Controlling Charge Carrier Concentration in
Organic Semiconductors
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
Institut für
Angewandte Photophysik
Outline
• Basics of organic semiconductors
• Charge carrier control: Doping of organic semiconductors
- Electrical Characterisation
- UPS/XPS Spectroscopy: space charge layers
•
Application in devices:
Organic light-emitting diodes
Organic solar cells
Organic Semiconductors:
a new class of materials
What is an organic semiconductor…..?
Why organic semiconductors…..?
Light emitters
Photovoltaic cells
Integrated circuits
Conjugated π-electron systems
Sp2-hybridised Carbon:
p z -o rb ita l
p la n e o f th e
s p 2 -o rb ita ls
π -b o n d
p z -o rb ita l
• Spatially extended electron
systems:
σ -b o n d
π -b o n d
• Large dipole moments
σ∗
pz
π∗
• Tunability of energy levels
pz
π
sp 2
sp 2
σ
• Good coupling between
molecules in solid state
Molecules with conjugated π-electron system
• saturated π-electron systems
• VdW crystals
• small π−π-overlap, narrow bands
d e lok a lisie rte
delocalized
π − E le k tro n en
π-electrons
σ∗
6x
pz
LUMO (π*) => EC
HOMO (π) => EV
18 x sp 2
Film preparation technology:
vapor deposition
Polymers with conjugated π-electron system
• broad bands
• transport limited by
interchain hops
n
σ∗
n6xx pzp z
CB (π*)
VB (π)
18sp
x sp 2
2
Film preparation technology:
solution processing
Inorganic vs. Organic Semiconductors
LUMO
CB
ED
EA
VB
Inorganic Semiconductor
Band
102-103
1015-1018 (doping controlled)
<<1015
HOMO
Organic Semiconductor
Transport Mechanism
Hopping
10-6-1 (typ. 10-3)
RT mobility (cm2/Vs)
Charge carrier concentration 1010-1016 (injection controlled)
Electr. Active impurit. (cm-3)
≈ 1017
Outline
• Basics of organic semiconductors
• Charge carrier control: Doping of organic semiconductors
- Electrical Characterisation
- UPS/XPS Spectroscopy: space charge layers
•
Application in devices:
Organic light-emitting diodes
Organic solar cells
Basics of the doping mechanisms: p-type doping
Inorganic
• broad bands
• small correlation energies (e-h ≈ 4meV)
• hydrogen model works
Organic
• hopping transport
• large correlation (e-h ≈ 1 eV)
• polaron effects important
Co-evaporation of doped films
Substrate
p ≈ 10-4 Pa
Tevap= 100..400 oC
TSubs= -50..150 oC
d = 25..1000 nm
rM≈ 1 Å/s
Quartz monitors
Dopant/Matrix ratio of 1:2000 achieved
Dopant
Matrix
Institut für
Angewandte Photophysik
Model system for p-doping: ZnPc:F4-TCNQ
Organic Matrix:
Zinc-Phthalocyanine (ZnPc)
monoclinic
a = 26.3 Å, b = 3.8 Å, c = 23.9 Å,
γ = 94.6o, Z = 4
-
HOMO ≈ -5.3eV
e
Organic Acceptor:
Tetrafluoro-tetracyano-quinodimethane
(F4-TCNQ)
LUMO ≈ -5.0eV
Institut für
Angewandte Photophysik
N
F
F
N
N
F
F
N
N
F
F
N
N
F
F
N
ZnPc-F4-TCNQ charge transfer by FTIR
Degree Z of the charge-Transfer:
IR-spectra of a F4-TCNQ - KBr - sample
60
C
>C=C<
N
b2uν32-peak for
50
transmission [%]
b1uν1 8-peak for
40
Z=
ν0 − ν x
ν 0 − ν1
-1
Z=1 at 2173 cm
-1
Z=1 at 2194 cm
b1uν19-peak for
-1
Z=1 at 1501 cm
ν0 : neutral molecule
ν1 : charged molecule
νx : doped layer
30
b2uν32-peak
20
for Z=1 at 2214 cm
10
b1uν18-peak for
Result in Pc
films: Z=1 !
-1
b1uν1 9-peak
-1
Z=0 at 2228 cm
for Z=0 at 1550 cm
-1
0
2240
2220
2200
2180
2160 1600 1580 1560 1540 1520 1500
-1
wave number [cm ]
Conductivity σ [S/cm]
Conductivity vs. Doping Concentration
Nominally undoped ZnPc:
T= 30°C
ZnPc series 1
ZnPc series 2
-2
10
≈10-10 S/cm in vacuo
⇒ Doping increases conductivity
by orders of magnitude
-3
10
linear dependence
-4
10
-3
10
-2
10
-1
10
Molar Doping Ratio F4-TCNQ : ZnPc
• Superlinear behavior ?
• Shallow acceptors ? Coulomb energy too large….
Hole densities of ZnPc:F4-TCNQ
Temperature [K]
380
340
320
300
12.5:1
35:1
50:1
20
10
19
10
18
-2
p/Nµ
10
10
100:1
200:1
10
-3
400:1
10
p (T ) = N µ
E F − Eµ
kT
-3
-1
hole density p (cm )
10
360
Hole density is not
thermally activated !
-4
2.6
2.8
3.0
3.2
3.4
1000/T (K)
M. Pfeiffer et al., Adv. in Sol. State Physics 39, 77 (1999).
Institut für
Angewandte Photophysik
Conduction in exponential band tails: Explains superlinear doping efficiency
Theory: Extension of M.C.J.M. Vissenberg, M. Matters, Phys. Rev. B 57, 12964 (1998)
Density of states:
g(E) = g0exp(-E/kBTc)
f(E)
Variable Range Hopping
T < Tc
EF >> kBTc
g(E)
Percolation model: σ = σ0 Zc-1
EF
with Zc-1 threshold conductance
Assuming shallow acceptors
σ ~ (NA)To/T
; T0 > T
Application to field-effect and conductivity data:
1E-4
1E-5
1E-6
experimental data:
1:50
1:100
1:200
1:500
1E-7
360 320
280
polycrystalline ZnPc
240
1E-3
1E-4
1E-4
1E-5
experimental data:
1:150
1E-5
1E-6
1E-6
1E-7
polycrystalline ZnPc
360 320 280
200
temperature (K)
240
200
160
temperature (K)
material
Basic parameters are independent
of specific material:
1E-3
2
conductivity (S/cm)
conductivity (S/cm)
1E-3
theoretical curves
0.01
σ (S/cm)
Eact (eV) σo(105S/m) To (K)
-1
α (Å )
-3
0.18
12 3
485 15
0.37 0.01
-4
0.23
11 6
455 15
0.64 0.02
-5
0.32
6 1
485 15
1.00 0.04
-7
0.34
3 1
515 15
1.39 0.03
ZnPc (polycr.) 5.8 * 10
ZnPc (amorp.)
mobility (cm /Vs)
theoretical curves
0.01
4 * 10
VOPc (polycr.) 2.3 * 10
TDATA (am.) 5.9 * 10
σ and Eact for dopant density 1% and T=25°C
N-type doping: conventional approach
Matrix:
Naphthalin-tetracarboxylicdianhydride
(NTCDA)
Electron transfer from high-lying
HOMO to matrix LUMO:
Donor:
Bisethylendithiotetrathiafulvalene
(BEDT-TTF)
charge transfer
HOMO:
Donor
LUMO
HOMO
A. Nollau et al., J. Appl. Phys. 87, 4340 (2000)
N-type doping: novel approach using cationic dyes
dye radical
LUMO
created in situ
from cation
charge transfer
HOMO
donor
N
+
O
conductivity
[S/cm]
reduced
pyronin B
very strong
organic donor
10
-4
10
-5
undoped NTCDA
σ=3.3*10
N
Cl-
Pyronin B chloride
donor precursor
1E-3
-8
S/cm
0.01
m olar doping ratio
A. Werner et al., App.Phys.Lett. 82, 4495 (2003)
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
matrix
dopant
N
N
N
N
Zn
N
N
N
N
N
F
F
N
F4-TCNQ
ZnPc
N
F
F
N
with N. Armstrong, D. Alloway, P. Lee, Tucson
J. Blochwitz et al., Organic Electronics 2, 97 (2001)
Institut für
Angewandte Photophysik
Creation of ohmic contacts by doping
undoped : blocking
doped : ohmic
J. Blochwitz et al., Organic Electronics 2, 97 (2001)
Institut für
Angewandte Photophysik
Outline
• Basics of organic semiconductors
• Charge carrier control: Doping of organic semiconductors
- Electrical Characterisation
- UPS/XPS Spectroscopy: space charge layers
•
Application in devices:
Organic light-emitting diodes
Organic solar cells
OLED cooperation in Dresden
Institut für Angewandte Photophysik (www.iapp.de)
• Basic research on novel OLED device concepts
• Organic solar cells
Fraunhofer-IPMS Dresden (www.ipms.fraunhofer.de)
• Stability of doped organic LED
• OLED-Inline-deposition set-up (30x40cm Substrate)
Novaled GmbH (www.novaled.com)
• Holds University patents
• New Materials for stable doping systems
Institut für
Angewandte Photophysik
Organic Light Emitting Diodes
OLEDs: Basic Principles
A
Cathode
V
Emissive layer
Transparent anode
Glass substrate
Light emission
Device structure
LUMO
E
-
- -
Cathode
Anode
+
+
+
HOMO
x
Device energy diagram
OLED Benefits
Thinness
Viewing Angle
Brightness and colour
Source: Kodak
OLED Performance
Cost
Power Consumption
Suitable for Mobile
Thin
Suitable for TV and Movies
Wide View Angle
Light Weight
High Response
High
Resolution
a-Si TFT LCD
Lifetime
Color Quality
p-Si TFT LCD
Brightness
Source: Toshiba 2002
Contrast
p-Si TFT OLED
Further improvements
expected
OLED products on the market
Passive Matrix
Active Matrix
Phone
SNMD
RGB PM
Sanyo-Kodak
Car Audio
Pioneer
Car Audio TDK
• 2003 market: approx 250 mio US$
• 98% small molecule devices
Sony 13” full color display
Comparison: Inorganic vs. Organic LED
Inorganic LED (e.g., GaAs/AlGaAs)
Organic LED
CB
EFe
p
Metal
n
ETL
HTL
EFh
VB
• Exponential current-voltage relation
• space charge limited currents
• Flat-band under operation
• low work function metals needed
ITO-preparation necessary
• Low work-function contacts not needed !
Ideal OLED: pin-heterostructure
Steps towards this design:
Step 1
p
p-type doping
n
i
-
Step 2
blocking layers for
high efficiency
+
+
+ +
+
-
-
- -
LUMO
EF
+
HOMO
Step 3
n-type doping
Institut für
Angewandte Photophysik
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)
N
N
N
N
conductivity σ (S/cm)
Starburst = TDATA
4,4',4''-tris(N,N-diphenylamino)
triphenylamin
doping
ratio
(mol%)
1E-5
6,86%
5,46%
3,87%
2.82%
2.18%
1.35%
1.02%
1E-6
Eact=0,3eV
2,8
2,9
3,0
3,1
3,2
3
3,3
3,4
3,5
-1
10 /T (K )
• undoped TDATA: 10-10 S/cm
Dopant: F4-TCNQ
• doped TDATA: up to 10-5 S/cm
=> low ohmic losses: 0.1V/100nm@100mA/cm2
Structure of pin-OLED
N-doping: Batho-Phenanthroline doped with Li (developed by Kido group)
Institut für
Angewandte Photophysik
Performance of p-i-n structure
5
10
4
10
2
Luminance [cd/m ]
2
10,000 cd/m @5.2V
3
Current efficiency:
5.27 cd/A @1,400cd/m2
(pure Alq3 emitter)
10
2
1,000 cd/m @2.9V
2
10
2
100 cd/m @2.55V
1
10
0
10
2
3
4
5
6
7
8
9
10
Voltage [V]
Lowest voltages reported in literature for small-molecule devices
J. Huang et al., Appl. Phys. Lett. 80, 139 (2002).
Institut für
Angewandte Photophysik
What determines OLED efficiency ?
η external
hν
= bI ×
×η recomb ×η optical
eU
• bI: Electron and hole current balance: 1 can be reached
• eU: Operating voltage should be ≈ photon energy 3
• 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 !
Institut für Angewandte Photophysik
Technische Universität Dresden
Highly efficient PIN Triplet OLED
100000
2
10000 cd/m @ 3.79V
52.4 cd/A; 43.5 lm/W
10000
2
Luminance (cd/m )
2
2
1000 cd/m @ 3.21V
62.4 cd/A; 61.1 lm/W
10000 cd/m @ 4.2V
26 cd/A; 19 lm/W
1000
2
1000 cd/m @ 3.4V
35 cd/A; 33 lm/W
2
100 cd/m @ 2.95V
65.3 cd/A; 69.4 lm/W
100
2
100 cd/m @ 3.1V
44 cd/A; 44 lm/W
10
Standard
D-doping
1
2
3
4
Voltage (V)
5
Highly efficient PIN Triplet OLED
0,01
0,1
1
10
70 lm/W
100
1000
100
30 lm/W @
2
100 mA/cm
19.5%
10
10
14.5% @
2
100 mA/cm
TCTA5TAZ10
TCTA10TAZ10
TCTA10TAZ5
1
1E-3
0,01
0,1
1
10
2
Current density (mA/cm )
100
1
1000
Power efficiency (lm/W)
Quantum efficiency (%)
1E-3
100
LED performance vs. time: linear plot
120
120
AlInGaP Red/Yellow
100
OLED
100
Fluorescent Lamp
Power Efficiency
80
80
60
InGaN
green
60
40
PLED
40
20
20
Tungsten Bulb
(unfiltered)
Red Filtered
0
0
1970
1975
1980
1985
1990
1995
2000
2005
Year
Institut für Angewandte Photophysik
Technische Universität Dresden
What determines OLED efficiency ?
η external
hν
= bI ×
×η recomb ×η optical
eU
• bI: Electron and hole current balance: 1 can be reached
• eU: Operating voltage should be ≈ photon energy 3
• ηrecomb: 75% Triplet, 25% Singlet excitons: 0.25 for fluorescent emitter
Use phosphorescent (triplet) emitter 3
• ηoptical: about 20% in flat structure: 80% lost to wave guide modes:
optimize optical outcoupling !
Institut für Angewandte Photophysik
Technische Universität Dresden
Optical outcoupling in OLED
•
Three modes:
•
External (≈ 20%)
•
Substrate (≈ 35%)
•
ITO/org (≈ 45%)
Outline
• Basics of organic semiconductors
• Charge carrier control: Doping of organic semiconductors
- Electrical Characterisation
- UPS/XPS Spectroscopy: space charge layers
•
Application in devices:
Organic light-emitting diodes
Organic solar cells
Problem I: Exciton Separation
LUMO
• Absorption dominated by
Frenkel excitons
• Large exciton binding energy:
approx. 0.5eV
HOMO
• Free carrier generation
requires high fields (=> use in
photocopiers)
Solution: Donor-Acceptor-Heterojunction
Acceptor
Donor
LUMO
hν
HOMO
Absorption and exciton formation
Exciton
diffusion
Exciton
diffusion
Exciton
separation at the interface
Carrier drift in the electric
field of a pn-heterojunction
Problem II: Exciton Diffusion
Acceptor:
n-type
C.W. Tang (Kodak)
(1986)
•
•
Efficiency 1%
Problems:
- Too low exciton diffusion length
- low voltage, no controlled doping
Donor: p-type
Cells with interpenetrating network: C60 as
electron acceptor
ultrafast electron transfer
ITO
EC
polymer
-C 60
EF
EC
EV
EV
+
Al
The piin-heterojunction concept with electron acceptor
p
i1
i2
n
• Photoactive interface
between two i-layers
• Wide gap transport
layers
EF
• Highly doped layers to
maximize Vbi
• Electron acceptor
C60 energy levels
M. Pfeiffer
Evolution of the organic solar cell concept
Tang 86
p-i-n cell
n
photoactive
region
d
ZnPc*C60
p
Optical mode can be optimized as well
Pin-structure solar cell
2
current density [mA/cm ]
60
40
p-i-n Zelle mit 60nm ZnPc*C 60 (1:2)
U OC= 0.51 V
J SC= 16.7 mA/cm
FF= 36.4 %
η =2.3%
2
2
20
133 mW/cm
simulated AM 1.5
0
-20
-1,0
-0,5
0,0
0,5
voltage [V]
• ca. 80% internal quantum efficiency
• Fill factor has to be improved
1,0
Tandem Solar Cell Structure
Al
n-C60
• Full sun spectrum can be used
ZnPc:C60
p-HTL
recombination center:
metal cluster
(Au)
n-C60
ZnPc:C60
p-HTL
ITO
• Higher voltage, lower current
than single devices
Tandem solar cell: Doping is crucial for low losses
metal
ITO
undoped
Au
n
EQF,h
EQF,e
p
2
current density [mA/cm ]
Tandem cell: influence of doping
0.8
n-C60/i-MeO
i-C60/p-MeO
n-C60/p-MeO
0.6
0.4
Au
(p)-MeOTPD
0.2
(n)-C60
0.0
-0.2
ZnPc:C60
30°C
2
@ 9 mW/cm
-0.4
-0.4
-0.2
0.0
0.2
0.4
p-MeOTPD
0.6
voltage [V]
• with Doping and Gold-Cluster: quasi-ohmic behavior
ITO
Tandem solar cell with 3.6% efficiency
Tandemzelle: optimale Struktur mit 140 nm Spacer
Einzelzelle:
U OC= 0.51 V
Tandem:
U OC= 0.99 V
2
current density [mA/cm ]
40
20
J SC= 10.4 mA/cm
FF= 46.7 %
ηη=3.6%
=3.5%
JSC= 16.7 mA/cm
FF= 36.4 %
η =2.3%
2
0
2
133+/-3 mW/cm
simulated AM 1.5
-20
-0.9
0.0
voltage [V]
0.9
2
Next step:
3-layer
cell
Tandem Zellen:
Der
nächste
Schritt
glass/ITO
p-i-n
gap 2 eV
p-i-n
gap 1.5 eV
p-i-n
gap 1.5 eV
Simulation by H. Hoppe (Uni Linz)
VOC
0.6 V
0.5 V
∑ = 1.5 ... 1.6 V ???
0.5 V
η = 5-6%
Al
Conclusions:
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 für
Angewandte Photophysik
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