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Electronic and Optoelectronic Polymers
Wen-Chang ChenDepartment of Chemical Engineering
Institute of Polymer Science and EngineeringNational Taiwan University
History of Conjugated Polymers
Electronic Structures of Conjugated PolymersBrief introduction
Chemistry of Conjugated Polymers
Doping concepts of Conjugated Polymers (Conducting Polymers)
Polymer Light-emitting Diodes
Polymer-based Thin Film Transistors
Polymer-based Photovoltaics
Outlines
Electronic Parameters of Conjugated Polymers
EA
IP
Eg
Energy
LUMOπ*
HOMOπ
Vacuum
Valence band
Conduction bandBW
BW
Eg: Band gap = IP - EA
IP: Ionization potential
EA: Electron affinity
BW: Bandwidth
LUMO: Lowest Unoccupied Molecular Orbital
HOMO: Highest Occupied Molecular Orbital
Ionization Potential (IP)
The energy required for the ionization reaction. The minimum energy needed to remove an electron from the top of the valence band tovacuum level
P P+‧ e-+Thus, the IP values for a polymer indicate the susceptibility or ease of removing electrons from the polymer by a suitable electron acceptor. IP values measure the dopability of a polymer to a p-type conductor or ease of hole injection .
Electronic Parameters of Conjugated Polymers
Electron Affinity (EA)
The energy needed to add an electron to the bottom of the conduction band or LUMO level from the vacuum level.
P P-‧e-+
EA measures the ease of electron injection or n-type dopability
Bandgap or energy gap (Eg) = the smallest energy gap transition between π and π* bands = HOMO-LUMO energy gap
π-Conjugated MoleculesHave been around a long time with numerous examples from organicand biological chemistry
Molecules composed of alternating single and double bonds
Towards Organic Semiconductors
Electronic Structures of Atomic Carbon in Conjugated Molecules
ground state excited state hybridized state
sp2 hybirds
unhybirzedPz orbital
mix 3 orbitals, get a set of 3 sp2 orbitals
the other p orbital remains unaffected
each sp2 hybrid and p orbital contains a single unpaired electron
2s 2px+2py 3 sp2 orbitals
used to form π bond
both C are sp2 hybridized. 4 C-H s bonds are made by the interaction of C sp2 with H1s orbitals (see red arrows) 1 C-C s bond is made by the interaction of C sp2 with another C sp2 orbital (see green arrow) 1 C-C p bond is made by the interaction of the C p with the other C p orbital (see black arrows
Molecular Orbital (MO) Theory of Conjugation
Formation of the Molecular Orbitals for ethylene
Evolution of a Semiconductor Bandgap with Increasing Chain Length
Energy
π1
π2*π3*
π4*
π1
π2
π3
π4*
π5*
π6*
π2
π1
Empty π* band or conduction band (CB)
Filled πband or valence band (VB)
π to π* Energy Gap in a Series of Polyenes of Increasing Chain Length
n
The Hypothesis of Alternation -Peirels Instability
Band structures of trans-polyacetylene
Why is Eg≠0 as n ∞The reason of a finite band gap in is instability of the ground state of a chain of equal bonds with respect to nuclear shifts that create a configuration having alternating lengths. Bond length alternations is due to gain in electronic energy that compensates the loss of elastic energy.
Five Main Contributions to Electronic Structures of Conjugated Polymers
Eg = Eδr + Eθ + ERes + ESub + EInt
Eδr: the energy related to Bond Length Alternation (BLA).Eθ: the mean deviation from planarity.ERes: the aromatic resonance energy of the cycle. ESub: the inductive or mesomericelectric effects of eventual substitution.EInt: the intermolecular orinterchain coupling in the solid state.
Jean Roncali, Chem. Rev., 1997, 97, 173.
Mesomeric structures Fused six-membered ringLadder polymersZwitterionic structures
Polysulfur nitride, shows metallic conductivity
Egopt = 1.55 eV
van Mullekom et al., Mater. Sci. Eng. A, 2001, 32, 1.
Ajayaghosh, Chem. Soc. Rev., 2003, 32, 181.
Egopt(solution) = 1.1 eV
How to Minimize the BLA Along the Backbone for Small Eg?
Ladder type → twist inhibitionAvoid steric hindranceAromatic versus quinoid form (sp3 versus sp2)Intermolecular hydrogen bonding
H
N
C
S
PTTI
PTPI
) ( Twist angle: 24°
Eg = 2.35 eV
Twist angle: 0.5°
Eg = 1.42 eV
Orbital overlap varies nearly with the cosine of the twist angle, any departure from coplanarity will result in an increase of Eg (Eθ).
How to Maintain the Planarity Along the Backbone for Small Eg?
Aromaticity results in a competition between π -electron confinement within the rings and delocalization along the chain.
Aromatic and quinoid forms are not energetically equivalent.
Aromatic form Egcal = 2 eV
Quinoid form Egcal = 0.26-0.47 eV
How to Control the Aromaticity Along the Backbone for Small Eg?
How to Control the Substituents for Small Eg?
n n
OCH3
H3COn
OCH3
H3CO NCn
NCn
CN
Structure (IP, EA, Eg) eV
5 . 5
5 . 0
4 . 5
4 . 0
3 . 5
3 . 0
2 . 5
E (e
V)
5.05
2.73
4.72
2.65
5.27
3.10
5.15
2.913.15
5.12
methoxy group will decrease the IP (destabilization of VB)
cyano group will increase the EA (stabilization of CB)
JL Bredas, AJ Heagger Chem. Phys. Lett, 1994, 217, 507.
Experimental Measurements of Electronic Structure Parameters of Conjugated Polymers
Bandgap (Eg)
IP & EA
Photoelectron spectroscopy
Need ultrahigh vacuum techniques
UPS (from UV)
XPS (ESCA, from X-ray)
Accurate but more difficult to do
Estimate from electrochemical redox potential
IP = Eoxonset + 4.4 eV
EA= Eredonset + 4.4 eV
Optical absorption spectrocopy(UV-Vis-NIR)
Electrochemical redox potential (cyclic voltammetry)
Eredonset
Eoxonset
Reduction wave
Oxidation wave
Computational Techniques for Conjugated Polymers
Applicable to such complex conjugated systems
Amenable to geometry optimization and should yield good calculation geometrical parameters
Able to produce good electronic structure characteristics, such as bandgap, ionization potential (IP), electron affinity (EA), charge distribution and wave functions
Capable of yielding good results for other calculated physical observables , such as force constants, transition dipole, etc
Employ for the study of the electronic structure of π-conjugated ploymersshould be :
Computational Techniques for Conjugated Polymers
Molecular (Oligomer) method
PBC (periodic boundary condition
Extrapolating the linear curve of the HOMO-LUMO gap of the conjugated oligomer against the reciprocal of the number of monomer units (1/n) affords a prediction of the band gap
Single, infinite, gaseous state, one-dimensional polymer chain was treated.
Linear prediction
2nd order polynomial
Unit cell for polythiophene
Computational Techniques for Conjugated Polymers
DFT/B3LYP/6-31G*
DFT: density functional theory reliable for the non-planar structuresgiving reasonable gap values for most conjugated polymer system
B3LYP: hybrid functional6-31G*: basis set
Performed on Gaussion 03 program
Methodology
Analyzed Geometry Parameters and Properties
Torsional angle = tilt between two conjugated planes
Bond length alternation (BLA) = L(C-C)- L(C=C)
Bridge length (LB) = length between two conjugated units
Intramolecular charge transfer = net Mulliken charge distribution
⎟⎟⎠
⎞⎜⎜⎝
⎛= 2
22
ddmκε
η*Electron effective mass
Optoelectronic Polymer Lab, NTU
Cheng-Liang Liu,1 Fu-Chuan Tsai,1 Chao-Chung Chang,1
Wen-Chang Chen,1,2 and Hsi-Hsin Shih3
1: Department of Chemical Engineering and 2: Institute of Polymer Science and Engineering, National Taiwan University3: Union Chemical Laboratory, Industrial Technology Research
Institute
Theoretical Analysis on New Conjugated Poly(azomethine)s
For Thin Film Transistors
Research Objectives
Optoelectronic Polymer Lab, NTU
Discuss the relationships among the optimized geometries, electronic structures and polymer structures of conjugated polymers
Influences of different linkage on electronic structures Influences of replacement of phenyl ring on PPI by thiopheneInfluences of electron Donor/Acceptor six-member ring on the electronic properties of poly(azomethine)sInfluences of electron Donor/Acceptor five-member ring on the electronic properties of coplanar poly(azomethine)s
Molecular designs of coplanar poly(azomethine)s for OTFT applications
Theoretical Analysis
unit-cell
Methodology
Periodic Boundary Condition (PBC)– Single, infinite, gaseous state, one-dimensional polymer chain was treated.
DFT/B3LYP/6-31GDFT: density functional theory
reliable for the non-planar structuresgiving reasonable gap values for most conjugated polymer system
B3LYP: hybrid functional6-31G: basis set
Performed on Gaussion 03 program
Optoelectronic Polymer Lab, NTU
widely used for the calculation
Geometry of Azomethine model compound
N
Twisted nature of azomethine model compound:the X-ray diffraction results of the small molecule
trans-N-benzylideneaniline
Optoelectronic Polymer Lab, NTU
Influences of Linkages on Electronic Structures
Optoelectronic Polymer Lab, NTU
Geometries & Electronic Structures of PPV, PPI and PPN
) (
PPV
PPI
H
N
C
1
2 3
4
PPN
Φ
Optoelectronic Polymer Lab, NTU
Eg
(eV)EA(eV)
IP(eV)
Φθ (1,2,3)r
(2,3)r
(1,2)
2.472.314.780.0127.0C-C1.46
2
C=C1.35
5
2.313.615.920.0115.7C-N1.41
8
N=N1.28
2
2.832.635.4731.9122.5C-N1.41
0
C=N1.29
3
Geometries & Electronic Structures
Optoelectronic Polymer Lab, NTU
Order of IP/EA :
PPV (C=C) < PPI (C=N) < PPN (N=N)
Order of Eg :PPN (N=N) < PPV (C=C) < PPI (C=N)
e-accepting
e-donating
planar conformationtwisted conformation
Influences of Replacement of Phenyl Ring on PPI by Thiophene
Optoelectronic Polymer Lab, NTU
Ar1
N N
(
)n
Ar1=
Ar2=S
( )
( ) (PPI)
(PTPI)
Ar1
N N
(
)n
Ar1=
Ar2=S
( )
( ) (PPI)
(PTPI)
Optimum Geometries & Electronic Structures of PTPI
Optoelectronic Polymer Lab, NTU
0.138
0.176
Δr(r1-r2)
2.352.97(↓)
5.32 (↑)24.01.2971.435
2.832.635.4730.41.2931.463
Eg
(eV)EA(eV)
IP(eV)
Φr2
(C=N)r1
(C-C)PPI
PTPI
H
N
C
S
Eg (PTPI) < Eg (PPI):
smaller Φ in PTPI
smaller Δr in PTPI
extensive π-electron delocalization of thiophene ring in PTPI
↓: HOMO/LUMO stabilization
↑: HOMO/LUMO destabilization
∵
Influences of Electron Donor/Acceptor Six-member Ring on Electronic Properties of Poly(azomethine)s
Optoelectronic Polymer Lab, NTU
CH N Ar N CHnS
( )Ar= (PTPI)
N
( )Ar= (PTPyI)
Ar= (PTNI)
(
)
2.352.975.3224.0
Eg
(eV)EA(eV)
IP(eV)
Φ (o)
2.50(+0.15)
2.66(↑)
(-0.31)
5.16(↑)
(-0.16)
30.6(↑)
(+6.6)
2.40(+0.05)
3.16(↓)
(+0.19)
5.57(↓)
(+0.25)
30.2(↑)
(+6.2)
H
N
C
S
PTPI
PTPyI
PTNI
planar
Optimum Geometries & Electronic Structures ofPTPI, PTPyI and PTNI
Optoelectronic Polymer Lab, NTU
The replacement of phenyl ring either by the e-accepting pyridine or by the e-donating naphthalene leads to more twisted conformation and thus higher Eg obtained.
The slightly decreased Eg of PTPyI in comparison with PTNI is due to the small intra-charger transfer along the polymer chain induced by the e-accepting pyridine.
Optoelectronic Polymer Lab, NTU
Optimum Geometries & Electronic Structures ofPTPI, PTPyI and PTNI
Influences of Electron Donor/Acceptor Five-member Ring on Electronic Properties of Coplanar Poly(azomethine)s
Optoelectronic Polymer Lab, NTU
Ar CH N Ar N CHn
NH
S
NN
S
S
OO
O
Geometry of Azomethine model compound
Optoelectronic Polymer Lab, NTU
A planar configuration of the monoazomethine
showed by X-ray crystal analysis
Polymer Reprint 2004, 45, 253
Optimum Geometries of PEEI and PYYI
Optoelectronic Polymer Lab, NTU
PEEIPYYI
Optimum Geometries of PTTI, PThThI, and PFFI
Optoelectronic Polymer Lab, NTU
PTTI
PThThI
PFFI
Geometries & Electronic Structures ofCoplanar Poly(azomethine)s
Optoelectronic Polymer Lab, NTU
2.474.727.190.80.60.0401.3631.2961.4362e (PThThI)
1.423.615.030.30.30.0181.3571.3081.4232d (PTTI)
1.563.334.890.10.30.0251.3481.3091.4242c (PFFI)
1.672.734.4012.11.80.0061.3711.3091.4272b (PYYI)
1.113.144.250.60.20.0231.3451.3141.4142a (PEEI)
2.832.635.4730.40.90.0211.4101.2931.4631 (PPI)
(eV)(eV)(eV)(Å)(Å)(Å)(Å)
EgEA IP Φ2(degre
e)
Φ1 (degre
e)
δ RC3-NRC2=NRC1-C2
Electronic Structures of The Studied Poly(azomethine)s
Optoelectronic Polymer Lab, NTU
PEEI < PYYI < PFFI < PTTI < PPI < PThThI
PPI < PYYI < PFFI < PEEI < PTTI < PThThI
IP increases in the following order:
EA increases in the following order:
PEEI < PTTI < PFFI < PYYI < PThThI < PPI
Eg increases in the following order:
e-donating
e-accepting
coplanar conformation twisted conformation
Donor/Acceptor system
Band Structures of Organic Thin Film Transistor Materials
Optoelectronic Polymer Lab, NTU
*meτμ =
effective mass⎟⎟⎠
⎞⎜⎜⎝
⎛= 2
22
ddmκε
η*
Drude form for mobility
energy near the band
wavevector
life time of the carriers
εκτ
valance (conductive) band effective mass
hole (electron) mobility
effective mass π electron delocalization mobility
J. Mater. Chem. 1995, 5, 1179
Band Structures of Coplanar Poly(azomethine)s
Optoelectronic Polymer Lab, NTU
2.313.615.92405(0.464 me)
347(0.474 me)
4 (PAZ)
2.472.314.78392(0.446 me)
373(0.466 me)
3 (PPV)
2.474.727.19287(0.504 me)
318(0.530 me)
2e (PThThI)
1.423.615.03583(0.275 me)
572(0.278 me)
2d (PTTI)
1.563.334.89556(0.343 me)
590(0.376 me)
2c (PFFI)
1.672.734.40552(0.403 me)
562(0.406 me)
2b (PYYI)
1.113.144.25644(0.223 me)
613(0.233 me)
2a (PEEI)
2.832.635.47298(0.688 me)
247(0.742 me)
1 (PPI)
Eg(eV)
EA (eV)
IP (eV)
Conductionbandwidth
(meV)and
effective mass
Valence bandwidth
(meV)and
effective mass
Conclusions
Optoelectronic Polymer Lab, NTU
The twisted conformation of aromatic poly(azomethines) is attributed to the repulsion force between the adjacent hydrogen atoms on the C=N linkage and the N-phenylene.The coplanar geometry of PPV and PAZ result in a smaller Eg than that of PPI. The IP and EA of PPI are in the intermediate between PPV and PAZ. The coplanar configuration or donor-acceptor intrachain charge transfer resulted in enhanced electronic properties of PEEI PYYI, PFFI, and PTTI in comparison with PPI, including lower Eg, higher BW, and lower effective mass. The proposed coplanar poly(azomethine)s for OTFT applications.