Synthesis of semi-conducting polymer

Assoc. Prof. Dr. Jatuphorn Wootthikanokkhan

School of Energy and Materials, KMUTT

Some interesting semiconducting polymers

Polyphenylene vinylene (PPV)

Polythiophene (PTh)

Polypyrole

Polyaniline

Polyacetylene (PA)

Structures of some electrical conducting polymers

Polyacetylene

Poly(p-phenylene)

NH

NH

Polyaniline

HN

NH

HN

NH

Polypyrole

S

S

S

S

Polythiophehe

Application of semiconducting polymers

Example of the donor materials include (MEH)PPV, and P3HT

Examples of the acceptor materials include C60 and PCBM

OMe

O

S

C 6 H 13

n

P 3 H T O

On

n

P o l y p a r a p h e n y l e n e v i n y l e n e ( P P V )MEH-PPV

Fullerene [C60]

PCBM 4

Structure of polymer solar cell devices

1. Bi-layer hetero-junction solar cell.

2. Bulk hetero-junction solar cell.

Bulk heterojunction, BHJ

Bilayer heterojunction

Idealized BHJ morphology;

The donor and acceptor phases are interspaced by 10–20nm.

The inter-digitated pathways ensure unhindered charge carrier

transport

[Hope et al. 2007] 5

การสังเคราะห์พอลอิะเซธิลนี

Synthesis of polyacetylene

Shirakawa route

Luttinger route

Precursor

polymer route

Shirakawa route

By using Natta catalysts, in a solution. The above product was air sensitive, infusible, and insoluble.

In 1970s, Shirakawa [Polymer J.,

2(1971)231] modified the above method by carrying out the reaction on the internal surface of the reactor, wetted by a solution of initiator in an inert solvent.

Good quality PA film was obtained.

Luttinger route

Similar idea to the Shirakawa route but using different catalyst. (The best is Co(NO3)2 and NaBH4)

PA powder was obtained (which could be suspended in an inert solvent and casted

into film).

The reaction is less sensitive to moisture and oxygen.

Precursor route Further efforts were made to prepared workable PA by using different

approaches.

Originally, attempt was made to prepare PA by de-hydrochlorination of PVC. But the product was of short conjugation length and partially crosslinked.

Another approach involve the use of Ring Opening Metathesis Polymerization [ROMP] of cyclic monomers.

The product is a precursor which is soluble in many organic solvents.

It can be converted into PA by heat treatment.

Polymer, 21(1980)595

การสังเคราะห์พอลฟีินิลนีไวนิลนี (PPV)

Early attempts utilized dehydrohalogenation resulted in an intractable oligomer powders.

Wessling precursor route (using bis-sulfonium salt monomer) is more preferable.

To avoid a premature formation of C=C, the reaction should be carried out at low T,

in a dilute monomer conc.

and equimolar (or lower) base/monomer. (thermal or base induced elimination of side chains)

[J.Polym Sci. Part A: Polym Chem., 26(1988)3241]

Sulfonium polymer precursor (white viscous solution)

การสังเคราะห์พอลฟีินิลนีไวนิลนี (PPV)

The reaction can be terminated by addiing diluted HCl, and then dialyzed to separate polymer from ions, monomer, and residues.

The precursor polymer is water soluble.

Further heat treatment results in a formation of PPV.

Thermal treatment of the PPX precursor to form PPV

PPX PPV

O M e

y y

2 0 0 C v a c u u m , 2 h .

การสังเคราะห์พอลอิลัคลิไธโอฟีน (PAT)

Synthesis of polythiophene

Electro-synthesis Chemical-synthesis

Oxidative coupling

Organo-metallic mediated synthesis

(Cross-coupling)

McCullough’s route

Reike’s method

กลไกการสังเคราะห์แบบ coupling

Oxidative coupling

Radical mechanism

Carbo-cation mechanism

Radical cation mechanism

Anderson (Macromolecules,

27(1994

Neimi et al. 1992. (Olinga and François, 1995)

Three mechanisms have been proposed (still remain controversial).

A “radical cation” analogue to that of the electro-synthesis has been proposed and is generally accepted as the most likely synthesis route.

Oxidative coupling of thiophene

http://en.wikipedia.org/wiki/Polythiophene

Radical cation mechanism:

1. Oxidation of the monomer to a radical cation. 2. Dimerization (coupling) of the radical cations

3. Proton loss to yield a neutral dimer

4. Oxidation of the dimer to its radical cation

5. Reaction of the dimer radical cation with

another radical cation, …..,

6. Oxidized conducting polymer

การสังเคราะห์พอลอิลัคลิไธโอฟีน (PAT)

Regio-regularity is an important issue for

this product.

Head-to-head is sterically unfavorable for

coplanarity, causing loss of conjugation

length.

Head-to-tail coupling does not limit conjugation.

Advantage of the electro-synthesis route

Convenient

Advantages of the chemical-synthesis route

A greater selection of monomers

The ability to synthesize perfectly regio-

regular substituted PTs.

Conjugated π-orbitals of a coplanar and a twisted substituted PT.

http://en.wikipedia.org/wiki/Polythiophene

Cross-coupling mechanism

Organometallic mediated: McCullough’s route

High regioregular polymer (over 98% H-to-T) was reported.

Oxidative coupling of thiophene

Andersson et al. (Macromolecules, 27(1994)6503) studied the polymerization of 3-

(4-octylphenyl)thiophene with ferric chloride,

Found a high degree of regioregularity when the catalyst was added to the

monomer mixture slowly.

It was concluded that, given the selectivity of the couplings, and the strong

oxidizing conditions, the reaction could proceed via a carbocation mechanism

Chemical and Regioselective Polymerization of Poly(octyphenyl)thiophenel

De-doping

De-doping = removal of residual Fe and Cl

Polymers De-doping chemicals Ref

Copolymer of 3 hexylthiophene and theinylacetate

aqueous ammonia Materials Science, 40(2005)3423

Poly (4-octylphenyl)thiophene ammonia Macromolecules, 27(1994)6503

Poly(alkyl thienyl)phenylene hydrazine hydrate and water (1:2) Synthetic metals,

92(1998)33-37

Table I. Percentage H-T of P3HT and P3OT polymerized at 23 C for different time.

Polymer Reaction time (h) H-T (%)

P3HT 24 76.1

48 76.5

72 79.9

P3OT 24 68.9

48 76.7

72 79.8

Table II. Percentage H-T of P3HT and P3OT polymerized at 0 C for different time.

Polymer Reaction time (h) H-T (%)

P3HT 6 79.0

24 73.0

P3OT 6 79.0

24 68.0

Table III. Percentage H-T of P3HT and P3OT polymerized at 0 C and (-22) C for 6 h.

Polymer Reaction Temp (C) H-T (%)

P3HT 0 79.0

-22 75.0

P3OT 0 79.0

-22 72.0

Basic apparatus for carrying out an electropolymerization

1. Power source.

2. Ammeter

3. Voltmeter.

4. Anode (working electrode).

5. Reference electrode.

6. Cathode (Auxiliary

electrode)

7. Electrolyte solution.

8. Conducting polymer film forming.

ในกรณีท่ีต้องการท าการวัดศักย์ไฟฟ้าด้วยจะต้องมีการเพ่ิมขัว้อ้างอิง (reference electrode) เข้าไปด้วย ซ่ึงในกรณีนีจ้ะเรียกเซลล์ไฟฟ้าเคมีดงักล่าวว่า three electrode cell

ขั้วท างานจะเป็นขั้วท่ีเกิดปฏิกิริยาพอลิเมอรไรเซชนั ส่วนขั้วช่วยจะมีหนา้ท่ีช่วยท าใหว้งจรสมบูรณ์และเกิดการไหลของกระแส

Basic apparatus for carrying out an electropolymerization

วสัดุท่ีใชท้ าขั้วท างานไดแ้ก่ พลาตินมั ITO แกว้ stainless steel และ glassy carbon (การใช ้glassy carbon จะช่วยใหส้ามารถลอกฟิลมอ์อกไดง่้ายโดยผิวอิเลคโทรดไม่เสียหาย)

ส่วนขั้วอา้งอิงจะใช ้Ag/AgCl หรือ saturated calomel electrode หรือลวดเงิน (silver wire)

ในขณะท่ีขั้วช่วย จะนิยมท าจากโลหะท่ีเฉ่ือยต่อการเกิดปฏิกิริยาออกซิเดชนั (ส าหรับโลหะอยูใ่นข่ายท่ีเฉ่ือยต่อการเกิดออกไซดไ์ดแ้ก่ Pt, Au, และ glassy carbon) โดยจะมีการใชส้ะพานเกลือ (salt bridge) เป็นตวัเช่ือมระหวา่งขั้วอา้งอิงกบัสารละลายโมโนเมอร์เพ่ือหลีกเล่ียงการปนเป้ือนของสารละลายโมโนเมอร์จากสารท่ีบรรจุอยูใ่นขั้วดงักล่าว (เช่น Ag/AgCl)

Typical electrochemical cell used for the polymerization

Typical electrochemical cell used for the polymerization

Electropolymerization of thiophene

Homogeneous and stable film of the polymer is deposited in its

oxidized conducting form onto the electrode.

A regiorandom structure obtained.

J.Chem.Rev., 92(1992)711

Some advantages and disadvantages of the Electro-Polymerization

Adv.

Easiness of operation

Controllability of doping conc., film shape and thickness

Disadv.

Formed film size depends on

the electrode area

Usually, a regiorandom

structure obtained

Properties (electrical,

morphological, physical)

depend on the composition of

the polymerization system

(chemical species and

concentration of solvent, electrolyte, monomer)