Transcript 投影片 1
Electronic and Optoelectronic Polymers
Wen-Chang Chen
Department of Chemical Engineering Institute of Polymer Science and Engineering National Taiwan University
Outlines
History of Conjugated Polymers
Electronic Structures of Conjugated Polymers
Polymer Light-emitting Diodes
Polymer-based Thin Film Transistors
Polymer-based Photovoltaics
Optical Absorbance
Absorption of light and the excited states of molecules Beer-Lambert Law
A = 2 - log10 %T
A is absorbance
I 0
C is concentration is intensity of incident light λ is wavelength of light
I 1
is intensity after passing through the materials l is path length k is extinction coefficient α is molar absorptivity or absorption coefficient α is a measurement of the chromophore’s oscillator strength or the probability that the molecule will absorb a quantum of light during its interaction with a photon
Jablonski Diagram
Photophysics Process
Non-Radiative Process Internal conversion (IC): electron conversion between states of identical multiplicity Intersystem conversion (ISC): electron conversion between states of different multiplicity singlet state : all electrons are paired ( )with opposite spins Triplet state : same spins pairing of electrons ( )
From Quantum Statistics
Photophysics Process
Excited state Ground state Singlet Triplet 75% 25% 1/√2 ( 1/√2 ( + Triplet state (symmetric) Spin unpaired, S=1 ) ) Singlet state (anit-symmetric) Spin paired, S=0
Radiative Process
Photophysics Process
(S 1 S 0 ) (T 1 >100ns S 0 ) 0.1~10ns Absorption or excitation spectroscopy is used to probe electronic structure and properties ground state Emission or luminescence spectroscopy is used to probe electronic structure and properties excited state
Photophysics Process
Fluorescence : spontaneously emitted radiation ceases immediately after exciting radiation is extinguished Phosphorescence : spontaneously may persist for long period mirror image
Excitons (bounded electron-hole paies)
Excited States are produced upon light absorption by a conjugated polymers Charge Transfer (CT) Exciton : typical of organic materilas Molecular picture Ground state Excited state binding energy ~1eV Diffusion radius ~10Å Treat excitions as chargeless particles capable of diffusion and also view them as exited stated of the molecules
Why PLEDs ?
Easy and low-cost fabrication Solution processibility Light and flexible Easy color tuning Spin coating and inject printing
History of Organic Light Emitting Diodes
1963 First organic electroluminescene based on anthracene single crystal Low quantum efficiency and high operating voltage (>100V) 1987 The first efficient, bright, and thin film organic light emitting diode (OLED) was reported by C. W. Tang et al. Appl Phys Lett 1987, 51, 913 (Kodak Research Labs, Rochester, NY) quantum efficiency (~1%) and low operating voltage (~10V) 3 cd/A (green) 1990 Conjugate polymers LEDs (PPV) were first reported by R. H. Friend and coworkers Nature 1990, 347, 539 (Univ. of Cambridge, England)
Quantum efficiency ~0.05% Green yellow Light
Progress of Light Emitting Diodes (LEDs) Performance
Geometry & Mechanism of PLEDs
Mechanism of PLEDs
Schematic of PLED operations
Mechanism and Design of PLEDs
Single-layer LED Structure V Light Anode h + EL Material e Cathode Energy Level Diagram The problem of charge injection
Φ
anode IP EA LUMO Barrier to electron injection Barrier to hole injection Anode
Φ
cathode HOMO EL Material Cathode Vacuum Level
Scheme of Multilayer PLEDs
Fabrications of Organic Light Emitting Diodes
Electron Transport Layer:
Vacuum Evaporation of Dyes/Oligomers
Spin Coating of Polymers Emissive Layer:
Vacuum Evaporation of Dyes/Oligomers
Spin Coating of Polymers
Layer-by-layer Self assembly Cathode:
Metal (Al, Mg, Ca) by Vacuum Evaporation V Cathode Electron Transport Layer Emissive Layer Hole Transport Layer Anode Substate Transparent substrate
Plastic Glass Anode
ITO (sputter)
Conducting Polymer (spin coating) Hole Transport Layer:
Vacuum Evaporation of Dyes/Oligomers
Spin Coating of Polymers Emitters 50~150nm CTL 5~50nm Cathode 100~400 nm ITO 100~500 nm
Device Preparation and Growth (use thermal coater) Glass substrates precoated with ITO - 94% transparent 15 Ω/square Precleaning Tergitol, TCE Acetone, 2-Propanol Growth - 5 x 10
-
7 Torr - Room T 20 to 2000 Å layer thickness
Hole Transport Materials (HTM) in PLEDs
Triarylamine as functional moiety Poly (9,9-vinlycarazole) (PVK)
H 2 C CH n N
IP between ITO (φ=4.7) and emitters Typically IP~ 5.0eV
Electron Transport Materials (ETM) in PLEDs
EL mechanism Energy level diagram Exciton recombination PLED architectures with ETM Control charge injection , transport , and recombination by ETM
lower barrier for electron injection
μ e > μ h in ETM
Larger
△
IP to block hole
SA Jenekhe et al, Chem Mater 2004 ,
16
, 4556
Electron Transport Materials (ETM) an Electrode in PLEDs
Cathode Electrode Small work function of metal Commonly used in Electron transport materials
Reversible high reduction potential Cathode Materials
Suitable EA & IP for electron injection and hole block
High electron mobility Protective layer
High Tg and thermal stability
Processability (vacuum evaporation or spin casting)
Amorphous morphology (prevent light scattering) Nitrogen -contaning heterocyclic ring Electron withdrawing in main backbone or substituents Anode Electrode Large work function (ITO, φ a =4.7~4.8 eV)
SA Jenekhe et al, Chem Mater 2004 ,
16
, 4556
Electron Transport Materials in OLEDs
Oxadiazole Molecules and Dendrimers Benzothiadiazole Polymers Triazines Polymeric Oxadiazole Azobased Materials Metal Chelates Polybenzobisaoles Pyridine-based Materials
SA Jenekhe et al, Chem Mater 2004 ,
16
, 4556
Electron Transport Materials in OLEDs
Quinoline-based Materials Phenanthrolines Anthrazoline-based Materials Siloles Cyano-containing Materials Perfluorinated Materials High EA ~3eV High degree of intermolecular π- π stacking Enhanced EQE & brightness & luminance yield
SA Jenekhe et al, Chem Mater 2004 ,
16
, 4556
Visible Spectrum & Color & CIE 1931 Coordinate
Emissive Materials in PLEDs
Blue emitters White emitters ~436nm (0.15,0.22) Green emitters ~546 nm (0.15,0.60) Red emitters (0.33,0.33) cover all visible region ~700nm (0.65,0.35)
Experimental setup for direct measurement of EQE
Efficiency
External Quantum Efficiency (EQE) N p phonon number N e electron number Definition of efficiency
Mechanism and Design of PLEDs
Cathode V Electron Transport Layer Emissive Layer Hole Transport Layer Anode Substate Key Process in EL Devices Double Charge (electrons and holes) Injection (At interface) γ = injection efficiency if ohmic contact, γ = 1 Charge Transport/Trapping Excited State Generation by Charge Recombination η = singlet exction generation efficiency~ 0.25?
Radiative Decay of Excitons φ = Fluorescence efficiency
Towards Improved PLEDs
Better Efficiency (> 5%) High Luminance (>10 6 cd/cm 2 ) Stability with Packaging (5000~25000 hrs) Low operating Voltage (3~10V) Charge Injection (choose suitable work function electrode) Charge Transport (choose high electron and hole mobility)
Flexible Internet Display Screen
THE ULTIMATE HANDHELD COMMUNICATION DEVICE UDC, Inc.
Cambridge Display Technology (CDT) Full color display - Active matrix - 200 x 150 Pixels - 2 inch diagonal
Eletrophosphorescence from Organic Materials
Excitons generated by charge recombination in organic LEDs 2 P +
‧
+ 2 P -
‧
1 P* + 3 P* Singlet :electroluminescence Triplet: electrophosphorescence Spin statistics says the ratio of singlet : triplet, 1 P* : 3 P*= 1 : 3 To obtain the maximum efficiency from an organic LED, one should harness both the singlet and triplet excitations that result from electrical pumping
Eletrophosphorescence from Organic Materials
The external quantum efficiency (η ext ) is given by η ext = η int η ph = (γ η ex φ p )η ph η ph = light out-coupling from device η ex = fraction of total excitons formed which result in radiative transitons (~0.25 from fluoresent polymers) γ = ratio of electrons to holes injected from opposite contacts φ p = intrinsic quantum efficiency for radiative decay If only singlets are radiative as in fluorescent materials, η ext ~ 5%, assuming η ph is limited to ~ 1/2n 2 ~ 20 % for a glass substrate (n=1.5) By using high efficiency phosphorescent materials, η int 100 %, in which case we can anitcipate η ph ~ 20 % can approach
High Efficiency LEDs from Eletrophosphorescence
Organometallic compounds which introduce spin-orbit coupling due to the central heavy atom show a relatively high ligand based phosphorescence efficiency even at room temperature All emission colors possible by using appropriate phosphorescent molecules From S. R. Forrest Group (EE, Princeton University) Maximum EQE Blue emitters Green emitters Red emitters 7.5
±
0.8 % APL 2003, 82, 2422 15.4
±
0.2 % Nature, 2000, 403, 750 7
±
0.5% APL, 2001, 78, 1622
http://www.cibasc.com/pic-ind-pc-tech-protection-lightstabilization2.jpg
As DCM2 acts as a filter that removes singlet Alq 3 excitons, the only possible origin of the PtOEP luminescence is Alq 3 triplet states that have diffused through the DCM2 and intervening Alq3 layers.