Highly efficient organic devices

Highly Efficient Organic Devices

Karl Leo*

Institut für Angewandte Photophysik,

TU Dresden, 01062 Dresden, Germany, www.iapp.de* currently: KAUST, Thuwal, Saudi-Arabia

XIII Brazilian MRS Meeting 2014

João Pessoa

30.9.2014

Acknowledgments

• Johannes Widmer• Christian Körner• Chris Elschner• Christoph

Schünemann• Wolfgang Tress• Martin Hermenau• Toni Müller• Max Tietze• Selina Olthof• Malte Gather• Simone Hofmann• Tobias Schwab• Moritz Riede

Hong-Wei Chang

Chung-Chih Wu

Xuanhua Li

Fengxian Xie

Wallace Choy

Martin Pfeiffer

Karsten Walzer

Christian Uhrich

Roland Fitzner

Egon Reinold

Peter Bäuerle

University of UlmDepartment Organic

Chemistry II

King Abdullah University of Science and Technology - KAUST

King Abdullah University of Science and Technology (KAUST)

Solar and Photovoltaics Engineering Research Center (SPERC)

Outline

• Introduction to Organic Semiconductors

• Doping of Organic Semiconductors

• Organic Light Emitting Diodes (OLED)

• Organic Solar Cells

Photovoltaic cells

Organic materials

Transistors and memory

• Large area & flexible substrates possible

• Large variety: millions of molecules, mostly carbon

• Low cost: approx. 1g/m2 active material

Organic light emitting diodes

Organic Semiconductors

Polymers vs small molecules

• Polymers: deposition from solution

• Small molecules (oligomers): vacuum or solution

• OLED: Polymer lost the race (for the moment…)

• Solar Cells: Polymer and small molecules on par

Some people drink organic semiconductors…..

350 400 450 500 550 600 650 7000,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

2,2

2,4

2,6

2,8

3,0

3,2

3,4

Abs

orpt

ion

Wellenlänge

Carbon: the influence of dimensionality

Source: Castro Neto, Geim et al.

Van der Waals-coupling:Narrow bands

mob

ility

0D

2D covalent broad bands

2D

10-2

101

104

1D

1D covalent:broad bands

Source: IBM J. Res. Dev.

Typical OLED today!

Mobility in Organic Semiconductors

Single crystal electroluminescence

• Williams&Schadt 1969• 100μm Anthracene crystal, 100V voltage

First OLED

C.W. Tang and S.A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987)

First White OLED

J. Kido et al, Appl. Phys. Lett. 64, 813 (1993)

Time

1st wave: small OLED Display

Progression of Organic Products

3rd wave: OLED lighting

2nd wave: OLED TV

4th wave: OPV

5th wave: Organic Electronics

Passive Matrix

• 2013 market: Approx. 10 billion $ (Idtechex)

• 100% small molecule OLED

• 99% Asian Manufacturers

OLED Displays on the Market

Philips OLEDShaver

Active Matrix

Samsung phone

LG OLED TV

Nokia phone

Kodak Camera

iWatch: flexible OLED display

Oled-display.net

• OLED: ideal for flexible devices

• Thin-film encapsulation is challenging

• Usual approach: Plastic film coated with multilayer encapsulation system

• Diffusion rates must be 106 times lower than for food encapsulation

Outline

• Introduction to organic semiconductors

• Doping of Organic Semiconductors

• Organic Light Emitting Diodes (OLED)

• Organic Solar Cells

The pin-OLED structure

• Device operates in flat-band condition• Carriers are injected through thin space-charge layers

p-H

TL

Ele

ctr

on

Blo

cker

Hole

Blo

cker

Em

itte

r

Anode

Cathode

n-E

TL

p i n

AOBFF

F F

N

N

N

N

F4-TCNQ

NN

N

N

N

N

NN Zn

S

CN

CNS

S

Bu Bu

S

S

BuBu

CN

CN

DCV5T-Bu

ZnPc

C60

Anode

p-doped HTL

Photovoltaicactive Layer

n-doped ETL

Cathode

p

i

n

B. Maennig et al., Appl. Phys. A 79, 1 (2004)M. Riede et al., Nanotechnology 19, 424001 (2008)

4P-TPD

Di-NPD

2-TNATA

The p-i-n Concept forOrganic Solar Cells

broad bands small correlation energies (e-h 4meV) hydrogen model works

Inorganic Organic

hopping transport large correlation (e-h 0.5 eV) polaron effects important

Basics of Doping: p-doping

Dopant Matrix

Quartz monitors

Substrate

p 10-4 Pa

Tevap= 100..400 oC

TSubs= -50..150 oC

d = 25..1000 nm

rM1 Å/s

Dopant/Matrix ratio of 1:2000 achieved

Co-evaporation of doped films

UPS/XPS study of doping process

• MeO-TPD doped with F4-TCNQ• Molar doping ratio is varied

S. Olthof et al. J. Appl. Phys. 106, 103711 (2009)

Fermi level shift and conductivity change

• MeO-TPD doped with F4-TCNQ

• Fermi level shift observed in UPS and XPS

• Fermi level shifts first very quickly, slope >>kT

• Then saturation

S. Olthof et al. J. Appl. Phys. 106, 103711 (2009)

Origin of Saturation: tail states

• Fermi level shift is caused by tail states of Gaussian

• Distance to HOMO level depends on material

• Distance correlates with disorder: smaller in ZnPC, larger in amorphous materials

S. Olthof et al. J. Appl. Phys. 106, 103711 (2009)

Model assuming deep traps

M. Tietze et al., Phys. Rev. B86, 025320 (2012)

• Deep traps with concentration N

t

• Energy Et

• NA<<N

t: only traps are

filled

• NA>>N

t: Normal doping

Trap model and experiment

M. Tietze et al., Phys. Rev. B86, 025320 (2012)

• Model describes Fermi level shift reasonably well

• Experiment more “smeared out”: broadening of trap state

• Concentration and energy of traps can be determined precisely

NN

N

N

N

N

NN Zn

NN

4

4.5

3.5

3

Electron Affinity [eV]

OLED

OSC

C60NTCDA

ZnPc

BPhen

TCNQ

require

stro

nger donors

air sensitive donors

air stable donors

air sensitive donors

Dopand

Matrix

Alternative solution: metallic dopants Li, Cs (Kido et al.): unstable at higher temperature

Molecular n-type doping: a challenge

P. Wei et al., JACS, dx.doi.org/10.1021/ja211382x

Air stable n-dopants

• Usual n-dopant are not stable in air

• Here: Iodine splits off when evaporated

• Strong n-dopant in C60

Best devices: 1.89V ≈ thermodynamic limit + 20%

All-organic device: Red pin OLED at 2.4V

Outline

• Introduction to organic semiconductors

• Doping of Organic Semiconductors

• Organic Light Emitting Diodes (OLED)

• Organic Solar Cells

•The quantum efficiency of OLEDs is given by

•The luminous efficacy is defined as

Highly Efficient OLEDs

[1] Meerheim, PhD Thesis 2009

Charge Balance

Singlet/Triplet ratio

Rad. efficiency

Outcouplingefficiency

Driving voltage

Except for outcoupling, everything is close to optimum!

• e-h-recombination: 75% triplet- and 25% singlet-excitons

• Phosphorescent emitters: triplets are used as well due to spin-orbit

coupling by heavy metals (Ir, Pt, Cu…)

• ≈ 100% internal quantum efficiency reached

+

+

+

+

hole electron exciton

Triplet

Triplet

Triplet

Singlet

Spin Statistics: Phosphorescent Emitters are needed (Thompson & Forrest)

Outcoupling Efficiency

•Different index of

refraction of organic,

glas and air•Total reflection at

interfaces•80% of all light is

trapped in flat device:

ξ≈0.2

Distribution of Power in Modes

•Outcoupled modes•Substrate modes (1)•Organic modes (2)•Plasmonic losses (3)

3

Source: TemiconSource: Temicon

3

Substrate Modes: Outcoupling easily achieved

Waveguide Modes

Cathode

Organics

ITO

Glass

Emitting Center

M. Furno et al.

Surface Plasmon Modes

Cathode

Organics

ITO

Glass

Emitting Center High Losses due to Coupling to Metal!

M. Furno et al.

Distribution of power into different modes

• Calculations by Mauro Furno (M. Furno et al. Proc. SPIE 7617, 761716 (2010); Phys. Rev. B 85, 115205 (2012))

• Model includes Purcell effect

• Model can be tested by variation of electron transport layer thickness

R. Meerheim et al., Appl. Phys. Lett. 97, 253305 (2010)

50 100 150 200 2500

20

40

60

80

100

Measu

rem

ents

bottom emission on high index glass

Outcoupled

Surface Plasmons

Non-radiative losses

Absorption

Electrical + Half sphere losses

Qua

ntum

effi

cien

cy (

%)

ETL thickness (nm)

MeO-TPD (36)NDP-2

Spiro-TAD (10)

BAlq (10)

Bphen (x)Cs

ITO (90)

Ag (100)

NPB:Ir(MDQ)2(20)

High-n (HI) glass

R. Meerheim et al., Appl. Phys. Lett. 97, 253305 (2010)

Experiment: High Index Glass

MeO-TPD (36)NDP-2

Spiro-TAD (10)

BAlq (10)

Bphen (x)Cs

ITO (90)

Ag (100)

NPB:Ir(MDQ)2(20)

High-n (HI) glass

R. Meerheim et al., Appl. Phys. Lett. 97, 253305 (2010)

Experiment: High Index Glass

Up to 54 % EQE (104 lm/W) reached for red OLEDs

Fabrication of gratings

Wallace Choy et al., University of Hongkong

OLED on periodically structured substrates

1D grating

Bottom- and top-emitting OLED

TobiasSchwab

Efficiency Enhancement for Bottom-Emitting OLEDs

EQE increase: Λ = 0.7µm → 1.26 x EQEplanar

increased luminance comparable leakage

[1]

Fuchs et al., Optics Express, Vol. 21, Issue 14, pp. 16319-16330 (2013)

Bragg Scattering: Theory

periodic structure → lattice constant

additional intensity to air cone:

→ reciprocal lattice constant

high order m large G

[1]

[1] Salt et al., PRB (2000) Cornelius Fuchs

Mode analysis for p-polarization

Mode analysis for p-polarization

Mode analysis for p-polarization

Mode analysis for p-polarization

Outcoupling with nanoparticle layers

• Polymer film with TiO2 scattering particles

• Easy and low-cost preparation

• Comparatively smooth layers (RMS=4.5nm) integrated below ITO electrode

• Reasonable overlap with waveguide mode (blue)

• Small overlap with plasmon mode (red)

Hong-Wei Chang et al., J. Appl. Phys. 113, 204502 (2013)

• White OLED tandem stack

• Blue-red triplet harvesting unit

• Combined with green phosphorescent unit

Hong-Wei Chang et al., J. Appl. Phys. 113, 204502 (2013)

White translucent OLED with NP scattering

Hong-Wei Chang et al., J. Appl. Phys. 113, 204502 (2013)

• Outcoupling without NP layer: EQE 22% / 32 lm/W

• With NP layer: 33% EQE / 46 lm/W

• With NP and outcoupling sphere: 46% EQE / 62 lm/W

White translucent OLED with NP scattering

O with NP & sphere

● with NP

■ w/o NP

Improved angular dependence

• OLED with nanoparticles: Emission spectrum virtually angle-independent

• Emitter power smoothed to Lambertian distribution

• Nanoparticle layer ideal for white devices!

Hong-Wei Chang et al., J. Appl. Phys. 113, 204502 (2013)

● with NP

■ w/o NP

All-phosphorescent white OLED

• S. Reineke et al., Nature 459, 234 (2009)

• Novel emitter layer design

• High-index substrate and higher-order electron transport layer

ETL

ITO

Ag

HTL

High-n (HI glass)

S. Reineke et al., Nature 459, 234-238 (2009)

Efficacy for white OLED

Outline

• Introduction to organic semiconductors

• Doping of Organic Semiconductors

• Organic Light Emitting Diodes (OLED)

• Organic Solar Cells

© Heliatek

Organic Photovoltaics

Homogeneous Surface

Novel applications possible

Source: Solartension

Elementary processes in organic solar cells

absorption

exciton diffusion

exciton separation

charge transport

charge extraction

• Absorption leads to tightly bound (0.2 … 0.5 eV) excitons

• Separation in electric field inefficient

• Usual solar cell structure does not work

The organic exciton separation problem

S. E. Gledhill et al. J. Mat Res. 20, 3167 (2005)

P. Würfel, CHIMIA 61, 770 (2007)

GaAs exciton

Organic exciton

Exciton separation at a heterojunction

C. W. Tang, Appl. Phys. Lett. 48, 183 (1986)M. Hiramoto et al., Appl. Phys. Lett. 58, 1062 (1991)J. J. Hall et al., Nature 376, 498 (1995)G. Yu et al. Science 270, 1789 (1995)

Flat heterojunction (FHJ) bulk heterojunction (BHJ)

Exciton diffusion length

Exciton diffusion length LD = (10 ±1) nm

Exciton separation at a heterojunction

C. W. Tang, Appl. Phys. Lett. 48, 183 (1986)M. Hiramoto et al., Appl. Phys. Lett. 58, 1062 (1991)J. J. Hall et al., Nature 376, 498 (1995)G. Yu et al. Science 270, 1789 (1995)

Flat heterojunction (FHJ) bulk heterojunction (BHJ)

Energy loss is unavoidable!

Bulk heterojunction: Morphology control

• Heterojunction is characterized by complex morphology

• Ideally: columnar structure

• Reality: disordered mixture with nanodomains

• Multi-scale approach needed for materials development• Connection between molecular structure and device

performance very complex

D. Andrienko

How to find the “right” molecule?

The thiophene zoo...

3T 4T 5T 6T

University of UlmDepartment Organic

Chemistry II

Energy Levels vs. backbone length

DCVnT: Fitzner et al., AFM 21, 897 (2011)DCVnT-Bu: Schüppel et al., PRB 77, 085311 (2008)

# thiophene units

Influence of side chains on energy levels

- Significant Energy shifts in thin films

- Only weak effects of side chains in solution

The thiophene zoo...

3T 4T 5T 6T

University of UlmDepartment Organic

Chemistry II

DCV5T-Me: small differences, big effects

DCV5T-Me(3,3) [D33] DCV5T-Me(1,1,5,5) [D15]

- almost identical molecular structure- identical stack

6.9% 4.8%

Chris Elschner

GIWAXS single layersglass / DCV5Ts (30 nm)

[D33] [D15]

- broadened out of plane reflections @ RT

- orientation of crystals spreads out, crystal size grows @ 110°C

single layer pattern very similar !

Tsubstrate

RT 80°C 110°C 140°C

[D15]

[D33]

D33 (top): best OSC @80°C, crystallization @110°CD15 (bottom): best OSC @≈110°C (?), crystallization @140°C

GIWAXS blendsglass / DCV5Ts : C60 (30 nm, 2:1)

Interpretation

RT intermediate temp. high temp.

- nanoscale mixing of donor and C60- low crystallinity- smooth surface

0 5 10 15 20 25 30 35 40

0

100

200

300

400

500

inte

nsi

ty (

cps)

2(°)

glass \ D15:C60 (2:1) RT glass \ D15:C60 (2:1) 90°C

Tsubstrate

Interpretation

RT intermediate temp. high temp.

- nanoscale mixing of donor and C60- low crystallinity- smooth surface

- morphology changes: - crystallinity - roughness - OSC efficiency

0 5 10 15 20 25 30 35 40

0

100

200

300

400

500

inte

nsi

ty (

cps)

2(°)

glass \ D15:C60 (2:1) RT glass \ D15:C60 (2:1) 90°C

Tsubstrate

Interpretation

[D15] > 110°C[D33] > 80°C

RT intermediate temp. high temp.

- nanoscale mixing of donor and C60- low crystallinity- smooth surface

- morphology changes: - crystallinity - roughness - OSC efficiency

0 5 10 15 20 25 30 35 40

0

100

200

300

400

500

inte

nsi

ty (

cps)

2(°)

glass \ D15:C60 (2:1) RT glass \ D15:C60 (2:1) 90°C

- surface segregation of DCV → crystallinity → roughness - OSC efficiency

5 10 15 200

50

100

150

200

250

300

350 glass / D15:C60 (2:1) 140°C glass / D15:C60 (2:1) 110°C

inte

nsi

ty (

arb

. u

nits

)

2 (°)

critical

Tsubstrate

Interpretation

RT intermediate temp. high temp.

- nanoscale mixing of donor and C60- low crystallinity- smooth surface

- morphology changes: - crystallinity - roughness - OSC efficiency

0 5 10 15 20 25 30 35 40

0

100

200

300

400

500

inte

nsi

ty (

cps)

2(°)

glass \ D15:C60 (2:1) RT glass \ D15:C60 (2:1) 90°C

- surface segregation of DCV → crystallinity → roughness - OSC efficiency

[D15] > 110°C[D33] > 80°C

Tsubstrate

8.3% certified DCV5T cell

R. Meerheim et al., Appl. Phys. Lett. 105, 063306 (2014)

Rico Meerheim

Christian Körner

8.3% certified DCV5T cell

R. Meerheim et al., Appl. Phys. Lett. 105, 063306 (2014)

Rico Meerheim

Christian Körner

T. Mueller et al.

Efficiency Outlook Single Cells

Main assumptions: EQE 60% FF 60%

Max efficiency about 15%:10-12% in module

Higher Efficiency for Multijunction Cells

M. Graetzel et al., Nature 488, 304 (2012)

31

Shockley-Queisser limit for single junction: 31%

Major gains only for

Tandem junction: 42%

Triple junction: 49%

Lower currents/higher voltages reduce electrical losses

42

first cell second cell

e.gap 1.9eV 1.25eV ~21%o.gap ~770nm ~1300nm

e.gap 2.1eV 1.5eV ~20%o.gap ~690nm ~1030nm

e.gap 2.225eV 1.7eV ~19%o.gap ~645nm ~890nm

T. Mueller et al.

Efficiency Outlook for Tandem Cells

Main assumptions: EQE 60% FF 60%

>20% for tandem possible!

P-i-n tandem cells:

• Pn-junction is ideal recombination contact

• optimizing interference pattern with conductive transparent layers

=>optical engineering on nanometer layer thickness scale

photoactive layer 1

photoactive layer 2

substrate foil

-+

p

np

n

+

-

Pin-tandem cells: doped layers are critical for optical optimization

J. Drechsel et al., Appl.Phys.Lett. 86, 244102 (2005)

High-efficiency thiophene cells

Jsc (mA/cm²) 4.80

Voc (V) 2.79

FF (%) 72.4

PCE (%) 9.7

Triple

Jsc (mA/cm²) 7.39

Voc (V) 1.88

FF (%) 69.0

PCE (%) 9.6

Tandem

Jsc (mA/cm²) 13.20

Voc (V) 0.96

FF (%) 65.8

PCE (%) 8.3

Single

R. Meerheim et al., Appl. Phys. Lett. 105, 063306 (2014)

EQE of triple cell (9.7%)

R. Meerheim et al., Appl. Phys. Lett. 105, 063306 (2014)

Small-Molecule OPV Record > 1cm²

diagram available under www.orgworld.de

Development of OPV Efficiencies

diagram available under www.orgworld.de

Perovskites: the new kid on the block...

Development of OPV EfficienciesDevelopment of OPV Efficiencies

Perovskite „record“ cell

H. Zhou et al., Science 345, 542 (2014)

Strong hysteresis effects

Forward „efficiency“ : 13.08%Reverse „efficiency“: 16.79%H. Zhou et al., Science 345, 542 (2014)

Perovskite cells: variation of HTL

• p-doped HTL with different alignment

• First fully vacuum processed cells: no hysteresis

• L. Polander et al., Appl. Phys. Lett. Mat. 2, 081503 (2014)

Lauren Polander

Solar cell parameters

• Optimum molecule: Spiro-MeO-TPD

• No hysteresis observed

• L. Polander et al., Appl. Phys. Lett. Mater. 2, 081503 (2014)

Lifetime of ZnPc:C60

lab cells

• Pin structures

• Glass-glass encapsulated

• Measured unter 2 suns

(Roughly) extrapolated lifetime: 37 years!

Christiane Falkenberg, PhD thesis, TU Dresden

• Heliatek’s foil-encapsulated solar films withstand lifetime tests well above PV industry standard

• Degradation after damp-heat stress (85°C, 85% RH): below 3%

• Based on commercially available barrier foils

• Heliatek propriety encapsulation and sealing process

• IEC standard damp heat test

Management Presentation

Heliatek reliability lab measurement of BDR-based stack, 80 cm² active area

Lifetime of flexible module

Outdoor test: Singapore

Courtesy: Heliatek

Material Efficiency kWh/kWp

Ratio toCIGS

Ratio toc-Si

CIGS 9.3% 136 1

c-Si 15.2% 147 1.20 1

mc-Si 8.5% 156 1.27 1.06

Organic 8.6% 187 1.38 1.27

February to April 2012

300 tilt, NW orientation

O-Factor: 27% relative to c-Si

C.J. Mulligan et al. / Solar Energy Materials & Solar Cells 120 (2014) 9–17

Cost Calculation: Mass Production

• 60m2/min production: ≈ 3 GW/year

• P3HT active material, C60

(PCBM)

• Ag/Pedot anode

• Al cathode

• 100% production yield

Total cost: 7.80 (±2) US$/m2 ≈ 0.05US$/Wattpeak

≈ 0.02US$/kWh*

Cost distribution

C.J. Mulligan et al. / Solar Energy Materials & Solar Cells 120 (2014) 9–17

* if system cost can be scaled similarly

14 Linear Organic Evaporators

DC-Magnetron

Lineare Ion Source

2 Metal Evaporators

Substrate Winder

Interleaf Winder

Port for Inert Substrate Load Lock

cathode

EBL

HBL

EMLred

EMLgreen

EMLblue

HTL

ETL

BL

BL

3-color-white pin OLED

Organic Roll-to-Roll Coater

• Organic semiconductors: low mobility, but excellent optoelectronic properties

• Organic LED have made tremendous progress; established product for smartphone displays

• Remaining challenge for higher efficiency: Optical outcoupling

• Internal modes can be outcoupled with suitable scattering structures

• Organic solar cells: Efficiencies have grown dramatically

• Tandem cells can be easily realized

Conclusions

• L. Burtone, C. Elschner, L. Fang, A. Fischer, J. Fischer, H. Froeb, M. Furno, M. Gather, S. Hofmann, F. Holzmüller, D. Kasemann, C. Körner, B. Lüssem, R. Meerheim, J. Meiss, T. Menke, T. Meyer, T. Mönch, L. Müller-Meskamp , D. Ray, K. Vandewal, S. Reineke, M.K. Riede, C. Sachse , T. Schwab, N. Sergeeva, J. Widmer, S. Ullbrich (IAPP)

• K. Fehse C. May, C. Kirchhof, M. Toerker, M. Hoffmann, S. Mogck, C. Lehmann, T. Wanski (FhG-COMEDD)

• J. Blochwitz-Nimoth, J. Birnstock, T. Canzler, M. Hummert, S. Murano, M. Vehse, M. Hofmann, Q. Huang, G. He, G. Sorin (Novaled)

• M. Pfeiffer, B. Männig, G. Schwartz, T. Müller, C. Uhrich, K. Walzer (Heliatek)• J. Amelung, M. Eritt (Ledon)• D. Gronarz (OES)

• R. Fitzner, E. Brier, E. Reinold, A. Mishra, P. Bäuerle (Ulm)• D. Alloway, P.A. Lee, N. Armstrong (Tucson)• K. Schmidt-Zojer (Graz), J.-L. Bredas (Atlanta)• C. Tang (Rochester)• R. Coehoorn, P. Bobbert (Eindhoven)• T. Fritz (Jena)• P. Wei, B. Naab, Z. Bao (Stanford)• D. Wöhrle (Bremen), J. Salbeck (Kassel), H. Hartmann (Merseburg/Dresden)• C.J. Bloom, M. K. Elliott (CSU)• P. Erk et al. (BASF)• BMBF, SMWA, SMWK, DFG, EC, FCI, NEDO

Acknowledgment

Prof. Dr. Karl LeoInstitut für Angewandte PhotophysikTechnische Universität Dresden01062 Dresden, Germanyph: +49-351-463-37533 or mobile: +49-175-540-7893 Fax: +49-351-463-37065 email: leo@iapp.deWeb page: http://www.iapp.de