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 Ulm
Department 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

Organic Semiconductors
• Large area & flexible substrates possible
• Large variety: millions of molecules, mostly carbon
• Low cost: approx. 1g/m2 active material
Photovoltaic
cells
Organic
materials
Transistors
and memory
Organic light
emitting
diodes

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…..
350 400 450 500 550 600 650 700
3,4
3,2
3,0
2,8
2,6
2,4
2,2
2,0
1,8
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
Absorption
Wellenlänge

Carbon: the influence of dimensionality
Source: Castro Neto, Geim et al.
Van der Waals-coupling:
Narrow bands
104
m ob ility
0D
2D covalent
broad bands
2D
101
10-2
1D covalent:
broad bands
1D

Mobility in Organic Semiconductors
Typical OLED
today!
Source: IBM J. Res. Dev.

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

OLED Displays on the Market
Passive Matrix
Philips OLED
Shaver
• 2013 market: Approx. 10 billion $ (Idtechex)
• 100% small molecule OLED
• 99% Asian Manufacturers
Active Matrix
Kodak Camera
Samsung phone
Nokia phone
LG OLED TV

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

The pin-OLED structure
p i n
p-HTL
Electron Blocker
Hole Blocker
Emitter
Anode
Cathode
n-ETL
• Device operates in flat-band condition
• Carriers are injected through thin space-charge layers

AOB
F F
F F
N
N
N
N
F4-TCNQ
N
N
N
N
N
N
N
N Zn
S
CN
CN
S
Bu Bu
S
C60
ZnPc
S
Bu Bu
S
CN
CN
DCV5T-Bu
Cathode
n-doped ETL
Photovoltaic
active Layer
p-doped HTL
Anode
n
i
p
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 for
Organic Solar Cells

Basics of Doping: p-doping
Inorganic Organic
· broad bands
· small correlation energies (e-h » 4meV)
· hydrogen model works
· hopping transport
· large correlation (e-h » 0.5 eV)
· polaron effects important

Quartz monitors
Co-evaporation of doped films
Substrate
Dopant Matrix
p » 10-4 Pa
Tevap= 100..400 oC
TSubs= -50..150 oC
d = 25..1000 nm
rM» 1 Å/s
Dopant/Matrix ratio of 1:2000
achieved

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

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

Model assuming deep traps
• Deep traps with
concentration Nt
• Energy Et
• NA<<Nt: only traps are
filled
• NA>>Nt: Normal doping
M. Tietze et al., Phys. Rev. B86, 025320 (2012)

Trap model and experiment
• Model describes Fermi
level shift reasonably well
• Experiment more “smeared
out”: broadening of trap
state
• Concentration and energy
of traps can be determined
precisely
M. Tietze et al., Phys. Rev. B86, 025320 (2012)

Molecular n-type doping: a challenge
N
N
N
N
N
N
N
N Zn
N N
3
3.5
Electron Affinity [eV]
4
4.5
OLED
OSC
C60
NTCDA
ZnPc
BPhen
TCNQ
require stronger donors
air sensitive donors
air stable donors
Dopand
Matrix
Alternative solution: metallic dopants Li, Cs (Kido et al.): unstable at higher temperature

Air stable n-dopants
• Usual n-dopant are not
stable in air
• Here: Iodine splits off
when evaporated
• Strong n-dopant in C60
P. Wei et al., JACS, dx.doi.org/10.1021/ja211382x

All-organic device: Red pin OLED at 2.4V
Best devices: 1.89V ≈ thermodynamic limit + 20%

Outline
• Introduction to organic semiconductors
• Organic Light Emitting Diodes
(OLED)

Highly Efficient OLEDs
Singlet/Triplet ratio
Rad. efficiency
•The quantum efficiency of
OLEDs is given by
•The luminous efficacy is
defined as
[1] Meerheim, PhD Thesis 2009
Charge Balance
Outcoupling
efficiency
Driving voltage
Except for outcoupling, everything is close to optimum!

Spin Statistics: Phosphorescent Emitters are needed
(Thompson & Forrest)
hole electron exciton
+
+
+
+
Triplet
Triplet
Triplet
Singlet
• 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

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

Substrate Modes: Outcoupling easily achieved
SSoouurrccee:: TTeemmiiccoonn
3

Waveguide Modes
Glass
ITO
Organics
Cathode
Emitting Center
M. Furno et al.

Surface Plasmon Modes
Glass
ITO
Organics
Cathode
M. Furno et al.
Emitting Center High Losses due to Coupling to Metal!

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)

Ag (100)
Bphen (x)
Cs
BAlq (10)
NPB:Ir(MDQ)2(20)
Spiro-TAD (10)
MeO-TPD (36)
NDP-2
ITO (90)
High-n (HI) glass
Experiment: High Index Glass

Ag (100)
Bphen (x)
Cs
BAlq (10)
NPB:Ir(MDQ)2(20)
Spiro-TAD (10)
Up to 54 % EQE (104 lm/W) reached for red OLEDs
MeO-TPD (36)
NDP-2
ITO (90)
High-n (HI) glass
Experiment: High Index Glass

Fabrication of gratings
Wallace Choy et al., University of Hongkong

OLED on periodically structured substrates
 1D grating
 Bottom- and top-emitting OLED
Tobias
Schwab

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

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 translucent OLED with NP scattering
• White OLED tandem stack
• Blue-red triplet harvesting unit
• Combined with green phosphorescent unit

White translucent OLED with NP scattering
• 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
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!
● 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

Ag
ETL
HTL
ITO
High-n (HI glass)
Efficacy for white OLED
S. Reineke et al., Nature 459, 234-238 (2009)

Outline
• Introduction to organic
semiconductors

© 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

The organic exciton separation problem
GaAs exciton
• Absorption leads to tightly bound
(0.2 … 0.5 eV) excitons
• Separation in electric field
inefficient
• Usual solar cell structure does
not work
Organic exciton
S. E. Gledhill et al. J. Mat Res. 20, 3167 (2005)
P. Würfel, CHIMIA 61, 770 (2007)

Exciton separation at a heterojunction
Flat heterojunction (FHJ) bulk heterojunction (BHJ)
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)

Exciton diffusion length
Exciton diffusion length LD = (10 ±1) nm

Exciton separation at a heterojunction
Flat heterojunction (FHJ) bulk heterojunction (BHJ)
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)
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 Ulm
Department Organic
Chemistry II

Energy Levels vs. backbone length
# thiophene
units
DCVnT: Fitzner et al., AFM 21, 897 (2011)
DCVnT-Bu: Schüppel et al., PRB 77, 085311 (2008)

Influence of side chains on energy levels
- Only weak effects of side chains in solution
- Significant Energy shifts in thin films

DCV5T-Me: small differences, big effects
DCV5T-Me(3,3) [D33] DCV5T-Me(1,1,5,5) [D15]
6.9% 4.8%
- almost identical
molecular structure
- identical stack
Chris Elschner

GIWAXS single layers
glass / 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
GIWAXS blends
glass / DCV5Ts : C60 (30 nm, 2:1)
RT 80°C 110°C 140°C
[D15]
[D33]
D33 (top): best OSC @80°C, crystallization @110°C
D15 (bottom): best OSC @≈110°C (?), crystallization @140°C

Interpretation
RT intermediate temp. high temp.
- nanoscale mixing
of donor and C60
- low crystallinity
- smooth surface
g l a s s D 1 5 : C 6 0 ( 2 : 1 ) R T
g l a s s D 1 5 : C 6 0 ( 2 : 1 ) 9 0 ° C
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0
5 0 0
4 0 0
3 0 0
2 0 0
1 0 0
0
i n t e n s i t y ( c p s )
2 q ( ° )
Tsubstrate

Interpretation
- nanoscale mixing
of donor and C60
- low crystallinity
- smooth surface
- morphology changes:
- crystallinity
- roughness
- OSC efficiency
g l a s s D 1 5 : C 6 0 ( 2 : 1 ) R T
g l a s s D 1 5 : C 6 0 ( 2 : 1 ) 9 0 ° C
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0
5 0 0
4 0 0
3 0 0
2 0 0
1 0 0
0
2 q ( ° )
Tsubstrate

Interpretation
[D15] > 110°C
[D33] > 80°C
- nanoscale mixing
of donor and C60
- low crystallinity
- smooth surface
- crystallinity
- roughness
- OSC efficiency
g l a s s D 1 5 : C 6 0 ( 2 : 1 ) R T
g l a s s D 1 5 : C 6 0 ( 2 : 1 ) 9 0 ° C
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0
5 0 0
4 0 0
3 0 0
2 0 0
1 0 0
0
2 q ( ° )
- surface segregation of
DCV → crystallinity
→ roughness
- OSC efficiency
3 5 0 g la s s / D 1 5 : C 6 0 ( 2 : 1 ) 1 4 0 ° C
q < q c r i t i c a l
5 1 0 1 5 2 0
3 0 0
2 5 0
2 0 0
1 5 0
1 0 0
5 0
0
g la s s / D 1 5 : C 6 0 ( 2 : 1 ) 1 1 0 ° C
i n t e n s i t y ( a r b . u n i t s )
2 q ( ° )
Tsubstrate

Interpretation
- nanoscale mixing
of donor and C60
- low crystallinity
- smooth surface
- crystallinity
- roughness
- OSC efficiency
g l a s s D 1 5 : C 6 0 ( 2 : 1 ) R T
g l a s s D 1 5 : C 6 0 ( 2 : 1 ) 9 0 ° C
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0
5 0 0
4 0 0
3 0 0
2 0 0
1 0 0
0
2 q ( ° )
- surface segregation of
DCV → crystallinity
→ roughness
- OSC efficiency
[D15] > 110°C
[D33] > 80°C
Tsubstrate

8.3% certified DCV5T cell
Rico Meerheim
Christian Körner

Efficiency Outlook Single Cells
T. Mueller et al.
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)
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
31

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
n
-
photoactive layer 2
+
-
p
n
photoactive layer 1
p
+
substrate foil
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

EQE of triple cell (9.7%)

Small-Molecule OPV Record > 1cm²

Development of OPV Efficiencies
diagram available under www.orgworld.de

Perovskites: the new kid on the block...
diagram available under www.orgworld.de

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
Heliatek reliability lab measurement of BDR-based stack, 80 cm² active area
Management Presentation
Lifetime of flexible module

Outdoor test: Singapore
Courtesy: Heliatek
Material Efficiency kWh/kWp Ratio to
CIGS
Ratio to
c-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

Cost Calculation: Mass Production
• 60m2/min production: ≈ 3 GW/year
• P3HT active material, C60 (PCBM)
• Ag/Pedot anode
• Al cathode
• 100% production yield
C.J. Mulligan et al. / Solar Energy Materials & Solar Cells 120 (2014) 9–17

Cost distribution
Total cost: 7.80 (±2) US$/m2 ≈ 0.05US$/Wattpeak ≈ 0.02US$/kWh*
* if system cost can be scaled similarly
C.J. Mulligan et al. / Solar Energy Materials & Solar Cells 120 (2014) 9–17

Organic Roll-to-Roll Coater
14 Linear Organic Evaporators
BL
DC-Magnetron
Lineare Ion Source
2 Metal Evaporators
Substrate Winder
Interleaf Winder
Port for Inert Substrate Load
Lock
cathode
EBL
HBL
EML
red
EML
green
EML
blue
HTL
ETL
BL
3-color-white pin OLED

Conclusions
• 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

Acknowledgment
• 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

Prof. Dr. Karl Leo
Institut für Angewandte Photophysik
Technische Universität Dresden
01062 Dresden, Germany
ph: +49-351-463-37533 or mobile: +49-175-
540-7893
Fax: +49-351-463-37065
email: leo@iapp.de
Web page: http://www.iapp.de

Highly efficient organic devices.

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