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)
A semiconductor in which chain conformation ( structure ) impacts band gap
(after thermal conversion)
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.
Plots of I D (drain) versus V G (gate) and (I D ) 1/2 versus V G (gate) (Inset) Schematic diagram of organic IGFET.
Annual gains in room temperature mobilities 10 1 10 -1 10 -2 10 -3 10 -4 10 -5 '86 '88 '90 '92 Year '94 '96
Conventional Semiconductors: The top down approach •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. •Conventional devices configurations are applicable to p conjugated polymers a) b) A FET (field effect transistor) Top-contact device, with source and drain electrodes evaporated onto organic layer.
Bottom-contact device with organic deposited onto source and drain.
Conventional Semiconductors at the atomic level
n
-type doping
Phosphorous has 5 valence electrons
Si Si e P +
CB
Si Si E gap + + E donor
0
p
-type doping Si
CB
Si Al Si Si Si An unbonded electron Si Al + (hole in valence band)
VB
Si Si
0
+ E acceptor hole in valence band At room temperature electron is delocalized in conduction band (CB)
VB
CB CB At room temperature hole is delocalized in valence band (VB)
E donor
VB e
f
e
f
VB Band structure is essentially
E acceptor
rigid Mobility is everything
When charge is moving the key word is mobility, mobility, mobility (cm 2 V –1 s -1 ) 1986-2000 Device characteristics for a DH6T OTFT having an aluminum gate electrode, a 3700 Å vapor-deposited parylene-C gate insulating layer, and gold source and drain electrodes with L=137 m m and W=1.5 mm:(a) and (b). Drain current I D V D versus drain voltage for a range of gate voltage values V G plotted (a) linearly and (b) semi-logarithmically; (c) I D versus V G at V D = 2 V. Fitting this data yielded a linear regime
Current mobilities are good enough for many device
2 V -
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 X-ray diffraction Microstructure Mobility
Structure matters
Consequences of molecular morphology in poly(3-alkylthiophenes) Solution cast a) Processing can impact crystal orientation and, thereby, the mobility.
S S
Mobilities
S S S Solution cast S
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).
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 Manifold Diaphragm
Poled PZT “E” field applied PZT Response
Piezoelectric transducer Ink droplet Ink Channel
•
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 Rigid Shell (Shear Mode Piezo)
Electrodes GND GND E GND + E + GND GND
“Enabling Technology for PLED “
•
Capable of jetting high molecular weight polymers
•
Long Life 10’s of liters
•
Multi nozzle / low frequency allows high substrate throughput with low mechanical stress and vibration
•
Ink conductivity and solvent system choice are virtually unrestricted with several suppliers of PZT heads.
Shearing of PZT occurs
Other applications: LEDs
PPV
Photoluminescence
15 K
Absorbance
T=77 K 4.8 eV Polymer LEDs O O S n PEDOT n 2.7 eV n PSS SO 3 H 5.0 eV PPV 5.2 eV 2.8 eV calcium cathode indium-tin oxide anode semiconducting polymer 1.5
2.5
Energy (eV) Construction of Polymer LEDs
Burroughes et al., Nature, 347, 539 (1990), US patent 5,247,190 3.5
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.
Friend, Burroughes and Tatsuya, Physics World (Vol. 12) p35-40 (1999) LED design strategy indium tin oxide poly(
p
-phenylene vinylene) n aluminum, magnesium, or low-workfunction metal External Circuit glass substrate O Poly(2-methoxy-5-(2'-ethylhexoxy)-phenylene) O O Side chain structure reduces side chain crystallization and frustrates packing O Poor interchain p overlap enhances Electron and hole mobilities are now an issue 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 Absorption 0-3 0-2 0-1 0-0 E f 3 2 1 0 0 1 2 3 p* Emission 0-3 0-2 0-1 0-0 p E 0-0 = E 0-0 (gas phase) Conjugated polymers display inhomogeneous broadening 15 K PPV Photoluminescence Absorbance T=77 K Dilute solutions in solvent or the solid-state: Emission Stokes shift Absorption 0-0 0-1 0-0 0-1 0-2 0-2 0-3 0-3 Photon Energy 1.5
2.5
Energy (eV) 3.5
(from R.H. Friend et al.) 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 represents a distribution of effective conjugation lengths.
Different crystalline domains Particle in a box 0 L n=2 (p*) n=1 (p) Emission 0 n=2 L' n=1 a n=2 0 L'' n=1 Energy eigenvalues are proportional to L -2 1 Absorption can potentially occur at all sites (if E>E gap ) 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: a) S S S S S S b) H N c) N H H N N H O O O O a) Regioregular poly(3-hexyl thiophene) or r-P3HT 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)
For example: Self-assembly leads to ordered (lamellar) phases
(200)
l=0
x1 (002) (102) (202)
l=2
x2 (311) (111) (411)(611)
X-Ray Diffraction of Poly(3-
x2 (220) (120) (320)
n
-octylthiophene)
(020) (420)
0.0
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
UV absorption In the ordered state
50 C 45 C 40 C 35 C 30 C 20 C 15 C 5 C 0 C -20 C -35 C isosbestic point
X-ray powder diffraction
250 300 350
Wavelength (nm)
400
A Simple Free Energy Diagram Order-Disorder Transition Temperature
25 C (ordered) 45 C (disordered)
B Ordered state A B
is the path on cooling
A Thermotropic columnar mesophase
10 2 q 20 (deg.) 30 Photoluminescence is also strongly impacted!
“Self-assembly” leads to formation of helical phases Si-Si-Si-Si dihedral angle is ~170 ° Repeat unit: - (SiR 2 ) – and R= (CH 2 ) 3 CH 3 Conformational energy surface of an all transoid (15/7 helix) in poly(di-
n
-butylsilane) oligomer 15/7 helix C-Si-C-C dihedral (deg.) Topology of the surface is well defined with one C-C-Si-C dihedral (deg.) 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 Combined PL and Abs 0.30
0.30
0.25
0.10
0.20
0.20
0.00
Full Set 300 350 40 0 0 sec 10 sec 35 sec 77 sec 121 sec 164 sec 622 sec 0 sec 5 sec 41 sec 84 sec 127 sec 172 sec 617 sec 200 l
Ex
=305
nm
150 100 0.15
0.10
50 0.05
0.00
300 350 400 325 350 375 Energy migrates between the two structural phases 0 400 (nm)
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 Nucleation then growth Substrate • 1 st : Low absorption in ordered phase but efficient PL • 2 nd : High absorption in disordered phase and exciton migration to the ordered region (I.e., red arrows ) • One-dimensional growth increases relative PL from 1 st process
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 S 0 is the ground (singlet) state; S 1 first excited triplet; and T i and T n is the first excited singlet; T are higher lying triplet states.
1 the
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 mixed mixed triplet triplet triplet singlet triplet triplet 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 (H ü ckel theory) invoking the Born-Oppenheimer approximation.
Schrödinger Equation: Of course y is the wavefunction,
m
is the electron mass and
U
is the analytic if we use a H 2 + ion.
E bonding < E antibonding
Bonding orbitals occur when symmetric solution is used y H
(r)
+ y H
(r + R)
Anti-bonding orbitals occur when asymmetric solution is used y H
(r)
- y H
(r + R)
+ +
Tight-binding for p electrons and semiconducting polymers For carbon in a diamond lattice:
C C C C C
sp 3
hybridization In conjugated polymers the carbon has
sp 2 +
p bonding Now the mix is
Extending the chain H p * LUMO band C p z p z C C H H sp 2 hybridization Isolated orbitals p Paired electrons Linear chain (1-D band) HOMO 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:
R
E A is the energy of a single atomic orbital A(
R
) is an overlap integral
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) = E
A
– 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 ~ D 2
ln
(t/ D ) Elastic energy is increased ~ D 2 At low temperature dimerization always favorable
k Fermi
d = 4a
is still p
/2a
and since
d = 2
p/
k ,
(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 bonding C C C C C
s
Si bonding Si Si Si
geminal
Si Si C C C C C C
vicinal
Si Si
3. Rotation breaks conjugation Structural instabilities and disorder are extremely important (at all length scales)
Simple picture of polyacenes and poly-p-phenylenes Benzene
Free electron perspective: (works best for polyacenes)
Energy levels a a a a a a A circular ring of length L = 6a 6 unpaired p z electrons (1 per C) -2 +2 -1 0 +1
k
= p /
a
Now from the tight binding perspective E(k) = E A a – 2
t
cos
ka
, y(f) = c
e
im f m = 0, 1, 2, 3 (only six states)
k
= m p /3
a k
= 0
k
= 2 p /3
a k
= p /3
a
Now consider: Poly(
p
-phenylene)
Polarization direction Poly-p-phenylene from tight binding Weak interactions Strong interactions E gap 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 I II III IV 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 Poor Exciton wavefunction 3.1 ev 4.8 ev Better 6.5 ev In a photocell it is advantageous to efficiently separate charge Molecular wave function is important Best
Separating electrons and holes: A prerequisite for photovoltaic applications Blending C 60 and p -conjugated polymer Recombination of P+ and P reduces 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 BP 2+ 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 C 60
E gap
4.7 eV ITO 5.0 eV
p
MEH-PPV HOMO C 60 6.1 eV
p*
MEH-PPV LUMO C 60 4.3 eV Al
Carbon-60 LUMO is well positioned to accept electrons Simple design strategy
ITO
p
MEH-PPV HOMO C 60 Al
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