1. UGC – HRDC, Academic Staff College, University of Mumbai
Kalina Campus, Mumbai.
Refresher course in Chemistry
-
‘Recent Development in Applied Chemistry’
2. Content of Presentation
Introduction.
Types of light emitting devices.
What is OLED?
Basic structure of OLEDs.
Working principle of OLEDs.
Parts of OLEDs.
Advantageous of OLEDs.
Feature prospective
3. Introduction
Source of Light: Lighting technologies are substitutes for sunlight.
Natural: Sun, stars, Bioluminescent (insects), etc.
Synthetic/Artificial: Burning of coal/ Wood/ scandal, lamps, LED, OLED, etc.
Solid state lighting - alternative lighting achieved by an eco-friendly, energy
efficient, new green technology, where illumination is obtained through
semiconductor devices like light-emitting diodes (LEDs), organic light-emitting
diodes (OLEDs) or light-emitting polymers (LEPs).
The traditional technologies developed so far include in candescence and
fluorescence. These technologies have all made significant progress over the
past 200 years, but appear to be saturating at efficiencies in the 1 – 25%
range.
Newly developed technology - solid-state lighting (SSL), has the potential to
reduce lighting energy usage by nearly one half.
4. There are many challenges for efficiently creating white light from semiconductor materials
with band-gaps that span the visible spectrum is extremely challenging.
Currently used lighting systems are briefly discussed below.
Types of light emitting devices
Incandescent lamps - tungsten filament lamps, which contain
vacuum.
Tungsten- halogen - tungsten filament just like a regular incandescent
lamp; how ever the bulb is filled with halogen gas.
Fluorescent lamps - low pressure of mercury vapor and emits a
small amount of blue/green radiation, but the majority is in the UV at
253.7 nm and 185 nm.
LED lamps emit visible light in a very narrow spectral band; generate
white light.
Technology Average Life time (h) Efficiency (lumen/W)
Incandescent lamp 750-1500 12-18
Halogen lamp 2000-4000 16-29
Linear fluorescent
lamp
20000 80-100
Compact fluorescent
lamp
6000-10000 60-70
White OLEDs 10000 64 OLED
LED
Fluorescent lamp
5. OLEDs: the future of light1
1. Chen Z-K, Nancy HSL, Wei H, Xu Y-S, Yong C. Macromolecules 2003;36:1009–20; Seung WK, Byung JJ, Taek A, Hong KS.
Macromolecules 2002;35:6217–2; Neef CJ, Ferraris JP. MEH-PPV: Macromol. 2000;33:2311–4; Ide N, Tsuji H, Ito N, Sasaki H,
Nishimori T, Kuzuoka Y, etal. Proc.SPIE 2008;7051:705119–21; Van Elsbergen V, Boerner H, Lobl H-P, Goldmann C, Grabowski SP,
Young E, et al. Proc.SPIE 2008;7051:70511A-1; Huang Q, Walzer K, Pfeiffer M, Lyssenko V, He G, Leo K. Appl.Phys.Lett.
2006;88:113515; Schwartz G, Reineke S, Rosenow TC, Walzer K, Leo K. Adv.Funct.Mater. 2009;19:1.
7. Basic structure of OLEDs
A single layer OLED consists of an organic layer sandwiched between two electrodes.
This organic layer performs three main functions: hole transport (ETL), electron
transport (HTL) and emission2.
The interface provides an efficient site for the recombination of the injected electron–
hole pair and results in electroluminescence - the emitter material is doped in one of the
two layers.
2. Kido J, Nagai K, Okamoto Y. Chem Lett 1990;13:657.
The basic OLED structure consists
mainly of
Indium tin oxide (ITO)
Hole transport layer
Electron transport layer
Emitting layer
Glass substrate
Metallic cathode
8. Working Principle of OLEDs
Organic molecules are electrically conductive - delocalization of pi-electrons/non-
bonding electrons caused by conjugation over all or part of the molecule.
At applied voltage - the anode is positive and cathode is negative.
Electron injected into the LUMO of the organic layer at the cathode and withdrawn from
the HOMO at the anode.
Intensity or brightness of the light depends on the amount of electrical current applied.
Frequency of emission - band
gap of the emission material
positive
HOMO
LUMO
10. Anode of OLEDs
Anode of an OLED must be
transparent in order to inject holes into organic layers
highly conductive in order to achieve a device with
high performance and efficiency.
Indium tinoxide (ITO) - used anode material with low
roughness and high work function (ΦW = 4.5 to 5.1 eV),
which is high enough to inject holes into the highest
occupied molecular orbital (HOMO) of the organic
materials.
Good electrical conductivity, high transparency (90%)
to visible light and excellent adhesion to the substrates.
In efficient flexible OLEDs with modified graphene
anode.
11. Hole injection layer (HIL)
The materials with high positive charge mobility, electron blocking capacity
and high glass transition temperature.
Examples: 4,4’,4’’-tris(N-3-methylphenyl-N-phenylamino) triphenylamine (m-
MTDATA) and copper phthalocyanine (CuPc) are the examples of materials used
for hole injection layer.
copper phthalocyanine 4,4’,4-tris(N-3-methylphenyl-N-phenylamino)
triphenylamine
12. Hole transport layer (HTL)
Materials having low ionization potential, low electron affinities and high hole mobility.
Function as hole transporting materials by accepting and transporting hole carriers with a
positive charge.
Examples: N,N’-diphenyl-N,N’-bis(3-methylphenyl)1,1’-biphenyl-4, 4’-diamine (TPD),
N,N’-diphenyl-N,N’-bis(1-naphthylphenyl)-1,1’-biphenyl-4,4’-diamine (NPB) and 1,10-
bis(di-4-tolylaminophenyl)cyclohexane (TAPC).
TPD NPB TAPC
13. Electron transport layer (ETL)
Materials having good electron transporting and hole blocking properties, high
electron affinities together with high ionization potential.
• electron conductive pathway for negative charge carriers to migrate from the cathode
into the emission layer. Most common ETL materials are Aluminum tris-8-
hydroxyquinoline (Alq3) and 9,10-di(2-napthyl)anthracene (ADN).
Aluminum tris-8-hydroxyquinoline 9,10-di(2-napthyl)anthracene
14. Emissive layer (EML)
A layer in between HTL and ETL - emitter of visible photons - emissive layer (EML).
Organic molecules or polymers or dendrimers with high efficiency, life time and colour
purity, a high glass-transition temperature to obtain devices with longer lifetime.
Colour required – depends on energy gap i.e., the distance between HOMO and LUMO
lies such that the energy released during recombination will be within the desired
wavelength.
Examples: 4,4,N,N’-dicarbazolebiphenyl (CBP) and 1,3-bis(9-carbozoyl)benzene (mCP)
are recently developed.
CBP
mCP
20. OLEDs: future perspectives
Cell Phone screens
Keyboards (Optimus Maximus)
Light
OLEDs: Applications
Wallpaper lighting defining
new ways to light a space
Scroll laptop
Cell phones
21. Many research groups around the world are investigating various organic LEDs:
Flexible polymer OLEDs
Devices based on dendrimers
Devices into display by easy and cheap fabrication techniques.
Introduction of graphene as the device substrate, ITO is going to be replaced
by graphene.
Currently more than 80 companies, nearly 70 universities and other non-industrial
laboratories worldwide are engaged in the field of OLEDs.
OLEDs: Current Status of Research
24. Efficiency: transportation and recombination of electrons and holes in organics
constitute a current. The internal quantum efficiency (ηint) of
an OLED is defined as the number of photons produced within the device (Nin) over
the number of electrons injected (Nelectron) per unit time.
The external quantum efficiency (EQE) is one of the most important figure of merit of
OLEDs; it is defined as the ratio of photons out coupled from the device to the electrons
generated within the device. The best OLEDs have an EQE of about 25%.
The light generation mechanism in OLEDs is due to the radiative recombination of excitons
on electrically excited organic molecules. Light is generated from thin organic emitting layer
spontaneously in all the directions and propagates via various modes, that is, external
modes (escape from the substrate surface), substrate-, and ITO/organic wave guided modes
due to total internal reflection.
Light out coupling
25. efficiency of such a device strongly depends on how good holes and
electrons can be conducted. If either electrons or holes are trapped by
defects, meaning that they cannot contribute to the current anymore,
then an excess of one type of charge exists. For example, in the case that
holes are trapped, there are more electrons than holes, meaning only a
part of the electrons can create light and the efficiency of the OLED is
reduced.
Charge transfer efficiency of HIL, HT, ET:
time of flight of OLED
Time of flight. To measure mobility you create a thick film of the organic material,
place it over the top of a transparent electrode and on the side place a collection
electrode. Then fire a laser at a wavelength that will photogenerate charge carriers.
