Controlling Charge Carrier Concentration in Organic Semiconductors
Institut fürAngewandte Photophysik
M. Pfeiffer, J. Blochwitz-Nimoth, G. He, X. Zhou, J. Huang, J. Drechsel, A. Werner, B. Männig, K. Leo
Institut für Angewandte Photophysik, TU Dresden, 01062 Dresden, Germany
www.iapp.de
• Charge carrier control: Doping of organic semiconductors
- Electrical Characterisation- UPS/XPS Spectroscopy: space charge layers
• Application in devices:
Organic light-emitting diodesOrganic solar cells
Outline
• Basics of organic semiconductors
What is an organic semiconductor…..?
Why organic semiconductors…..?
Photovoltaic cellsLight emitters Integrated circuits
Organic Semiconductors: a new class of materials
pzpz
sp2 sp2
σ
σ∗
π
π∗
p lane of thesp 2-orb ita ls
pz-orb ita l π -bond
π -bond
σ -bond
pz-orb ita l
Conjugated π-electron systems
Sp2-hybridised Carbon:
• Spatially extended electronsystems:
• Large dipole moments
• Tunability of energy levels
• Good coupling betweenmolecules in solid state
de loka lis ie rte π −E lektronen
pz
σ∗
sp2
6 x
18 x
LUMO (π*) => EC
HOMO (π) => EV
• saturated π-electron systems• VdW crystals• small π−π-overlap, narrow bands
delocalizedπ-electrons
Molecules with conjugated π-electron system
Film preparation technology: vapor deposition
pz
σ∗
sp2
6 x
18 x
n
n x pz
sp2
VB (π)
CB (π*)
• broad bands• transport limited by
interchain hops
Polymers with conjugated π-electron system
Film preparation technology: solution processing
Inorganic vs. Organic Semiconductors
CB
VB
ED
EA
LUMO
HOMO
≈ 1017Electr. Active impurit. (cm-3)<<10151010-1016 (injection controlled)Charge carrier concentration1015-1018 (doping controlled)
10-6-1 (typ. 10-3)RT mobility (cm2/Vs)102-103HoppingTransport MechanismBand
Organic SemiconductorInorganic Semiconductor
• Charge carrier control: Doping of organic semiconductors
- Electrical Characterisation- UPS/XPS Spectroscopy: space charge layers
• Application in devices:
Organic light-emitting diodesOrganic solar cells
Outline
• Basics of organic semiconductors
• broad bands• small correlation energies (e-h ≈ 4meV) • hydrogen model works
Inorganic Organic
• hopping transport• large correlation (e-h ≈ 1 eV)• polaron effects important
Basics of the doping mechanisms: p-type doping
Dopant Matrix
Quartz monitors
Substrate
p ≈ 10-4 Pa Tevap= 100..400 oCTSubs= -50..150 oCd = 25..1000 nmrM≈ 1 Å/s
Dopant/Matrix ratio of 1:2000 achieved
Institut fürAngewandte Photophysik
Co-evaporation of doped films
Model system for p-doping: ZnPc:F4-TCNQ
Organic Matrix:Zinc-Phthalocyanine (ZnPc)monoclinica = 26.3 Å, b = 3.8 Å, c = 23.9 Å,γ = 94.6o, Z = 4HOMO ≈ -5.3eV
Organic Acceptor:Tetrafluoro-tetracyano-quinodimethane(F4-TCNQ)LUMO ≈ -5.0eV
Institut fürAngewandte Photophysik
e-
ZnPc-F4-TCNQ charge transfer by FTIR
2240 2220 2200 2180 2160
0
10
20
30
40
50
60 >C=C<C Nb2uν32-peak forZ=1 at 2173 cm-1
b2uν32-peakfor Z=1 at 2214 cm -1
b1uν18-peak forZ=0 at 2228 cm-1
b1uν18-peak forZ=1 at 2194 cm-1
trans
mis
sion
[%]
wave number [cm-1]1600 1580 1560 1540 1520 1500
b1uν19-peak for Z=1 at 1501 cm-1
b1uν19-peakfor Z=0 at 1550 cm-1
IR-spectra of a F4-TCNQ - KBr - sample
FF
F F
N
N
N
N
FF
F F
N
N
N
N
10
0
ν−νν−ν
= xZ
Degree Z of the charge-Transfer:
ν0 : neutral moleculeν1 : charged moleculeνx : doped layer
Result in Pc films: Z=1 !
Nominally undoped ZnPc:
≈10-10 S/cm in vacuo
⇒ Doping increases conductivityby orders of magnitude
10-3 10-2 10-110-4
10-3
10-2 ZnPc series 1 ZnPc series 2
T= 30°C
linear dependenceCon
duct
ivity
σ [S
/cm
]
Molar Doping Ratio F4-TCNQ : ZnPc
Conductivity vs. Doping Concentration
• Superlinear behavior ?
• Shallow acceptors ? Coulomb energy too large….
2.6 2.8 3.0 3.2 3.410-4
10-3
10-2
10-1
400:1
100:1
50:135:1
12.5:1
200:1
p/N
µ
1000/T (K)
380 360 340 320 300Temperature [K]
1018
1019
1020
hol
e de
nsity
p (c
m-3) kT
EENTp F µ
µ
−=)(
Hole density is not thermally activated !
M. Pfeiffer et al., Adv. in Sol. State Physics 39, 77 (1999).Institut für
Angewandte Photophysik
Hole densities of ZnPc:F4-TCNQ
Theory: Extension of M.C.J.M. Vissenberg, M. Matters, Phys. Rev. B 57, 12964 (1998)
EF
f(E) g(E)
Density of states:g(E) = g0exp(-E/kBTc)
Variable Range HoppingT < TcEF >> kBTc
Percolation model: σ = σ0 Zc-1
with Zc-1 threshold conductance
Conduction in exponential band tails: Explains superlinear doping efficiency
Assuming shallow acceptors
σ ~ (NA)To/T ; T0 > T
360 320 280 240 2001E-7
1E-6
1E-5
1E-4
1E-3
0.01 theoretical curves
polycrystalline ZnPc
cond
uctiv
ity (S
/cm
)
temperature (K)
experimental data: 1:50 1:100 1:200 1:500
360 320 280 240 200 160
1E-6
1E-5
1E-4
1E-3
0.01
experimental data: 1:150
cond
uctiv
ity (S
/cm
)
temperature (K)
1E-7
1E-6
1E-5
1E-4
1E-3 theoretical curves
polycrystalline ZnPc
mob
ility
(cm
2 /Vs)
material σ (S/cm) Eact (eV) σo(105S/m) To (K) α (Å-1)
ZnPc (polycr.) 5.8 * 10-3 0.18 12 3 485 15 0.37 0.01
ZnPc (amorp.) 4 * 10-4 0.23 11 6 455 15 0.64 0.02
VOPc (polycr.) 2.3 * 10-5 0.32 6 1 485 15 1.00 0.04
TDATA (am.) 5.9 * 10-7 0.34 3 1 515 15 1.39 0.03
σ and Eact for dopant density 1% and T=25°C
Application to field-effect and conductivity data:
Basic parameters are independent of specific material:
Matrix: Naphthalin-tetracarboxylic-dianhydride(NTCDA)
A. Nollau et al., J. Appl. Phys. 87, 4340 (2000)
Donor: Bisethylendithio-tetrathiafulvalene(BEDT-TTF)
N-type doping: conventional approach
charge transfer
HOMO: Donor
LUMO
HOMO
Electron transfer from high-lyingHOMO to matrix LUMO:
ON+
N
Cl-
Pyronin B chloridedonor precursor
charge transfer
reducedpyronin B
donor
HOMO
LUMO
1E-3 0.01
10-5
10-4
undoped NTCDA
σ=3.3*10-8 S/cm
conduct
ivit
y[S
/cm
]
molar doping ratio
A. Werner et al., App.Phys.Lett. 82, 4495 (2003)
dye radicalcreated in situ
from cation
very strongorganic donor
N-type doping: novel approach using cationic dyes
UPS/XPS study of organic/inorganic interfaces
Interested in:• Space charge layers at junctions and their influence on injection
• Barriers and their possible dependence on doping
• Direct observation of Fermi level shift
Institut fürAngewandte Photophysik
matrix dopant
NN
N
N
N
N
NN Zn
FF
F F
N
N
N
NZnPc F4-TCNQ
with N. Armstrong, D. Alloway, P. Lee, Tucson
J. Blochwitz et al., Organic Electronics 2, 97 (2001)
undoped doped
Institut fürAngewandte PhotophysikJ. Blochwitz et al., Organic Electronics 2, 97 (2001)
Creation of ohmic contacts by doping
: blocking : ohmic
• Charge carrier control: Doping of organic semiconductors
- Electrical Characterisation- UPS/XPS Spectroscopy: space charge layers
• Application in devices:
Organic light-emitting diodesOrganic solar cells
Outline
• Basics of organic semiconductors
Institut fürAngewandte Photophysik
Institut für Angewandte Photophysik (www.iapp.de)
• Basic research on novel OLED device concepts
• Organic solar cells
OLED cooperation in Dresden
Novaled GmbH (www.novaled.com)
• Holds University patents
• New Materials for stable doping systems
Fraunhofer-IPMS Dresden (www.ipms.fraunhofer.de)
• Stability of doped organic LED
• OLED-Inline-deposition set-up (30x40cm Substrate)
Organic Light Emitting Diodes
OLEDs: Basic Principles
Device structure
Glass substrate
V
A
Transparent anode
Emissive layer
Cathode
Light emission
Device energy diagram
E
x
Cathode
LUMO
HOMO
Anode+
+ +
---
Thinness
Viewing Angle
Brightness and colour Source: Kodak
OLED Benefits
CostThin
Light Weight
HighResolution
Lifetime
ContrastBrightness
Color Quality
High Response
Wide View Angle
Power Consumption
Suitable for TV and Movies
Suitable for Mobile
a-Si TFT LCD
p-Si TFT LCD
p-Si TFT OLED
Further improvementsexpectedSource: Toshiba 2002
OLED Performance
OLED products on the market
Car Audio Pioneer
Car Audio TDK
Phone SNMD
RGB PM
Passive Matrix
Active Matrix
Sanyo-Kodak
• 2003 market: approx 250 mio US$
• 98% small molecule devices
Sony 13” full color display
• Exponential current-voltage relation
• Flat-band under operation
• Low work-function contacts not needed !
p n
Comparison: Inorganic vs. Organic LED
Inorganic LED (e.g., GaAs/AlGaAs) Organic LED
CB
VB
EFe
EFh
Metal
• space charge limited currents
• low work function metals neededITO-preparation necessary
HTLETL
p-type doping
blocking layers for high efficiency
n-type doping
Ideal OLED: pin-heterostructure
Step 1
Step 2
Steps towards this design:
Step 3HOMO
LUMO
p i n
EF
+ + + + +
-----
+
Institut fürAngewandte Photophysik
N
N
N
N
Starburst = TDATA 4,4',4''-tris(N,N-diphenylamino) triphenylamin
2,8 2,9 3,0 3,1 3,2 3,3 3,4 3,5
1E-6
1E-5
6,86%
5,46%3,87%2.82%2.18%1.35%1.02%
cond
uctiv
ity σ
(S
/cm
)
103/T (K-1)
dopingratio
(mol%)
Eact=0,3eV
• undoped TDATA: 10-10 S/cm
• doped TDATA: up to 10-5 S/cm=> low ohmic losses: 0.1V/100nm@100mA/cm2
Dopant: F4-TCNQ
Step 1: Doping of amorphous wide gap materials
X.Q. Zhou et al., Adv. Funct. Mat.11, 310 (2001)
Y. Shirota et al., APL 65, 807 (1994)
Structure of pin-OLED
Institut fürAngewandte Photophysik
N-doping: Batho-Phenanthroline doped with Li (developed by Kido group)
Lowest voltages reported in literature for small-molecule devicesJ. Huang et al., Appl. Phys. Lett. 80, 139 (2002).
Performance of p-i-n structure
2 3 4 5 6 7 8 9 10
100
101
102
103
104
105
10,000 cd/m2 @5.2V
1,000 cd/m2 @2.9V
100 cd/m2 @2.55V
Lum
inan
ce [c
d/m
2 ]
Voltage [V]
Current efficiency: 5.27 cd/A @1,400cd/m2
(pure Alq3 emitter)
Institut fürAngewandte Photophysik
Institut für Angewandte PhotophysikTechnische Universität Dresden
What determines OLED efficiency ?
opticalrecombIexternal eUhb ηηνη ×××=
• bI: Electron and hole current balance: 1 can be reached
• eU: Operating voltage should be ≈ photon energy
• ηrecomb: 75% Triplet, 25% Singlet excitons: 0.25 for fluorescent emitter
Use phosphorescent (triplet) emitter
• ηoptical: about 20% in flat structure: 80% lost to wave guide modes:
optimize optical outcoupling !
Highly efficient PIN Triplet OLED
2 3 4 5
1
10
100
1000
10000
100000
10000 cd/m2 @ 4.2V26 cd/A; 19 lm/W
1000 cd/m2 @ 3.4V35 cd/A; 33 lm/W
100 cd/m2 @ 3.1V44 cd/A; 44 lm/W
10000 cd/m2 @ 3.79V52.4 cd/A; 43.5 lm/W
1000 cd/m2 @ 3.21V62.4 cd/A; 61.1 lm/W
100 cd/m2 @ 2.95V65.3 cd/A; 69.4 lm/W
Lum
inan
ce (c
d/m
2 )
Voltage (V)
Standard D-doping
Highly efficient PIN Triplet OLED
1E-3 0,01 0,1 1 10 100 10001
10
1001E-3 0,01 0,1 1 10 100 1000
1
10
100
14.5% @ 100 mA/cm2
19.5%
TCTA5TAZ10 TCTA10TAZ10 TCTA10TAZ5
Qua
ntum
effi
cien
cy (%
)
Current density (mA/cm2)
30 lm/W @ 100 mA/cm2
70 lm/W
Pow
er e
ffici
ency
(lm
/W)
Institut für Angewandte PhotophysikTechnische Universität Dresden
LED performance vs. time: linear plot
1970 1975 1980 1985 1990 1995 2000 20050
20
40
60
80
100
120
0
20
40
60
80
100
120
Red Filtered
Tungsten Bulb (unfiltered)
Fluorescent Lamp
InGaNgreen
OLED
PLED
AlInGaP Red/Yellow
Pow
er E
ffici
ency
Year
Institut für Angewandte PhotophysikTechnische Universität Dresden
What determines OLED efficiency ?
opticalrecombIexternal eUhb ηηνη ×××=
• bI: Electron and hole current balance: 1 can be reached
• eU: Operating voltage should be ≈ photon energy
• ηrecomb: 75% Triplet, 25% Singlet excitons: 0.25 for fluorescent emitter
Use phosphorescent (triplet) emitter
• ηoptical: about 20% in flat structure: 80% lost to wave guide modes:
optimize optical outcoupling !
• Three modes:
• External (≈ 20%)
• Substrate (≈ 35%)
• ITO/org (≈ 45%)
Optical outcoupling in OLED
• Charge carrier control: Doping of organic semiconductors
- Electrical Characterisation- UPS/XPS Spectroscopy: space charge layers
• Application in devices:
Organic light-emitting diodesOrganic solar cells
Outline
• Basics of organic semiconductors
LUMO
HOMO
• Absorption dominated byFrenkel excitons
• Large exciton binding energy:approx. 0.5eV
• Free carrier generation requires high fields (=> use in photocopiers)
Problem I: Exciton Separation
Exciton diffusion
LUMO
HOMO
hν
Absorption and exciton formationExciton diffusion
Exciton separation at the interfaceCarrier drift in the electricfield of a pn-heterojunction
DonorAcceptor
Solution: Donor-Acceptor-Heterojunction
Donor: p-typeAcceptor:n-type
C.W. Tang (Kodak)(1986)
• Efficiency 1%• Problems:
- Too low exciton diffusion length- low voltage, no controlled doping
Problem II: Exciton Diffusion
EC
EV
EF
EC
EV
polymer
C60Al
ITO--
+
ultrafast electron transfer
Cells with interpenetrating network: C60 as electron acceptor
p ni1 i2
EF
The piin-heterojunction concept with electron acceptor
• Photoactive interfacebetween two i-layers
• Wide gap transportlayers
• Highly doped layers to maximize Vbi
• Electron acceptor
C60 energy levelsM. Pfeiffer
photoactiveregion
Tang 86
ZnPc*C60
Evolution of the organic solar cell concept
p
n
p-i-n cell
d
Optical mode can be optimized as well
-1,0 -0,5 0,0 0,5 1,0
-20
0
20
40
60
133 mW/cm2
simulated AM 1.5
p-i-n Zelle mit 60nm ZnPc*C60 (1:2)UOC= 0.51 VJSC= 16.7 mA/cm2
FF= 36.4 %η=2.3%
curr
ent d
ensi
ty [m
A/c
m2 ]
voltage [V]
• ca. 80% internal quantum efficiency• Fill factor has to be improved
Pin-structure solar cell
recombinationrecombination centercenter::metal metal clustercluster
(Au)(Au)
Al
n-C60
ITO
p-HTL
ZnPc:C60
p-HTL
n-C60
ZnPc:C60
Tandem Solar Cell Structure
• Full sun spectrum can be used
• Higher voltage, lower currentthan single devices
ITO metal
n
pAu
EQF,e
EQF,h
undoped
Tandem solar cell: Doping is crucial for low losses
-0.4 -0.2 0.0 0.2 0.4 0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
30°C@ 9 mW/cm2
n-C60/i-MeO i-C60/p-MeO n-C60/p-MeO
curr
ent d
ensi
ty [m
A/cm
2 ]
voltage [V]
Au
(n)-C60
ITO
p-MeOTPD
ZnPc:C60
(p)-MeOTPD
• with Doping and Gold-Cluster: quasi-ohmic behavior
Tandem cell: influence of doping
-0.9 0.0 0.9
-20
0
20
40
133+/-3 mW/cm2
simulated AM 1.5
Tandemzelle: optimale Struktur mit 140 nm Spacer
Tandem:UOC= 0.99 VJSC= 10.4 mA/cm2
FF= 46.7 %η=3.5%
Einzelzelle:UOC= 0.51 VJSC= 16.7 mA/cm2
FF= 36.4 %η=2.3%
curr
ent d
ensi
ty [m
A/c
m2 ]
voltage [V]
η=3.6%
Tandem solar cell with 3.6% efficiency
Tandem Zellen: Der nächste SchrittTandem Zellen: Der nächste Schritt
glassglass/ITO/ITO pp--ii--nngapgap 2 2 eVeV
pp--ii--nngapgap 1.5 1.5 eVeV
pp--ii--nngapgap 1.5 1.5 eVeV
0.6 V0.6 V 0.5 V0.5 V 0.5 V0.5 V
AlAl
∑∑ = 1.5 ... 1.6 V ???= 1.5 ... 1.6 V ???VVOCOC
ηη = 5= 5--6%6%
Simulation by H. Hoppe (Uni Linz)
Next step: 3-layer cell
Conclusions:
• Organic semiconductors can be efficiently doped
• Microscopic physics not well understood
• Doping very useful for devices
Future work:
• Molecular n-doping
• Microscopic understanding of doping necessary
• Make solar cells as good as OLED already are
Institut fürAngewandte Photophysik
Conclusions
Acknowledgment
• M. Pfeiffer, J. Blochwitz-Nimoth, G. He, X. Zhou, J. Huang, A. Werner, J. Drechsel, B. Männig, K. Harada, T. Fritz
• J. Amelung, W. Jeroch, M. Schreil, U. Todt (FhG-IMS)
• D. Alloway, P.A. Lee, N. Armstrong, Tucson (XPS/UPS)
• N. Karl (Stuttgart)
• D. Wöhrle (Bremen), J. Salbeck (Kassel), H. Hartmann (Merseburg)
• BMBF, DFG, FCI, SMWK