Electronic and Optoelectronic Polymers - 國立臺灣大學homepage.ntu.edu.tw/~ntuipse/File/class lecture (chap 2).pdf · Electronic and Optoelectronic Polymers ... zElectronic Structures

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 :


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


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


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


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


Influences of Linkages on Electronic Structures


Geometries & Electronic Structures of PPV, PPI and PPN

) (

PPV

PPI

H

N

C

1

2 3

4

PPN

Φ


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


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


Ar1

N N

(

)n

Ar1=

Ar2=S

( )

( ) (PPI)

(PTPI)

Ar1

N N

(

)n

Ar1=

Ar2=S

( )

( ) (PPI)

(PTPI)

Optimum Geometries & Electronic Structures of PTPI


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


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


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.


Optimum Geometries & Electronic Structures ofPTPI, PTPyI and PTNI

Influences of Electron Donor/Acceptor Five-member Ring on Electronic Properties of Coplanar Poly(azomethine)s


Ar CH N Ar N CHn

NH

S

NN

S

S

OO

O

Geometry of Azomethine model compound


A planar configuration of the monoazomethine

showed by X-ray crystal analysis

Polymer Reprint 2004, 45, 253

Optimum Geometries of PEEI and PYYI


PEEIPYYI

Optimum Geometries of PTTI, PThThI, and PFFI


PTTI

PThThI

PFFI

Geometries & Electronic Structures ofCoplanar Poly(azomethine)s


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


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


*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


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


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.

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Electronic and Optoelectronic Polymers - 國立臺灣大學homepage.ntu.edu.tw/~ntuipse/File/class lecture (chap 2).pdf · Electronic and Optoelectronic Polymers ... zElectronic Structures