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Molecular Opto-Electronic: Materials and Device Applications
Stelios ChoulisDepartment of Mechanical Engineering and Material
Science and Engineering Cyprus University of Technology
email: [email protected]
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Outline
Introduction (Fundamentals of conventional electronic materials and devices) Why Molecular (Organic) Electronic materials and Optoelectronic devices.Towards new productsOrganic Materials and Device Structure Operation PrinciplesExamples of molecular electronic devicesSummary
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Electronic materials and devices
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Electronic Materials and devices
Structure
Properties
Processing
Application/Devices
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•
Scanning electron microscope images of an IC:
•
A dot map showing location of Si (a semiconductor):--
Si shows up as light regions.
•
A dot map showing location of Al (a conductor):--
Al shows up as light regions.
Fig. (a), (b), (c) from Fig. 18.0, Callister 7e.
Fig. (d) from Fig. 18.27 (a), Callister 7e. (Fig. 18.27 is courtesy Nick Gonzales, National Semiconductor Corp., West Jordan, UT.)
(b)
(c)
View of an Integrated Circuit
0.5mm
(a)(d)
45μm
Al
Si (doped)
(d)
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Classification of technologically useful electronic materials
Electronic materials
Superconductors(very low temp) Conductors Semiconductors Insulators
Metals
Ceramics
metals Inorganicsemiconductors
Organic semiconductors Ceramics
polymersElemental(Si, Ge)
Compound(GaAs, GaN)
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rubber
polyethylene Polyester (PET)
polystyrene
PTFE -
teflon latex
silicone
Polyamide (nylon)
polybutadiene
Conventional applications Polymer Insulators
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New applications [Organic (Polymer and small molecule Semiconductors)]
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Electronic/Electrical Properties
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Electrical Conduction
•
Resistivity, ρ
and Conductivity, σ:
E: electricfieldintensity
resistivity
J: current density
conductivity
ρ=Δ
AI
LV
σ =
1ρ
•
Resistance:
σ=
ρ=
AL
ALR
•
Ohm's
Law:ΔV = I R
voltage drop (volts = J/C)C = Coulomb
resistance (Ohms)current (amps = C/s)
Ie-A
(cross sect. area) ΔV
L
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Band Structure of Solids
Semiconductor or conductor: electrical properties are a collective behavior
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Mg
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Review of Band Structure
Note difference in the band gap depending on the book 4 or 2 eVIs used (see next slides)
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Conduction & Electron Transport
•
Metals (Conductors):--
Thermal energy putsmany electrons intoa higher energy state.
•
Energy States:--
for metals nearbyenergy statesare accessibleby thermalfluctuations.
+-
-
Energy
partly filled valence band
empty band
GAP
fille
d st
ates
Energy
filled valence band
empty band
fille
d st
ates
General Picture Conductor
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Electrons are scattered from atoms or defects as they move through a conductor
Average drift velocity is v
Conductivity: Conductors
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Energy States: Insulators & Semiconductors
•
Insulators:--
Higher energy states notaccessible due to gap (> 2 eV).
Energy
filled band
filled valence band
empty band
fille
d st
ates
GAP
•
Semiconductors:--
Higher energy states separated by smaller gap (< 2 eV).
Energy
filled band
filled valence band
empty band
fille
d st
ates
GAP?
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Charge Carriers in Semiconductors
Two charge carrying mechanisms
Electron – negative chargeHole – equal & opposite
positive charge
Move at different speeds - drift velocity
Higher temp. promotes more electrons into the conduction band
Electrons scattered by impurities, etc.
Adapted from Fig. 18.6 (b), Callister 7e.
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 18.16 When a voltage is applied to a semiconductor, the electrons move through the conduction band, which the electron holes move through the valence band in the opposite direction.
Charge Carriers in Semiconductors
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Conduction in Terms of Electron and Hole Migration in Pure Semiconductors
Adapted from Fig. 18.11, Callister 7e.
electric field electric field electric field
•
Electrical Conductivity given by:
# electrons/m3 electron mobility
# holes/m 3
hole mobilityhe epen μ+μ=σ
•
Concept of electrons and holes:
+-
electron holepair creation
+-
no applied applied
valence electron Si atom
applied
electron holepair migration
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•
Extrinsic:--n ≠
p
--occurs when impurities are added with a different# valence electrons than the host (e.g., Si atoms)
Extrinsic Conduction (doping)
•
n-type
Extrinsic: (n >> p)
no applied electric field
5+
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+
Phosphorus atom
valence electron
Si atom
conductionelectron
hole
een μ≈σ
•
p-type
Extrinsic: (p >> n)
no applied electric field
Boron atom
3+
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+ hep μ≈σ
Adapted from Figs. 18.12(a) & 18.14(a), Callister 7e.
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•
Doping
Silicon with P for n-type semiconductors:•
Process:
1. Deposit P
richlayers on surface.
2. Heat it.
3. Result: Dopedsemiconductorregions.
silicon
silicon
Doping through PROCESSING USING DIFFUSION
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 18.19 When a dopant atom with a valence greater than four is added to silicon, an extra electron is introduced and a donor energy state is created. Now electrons are more easily excited into the conduction band.
n-type
n-type doping
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 18.21 When a dopant atom with a valence of less than four is substituted into the silicon structure, a hole is created in the structure and an acceptor energy level is created just above the valence band. Little energy is required to excite the holes into motion.
p-type
p-type doping
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•
Allows flow of electrons in one direction only
(e.g., usefulto convert alternating current to direct current.
•
Processing: diffuse P into one side of a B-doped crystal.• Results:
--No applied potential:no net current flow.
--Forward bias: carrierflow through p-type andn-type regions; holes andelectrons recombine atp-n junction; current flows.
--Reverse bias: carrierflow away from p-n junction;carrier conc. greatly reducedat junction; little current flow.
++ +
++
--
--
-
p-type n-type
+++
+
+
---
--
p-type n-type- +
P-N RECTIFYING JUNCTION (Transistors)
++
++
+
---
--
p-type n-type+ -
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1979Tandy –
TRS80
4k of memoryCassette tape storage
1981Osborne portable24 lbs
1983IBM Pc64k memory4.77MHz processor
19 billion instructions/sec
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Optical Properties
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Optical Properties
Light (Photons) has both particulate and wavelike properties
λ=ν=Δ
hchE
m/s) 10 x (3.00 light of speed c )sJ1062.6( constant sPlanck'
frequency wavelength energy
8
34
=
⋅=
=ν=λ=Δ
−xh
E
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The Electromagnetic Spectrum
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Four fundamental interactions of photons with matter
•Refraction•Reflection•Absorption•Transmission
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Light Absorption
tII ln0
α−=⎥⎦
⎤⎢⎣
⎡
t
II α−= e0 thicknesssample
cm][tcoefficienabsorptionlinear 1
===α −
t
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•Adapted from Fig. 21.5(a), Callister 7e.
Selected Absorption: Semiconductors
incident photon energy hν
Energy of electron
filled states
unfilled states
Egap
Io
blue light: hν
= 3.1 eVred light: hν
= 1.7 eV
•
Absorption by electron transition occurs if hν
> Egap
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Fundamentals for conventional Optoelectronic devices
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Generation of light in Semiconductors [light emitting diodes LEDs]
Excitation with energy higher than the band gap creates electrons in the CB and holes in the VB.Electron – negative chargeHole – equal & opposite
positive charge
The electron hole pair is called exciton.
Recombination of electron hole pairs provides light with energy approximate equal with the band gap/
Adapted from Fig. 18.6 (b), Callister 7e.
light
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LASER Light
How could we get all the light in phase? LASERS
LightAmplification byStimulatedEmission ofRadiation
Involves a process called population inversion of energy states
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Lasers: Population Inversion
What if we could increase most species to the excited state?
Fig. 21.14, Callister 7e.
•
Semiconductors:--
Higher energy states separated by smaller gap (< 2 eV).
Energy
filled band
filled valence band
empty band
fille
d st
ates
GAP?
For Laser you needMore electrons in the excited state
ee
eeeeee
Light
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Solar Cells: Photons in, electrons out
Photovoltaic energy conversion requires:photon absorption across an energy gapFormation of exciton (electron and hole pair)charge separation (break the exciton free electron and hole)charge transportCollection
ground state
excited state
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Solar Cells
•
p-n junction: •
Operation:--
incident photon produces hole-elec. pair.
--
transport or electrons and holes to the electrodes
--
creation of light.
•
Solar powered weather station:
polycrystalline SiLos Alamos High School weatherstation (photo courtesyP.M. Anderson)
n-type Si
P-type Sip-n junction
B-doped Si
Si
Si
Si SiB
hole
P
Si
Si
Si Si
conductance electron
P-doped Si
n-type Si
p-type Sip-n junction
light
+-
++ +
---
creation of hole-electron pair
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n type p type
+ -
E g
- +
Conventional photovoltaics
Semiconductor p-n junction (based on a one material Si):
Light produces electron-hole pairs throughout semiconductorExciton binding energy few meVp and n layers can break the exciton (to free electron hole pairs)
Light ⇒ photovoltage × photocurrent ⇒electric power
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Conventional Silicon Solar Cells
Light to power efficiency of best silicon solar cell ~ 25%
C-Si module efficiencies typically ~ 15%
J. Nelson IC tutorial 2009
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Applications of Materials Science and Engineering
New materials must be developed to make new & improved devices.
Light Emitting Diodes (LEDs)White light semiconductor sources
New semiconductorsMaterials scientists/engineers (& many others).Light emitting diodes, LASERs and Solar cells
Fig. 21.12, Callister 7e. Reproduced byarrangement with Silicon Chip magazine.)
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Why Molecular (Organic) Electronic materials and Optoelectronic devices.
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ElectronicMaterials: Present and Future
Inorganic materials
Hard, fragileMaterials
Organic materials
Soft , molecular, flexable
Material ProcessingVaccum
DepositionUltra high Temperatures
Solution processing, low temperature
Fabrication equipment
Highly specialized, expensive
Spin coated, printing
Cost
MBE, MOVPE
simple, inexpensive
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Some of the Major Electronic Material trends in the early 21 st Century
Molecular- Organic materials:Semiconducting polymers (P3HT, PPV, PFO….)Small molecules (pentacene, AlQ3…)
Buckyball, C60 based materialsFullerenes (PCBM) Carbon nano-tubes
Inorganic materialsNanoparticles of many type and shapes (Au, Ag, Si or CdTe nanorods)Metal Oxides (TiOx, ZnO)
Hybrid materialsUse a combination of different classes of materials and device structures to optimise device performance.
S n
O
O
S
Sx
y
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“The
electronics
of the
20th century
is
basedon semiconductor
physics. The
electronics
of
the
21st century
will be
based
on molecularchemistry/physics”F. L. Carter
Organic Semiconductors are molecular materials
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New applications (Polymer Semiconductors)
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Organic (Molecular) Materials … TWO GENERAL CLASSES
Alq3
MOLECULAR MATERIALS
Attractive due to:Attractive due to:•• Integrability with inorganic semiconductors(hybrids)•• Low cost)
•• Large area bulk processing possible•• Tailor molecules for specific
electronic or optical properties•• Unusual properties not easily attainable
with conventional materials
PPV
POLYMERS
n
But problems exist:But problems exist:•• Stability•• Thickness control of polymers•• Low carrier mobility
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Polymers
What is a polymer?
Poly mermany repeat unit
Adapted from Fig. 14.2, Callister 7e.
C C C C C CHHHHHH
HHHHHH
Polyethylene (PE)ClCl Cl
C C C C C CHHH
HHHHHH
Polyvinyl chloride (PVC)
repeatunit
repeatunit
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Making a Polymer from a Monomer
•Addition polymerization•Condensation polymerization
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Insulating Conventional Polymers
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Adapted from Fig. 14.7, Callister 7e.
Molecular Structures
Branched Cross-Linked NetworkLinear
s
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Copolymers
two or more monomers polymerized together random – A and B randomly vary in chainalternating – A and B alternate in polymer chainblock – large blocks of A alternate with large blocks of Bgraft – chains of B grafted on to A backbone
A – B –
random
block
graft
Adapted from Fig. 14.9, Callister 7e.
alternating
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Polymer Crystallinity
Polymers rarely 100% crystallineToo difficult to get all those chains aligned
•
%
Crystallinity:
% of material that is crystalline.--
Annealing causescrystalline regionsto grow. % crystallinityincreases.
Adapted from Fig. 14.11, Callister 6e.(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)
crystalline region
amorphousregion
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~ Polymer (Molecular) ~ Polymer (Molecular) Electronics ~Electronics ~
You need polymers with You need polymers with semiconductor properties semiconductor properties
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Alan J. Heeger, University of California at Santa Barbara, USA,
Alan G. MacDiarmid, University of Pennsylvania, Philadelphia, USA,
Hideki Shirakawa, University of Tsukuba, Japan
"for the discovery and development of conductive polymers"Plastic that conducts electricity
We have been taught that plastics, unlike metals, do not conduct electricity. In fact plastic is used as insulation round the copper wires in ordinary electric cables. Yet this year's Nobel Laureates in Chemistry are being rewarded for their revolutionary discovery that plastic can, after certain modifications, be made electrically conductive. Plastics are polymers, molecules that repeat their structure regularly in long chains. For a polymer to be able to conduct electric current it must consist alternately of single and double bonds between the carbon atoms. It must also be "doped", which means that electrons are removed (through oxidation) or introduced (through reduction). These "holes" or extra electrons can move along the molecule - it becomes electrically conductive. Heeger, MacDiarmid and Shirakawa made their seminal findings at the end of the 1970s and have subsequently developed conductive polymers into a research field of great importance for chemists as well as physicists. The area has also yielded important practical applications. Conductive plastics are used in, or being developed industrially for, e.g. anti-static substances for photographic film, shields for computer screen against electromagnetic radiation and for "smart" windows (that can exclude sunlight). In addition, semi-conductive polymers have recently been developed in light-emitting diodes, solar cells and as displays in mobile telephones and mini-format television screens. Research on conductive polymers is also closely related to the rapid development in molecular electronics. In the future we will be able to produce transistors and other electronic components consisting of individual molecules - which will dramatically increase the speed and reduce the size of our computers. A computer corresponding to what we now carry around in our bags would suddenly fit inside a watch.
Nobel Prize in Chemistry for 2000
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Light emission from Semi conducting (Conjugated) Polymers
Al electrode
PPVITOglass
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Extended conjugated system
Polymer with semiconductor properties Polymer with semiconductor properties due to the chemical structuredue to the chemical structure
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Polymer and Small Molecules with semiconductor properties
Exciton
binding energy (Eg) ~200-500 meV
(exciton
diffusion (LD) length~ 1-20 nm).
Molecular materials can have Semiconducting properties
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Towards new electronic device products
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Conjugated (Semi conducting) Polymer materials
n
OR
MeO n
RR n RR
Ar
n
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Organic light emitting diodes
The active part is very thin (< 1 μm)
Metal (0.1–0.5 μm)[Ca, Mg/Al]
Glass or PET (10 μm–5 mm)
Indium-Tin Oxide(0.1–0.3 μm)
Polymer(~ 0.1 μm)
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Ultra-thin Display by Sony
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Flexible video display by UDC
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OLED lighting
30 cm lighting panelsEfficiency of 40 lm/W blue demonstrated => > 100 lm/W Luminance at 4000 nits
Courtesy of Prof. Kido, Yamagata University
GE OLED 24”x24” light tiles 1,200 lumens of light Efficacy:15 lm/W.
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Plastic solar cells
printable polymer solar cells Siemens/Konarka 2004
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Organic (Molecular) Electronic Materials and Devices
•
Many application areas and materials systems:
Solar Cells
Lasers
NanoparticlesDisplays
Polymers
Dyes
sensors
TFTs
Metal Films
LightingMolecular s/c
BiomoleculesBlends
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Product requirements for any technology
Lifetime
Efficiency Costs
Parameters & technological goals
•
The final product is defined relevant to the above parameters
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Processing of molecular materials for electronic devices
Conjugated polymers can be processed at low temperatures by solution- Printed electronics!
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Substr.0
Electrode4
Packaging5
Thic
knes
s[µ
m]
Length
[mm]5 10 15 20 25 30 4540350 50
0
1
2
3
-1
Electrode1
Pedot2
Semicond.33
Processing of Molecular Optoelectronic Devices
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Lighth+ h+
h+
e- e-
Transparentsubstrate
Anode(ITO)
Conductingpolymer Emissive polymer
HOMO
LUMO
Cathode-layer (s)
ca. 100 nm 10 - >100 nm <100 nm
LUMO
HOMO
>100 nm
OLED device operation (energy diagram)
OLEDs rely on organic materials (polymers or small molecules) that give off light when tweaked with an electrical current
Light
Electrons injected from cathodeHoles injected from anodeTransport and radiative recombination of electron hole pairs at the emissive polymer.
OLED is the most mature technology of organic semiconductorsFirst reported from R. Friend and D. Bradley (Nature, 1991).
Organic Light emitting diodes-Fluorescence (OLEDs)
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Example of OLEDs application: Ultra- thin OLED Display by Sony
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1. Injection and Transport:• Efficient injection of carriers•
Balanced carrier mobilities
to avoid
recombination in the electrodes• High mobility to provide maximum current density
2. Understand photo-physics:
•Photophysics
of Decay (recombination dynamics properties)•Avoid non-radiative
decay.
•Avoid unfavourable interchain
interactions.
Towards High Efficiency OLEDs
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Organic semiconductors: Unipolar or Ambipolar transporters?
Transport Influenced by trap states- Highly purified organic material with
chemical regularity needed.
Transport by hoping mechanism. Can be analyses by temperature dependent studies and Theoretical models( Gaussian disorder model).
A. J
. Cam
pbel
l et a
l, JA
P. 8
2, 6
326
(199
7).
NEGATIVE CARRIER
TRANSPORT STATES
POSITIVE CARRIER
TRANSPORT STATES
POSITIVE CARRIER
TRAPS
Ener
gy
Density of States
NDOS
NDOS
Et
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300 400 500 60010-5
10-4
10-3
10-2
Electrons: P3HT
Holes: P3HT
μ (c
m2 V
-1s-1
)
E1/2(V1/2 cm-1/2)
Ambipolar and balanced Transport in Organic Semiconductors
Ambipolar transport intrinsic property of organic semiconductors. Weak field and temperature dependent electron transport in Polythiophene (P3HT), characteristic of highly ordered material.Mobility in the range of 10-3 cm2/Vsec is adequate for low current density applications (OLEDs and OPV) but not for high current density devices such as Lasers- Electrically operated organic laser has not achieved so far.
S n
P3HT: High purity and Packing of polymer chains
S.A.Choulis et al., Appl.Phys.Lett, 85, 3890,2004
350 400 450 500 550 600 650 700 75010-5
10-4
10-3
10-2
Electrons: RR-P3HT
μ e(cm
2 V-1 s
-1)
E1/2[(V/cm)1/2]
340 K 320 K 280 K 260 K 240 K
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Mobility studies: Towards Electrically operated polymer lasers
Organic lasers have been reported by optical excitation (ex., fig 1).
Electrically pumping Organic Laser has not achieved so far mainly due to low mobility of organic semiconductors.
Towards high Mobility organic semiconductors:
a) Control of polymer chain orientation depending on the device application (fig 2).
b) Polymer chains packing (fig 3).c) Control of the Chemical structure and
polymerization process.d) Development of new theoretical models to
describe materials with weak temperature dependence.
260 280 300 320 340 360 380 400 420 44010-5
10-4
10-3
10-2
10-1
0.1 1 10 10010-8
10-7
10-6
10-5
Pho
tocu
rren
t de
nsit
y (A
/cm2 )
time (μsec)
ttr=0.26 m secα i=0.86
αf=1.75
ITO /BP156/A lHoles at 295 KE=0.97 x105 (V/cm)
μ h(cm
2 V-1
s-1)
E1/2[(V/cm)]1/2
Holes: BP156, at 295 K
G Heliotis, S Choulis and D.D.C. Bradley et al, APL,88, 081104 (2006)
1)
J. L. Bredas
et al,
Proc. Nat. Acad. Sci. 99, 5804 (2002).
2)
3)
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Carrier Injection
Limited injection efficiency for a broad range of organic materials such as polyfluorenes (PFO).
Barrier for efficient hole injection from PEDOT:PSS (-5.0 eV) to Poly-fluorenes(HOMO=-5.8 eV).
Bridge the gap by another suitablePolymer interfacial layer (TFB, HOMO=-5.8 eV)
C8H17
n
H17C8
Homopolymer (PFO) Copolymer Copolymer R-moiety
C8H17n
H17C8
RN
SN
SS
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The concept of double step injection can be applied on the full range of organic electronic devices
0 2 4 6 8 10 12 14 160
2
Cur
rent
Den
sity
(mA
/cm
2 )
Diode Voltage (V)
with TFB without TFB
b)
Strong Increase in the current of hole only devices with TFB (Can be due to injection or transport).
Novel method to improve charge injection in organic electronic devices
S. A. Choulis et al, Adv. Fun. Mat., 16, 1075,
2006
300 350 400 450 500 550 600 650 7001E-6
1E-5
1E-4
without TFB with TFB
App
aren
t mob
ility
(cm
2 /Vse
c)
E1/2(V/cm)1/2
b)
Mobility is not affected by TFB. Increase in the current due to Improved injection.
0.0 0.5 1.0 1.5 2.0
-4.0x10-8
-2.0x10-8
0.0
2.0x10-8
4.0x10-8 Hole only diodes
Pho
tocu
rrent
(a.u
)
Diode Voltage (V)
Vbi
is not affected by TFB. Work Function of the anode remain Unchange.
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Fluorescence Polymer OLEDs
Some myths for organic semiconductors
Transport was low and only uni-polar. Low injection efficiencies. Degradation and low stability (dark spots)
But now on the market: Polymer based displays
Micro-displays products from OSRAM and other companies.
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A Diffusion problem from the Industry. Learn to solve scientific and engineering problems
Dark spots (on TVs, displays, photonic sources)
A diffusionProblem?
The term diffusion describes –
Atom and
Ion Movements in Materials
Dark Spots
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What Limits the Long Term Stability of PLEDs?
The origin of dark spots.
Improve the long term stability of OLEDs by eliminate the creation of dark spots
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Creation of dark spots
The acidity of PEDOT:PSS is etching the ITO (Indium Tin Oxide).
Under electrical operation and metal migration there are areas where Ca- and In+
species become close
A large electric field intensity and current density is created.
The enhanced local luminescence and heating leads to instability the polymer locally break down. The central dark spot is formed.
ITO
PEDOt:PSS
Ca/Al
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Growth of weakly-emissive areas
In the area of the dark spot the polymer has locally break down. This leads to instability of the polymer chain.
Subsequent electrochemical interaction between polymer and PEDOT:PSS responsible for the growth of weakly emissive areas around the central dark spot.
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Details of OLEDs Processing
The processing: ITO/glass substrates precoated with a (conductive acidic polymer layer) called PEDOT:PSS. The semiconductor polymer light emitting layer is coated on top of the PEDOT:PSS layer. The semiconductor polymer light emitting layer wasthen annealed at 130 °C for 30 min. Diodes were then completed by thermal evaporationof Ca (6 nm) and Al (200 nm).
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How you solve a problem
First you investigate the origin of the problem. Why dark spots are created ?You check the literature and learn for available details you investigate and understand the origin of the problem.You can not solve a problem if you do not understand the origin of the problem. You developed a project plan which summarize your model. This will help you to clarify your trials to solve the problem.
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General Advices for Engineering and Scientific Skills
understand the big picture, differentiate the important data from the irrelevant onesDefine your needs. Use the correct experimental, analytical techniquesbuild your own model, set your objectivespay attention to the detailsthink out of the box, you need to know what is outside the boxbe critical, ask yourself a lot of questionsBe able to accept your mistakes always be able to modify your model according to your progress do not stick on your initial model if your experimental results do not support it.
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Example of Patents- Eliminate dark spot formation
Before the Patent/ Trade Secret
Dark spots-
creation of non emissive areas in OLEDs-
Big problem for
Commercial applications
Dark spots: limit the LT and OLED micro-display applications
After the Patent/trade secret
Describe methodsyou will try to solve this problem: EliminateDark spot formation without reducing Device performance.
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Ultra-thin Display by Sony
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•Number of pixels 1280 x RGB x 768dots (W-XGA)
40”
OLED display by Seiko-Epson
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Phosphorescene OLEDs for Displays and Solid State Lighting
•
Understranding
Parameters determine the lifetime and efficiency of solution processible
Phosphorescence
OLEDs.
•
Aim:
Develop highly efficient and long lived polymer based OLEDs
for lighting applications.
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Phosphorescence OLEDs
Mix singlet and triplet states by incorporation of heavy metal atoms into organic-molecules.
M. A. Baldo et al, Nature (London) 395, 151, (1998)
Exciton emission rate is slow 1-10 μsec- Non radiativepaths can be created.
Singlet Manifold Triplet Manifold
S0
S1
S2
S3
S4
Abs. Fluor.
ISCT0
T1
T2
T3
Phosphorescence
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Why understanding of Molecular doping is important?
Pure phosphorescent dyes can not be used due to solubility problems and other losses.Conjugated polymers can not be used to host the dyes- due to low triplet confinement (quenching effects)Large band gap polymers such us PVK is the preferred choice in terms of energy levels for the host. Due to high triplet confinement quenching effects are eliminated.But PVK can not transport electrons.To achieve charge balance in the PHOLED device you need to apply molecular doping
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Application of molecular doping to PVK electrophosphorescence diodes
This is essentially a device which use molecular doping to achieve charge balance.
LEPITO
PEDOT
CathodePVK(host)
PBD (electron transporter)
TPD (hole transporter)
Irmppy3, emitter
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Adjust transport properties by molecular doping.
PEDOT:PSS
-5.1 eV
HOMO
PVK
-5.4 eV
TPD
PBD-6.2 eV
-5.8 eV
-2.4 eV
-2.2 eV
LUMO
CsF/Al
Ir(mppy)3
-5.5 eV
-2.3 eV
LEP
-2.4 eV
Changing TPD concentration changes effective hole mobility and injectionHigher mobility does not necessarily mean higher efficiency!
S. A. Choulis et al, Applied Physics Letters, 87, 113503, (2005).
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Direct injection for Green PHOLEDs
Device engineering modification to improve injection.Combination of TFB and PBD interfacial layers on the anode and cathode interfaces.
Achieve direct carrier injectionto the phosphorescence compound.
Green PHOLED with record 50 lm/W, 55 cd/A achieved.
S. A. Choulis et al, Applied Physics Letters, 88, 3501,(2006).
0.01 0.1 1 10
0
10
20
30
40
50
Control DevicePBD electron injecting layer PBD and TFB injecting layers
Lum
inan
ce E
ffica
cy (l
um/W
)
Current Density (mA/cm2)
LEPITO
PEDOT
Cathode
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Summary
Achieve direct carrier injection to the phosphorescence compound by incorporation of ultra-thin interfacial layers.The Hole injecting interfacial layer must have its HOMO level aligning with that of the phosphorescent dyes.The electron injecting interfacial layer must have its LUMO level aligning to that of the phosphorescence dye.
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Blue Solution Processed Device
Solution processed Blue PHOLED optimisation:Consider OXD-7 as electron transporterHigh efficiency of 14 lm/W, 22 cd/A for 30% OXD-7 in LEP.
Efficiency is comparable to highest published small molecule thermally evaporated multilayer devices but lifetime much shorter.
LEP
PVK
FIrpic
ITO
PEDOT
Cathode
OXD-7
FIrpic
M. Mathai
et al., Applied Physics Letters, 2006
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White Light from Organic light emitting diodes by down conversion method
Phosphor convert part of the blue light emitting by the OLED to yellow-orange. White emission is achieved by mixing of the emission colors.
B. Krummacher et al, Applied Physics Letters, 88, 113506,2006
Advantages of approach-Device architecture is maintainedColor is tuned by down-converting the blue lightRecord 25 lm/W cool white OLED achieved.
400 450 500 550 600 650 700 7500.0
0.2
0.4
0.6
0.8
1.0
CIE x/y = 0.26/0.40
inte
nsity
(nor
m.)
wavelength [nm]
measurement model
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OLED lighting
30 cm lighting panelsEfficiency of 40 lm/W blue demonstrated => > 100 lm/W Luminance at 4000 nits
Courtesy of Prof. Kido, Yamagata University
GE OLED 24”x24” light tiles 1,200 lumens of light Efficacy:15 lm/W.
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Low cost technologies by solution processing semiconductors
•Organic photovoltaics (OPV)
•
Hybrid photovoltaics (HPV)
Novel Solar cells
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Photons in, electrons out
Photovoltaic energy conversion requires:photon absorption across an energy gapcharge separationcharge transport
ground state
excited state
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O
O
PCBM: Eα
=3.7 eV, Ip
=6.1 eV (acceptor)
MDMO-PPVEα
=2.9 eV, Ip
=5.0 eVEg
=2.1` eV (donor)
O
O
n
RR-P3HTEα
=3.0 eV, Ip
=4.9 eV Eg
=1.9 eV (donor)
Why PCBM?•
Ultrafast electron transfer from
polymer to fullerene.• High solubility• Excellent transport properties
S n
Conjugated Polymers and Fullerenes: An Ideal Composite for Photo-Charge Generation
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Fundamental Limitation of Organic Solar Cells
Exciton binding energy (Eg) ~200-500 meV (exciton diffusion (LD) length~ 1-10 nm). Since LD<<1/α→ ultra-thin active layers to maximize exciton collection →while maximising absorption (relevant thick layers).
SolutionUse a bulk heterojunction
Length scale of heterojunctionswithin blend ~ exciton
diffusion length
Morphology within the blend critical for device performanceElectric field
photon
electron transport
hole transport
p contact n contact
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BULK HETEROJUNCTION SOLAR CELLS:
- Maximise the number of donor/acceptor interfaces
- Efficient charge separation
E
Acceptor
Donor
Transparentanode
Cathode
Self-assembled
nanoscale
materials
with
charge-
separating
junctions
everywhere!
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Towards high efficiency molecular solar cells
Challenges
Maximise light harvesting
Maximise charge separation/Minimise
recombination
Maximise charge transport
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Roadmap for Future OPV Applications
Flexible Low cost
Lightweight Printing production
Effic
iency
(%)
20031998
12
8
4
0
16
20
2008
UCSBCambridge
U. LinzSiemens
Konarka
Konarka
Siemens
2013
12
8
4
0
16
20
OPV single junction
OPV multiple junction
Year
Siemens/KonarkaOPV Prototype
www.konarka.com
Create a World without wires
Consumer
10-50 MW
10-30 MW
100 MW
10,000 MWOPV Targeted Markets
Total Accessible Market in megawatts (MW)
Initial applications
Brabec, MRS Bulletin (2005), Choulis & Brabec: Invited talks: TPE & Polydays Conferences, (2006 & 2007).Brabec & Durrant, MRS Bulletin (2008)Gaudiana & Brabec, Nature Photonics (2008).
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Towards 10 % OPV Efficiency
•
Bandgap: 1.2-2.0
eV
•
HOMO:
5.2-5.5 eV
• Mobilities: >10-3 cm2/Vsec
Identify methods to control the morphology within the polymer: fullerene blend photoactive layer.
2 ,0 03 ,00
4 ,00
5 ,00
6 ,00
7 ,00
8 ,0 0
2 ,00
1 ,00
9 ,0 010 ,0 0
2 ,8 2 ,4 2 ,0 1 ,6 1 ,2-3 ,0
-3 ,2
-3 ,4
-3 ,6
-3 ,8
-4 ,0
H O M O - 4 .8 eV
H O M O - 5 .8 eV
B and G ap [ eV ]
LUM
O L
evel
Don
or [
eV ]
01 ,002,003,004,005,006,007,008,009,0010 ,0011 ,0016 ,00
(Material Synthesis)
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Record Published OPV power conversion efficiency today
PCE 7.4-7.9 %
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Roll to roll processing
of organic
photovoltaics: • low
cost
• high volume• scalable
production
process
From Lab Cells to Mass Production by Printing Technology
Lab inkjet tool
Production Partner: LEONHARD KURZ GmbH
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Flexible electronics and printing technology
Transferring the
spin-coating
lab process
on glass
substrates
to a
printing
process
on flexible substrates
holds
scientific
challenges
which
have
not
been observed
or
reported
up to today.
•The attraction of organic electronics is their flexibility and printing roll to roll process.
S. A. Choulis, et al., Nanoletters,
2008.
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Outdoor Roof Testing of Flexible OPVs at Konarka
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
S O N D J F M A M J J A S O N D
Month
Nor
mal
ized
Pow
er o
utpu
t [a.
u.]
-20
-10
0
10
20
30
40
50
60
Tem
pera
ture
[°C
]
EfficiencyTemperature
Sep. 20th 2006 Nov. 7th 2007
Konarka Flexible Organic Solar Cells with more than 1 Year Outdoor Lifetime.
Long lived outdoor lifetimes needed for On-grid applications.
Hauch H et
al, Solar Energy Materials and Solar Cells, .92, 727, (2008).
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Summary
Molecular (Organic) materials show considerable promise for electronic and optoelectronic applications but…..
In comparison to conventional inorganic technologies performance of organic electronic devices must be further improved.
Deep understanding of the device Physics →Development of new materials and application of printed technology is needed to prove the full potential of these novel materials for advanced electronic applications.
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Images of novel organic electronics
PresentFuture
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The electronics of the 21 st century
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Achowlegements
Konarka
R&D group and Dr C. Brabec (CTO).
Prof Donal Bradley (FRS)Prof Jenny Nelson
Material and DevicesR&D Group, San Jose, CA, USA
Funding:From IPE, DOE (USA), BMBF (Germany), EPSRC (UK) and EU