highly efficient organic devices
DESCRIPTION
Plenary lecture of the XIII SBPMat (Brazilian MRS) meeting, given on September 30th 2014 by Karl Leo, professor of optoelectronics at Dresden University of Technology (Germany) and director of the Solar and Photovoltaic Engineering Research Center at KAUST (Saudi Arabia).TRANSCRIPT
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: [email protected] page: http://www.iapp.de