Transcript Document

Polymer electronics
Polymer Electronics
(A tutorial)
Polymer: A chemical compound or mixture of compounds formed by polymerization
and consisting essentially of repeating structural units
Electronics: A branch of physics that deals with the emission, behavior, and effects of
electrons (as in electron tubes and transistors) and with electronic devices
In constructing a polymer electronic device we are fundamentally interested controlling
the flow of energy
Where and how it is absorbed
 Where and how it flows
Where and how it is emitted
Also: How the device interacts with the environment around it
There are now many different types of polymer based electronics:
Transistors, Light Emitting Diodes, Photovoltaic, Sensors are just a few examples.
To fully understand and appreciate the difficulties with polymer based electrons we first
need to review more conventional device technologies (i.e., metals, oxides and
inorganic semiconductors).
General electrical properties
Polyacetylene (PA) or (CH)x is chemically the simplest
(as synthesized)
(after thermal conversion)
A semiconductor in
which chain
conformation (structure)
impacts band gap
Conducting polymers behave as semiconductors
Conductivity (Siemens/meter)
Poly(p-phenylene vinylene)
(A “highly” crystalline polymer host)
a
Silver
Polyacetylene
(After doping!)
Temperature (K)
Even when doped to a highly conductive state
most p-conjugated polymers behave as classic
semiconductors (VRH-variable range hoping is
the standard proposed mechanism)
+
Applications: FET transistors (no doping)
C. D. Dimitrakopoulos et al., Science 283, p. 822 (1999)
Copyright © 1999 Lucent Technologies.
Mobility (cm2/V-s)
10
Plots of ID (drain) versus VG (gate) and (ID)1/2 versus VG (gate)
(Inset) Schematic diagram of organic IGFET.
Annual gains in room temperature mobilities
1
10-1
-2
10
10-3
10-4
10-5
'86
'88
'90
'92
Year
'94
'96
Conventional Semiconductors: The top down approach
A FET (field effect transistor)
•Single crystal substrates (e.g., Si, GaAs)
•Expensive processing facilities (Billion
dollar fabrication plants)
•Generally invokes a series of process
steps in which a substrate is coated with a
photoresist, masked off, developed,
etched, vapor doped, and so on ad
infinitum.
•There are a very limited number of device
architectures which have been highly
optimized
•Despite the complexity silicon based
devices are good and aren’t going away
anytime soon.
a)
Top-contact device, with source and
drain electrodes evaporated onto
organic layer.
•Conventional devices configurations are
applicable to p-conjugated polymers
b)
Bottom-contact device with organic
deposited onto source and drain.
Conventional Semiconductors at the atomic level
n-type doping
p-type Si
doping
Phosphorous has
5 valence electrons
Si
Si
Si
An unbonded electron
Al
Si
Si
eP+
CB
Egap
+
Si
Si
Si
Edonor
+
0
Al -
CB
VB
Si
CB
CB
ef
Edonor
Si
hole in valence band
VB
At room temperature hole is
delocalized in valence band (VB)
Band structure is essentially
Eacceptor rigid
Mobility is everything
ef
VB
+
(hole in valence band)
E acceptor
0 +
At room temperature electron is
delocalized in conduction band (CB)
Si
VB
When charge is moving the key word is mobility,
mobility, mobility (cm2 V–1 s -1)
Device characteristics
for a DH6T OTFT
1986-2000
having an aluminum
gate electrode, a 3700
Å vapor-deposited
parylene-C gate
insulating layer, and
gold source and drain
electrodes with L=137
mm and W=1.5 mm:(a)
and (b). Drain current
ID versus drain voltage
VD for a range of gate
voltage values VG
plotted (a) linearly and
(b) semi-logarithmically;
(c) ID versus VG at VD= 2 V. Fitting this data
yielded a linear regime
Current mobilities are good enough for many
device
mobility
of 0.122 cm2 Vapplications but critical technology related 1issues
s-1. still remain.
Device costs are potentially very,very low ($0.05).
From C.D. Dimitrakopoulos and D.J. Mascaro, IBM J. Res. & Dev. (2001)
p-conjugated “polymers” (pentacene) at the molecular level
Mobility
X-ray diffraction
Microstructure
Structure matters
Consequences of molecular morphology in poly(3-alkylthiophenes)
a)
Solution cast
Processing can impact
crystal orientation and,
thereby, the mobility.
S
S
S
S
S
Mobilities
Solution cast
Thin-film X-ray diffraction
Spin coated
H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B.
M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P.
Herwig & D. M. de Leeuw, Nature 401 (1999).
S
The bigger picture
From J.M. Shaw and P.F. Seidler, IBM J. Res. & Dev. (2001)
Coming to a production facility near you …
Litrex 80L
200mm PLED System
shipping May 2001
Piezo multi-nozzle
selected technology
Orifice plate
Ink droplet
Ink Channel
Ink Manifold
• Piezo crystal deforms when electrical pulse applied,
moving diaphragm into ink channel
• Shear and Length mode piezoelectric response available
• Drop on Demand; Very little waste / complexity
• frequency response - 0.1 to 20kHz typical
• drop sizes 25 - 70 um diameter
Accoustic
Pulse
Diaphragm
“Enabling Technology for PLED “
Rigid Shell
Piezoelectric
transducer
• Capable of jetting high molecular
weight polymers
(Shear Mode Piezo)
• Long Life - 10’s of liters
+
Poled PZT
GND
Electrodes
“E” field applied
GND
E
GND
GND
+
• Multi nozzle / low frequency allows
high substrate throughput with low
mechanical stress and vibration
E
GND
PZT Response
Shearing of PZT occurs
• Ink conductivity and solvent system
choice are virtually unrestricted with
several suppliers of PZT heads.
Polymer LEDs
Other applications: LEDs
O
O
S n
PEDOT
PPV
Photoluminescence
15 K
Absorbance
T=77 K
n
4.8 eV
PSS
SO3H
5.0 eV
Friend, Burroughes and Tatsuya, Physics World
(Vol. 12) p35-40 (1999)
Construction of Polymer LEDs
Burroughes et al.,
Nature, 347, 539 (1990),
US patent 5,247,190
LED design strategy
Poly(2-methoxy-5-(2'-ethylhexoxy)-phenylene)
Side chain structure reduces side ch
poly(p -phenylene vinylene)
crystallization and frustrates packing
O
glass substrate
Electron and hole mobilities are now an issue
O
n
O
indium tin
oxide
semiconducting
polymer
40 nm
100 nm
The calcium injects electrons into
the polymer film, while the anode
injects holes. When an electron and
hole capture one another within the
PPV, they form neutral "excitons"
(bound excited states) that decay
by emitting a photon of light.
3.5
aluminum, magnesium,
or low-workfunction
metal
External
Circuit
calcium
cathode
O
2.5
Energy (eV)
n
PPV
5.2 eV
indium-tinoxide anode
1.5
2.8 eV
2.7 eV
Poor interchain p overlap enhances
photo- and electro- luminescence
Evolution of LED/OLED performance
From J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman,
and A. Stocking, Science 273, 884 (1996).
A litany of new materials
1: The prototypical (green) fluorescent
polymer is poly(p-phenylene vinylene)
2 & 3: Two best known (orange-red)
solution processible conjugated polymers
MEH-PPV (2) and ``OC1C10'' PPV (3).
4: Copolymers have been widely developed
because they allow color tuning and can
show improved luminescence.
5 & 6: Cyano-derivatives of PPV 5 and 6
show increased electron affinities and are
used as electron transport materials.
7: Blue emitters include high-purity
polymers such as poly(dialkylfluorene)s.
8 & 9: `Doped' polymers such as
poly(dioxyethylene thienylene), PEDOT
(8), doped with polystyrenesulphonic acid,
PSS (9), are widely used as hole-injection
layers.
A simple picture of photophysics
in isolated molecules
Conjugated polymers display
Absorption
0-3
0-2
0-1
0-0
3
2
1
0
p*
inhomogeneous broadening
Emission
Ef
3
2
1
0
0-3
0-2
0-1
0-0
PPV
p
E 0-0 = E 0-0
(gas phase)
Photoluminescence
15 K
Absorbance
T=77 K
Dilute solutions in solvent or the solid-state:
Stokes
shift
Emission
Intensity
0-0
0-1
Absorption
0-0
0-1
0-2
1.5
2.5
3.5
Energy (eV)
0-2
(from R.H. Friend et al.)
0-3
0-3
Photon Energy
Absorption occurs at all sites
but emission dominated by
longest conjugated segments.
A simple picture of intrachain photophysics for a conjugated polymer
Imagine a series of one-dimensional potential wells which
Particle in a box
represents a distribution of effective conjugation lengths.
n=2 (p*)
Different crystalline domains
n=1 (p)
0
L
n=2
Emission
0
n=1
L'
a
n=2
0
n=1
L''
Energy eigenvalues are
-2
proportional to L
1 Absorption can potentially occur at all sites (if E>Egap)
2 Exciton formation (bound "electron-hole“ pair) is rapid (subpicosecond)
3 Energy migration along chain to segment with lowest energy band (tens of picoseconds)
4 Photoluminescence is dominated by emission at longest conjugated segments.
Absorption (Abs) averages over all “chromophores” while
photoluminescence (PL) identifies a small subset
Engineering where the energy goes in and where it comes out
MEH-PPV loaded into a
mesoporous silica composite
1: A single polymer chain can be
loaded into each micropore
2: Part of the chain extends
beyond the micropore
3: Energy (light) is captured
everywhere but preferentially
transfers into extended chain
conformations in the pore
4: Emission is preferentially
polarized
From: Thuc-Quyen Nguyen, Junjun Wu, Vinh Doan,
Benjamin J. Schwartz, Sarah H. Tolbert, Science 288 (2001)
Keys to conducting polymer applications:
Synthetic control and processibility
 Addition of solubilizing side chains to “conducting” polymers
has created a myriad of new, processible polymers.
Examples:
b)
a)
S
S
S
O
S
S
S
c)
a) Regioregular poly(3-hexyl
thiophene) or r-P3HT
O
H
N
H
N
N
H
N
H
O
O
b) Poly(2-methoxy-5-(2'-ethylhexoxy)phenylene vinylene) or 2MehPPV
c) Polyaniline dodecylbenzene
sulfonate or DBSA-PANI
 Competing interactions and chemical incompatibilities
(i.e. polar vs. nonpolar) has given rise to new
structure/property relationships.
Self-Assembly (or … you I like, but you I hate)
X-ray Intensity (arb. units)
For example: Self-assembly leads to ordered (lamellar) phases
(200)
l=0
x1
l=2
x2
(002)
(102) (202)
l=1
x2
(311)
(111) (411)(611)
(400)
(600)
l=0
x2
X-Ray Diffraction of Poly(3-n-octylthiophene)
0.0
(220)
(120) (320)
(420)
(020)
6.0
12.0
18.0
24.0
30.0
New PHYSICS: Order-disorder transitions
(ODTs) of the alkyl side chains leads to
conformational changes along conjugated
polymer backbone
2q (deg.)
• This gives rise to new properties such as thermochromism
• Details are sensitive to the chemical architecture and physiochemical processing
Side chain behavior couples to main chain conformation:
Thermochromism, solvatochromism, ionochromism,…
This is the basis for sensor technologies
250
In the ordered state
isosbestic
point
X-ray powder
diffraction
300
350
Wavelength (nm)
400
A Simple Free Energy Diagram
Free Energy
Order-Disorder
Transition Temperature
B
Ordered state
A
B
is the path on cooling
Thermotropic
columnar
A
mesophase
Intensity (arb units.)
50 C
45 C
40 C
35 C
30 C
20 C
15 C
5C
0C
-20 C
-35 C
On warming
Absorbance (Arb. Units)
UV absorption
25 C (ordered)
45 C (disordered)
10
20
30
2q (deg.)
Photoluminescence is also strongly impacted!
“Self-assembly” leads to formation of helical phases
Repeat unit: - (SiR2 ) – and R= (CH2)3CH3
Conformational energy surface of an all transoid
(15/7 helix) in poly(di-n-butylsilane) oligomer
Energy (kcal/mol per dimer)
Si-Si-Si-Si
dihedral angle
is ~170°
15/7 helix
C-Si-C-C
dihedral
(deg.)
C-C-Si-C
dihedral
(deg.)
Topology of the surface is well defined with one clear minimum
Side chain ordering does not always work in your favor!
Two different atactic silicon
backboned (s-conjugation)
polymers in dilute solution
The polymer on right:
Minimal Stokes shift
Both absorption and emission are extremely sharp
From M. Fujiki, JACS 122, 3336 (2001)
Morphology impacts energy flow in subtle ways:
In the case of a bimodel distribution of conformations
Monitoring isothermally the Order-Disorder Transition of a thermochromic polymer
Sample: 0.2% poly(di-n-hexlysilane) in toluene spin coated (~300 Å) on quartz substrate
0.30
200
Full
Set
0.20
0.25
0.10
0.00
300
350
0.20
40
0
0 sec
0 sec
10 sec
35 sec
77 sec
121 sec
84 sec
127 sec
164 sec
622 sec
172 sec
617 sec
5 sec
41 sec
lEx=305 nm
150
100
0.15
0.10
50
0.05
0
0.00
300
350
400
325
350
375
Energy migrates between the two structural phases
400
(nm)
Photoluminescence (a.u.)
Absorbance (absolute)
Combined PL
and Abs
0.30
Model depicting energy transfer from disordered to
ordered phase
Formation of the ordered phase depends on nucleation and growth
For athermal nucleation (all nucleation occurs at t=0) n=1
Polymer
thin film
Substrate
• 1st: Low absorption in ordered phase but efficient PL
• 2nd: High absorption in disordered phase and exciton
migration to the ordered region (I.e., red arrows)
• One-dimensional growth increases relative PL from
1st process
Nucleation
then growth
Back to electronic properties….
Photoluminescence or electroluminescence
is a complex process involving the
formation of excitons, excimers, exiplexes
and other exotic charge states.
p-conjugated polymers have unusual charge excitations
Minding the gap
Electronic states
are split off from
the valence and
conduction bands
All charge excitations involve local self-consistent
structural distortions of the lattice
Schematic representations of recombination pathways
Excitons are bound “electron-hole” pairs which includes structural
relaxation of a single polymer chain (i.e., electron-phonon coupling)
Decay pathways for singlet and triplet decay
Radiative processes,
corresponding to absorption or
emission of light;
Non-radiative processes.
a: Fluorescence
b: Intersystem crossing (ISC)
c: Photoinduced triplet-triplet
absorption
d: Phosphorescence
S0 is the ground (singlet) state; S1 is the first excited singlet; T1 the
first excited triplet; and Ti and Tn are higher lying triplet states.
Recombination is spin dependant
LED’s require injection of holes at one electrode and electrons at the other.
If no spin polarization
An eigenstate requires:
triplet
triplet
triplet
mixed
mixed
triplet
triplet
triplet
singlet
singlet
Three states with s = 1 (triplet) and one state with s = 0 (singlet) or, in
the absence of other effects, 75% triplet and 25% singlet recombination
There is more to the singlet-triplet story
Singlet-Triplet Cross Section Ratio
Formation of singlet
excitons exceeds that
of the triplets and it is
dependent on the band
gap!
From: M. Wohlgenannt, Kunj Tandon, S. Mazumdar,
S. Ramasesha, Z. V. Vardeny, Nature 409 (2001)
Electronic band structure in one-dimension: A primer
Everything begins with directional wavefunction overlap in the context
of tight-binding (Hückel theory) invoking the Born-Oppenheimer
approximation.
Schrödinger Equation:
Of course y is the wavefunction, m is the electron mass and U is the
potential
The calculation is exact and
analytic if we use a H2+ion.
Ebonding < Eantibonding
Bonding orbitals occur when
symmetric solution is used
yH(r) + yH(r + R)
Anti-bonding orbitals occur
when asymmetric solution is
used yH(r) - yH(r + R)
+
+
Tight-binding for p-electrons and semiconducting polymers
For carbon in a diamond lattice:
C
C
C
C
sp3 hybridization
In conjugated polymers the carbon has sp2 + p bonding
Now the mix is
C
Extending the chain
p*
LUMO
band
p
HOMO
band
H
C
C
C
H
H
sp2 hybridization
pz
pz
Isolated
orbitals
Paired
Linear
electrons chain
(1-D band)
Interchain
stacking and overlap
(3-D band)
A simple picture of the one-dimensional band as the chain lengthens:
A one-dimensional chain (trans-polyacetylene)
From a tight-binding perspective:
EA is the energy of a
single atomic orbital
A(R) is an overlap
integral
R
After a simple approximation: [B(0) is approx. 1]
Overlap
intergral
For a linear chain only R = +a and –a are important
This gives E(k) = EA – a + 2 A cos ka and a half-filled band
t = A(R)
The Peierls instability (1939)
Competition: Electronic energy vs. Lattice (elastic) energy
(recall that band electrons move in a weak, periodic potential)
Electronic states moves lower in energy ~ D2 ln(t/D)
Elastic energy is increased ~ D2
At low temperature dimerization always favorable
kFermi is still p/2a and since d = 2p/k ,
d = 4a (charge density wave)
Conformational structure impacts electronic properties
1. Cis and trans polyacetylene, according to the tight binding picture
just presented, should have the same band gap but they don’t!
(as synthesized)
Question:
Why not?
(after thermal conversion)
2. Conjugation yields an energy/unit length which is
minimized with increasing backbone planarity
p
C
C
C
s
bonding
C
C
C
C
Si
C
C
C
bonding
C
Si
Si
Si
Si
C
Si
Si
Si
geminal
vicinal
3. Rotation breaks conjugation
Structural instabilities and disorder are extremely important (at all length scales)
Simple picture of polyacenes and poly-p-phenylenes
a
Benzene
a
a
a
a
a
Free electron perspective:
(works best for polyacenes)
Energy levels
+2
-2
+1
-1
0
A circular ring of length L = 6a
6 unpaired pz electrons (1 per C)
Now from the tight binding perspective
E(k) = EA - a – 2 t cos ka , y(f) = c eimf
m = 0, 1,  2, 3 (only six states) k = mp/3a
k = p/a
k = 2p/3a
k = p/3a
Now consider:
k= 0
Poly(p-phenylene)
Polarization
direction
Poly-p-phenylene from tight binding
Weak interactions
Strong interactions
Egap
Strong interactions
give bands and
tend to delocalize
charge
Impact of band structure on photocell device physics
Experiment
Theory
thick
I
Absorption (not perfect)
II
III
thin
IV
I
II
III
From: A. Kohler, D. A. dos Santos, D. Beljonne, Z. Shuai, J.-L.
Bredas, A. B. Holmes, A. Kraus, K.Mullen, R. H. Friend, Nature 392
(1998)
A closer look at the calculation
Efficiency
Exciton wavefunction
3.1 ev
Poor
4.8 ev
Better
Best
In a photocell it is advantageous to
efficiently separate charge
Molecular wave function is important
6.5 ev
Separating electrons and holes: A prerequisite for
photovoltaic applications
Blending C60 and p-conjugated polymer
Recombination of P+ and Preduces device efficiency
From: Christoph J. Brabec, N. Serdar
Sariciftci, and Jan C. Hummelen, Advanced
Functional Materials 11 (2001).
Nearly 100% electron transfer to the C60
Because of bipolaron BP2+ formation
minimal back transfer
Polymer has good hole mobilities
Design of interface and electron transport
are bottle necks
Blended device behavior
2.8 eV
3.7 eV
p* MEH-PPV
LUMO C60
Carbon-60 LUMO is well positioned
to accept electrons
Egap
4.3 eV
4.7 eV
5.0 eV
p MEH-PPV
Al
ITO
HOMO C60
6.1 eV
p* MEH-PPV
LUMO C60
ITO
p MEH-PPV
HOMO C60
Al
Simple design strategy
The future (assuming one has a good crystal ball)
 Better control of interface through intelligent
design
 Purer materials and more robust synthetic
procedures
 Control of both spin and charge
 Top down meets bottom up (passive and
active types of molecular self-assembly)