molecular opto-electronic: materials and device applications

Seite: 1

Molecular Opto-Electronic: Materials and Device Applications

Stelios ChoulisDepartment of Mechanical Engineering and Material

Science and Engineering Cyprus University of Technology

email: [email protected]

Stelios Choulispage:2

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

Seite: 3

Electronic materials and devices


Electronic Materials and devices

Structure

Properties

Processing

Application/Devices


•

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)


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)


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)]

Seite: 9

Electronic/Electrical Properties


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


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



Review of Band Structure

Note difference in the band gap depending on the book 4 or 2 eVIs used (see next slides)


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


Electrons are scattered from atoms or defects as they move through a conductor

Average drift velocity is v

Conductivity: Conductors


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?


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.



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


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


•

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.


•

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



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



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


•

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+ -


1979Tandy –

TRS80

4k of memoryCassette tape storage

1981Osborne portable24 lbs

1983IBM Pc64k memory4.77MHz processor

19 billion instructions/sec

Seite: 25

Optical Properties


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


The Electromagnetic Spectrum


Four fundamental interactions of photons with matter

•Refraction•Reflection•Absorption•Transmission


Light Absorption

tII ln0

α−=⎥⎦

⎤⎢⎣

⎡

t

II α−= e0 thicknesssample

cm][tcoefficienabsorptionlinear 1

===α −

t


•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

Seite: 31

Fundamentals for conventional Optoelectronic devices


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


LASER Light

How could we get all the light in phase? LASERS

LightAmplification byStimulatedEmission ofRadiation

Involves a process called population inversion of energy states


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


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


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


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

Stelios Choulispage:38 +

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


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.)

Seite: 40

Why Molecular (Organic) Electronic materials and Optoelectronic devices.


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


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


“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


New applications (Polymer Semiconductors)


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


Polymers

What is a polymer?

Poly mermany repeat unit


C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)ClCl Cl

C C C C C CHHH

HHHHHH

Polyvinyl chloride (PVC)

repeatunit

repeatunit



Making a Polymer from a Monomer

•Addition polymerization•Condensation polymerization


Insulating Conventional Polymers



Molecular Structures

Branched Cross-Linked NetworkLinear

s


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


alternating


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


~ Polymer (Molecular) ~ Polymer (Molecular) Electronics ~Electronics ~

You need polymers with You need polymers with semiconductor properties semiconductor properties


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


Light emission from Semi conducting (Conjugated) Polymers

Al electrode

PPVITOglass


Extended conjugated system

Polymer with semiconductor properties Polymer with semiconductor properties due to the chemical structuredue to the chemical structure


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


Towards new electronic device products


Conjugated (Semi conducting) Polymer materials

n

OR

MeO n

RR n RR

Ar

n


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)


Ultra-thin Display by Sony


Flexible video display by UDC


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.


Plastic solar cells

printable polymer solar cells Siemens/Konarka 2004


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


Product requirements for any technology

Lifetime

Efficiency Costs

Parameters & technological goals

•

The final product is defined relevant to the above parameters


Processing of molecular materials for electronic devices

Conjugated polymers can be processed at low temperatures by solution- Printed electronics!


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


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)


Example of OLEDs application: Ultra- thin OLED Display by Sony

Seite: 70

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


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


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


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)


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


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.


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.


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


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


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


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.


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).


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.


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.


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.


Ultra-thin Display by Sony


•Number of pixels 1280 x RGB x 768dots (W-XGA)

40”

OLED display by Seiko-Epson


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.


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


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


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


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).


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


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.


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


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


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.


Low cost technologies by solution processing semiconductors

•Organic photovoltaics (OPV)

•

Hybrid photovoltaics (HPV)

Novel Solar cells


Photons in, electrons out

Photovoltaic energy conversion requires:photon absorption across an energy gapcharge separationcharge transport

ground state

excited state


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


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


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!


Towards high efficiency molecular solar cells

Challenges

Maximise light harvesting

Maximise charge separation/Minimise

recombination

Maximise charge transport


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).


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)


Record Published OPV power conversion efficiency today

PCE 7.4-7.9 %


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


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.


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).


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.


Images of novel organic electronics

PresentFuture


The electronics of the 21 st century


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

molecular opto-electronic: materials and device applications

Documents