Chapter 4: Electroluminescence

Sylvania

ZnS/Cu/Cl/I/ Mn

100V 500 cd/m2

Fluorescence and Phosphorescence

Excimer Formation

Exciplex Formation

History of Organic Electroluminescence

1963 Pope 400V 10-20 um anthracene1965 Helfrich 100V 5 % efficiency1970 Williams1982 Vincett 30V 50 nm low efficiency1983 Partridge Polymeric materials

Basic Principle of Organic EL

Charge recombination leads to emission of fluorescence

ITO4.9-5.1 eV

Metal (eV)Ca 2.9Mg 3.7In 4.2Al 4.28Ag 4.6Cu 4.7Au 5.1

Fowler-Nordheim Equation: I = AF2exp(-k3/2/F)F: field strength, A: material constant, energy difference across the interface

Efficiency: = Number of photons emitted/Number of electrons injected

I/V relationship and B/V relationship

Tang etal, Kodak

ETLElectron TransportingLayer

HTLHole Transporting Layer

Hole Transporting Layer

Electron Transporting Materials

Criteria for the Materials of Emitting Layer

Matching of Energy Levels

TPD

ITO Surface Modification Layer for Hole Injection

S

OO

PEDOT.PSS

NH

NH

PANI

TPD

Addition of Hole Injection Layer

Fluorescence Dye as Dopant:A Yellowish Light Emitting Device

Rubene

Red light emitting materials

Dopant amounts and Performance of the EL device

Rubrene as a medium for energy transfer

Green emitters

Blue Light Emitting Device

460-480 nm, 4000 cd/m2

White Light OLED

White = Blue + Red

Blue Red

Device 1 Undoped; Device 2 Doped with 5% of red DCM2

Highly-bright white organic light-emitting diodes based on a single emission layer

C. H. Chuen and Y. T. Tao

Trilayer Device Structure

Recent advances on the Interfacial Problems

X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, and K. Leo APL 2001 410

They demonstrated the use of a p-doped amorphous starburst amine, 4, 48, 49-tris(N, N-diphenylamino triphenylamine )(TDATA), doped with a very strong acceptor, tetrafluorotetracyanoquinodimethane by controlled coevaporation as an excellent hole injection material for organic light-emitting diodes (OLEDs). Multilayered OLEDs consisting of double hole transport layers of p-doped TDATA and triphenyldiamine, and an emitting layer of pure 8-tris-hydroxyquinoline aluminum exhibit a very low operating voltage (3.4 V) for obtaining 100 cd/m2 even for a comparatively large (110 nm) total hole transport layer thickness.

Low voltage organic light emitting diodes featuring doped phthalocyanine as hole transport material

J. Blochwitz, M. Pfeiffer, T. Fritz, and K. Leo

Rough estimates lead to values of about 0.2% luminescence efficiency for the highest doped case. However, those devices use sophisticated multi-layer designs and low-work function contacts. We believe that the major reason for the lower efficiency of our diodes is that the simple two-layer design does not prevent negative carriers injected from the Al electrode from reaching the opposite electrode due to the missing energy barrier for electrons at the Alq3–VOPc interface. This limits the probability of exciton formation and their radiative decay.

Graded mixed-layer organic light-emitting devices

Anna B. Chwang,a) Raymond C. Kwong, and Julie J. Brown

Improved efficiency by a graded emissive region in organic light-emitting diodes

Dongge Ma, C. S. Lee, S. T. Lee, and L. S. Hung

Metal Complexes

Al Complexes

Organic light-emitting diodes using a gallium complex

2500 cd/m2 with LiF

210 cd/m2 with Al

Red Light Emitting DeviceBased on Eu Complexes

7-137 cd/m2

Thickness Effect

Better ET, 820 cd/m2

Hole Blocking Layer

Phosphorescent Devices

100000cd/m2

Shizuo Tokito APL 2003 569

Controlling Exciton Diffusion in Multilayer White Phosphorescent Organic Light Emitting DevicesBrian W. D'Andrade, Mark E. Thompson, Stephen R. Forrest* Adv. Mater. 2002

The color balance (particularly enhancement of blue emission) can be improved by inserting a thin BCP, hole/excitonblocking layer between the FIrpic and Btp2Ir(acac) doped layers in Device 2. Thislayer retards the flow of holes from the FIrpicdoped layer towards the cathode and thereby forces more excitons to form in the FIrpic layer, and it prevents excitons from diffusing towards the cathode after forming in the FIrpic doped layer. These two effects increase FIrpic emission relative to Btp2Ir-(acac).

Device 2 is useful for flat-panel displays since the human perception of white from the display will be unaffected by the lack of emission in the yellow region of the spectrum.

Electroluminescence in conjugated polymersR. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley,

D. A. Dos Santos, J. L. Bre¬ das, M. Lo» gdlund & W. R. Salaneck Nature 1999 397 121

Wessling Approach

Red Red

Green Blue

Solubilizing Groups

Figure 6 Energy levels for electroluminescent diodes. a±c, An ITO-PPV-Ca diode before contact between the three layers, illustrating the energies expected, a, from the metal Fermi energies, assuming no chemical interactions at the interface, b, after some `doping' of the interfacial layer of PPV by Ca, setting up bipolaron' bands within the PPV semiconductor gap (note that the Fermi energy for the `doped' PPV lies between the upper bipolaron level and the conduction band), and c, after interfacial chemistry which sets up a blocking layer at the interface (as expected in the presence of oxygen). d, Energy levels for the components of a two-layer heterojunction diode fabricated with PPVand CN-PPV.

Unexpectedly high efficiency