.5-5.1 ev (Indium diffusion problem) Au: 5.1 ev Pt: 5.7 ev Cathode EIL ETL EML HTL HIL Anode Substrate Light Cathode - Requirement of low work function (alkaline, alkaline earth materials) Ca: 2.9 ev Mg: 3.7 ev Al: 4.3 ev Ag: 4.3 ev Mg: Al alloys Ca: Al Alloys
HIL(hole injection layer) ITO surface treatments - O 2 plasma - CF 4 /O 2 plasma - UV-ozone - Organic contaminants removal - Work function increases up to near 5.0 ev Cathode EIL ETL EML HTL HIL Anode Substrate Ionization potential (HOMO) of common HTL materials is about 5.5 ev. Therefore, still lower than HTL about 0.5 ev. Light
HIL(hole injection layer) Requirements - Reduce the energy barrier between anode and HTL. ; Enhances charge injection at interface. ; Efficient hole injection from anode to EML - Starburst amine type materials, CuPc - Uniform film formation (no crystallization) - High thermal stability with high T g - Good contact (adhesion) with anode - Transparent in the RGB wavelength region CuPc PEDT/PSS TPD TF-TCNQ
HTL(hole transport layer) Cathode EIL ETL EML HTL HIL Anode Substrate - Matched anode work function and HOMO energy level of EML - High LUMO level : Electron blocking - No crystallinity and high T g - High hole mobility : 10-3 cm 2 /Vs - Aromatic amines Light
HTL(hole transport layer) Requirements - High hole mobility - Aromatic amine type materials (for thermal stability) - Intermediate HOMO between HIL and EML(~5.5eV) - Low LUMO level for electron blocking (<2.5eV) - Formation of stable radical cation - High triplet energy for exciton blocking(triplet devices) - Uniform film formation (no crystallization, amorphous) - High thermal stability with high T g - Transparent in the RGB wavelength region α-npd TPD TAPC Spiro-TPD More rigid & T g enhancement & higher pi electron density
ETL(electron transport layer) Cathode EIL ETL EML HTL HIL Anode Substrate - Matched cathode work function and LUMO energy level of EML - Low HOMO level : hole blocking - No crystallinity and high T g - High electron mobility : 10-6 ~ 10-5 cm 2 /Vs - Alq 3 derivatives, triazole, oxydiazole, silole (1) e-withdrawing moiety (2) Metal-organic complex (3) Silole derivatives (4) e-deficient center Light
ETL(electron transport layer) Requirements - Electron transport from cathode(or EIL) to EML - Reversible electrochemical reduction - Suitable LUMO values for electron transport(~3.0ev) - Suitable HOMO values for hole blocking(~6.0ev) - High triplet energy for exciton blocking(triplet devices) - Formation of stable radical anion - High electron mobility - High T g and amorphous - Processibility TAZ Alq 3 TPBI PBD
EML(Emissive layer) Cathode EIL ETL EML HTL HIL Anode Substrate Two principle branches (1) - Light-emitting polymers(leps) - Polymer Light Emitting Diode (PLEDs). - Using relatively large molecule (2) - Small Molecule Organic Light Emitting Diodes (SMOLEDs) - Using relatively small molecules (even monomers) Light
EML(Emissive layer) Small molecules - Thermally stable, - Highly luminescent in the solid state, - Thin-film forming upon vacuum deposition Polymers - Low cost - Low operating voltage - Large area, flat and flexible display - Easy fabrication - solvent orthogonality
EML(Emissive layer) Small Molecules Thermal Evaporation RGB/FMM Transfer LITI RIST LIPS Polymers Solution Process Coating Inkjet Printing Nozzle Printing Gravure Printing
OLED Structure 및발광원리 (Energy level) LUMO EML Cathode Carrier 이동도중발광층 (EML) 내에서재결합 Exciton 여기자 (Exciton) 형성 Anode Singlet State 형광 Triplet State 인광 HOMO Ground State
OLED 형광 / 인광 (Singlet & Triplet) Singlet Spin anti-symmetric Relaxation allowed Fast Efficient Singlet (25%) Excited State Triplet (75%) Ground state Triplet Spin symmetric Fluorescence OLED Phosphorescence OLED Relaxation disallowed Slow Inefficient
OLED 형광 / 인광 형광 인광 대표적인물질 특성 -Singlet 이용 ( 최대발광효율 25%) -Decay time ~ ns -Singlet + Triplet 이용 ( 이론적최대효율 100%) -Decay time ~ μs 장점 단점 - 내전류특성우수하여 Passive matrix 에유리 -Color 화양산공정가장안정됨 - 상용화선점 - 대형화에불리 (Shadow mask) - 고분자에비해장비고가 - 이론적양자효율 100% 로효율개선가능성높음 -Active matrix 에유리 - 대형화에불리 (Shadow mask) - 고분자에비해장비고가 - Blue 색순도, 수명낮음 <1,000 h 주요업체 LGE, SNMD, RiT, Pioneer, Sony, Sanyo... LGE, SNMD, RiT Pioneer, Sony, Sanyo...
EML(Emissive layer) Small molecules generally deposition process Ref) 한밭대, 노용영
EML(Emissive layer) - Small molecules (materials) Red Fluorescent Materials - Highly polar - pi-conjugated ; low luminance efficiency - Aggregation in solid state - Low doping concentration (in case of high doping, emission quenching problem) - Broad band wavelength (vibrational modes) ; Adopting amine unit (lowering HOMO level) ; Minimizing degree of substituent s freedom (rigid group) ; Using deep trap formation (1) Pyran-nased compounds (2) Perylene derivatives (3) Dicyano naphthalene compounds DCJTP Rubrene BSN Ref) 그라쎌, 김봉옥
EML(Emissive layer) - Small molecules (materials) Green Fluorescent Materials - Less polar than red emitting material - Non-planar structure with heteroatom - Aggregation in solid state ; Adopting amine unit 도입 ; Using bulky substituent (1) Coumarin-based compounds (2) Quinacridone-based compounds (3) Polyarene materials C545T Quinacridone Polyarene Ref) 그라쎌, 김봉옥
EML(Emissive layer) - Small molecules (materials) Blue Fluorescent Materials - Aromatic non-planar structure - Wide bandgap ( 3.0eV) - Thermal stability : high T g - High PL efficiency of host ; Using anthracene frame or distyryl arylene ; Many aromatic ring (1) Anthracene-based compounds (2) Distyrylarylene-based compounds (3) Polyphenyl / aromatic compounds (4) Metal-organic complexes DSA DNA/mADN/TBADN Spiro-oligo(phenylene) DSA-amine Ref) 그라쎌, 김봉옥
EML(Emissive layer) - Small molecules (materials) Phosphorescent Pt, Ir Complexes Ref)M.E.Thompson et al J.Am.Chem.Soc., 2001. 123. 4304-4312
고분자및저분자재료소자구조비교 ITO Patterning ITO Patterning 보조전극 HIL/HTL증착 EML/ETL증착 Cathode ETL EML HTL HIL Anode Cathode EML HTL Anode 보조전극 Spin Coating 발광층 Cathode Glass substrate Glass substrate Cathode Encapsulation 유기 EL ( 저분자 ) 유기 EL ( 고분자 ) Encapsulation 특성저분자고분자 박막층 (layer) 발광층형성법 표시특성 구동전압 소비전력 재료수명 7-8 층증착공정동일 6~7V - R,G >10,000, B <6,000 4 층 LITI, Ink Jet 등 printing process 동일 4~5V 저분자대비 40% 낮음 R>10,000, G<3,000,B <1,500
OLED 소자발광구조 Bottom Emission OLED Top Emission OLED Metal cap Getter Metal cathode Inert gas Semitransparent cathode Glass cap or Film type cap Sealant 투명 getter Organic Layer Passivation Layer Planarization layer Anode Planarization layer Glass Substrate Driving TR Glass Substrate Driving TR Pixel 구동회로부
OLED 소자발광구조 Top emission Bottom emission 개구율 A T > A B 효율 (cd/a) E T = E B 전류밀도 (J=A/m 2 ) J T < J B 수명 T T > T B 공정수 12 > 9 대면적화 투명 cathode 성능향상필요 대면적화용이 LG display
OLED 소자발광구조 Capping layer (Encapsulation layer) - 투명박막으로구성되어야함 - Metal Cap 및 desiccant 는적용어려움 Glass Cap을이용하거나투명한특성을가지는흡습제개발이필요 투명 Cathode - 대표적인투명전극 : ITO, IZO 일함수가높고스퍼터링공정법으로형성하여야하므로적용이어려움 - 얇은 metal 형성기술 금속을얇게형성하여투명한특성을가지게형성 (Optical skin depth) 항경우빛의투과가발생하지만투과율이매우낮음 새로운방식의투명전극형성기술개발이필요
Microcavity effect IN OUT M 2 ( 반투과막 ) L M 1 ( 반사막 ) Advantages -Spectral Narrowing: Color Purity 향상 -Luminance Enhancement Green Red Blue Sony J. Appl. Phys., 80 6954 (1996)
OLED 소자제작공정 Vacuum Thermal Evaporation ( 저분자 ) Laser transfer ( 고분자 + 저분자 ) Printing ( 고분자 + 저분자 ) 방식 기술적특징기계적위치정밀제어진공증착광학적위치제어레이저전사용액인쇄방식 기판크기 400 ⅹ 400 mm 550 ⅹ 650 mm 370 ⅹ 470 mm 대형화적용용이성 기술완성도 Remark 발광효율, 수명우수 고효율 / 색조절용이 고정세, 대면적기판대응어려움 Pioneer, NEC 등 수율, 발광효율저하 Donor film 재활용어려움 Thin thickness control 기술완성도미흡 Particle 오염우려 Sony(LIPS), SMD(LITI), Kodak(RIST) 수명, 발광효율낮음 재료개발필요 Head nozzle 막힘발생 ( 특히 IJP) Tact time 검증필요 DuPont (Pen), Seiko-Epson (IJP) LG display
OLED 소자제작공정 증착공정 Fine Metal Mask (FMM) Method - 대면적구현이어려움 - 기판및마스크쳐짐문제발생 단국대, 박진성
OLED 소자제작공정 Laser transfer Laser transfer Method Laser Induced Thermal Imaging (LITI) Laser-Induced Pattern-wise Sublimation (LIPS) - 기판대형화용이 - High resolution - 장비 / 공정개발필요 - 양산성검증필요 LG display
OLED 소자제작공정 Printing Printing method - 대면적구현용이 - 재료순도에의한효율저하 LG display
OLED 소자제작공정 Printing process for OLED < Inkjet printing > < Screen printing > < Flexo printing > < Gravure printing >
Full Color 구현기술 Shadow Mask (SM) 방식 Color Filter (CF) 방식 Color Change Media (CCM) 방식 구조 구성 R/G/B EL layer 각각 patterning 하여형성 백색유기 EL + Color filter 청색유기 EL + 녹색 / 적색형광색소 & 청색 color filter 발광효율 Color Reproduction Quality Merits 높은발광효율공정과정줄임공정과정줄임 Demerits 미세 pattern 된 R/G/B 형성이어려움 (Mask align 기술요구 ) 백색유기 EL 의 white balance 최적화필요 Blue EL 의효율향상및 CCM layer 의효율향상필요
Color Filter (CF) 방식 : White OLED + Color Filter OLED 소재로 R/G/B pixel 을따로구현하는대신흰색빛을내는 White OLED 상 Color Filter 를도입하여 R/G/B pixel 을구현하는소자 < 자료제공 :LG 디스플레이 > Color Filter 를통과하며휘도감소발생 White sub-pixel 추가 (WRGB)
White OLED Device structure < Single EML > < Multiple EML > < Color conversion > < Tandem > Structure Single EML Multiple EML Color conversion Tandem Efficiency X Lifetime X Process : Best, : Good, : Normal, X : Bad 백색발광을이용한 OLED 조명기술, 2011
White OLED 구분 White OLED RGB OLED 구조 < 자료제공 :LG 디스플레이 > < 자료제공 :LG 디스플레이 > 색재현성우수 (Color Filter 의순도에의존 ) 우수 ( 발광재료에의존 ) 발광효율 낮음 우수 제조공정 쉬움 어려움 가격 중간 높음 장점 단점 단순한유기막구조 R, G, B 형성용 Shadow Mask 가필요없음 대형화, 고해상도에유리 낮은전력소모 & 긴제품수명 4 개의 sub pixel 로섬세한화질과색상표현 Color Filter 통과한빛의최종광효율이 1/3 수준으로낮아고효율의재료가필요 White OLED (100%) 유기물사용증가 Color Filter 33% 각색의특징 ( 효율 ) 을최대한활용가능 휘도감소없음 Differential Degradation R, G, B 를형성하는유기물의수명이동일하지않아발생하는문제 서로다른수명에따라발현하는색이왜곡될소지 대형화 (FMM 휘어짐 ), 고해상도곤란 공정중오염증가 R, G, B OLED 소자모두개발할필요수율감소 Application 대형기판 / 중대형 Display 소형기판 / 소형 Display
OLED 소자구동원리 Passive Matrix (PM) Active Matrix (AM) Matrix 전극사이에 EL device Line 선택구간만 Emission Matrix 전극사이에 EL 구동 TFT Frame time 동안 Emission 가능
OLED 소자구동원리 Passive Matrix Data Lines (Anodes) + + + + + + Scan Lines (Cathodes) - 간단한구조및제작방법 - Small size panel - Cathode patterning 필요 - 높은구동전압 & 전력소비 & pulse drive 전류 - Cross-talk 또는 line defect 발생 (Shorted pixels) Programmable current source OEL C para
OLED 소자구동원리 Active Matrix VDD Data signal C s M 0 M 0 : current source C s : storage cap. OEL - Full color 구현가능 - 낮은구동전압 & 전류 & 전력소비 - 대형화용이 - Pixel 구현을위한 anode patterning 필요
OLED 소자구동원리 구동방식비교 Passive Matrix Active Matrix Driving Method Duty Driving (Scan 라인선택시점등 ) Static Driving ( 상시점등 ) 고휘도고정세화 - Scan Line 증가에따라높은순간휘도요구 - Scan Line 수의한계 - Scan Line 수에관계없이고휘도실현가능 저소비전력 - 순간휘도 = Scan Line 요구휘도 고전압구동 - 요구휘도의구동전압으로상시발광 저전압구동 소형화 - 구동 IC 를외부장착 - 구동회로 Panel 내부에내장 소자구조및 cost - Simple Process - Low Cost - LTPS * + OLED - 복잡한프로세스 *LTPS : Low Temperature Poly Silicon
OLEDs 효율 OLED 발광효율 = 내부발광효율 X 광추출효율
OLEDs 효율
OLEDs : 광추출효율 Cathode Organic Layers 내부광추출 ITO Internal Ext. Substrate External Ext. 외부광추출 Kanchan Saxena, Optical Materials (2009) 굴절률차이로인한전반사, 표면에서도파로모드발생 광손실 80% 외부로추출되는광효율 20% OLED 광효율향상을위해광추출기술이필요
OLED : 광추출기술 외부광추출기술 외부에추가적인구조를도입 기판내고립된빛을추출하는기술 최윤석교수 ( 한밭대학교 ) 마이크로렌즈어레이 (MLA) Surface roughness W. J. Hyun, S. H. Im, Org. Electron (2012) Texturing meshed H. Peng, Y. L. Ho Display technology (2005) B. W. D Andrade, J.J. Brown, Apply. Phys. Lett (2006) Y. H.Cheng, J. L. Wu, C. H. Chen, Apply. Phys. Lett (2007)
OLED : 광추출기술 내부광추출기술 ITO 와유기물층에고립된빛을추출하는기술 외부광추출기술보다높은광효율추출 최윤석교수 ( 한밭대학교 ) 광결정 (Photonic Crystals) Scattering layer 나노요철구조 Kanchan Saxena, Optical Materials (2009) H.-W. Chang, J. H. Lee, Appl. physics (2013) K. Hong, H. K. Yu,. Adv. Mater (2010)
OLED : 광추출기술 Low-haze for display device Nanoscale Corrugation by plasma treatment for Light Extraction (NCLE) a. NCLE 2 (Height 80nm, width 120nm) b. NCLE 3 (Height 110nm, width 130nm) c. NCLE 4 (Height 180nm, width 230nm) < SEM images of various NCLE on glass substrates formed > Haze = Total transmittance Specular transmittance Total transmittance 1. Top : Bare glass 2. Middle : NCLE glass 3. Bottom : glass packed MLA < Comparison of the resulting haze > < External quantum efficiency > Nanoscale, 7, 2723-2728 (2015)
OLED : 광추출기술 Low-haze for display device Randomly Dispersed Nanopillar Arrays Formed by Lateral Phase Separation of Polymer Blends 1. Bare glass ~ 0.18% 2. Nanopillar + 평탄화막 ~ 0.41% 3. 70μm 지름의 MLA ~ 84.84% < Comparison of haze > < Patterns of nanopores formed by phase separation of two-polymer blends > < Emission patterns similar to Lambertian emission > Small, 25, 3858-3863 (2013)
Encapsulation Inorganic layer (single layer) OLED encapsulation 의필요성 -외부오염인자인수분과산소는 OLED 소자의수명및신뢰성을저하시키는요인으로작용 외부로부터산소와수분을차단할수있는 layer 필요 OLED Requirement LCD Requirement H 2 O Permeation Rate (g/m 2 /day at 25 degrees) 각 display 소자별수분침투허용량 OLED 소자의수분존재 (3 %) 하에서의시간에따른발광상태변화
Encapsulation OLED encapsulation method Cover Glass method Multi layer method Organic/Inorganic layer Inorganic/inorganic layer Grated Organic/Inorganic layer Inorganic + Nanoparticles layer
Encapsulation Cover glass method Substrate ITO Epoxy Adhesive Encapsulation Housing Desiccant (Getter) Membrane OLED (Metal or Glass Cap) (BaO, CaO) Characteristics - 우수한 barrier properties & environmental stability - 높은투과특성 - 경량화가능 - expensive line/cost
Encapsulation Multi layer method (Organic/Inorganic layer) Characteristics - organic layer (Flexibility) 와 inorganic layer ( 침투억제 ) 의장점을동시에가짐 - Low process temperature - Too sensitive to particles - Huge investment - Penetration via side path Vitex
Encapsulation Multi layer method (Inorganic/Inorganic layer) SiN SiO SiN SiO SiN SiO SiN Characteristics Philips NON 구조및 SEM image - Inorganic layer ( 침투억제 ) 의장점을극대화 12층적층시 10-6 g/m 2 /day 수분투과율달성 - Too many layers - Too sensitive to particles - Huge investment Phillips
Encapsulation Multi layer method (Grated Organic/Inorganic layer) PECVD 를이용하여동일한 CVD 챔버내에서유 / 무기막을증착 경계부분에서유 / 무기막이혼합되는구조 Characteristics - 10-6 g/m 2 /day 정도의수분투과율달성 - 표면평탄도 0.6 nm - 투과도 82 % General electric, 2008
Encapsulation Multi layer method (Inorganic + Nanoparticles layer) Characteristics - 10-6 g/m 2 /day 정도의수분투과율달성 -Nanoparticle 이 defect sealing 역할을하여수분및산소침투억제및흡수 IMRE, 2009
Transparent OLED 구조 Transparent substrate, cathode and anode Bi-direction light emission Passive or Active Matrix OLED Useful for heads-up display Transparent projector screen Glasses Requirements for Transparent Conductive Oxide Good conductivity Bandgap energy Eg> 3 ev High mobility RT deposited film Amorphous film Non-toxic elements Good reliability Cheap processing
Transparent OLED 제품 OLED Display Panel OLED Display Products Fraunhofer 넥시안글레이즈 M-9090 Sony Ericsson Xperia Pureness
OLED 조명 조명용광원특성별비교
OLED 조명 OLED 조명특징 가볍고 flexible화가능 색상조절가능 눈부심없는확산조명 투명조명, 거울조명가능 고효율, 장수명, 저전압구동가능 넓은동작온도, 높은내구성 순간 on/off 대면적조명가능 유해물질없음
OLED 조명 OLED 조명기술현황
OLED 조명 향후전망
QLED Structure Anode HIL/HTL ETL EIL/ETL Cathode Substrate Anode HIL/HTL QDs EIL/ETL Cathode Substrate
QLED Structure Anode HIL/HTL ETL EIL/ETL Cathode Substrate Anode HIL/HTL QDs EIL/ETL Cathode Substrate
QLED 발광원리 (Mechanism) 양자사이즈효과나노구조속에갇힌전자가특정한조건하에서만존재하게되는효과로 Quantum Dots 에서는전자가 3 차원의어느방향으로나양자사이즈효과로갇혀, 어느방향으로나운동의자유도를갖지않는제로차원전자계가실현된다.
QLED 발광원리 (Mechanism) Four routes for generating excitons in QDs a. Optical excitation an exciton is formed in a QD by absorbing a high-energy photon b. Charge injection an exciton is formed by injection of an electron and a hole from neighbouring CTLs c. Energy transfer an exciton is transferred to a QD via FRET from a nearby donor molecule d. Ionization a large electric field ionizes an electron from one QD to another, thereby generating a hole. When these ionization events occur throughout a QD film, generated electrons and holes can meet on the same QD to form excitons
Quantum Dot-LCD using QD-Enhancement Film QD Photo-Luminescence Principle QD-LCD 장점 (QDEF) - 기존 LCD보다높은색순도 - OLED 보다저렴한제작비용 - LCD보다높은발광효율및에너지효율 QD-LCD 단점 (QDEF) - 중금속 ( 카드뮴 ) 사용 QD-LCD Structure 출처 : LG 캐미토피아, Optical Characterization of Group 12-16 (II-VI) Semiconductor Nanoparticles by Fluorescence Spectroscopy
QDEF-LCD Application QDEF-LCD TV panel LG Display QDEF-LCD VS 일반 LCD QDEF-LCD 일반 LCD 출처 : nanoco, 3M& 나노시스, LG display
QLED VS OLED Feature QLED OLED Efficiency Emission bandwidth(color saturation) Low (~18%, Nature Photonics 7, 13 23 (2013)) Narrow : FWHM<30nm High (~25%, Cho, Y. J., Yook, K. S. and Lee, J. Y. (2014)) Broad : FWHM>40nm Color Tunability Excellent : Change QD size Low : Different emitter Cost of Emitter Manufacturing Process Large display area Low one procedure for all RGB emitters Solution-based Yes High Vacuum deposition, Solution-based Yes Flexibility Yes Yes Near-IR emission(telecom munication sensor) Yes No Narrow, deep and controllable emission Vivid colors Wide color gamut