V VET TES EN NOV TAM TVM Electronic Structure of Organic Semiconductor Interfaces: Looking Back to the Future Antoine Kahn Department of Electrical Engineering Princeton University, Princeton, NJ 08540 Tel Aviv University October 10, 2004 S S S S S S c c c c N o o N N N O O N O N N Al
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Electronic Structure of Organic Semiconductor Interfaces: Looking Back to the Future
Antoine Kahn
Department of Electrical EngineeringPrinceton University, Princeton, NJ 08540
Tel Aviv University October 10, 2004
SS
SS
SS
c
c
c
c
N
o
o N
N
N
OO
N O
N
N
Al
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Outline
• Interest in organic materials and thin films
• Outstanding metal-organic interface issues
• Re-visiting key concepts– Surface photovoltage– Interface formation
o S-parameterso Defectso Induced density of interface states
• Doping as a means of enhancing charge injection
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• PTCDA (3,4,9,10 perylenetetracarboxylic dianhydride)
• Alq3 (tris(8-hydroxy-quinoline)aluminum) (ET material; EL in green)
• α-NPD (N,N’-diphenyl-N,N’-bis(l-naphthyl)-l,l’biphenyl-4,4’’diamine ( HT material)
OO
N O
N
N
AlNN
• MPcM-phthalocyanine(M=Cu, Zn)
C
C
C
C
o
o
o
o
o
o
IE = 5.5 eVEopt = 3.1 eV
IE = 5.8 eVEopt = 2.7 eV
IE = 5.1 eVEopt = 1.6 eV
IE = 6.7 eVEopt = 2.2 eV
C CNC
NC
NC
NC
F F
F F
• F4-TCNQ (tetrafluoro-tetracyano-quinodimethane); p-dopant
IE = 8.34 eV
ZnN
N
NN
NN
N N
1st layer
2nd layer
[10]
[01]
20 nm x 20 nm
V = -1.58 V, I = 0.10 nAS
~ 2 A
2 ML of PTCDA on Au(111)
I. Chizhov et al. J. Cryst. Growth 208, 449 (2000)
π-conjugated molecules
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patterned surface
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Organic Light-Emitting Device (OLED)
-
+
----
+
Cathode:low workfunction
EF
Anode:Indium-Tinoxide (ITO)transparent
HOMOLUMOVacuum Level
ITO/glass
α-NPD
Alq3
Mg:Ag (40:1)1000 Å
C.W. Tang and S.A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987)
Red, green and blue OLEDsHigh quantum efficiencyBrightness > 105 cd/m2 demonstrated (~ 100 cd/m2 for displays → 10~50 µA/mm2)low operating voltages (3 V @ 100cd/m2)stability: depends critically on how hard OLED is driven;
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SONY BEGINS MASS PRODUCTION OF FULL-COLOR ORGANIC LIGHT EMITTING DIODE (OLED) DISPLAYSNew Thin Screens for Mobile Devices Realize CRT-Quality Picture Clarity and Color Gamut
TOKYO, Japan, Sept. 14, 2004 - This month, Sony Corporation will commence mass production of a full-color Organic Light Emitting Diode (hereafter OLED) display……….
OLED display introduced in CLIE 'PEG-VZ90' handheld
Sony/Kodak prototype high resolution Display (2001)
2.2”; 512x218 AMOLED
Kodak Digital Camera (2003)
15”; 1280x720 white OLED
Organic Light-Emitting Devices (OLED)
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Organic Field Effect Transistor (OFET)
Pentacene OFET• Small molecule organic SC; Low-T deposition• Gate dielectric treated with octadecyltrichlorosilane• Thin film µ > 3cm2/V-s, ~1cm2/V-s typical (single transistor)• Ion/Ioff=108
Statistics on 1cm2 200 OFETs array• Average µ=0.81cm2/V-s; competitive with a-Si• Average Ion/Ioff=106-107
T.N. Jackson, ACS ProsPective, Jan. 04
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Organic Photovoltaics
CuPc
C60 BCP5.2 eV
3.5 eV
6.2 eV
4.5 eV
3.5 eV
7.0 eV
ITO4.8 eV
Al4.2 eV
5.3 eV
PEDOT
After Peumans et al., APL 79, 126 (2001)
ITO/PEDOT/200Å CuPc/400Å C60/150Å BCP/800Å Al
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• Quasi-infinite number of molecules and derivatives, with wide span of electronic properties
• Electronic and optical properties of the films determined in first approximation by the molecular moiety
• System more tolerant of defects (not electronically active); no dangling bonds!⇒ great flexibility on choice of substrates
• Control of molecular deposition down to the fraction of molecular plane• Unmatched freedom for device architecture
Advantages of π-conjugated molecular films
• Ability to modify materials with “optical” or “electrical” dopants for wave length shift or enhancing conductivity
• Excellent optical emission properties
ITO; metal; conducting polymer; insulator
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ITO α-NPD PtOEPin Alq3
BCP Alq3 Mg:Ag
0.25 eV 0.6 eV
EF
EF
0.15 eV
0.65 eV
Device engineering: the “hole-blocker“
EVACEVAC
+++++
- --- -
Hole injection Electron injection
Recombination zone
Blockingbarrier
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Molecular level alignment
HOMO
LUMO
IE
EA
EF
φM
∆= 0EvacEvac
Metal Semiconductor
EF
Do separately determined material parameters, e.g., φM, IE, EA, define the real energetics of metal-organic interfaces?
Schottky-Mott model
Metal
?
dipole barrier
φM
∆Evac(O)
EF
φBn
Evac
Metal
LUMO
HOMO
• Ishii et al, IEEE Trans. Electron Devices, 44, 1295 (1997)• Hill et al., Appl. Phys. Lett. 73, 662 (1998)
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Key questions concerning metal-organic interfaces
• Origin(s) of interface dipoles
• Mechanism(s) of molecular level alignment
• “Engineering” of metal-organic interfaces
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Experimental approach
• Growth of molecular layers and metal deposition in UHV (10-10 Torr)• Incremental build-up of interface and (mostly amorphous) films• ultraviolet photoemission spectroscopy (UPS) and inverse photoemission
spectroscopy (IPES) for valence and empty state• X-ray core level spectroscopy (XPS) for interface chemistry• Kelvin probe for contact potential difference (CPD) and surface photovoltage• In-situ current-voltage measurements (I-V)
Substrate (Au/Si)
Metal (20nm)
Evac (M)
φM
Dipole barrier ∆Evac (O)
EA
IE
LUMO
HOMO
φBe
φBh
EFUPS
IPES
CPD (with KP)
XPS (interface chemistry)
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B in d in g e n e rg y (e V )
-1 0 -8 -6 -4 -2 0 2 4 6 8
Inte
nsity
(arb
. uni
ts)
UPS/IPES picture of electronic structure
HOMO
LUMO
Evac
UPS: h+ transport levelUPS: vacuum levelIPES: e- transport level
HOMO LUMO
UPSoccupied states
IPESempty states
5.4eV
Alq3
EF
-18 -17 -16
hν
Onset of photoemission
Evac
IE
EA
Et
I.G. Hill et al., Chem. Phys. Lett. 317, 444 (2000)
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UPS on metal/organic interface
EF
LUMO
HOMO
0.76eV
5.14eV
0.90eV
5.28eV
Evac
Au
+-
ZnPcnarrower gap at metal interface due to polarization effect
-4 -3 -2 -1 0 1
Binding Energy to EF(eV)
128Å ZnPc
-18 -17 -16 -15
ZnPc on Auhv=21.2eV
clean Au
4Å ZnPc
8Å ZnPc
16Å ZnPc
32Å ZnPc
64Å ZnPc128Å ZnPc
0.76eV4Å ZnPc
8Å ZnPc
32Å ZnPc
64Å ZnPc
16Å ZnPc
EF
ZnPc on Auhv=21.2eV
clean Au
Binding energy to EF(eV)
metal-organic interface
• Is the interface dipole real?• Are the molecular levels flat away from the interface?• Do interface barriers correlate with current injection?
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Re-visiting old demons !Stiles and KahnPRL 60, 440 (1988)LT
RT
+adsorbates
GaAs (110)
EC
EV
EF
EVAC
Transition between single-metal-atom induced states and metal-induced states?
To some extend, yes; but not entirely!
Mao et al., JVST B9, 2083 (1991)
SPV
Alonso, Cimino et al.Surface Photovoltage!
Could SPV be affecting photoemission results on wide gap organic semiconductors?
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Re-visiting old friends !
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Interface dipole: UPS vs. CPD
• α-NPD on Au
• Excellent agreement between results of two techniques involving radically different measurement concepts
α-NPDAu
IE=5.4 eV
E vac
EF
5.2 eV
HOMO
LUMO
3.1 eV
1.4 eV
- +
Hole injection
barrier
Interface dipole∆
UPS
62 64 66 68 70 72 74 76 78 80 82
4Å α-NPD on Au
clean Au
Kinetic energy (ev)
10Å α-NPD on Au
20Å α-NPD on Au
α-NPD on Auhν= 82 eV
0 1 2 3 4 5clean Au
4 Å α-NPD/ Au
Kinetic energy (eV)
10 Å α-NPD/ Au
Inte
nsity
(arb
. uni
ts) 20 Å α-NPD/ Au
α-NPD on Auhν=82 eV
EF
∆
0 20 40 60 80 100 120 140 160 180
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
pure a-NPD on Au
φ re
lativ
e to
sub
stra
te (e
V)
Film Thickness (Å)
KP UPS
∆
Kelvin probe CPD
NN
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4.8 eV
0.3 eV
0.8 eV
Contaminated
Au α-NPD
5.3 eV5.4 eV
1.3 eV
1.2 eV
Sputtered clean
Au α-NPD
5.3 eV
Correlation between dipole, barrier and current
Evac Evac
EFEF
HOMO
HOMO
Au/Cr/Si400Å Au
α-NPD
400Å Au
NN
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Mechanisms of dipole formation(I) EA > φM
EFMLUMO
HOMO
EFMLUMO
HOMO
e-
Charge transfer from metal to empty molecular states; raises organic molecular levels to stop electron transfer
φM EA
Ex: F16CuPc on Mg and Al; PTCBI on Mg and Ag; PTCDA on Mg, In and Sn
-4 -3 -2 -1 0 1
Energy Relative to EF (eV)
PTCBI on AgHe I
EF
64Å32Å16Å8Å4ÅAg
PTCBIHOMO
-18 -17 -10 -5 0
Energy Relative to EF (eV)
EF
PTCBI on AgHe I, -3V bias
64Å32Å
16Å8Å4Å
Ag0.2 eV
Charge transfer
Polaron + bipolaronlocalized at the interface
I. Hill et al., Organic Electronics, 1, 5 (2000)
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-14 -12 -10 -8 -6 -4 -2 0 2
0.6 eV
x3
x3
Mg on Alq3He I (21.22eV)
E F
128A
64A
32A
16A
8A
4A2A0A
Inte
nsity
(a.u
.)
B inding Energy (eV)
Chemistry-induced gap states
C. Shen et al, J. Appl. Phys. 89, 449 (2001) S. Meloni et al., J. Am. Chem. Soc. 125, 7808 (2003)
Mg- (Al-) + Alq3 →organo-metallic complex
(II) EA < φM < IE and reactive interface
EFM
LUMO
HOMO
Chemical reaction induces filled and empty states which “pin” EFO, leading to charge exchange to align EFO and EFM
EFMLUMO
HOMO
EFO
e-
Ex: Alq3 and Mg or Al
“defects”
Mechanisms of dipole formation
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(III) EA < φM < IE and non-reactive interface
EFM
LUMO
HOMO
Compression of metal surface charge density; lowers the metal work function
EFMLUMO
HOMO
Ex: α-NPD on Au; Alq3 on Ag or Au; CBP on Ag or Au
+
r
-
+
r
-
surface surface
organic molecules
Ishii et al., Advanced Materials, 11, 605 (1999)X. Crispin, et al., J. Am. Chem. Soc. 124, 8131 (2002)
Large work function metals have strong surface dipole components
Mechanisms of dipole formation
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General approach for organic/metal interfaces
• Interfaces formed by gentle deposition of large organic molecules on clean metal surfaces
• Ideal to test models based on “intrinsic” aspects of interfaces, in particular the modification of the semiconductor interface electronic structure by the continuum of the metal.
Abrupt interface →
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S - interface parameter for metal-organics
0.80.60.50.370.25~ 0~ 0~ 0S
3.43.33.22.32.32.22.11.8Et (eV)
Alq3CBPα-NPDPentaceneZnPcPTCDAPTCBIF16-CuPcOrganic
dipole barrier
φM
∆Evac(O)
EF
φBn
Evac
Metal
LUMO
HOMOM e ta l w o r k fu n ctio n (e V )
Inte
rfa c
eE
Fpo
s itio
n( e
V)
Alq3IE = 5.8 eV
Au
MgS=0.8
3.5 4.5 5.5
1.0
3.0
2.0
0
AgAl
ZnPcIE = 5.2 eV
3.5 4.5 5.5
AuMgS~0.25
m
Bn
ddSφφ
=
• S = 0: Fermi level pinned• S = 1: Schottky-Mott limit
LUMO
HOMO
φBn
Interfacedipole
• Ishii et al, IEEE Trans. Electron Devices, 44, 1295 (1997)• Hill et al., Appl. Phys. Lett. 73, 662 (1998)
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Induced Density of Interface States (IDIS) model
• Proposed for Inorganic Semiconductor/metal interfaces.
Induced interface states pin the Fermi level
Charge Neutrality Level (CNL)
Aδ)D(Eeπ411
dΦdES
F2
M
F
+==
S ≡ Interface Slope Parameter:
V. Heine, Phys. Rev. 138, A1689 (1965)S.G. Louie and M.L. Cohen, Phys. Rev. B 13, 2461 (1976)C. Tejedor, F. Flores and E. Louis, J. Phys. C: Solid St. Phys. 10, 2163 (1977)J. Tersoff, Phys. Rev. Lett. 52, 465 (1984)
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Back of the envelope application to MO interfaces
~0~00.370.250.800.580.600.50Sexp
0.290.280.310.320.490.460.460.54Scal
2.562.202.601.943.704.704.004.00Eg (eV)
Dgs (cm-2eV-1)
PTCBIPTCDApentaceneZnPcAlq3BCPCBPα-NPD
121017.7 × 121068.9 × 121081.9 × 121014.9 × 12104.17 × 12102.18 × 12105.21 × 12105.20 ×
m
Bn
ddSφφ
=Slope parameter
density of interface states per unit energy at ECNLgsD
gsD = molecular area density4x energy gap
— S.G. Louie and M.L.. Cohen, Phys. Rev. B 13, 2461 (1976)
1
0
221
−
+=
εεδ
i
itgsDe
itδ − average molecule-metal distance
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IDIS and CNL for unreactive MO interfaces
1. Electronic structure of isolated molecule. Ab-initio DFT calculation + correlation.
2. Interaction with metal broadens molecular levels.
3. CNL: The induced density of states up to the CNL integrates to the number of electrons in the isolated molecule.
ΦM )(Eρ|T||c|π2
)Eδ(E|T|π2Γ
iαα,αj,
2αj,
2ji,
iυυ
2iυi
∑
∑=
=−=
N e- in isolated molecule
Ti,υ
IDIS
H. Vázquez et al., Europhys. Lett. 65 802 (2004)
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The CNL tends to align with the metal Fermi level
HOMOLUMO
π statesσ states ΦM (Au) EF CNL
S (ΦM – CNL)
PTCDA
Induced DOS, CNL and interface EF
H. Vázquez et al., Europhys. Lett. 65 802 (2004)
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S - Interface slope parameter at MO interfaces
Aδ)D(Eeπ411
dΦdES
F2
M
F
+==
0.25 (ZnPc)
0.25
CuPc/Au
0.6
0.5
CBP/Au
0.37
0.2
Pent/Au
0.00.0S (exp)
0.13 (d=3.0Å)0.2S (theory)
PTCDA/AuPTCBI/Au
PTCDAIE= 6.8 eV
AuMg In
3.5 4.5 5.5
Sn
S=0
Schottky-Mott limit
PTCBIIE= 6.2 eV
3.5 4.5 5.5
AuMg AgS~0
S=1
1.0
3.0
2.0
0
Inte
r fac
eEF
p osit
ion
(eV
)
Metal work function (eV)3.5 4.5 5.5
CBPIE = 6.2 eV
Au
MgAg S=0.6
ZnPcIE = 5.2 eV
3.5 4.5 5.5
AuMgS~0.25
PentaceneIE = 5.0 eV
3.5 4.5 5.5
AuS~0.37
Sm
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Modifying interface barriers
• Interface dipole approach: changing the work function of the substrate– I. Campbell et al., Appl. Phys. Lett. 71, 3528 (1997)
• Stepping-stone approach: ultra-thin interlayer– I.G. Hill et al. J. Appl. Phys. 86, 2116 (1999)
• Interface doping
-
+
-
+
-
+
-
+
-
+
α-NPD
CuPc
EF
Evac
EF
- +
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LUMO
HOMO
LUMO
HOMO
+
EF
EF
EF
EF
p-doped region
tunneling
++
Control of injection via electrical doping
+
Create a narrow depletion region for carrier tunneling
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Electronic structure of ZnPc and F4-TCNQ
W. Gao and A. Kahn, Appl.Phys.Lett., 79, 4040 (2001)
-1 0 -8 -6 -4 -2 0 2 4 6 8
ZnP c
IP E SU P SH e I In
tens
ity (a
.u.)
1 .9 4 e V
2 .9 8 e V
E n erg y R e la tive to E F (eV )
5.28eV
EF
3. 34eVLUMO
HOMO
Evac
Zn
N
N
NN
NN
N N
Zn
N
N
NN
NN
N N
ZnPc
Evac-10 -8 -6 -4 -2 0 2 4 6 8
F 4-TCNQ
4 .35eV
3 .10eV
IP E SU P SH eI
Inte
nsity
(a.u
.)
Energy R elative to E F(eV)
EF
5.24eV
LUMO
HOMO
8.34eV
Evac
C CNC
NC
CN
CNF F
F FC C
NC
NC
CN
CNF F
F F
F4-TCNQ
Evac
e-
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Depletion region at the doped ZnPc/Au interface
ZnPc on Au ZnPc:3% F4−TCNQ on Au
EF
LUMO
HOMO
0.76eV
5.14eV
0.90eV
5.28eV
Au
Evac0.48eV
EF
0.74eV0.56eV
5.24eV
0.18eV
< 32Å
5.14eV
Au
Evac0.55eV
EF
0.80eV0.38eV
5.24eV
0.42eV
< 128Å
5.14eV
Au
Evac
ZnPc:0.3% F4−TCNQ on Au
W. Gao and A. Kahn, Appl.Phys.Lett., 79, 4040 (2001) and Organic Electronics 3, 53 (2002)
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SPV at doped interface ?
LUMO
0.86eV
5.14eV
5.52eVAu
Evac
EF
HOMO
1.24eV
α-NPD
α-NPD:4% F4-TCNQ
0.68eV
EF
5.56eV
30-40Å
5.14eV
Au
1.10eV0.48eV
0.62eV
Evac
0 20 40 60 80 100 120 140 160 180
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
pure a-NPD on Au
φ re
lativ
e to
sub
stra
te (e
V)
Film Thickness (Å)
KP UPS
PES and CPD in perfect agreement ⇒ no SPV; fast recombination and low carrier mobility likely to prevent charge separation at the interface
0 10 20 30 40 50 60 70-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
a-NPD:0.5%F4-TCNQ on Au
φ re
lativ
e to
sub
stra
te (e
V)
Film Thickness (Å)
KP UPS
0.5% doped α-NPD on Au
Undoped α-NPD on Au
C. Chan et al, J. Vac. Sci. Technol. A 22, 1488 (2004)
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0.1 1 10
Cur
ren
Den
sity
(mA
/cm
2 )
Applied Voltage (V)
104
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
Auα-NPD
Au
substrate
Doping enhancement of hole injection in α-NPD
• Au/ 170nm α-NPD:0.5% F4-TCNQ /Au
α-NPD
Au/ 8nm α-NPD:0.5% F4-TCNQ + 162nm pure α-NPD /Au
α-NPD
Au/ 170nm pure α-NPD/Au
α-NPD
Hole injection enhancement of 4-7 orders of magnitude via tunneling through the depletion region
W. Gao and A. Kahn, J. Appl. Phys., 94, 359 (2003)
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Summary
• As for inorganic semiconductors, surface/interface physics has been highly instrumental in advancing basic understanding of organic interface electronic structure
• Organic molecular semiconductors depart in major ways from inorganic counterparts, but key interface concepts developed two or three decades ago are, to the least, extremely useful as guidingprinciples for understanding metal-organic interfaces
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NOVTAMTVM Ultra-flat, ultra high work function,
patterned surface
• Extremely high work function surface (meaning? hard work done on this surface or work function so high that nothing can escape)
• Essentially organic
• Perfectly clean; no apparent contamination, down to the sub-molecular level
• In spite of quasi-regular patterns, extremely small surface corrugation, nearly impossible to measure with scanning probes (Yossi can confirm!)
• Absolutely not able to reproduce in our labs and offices!
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Ideal ultra-flat high work function organic surface
Expanding the field of view does reveal some inhomogeneities
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