controlling charge carrier concentration in organic ...admol/presentations/karl_leo.pdf ·...
TRANSCRIPT
![Page 1: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/1.jpg)
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
![Page 2: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/2.jpg)
• 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
![Page 3: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/3.jpg)
What is an organic semiconductor…..?
Why organic semiconductors…..?
Photovoltaic cellsLight emitters Integrated circuits
Organic Semiconductors: a new class of materials
![Page 4: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/4.jpg)
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
![Page 5: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/5.jpg)
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
![Page 6: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/6.jpg)
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
![Page 7: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/7.jpg)
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
![Page 8: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/8.jpg)
• 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
![Page 9: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/9.jpg)
• 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
![Page 10: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/10.jpg)
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
![Page 11: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/11.jpg)
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-
![Page 12: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/12.jpg)
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 !
![Page 13: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/13.jpg)
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….
![Page 14: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/14.jpg)
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
![Page 15: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/15.jpg)
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
![Page 16: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/16.jpg)
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:
![Page 17: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/17.jpg)
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:
![Page 18: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/18.jpg)
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
![Page 19: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/19.jpg)
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)
![Page 20: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/20.jpg)
undoped doped
Institut fürAngewandte PhotophysikJ. Blochwitz et al., Organic Electronics 2, 97 (2001)
Creation of ohmic contacts by doping
: blocking : ohmic
![Page 21: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/21.jpg)
• 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
![Page 22: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/22.jpg)
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)
![Page 23: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/23.jpg)
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+
+ +
---
![Page 24: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/24.jpg)
Thinness
Viewing Angle
Brightness and colour Source: Kodak
OLED Benefits
![Page 25: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/25.jpg)
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
![Page 26: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/26.jpg)
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
![Page 27: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/27.jpg)
Sony 13” full color display
![Page 28: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/28.jpg)
• 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
![Page 29: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/29.jpg)
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
![Page 30: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/30.jpg)
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)
![Page 31: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/31.jpg)
Structure of pin-OLED
Institut fürAngewandte Photophysik
N-doping: Batho-Phenanthroline doped with Li (developed by Kido group)
![Page 32: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/32.jpg)
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
![Page 33: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/33.jpg)
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 !
![Page 34: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/34.jpg)
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
![Page 35: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/35.jpg)
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)
![Page 36: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/36.jpg)
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
![Page 37: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/37.jpg)
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 !
![Page 38: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/38.jpg)
• Three modes:
• External (≈ 20%)
• Substrate (≈ 35%)
• ITO/org (≈ 45%)
Optical outcoupling in OLED
![Page 39: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/39.jpg)
• 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
![Page 40: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/40.jpg)
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
![Page 41: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/41.jpg)
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
![Page 42: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/42.jpg)
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
![Page 43: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/43.jpg)
EC
EV
EF
EC
EV
polymer
C60Al
ITO--
+
ultrafast electron transfer
Cells with interpenetrating network: C60 as electron acceptor
![Page 44: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/44.jpg)
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
![Page 45: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/45.jpg)
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
![Page 46: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/46.jpg)
-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
![Page 47: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/47.jpg)
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
![Page 48: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/48.jpg)
ITO metal
n
pAu
EQF,e
EQF,h
undoped
Tandem solar cell: Doping is crucial for low losses
![Page 49: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/49.jpg)
-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
![Page 50: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/50.jpg)
-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
![Page 51: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/51.jpg)
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
![Page 52: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/52.jpg)
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
![Page 53: Controlling Charge Carrier Concentration in Organic ...admol/presentations/Karl_Leo.pdf · Controlling Charge Carrier Concentration in Organic Semiconductors Institut für Angewandte](https://reader030.vdocument.in/reader030/viewer/2022041109/5f0d84757e708231d43ac0a4/html5/thumbnails/53.jpg)
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