A brief description for next researches on poly(3 substituted)thiophenes and copolythiophenes

The electric properties of poly(3alkylthiophene)s (P3AT) are much depended on conformation of designed polymers as well as greatly affected by the regioregularity which can be expressed as the percentage of headtotail (HT) couplings 1. In fact, HT regioregular was demonstrated that electrical conductivity and PCE value are better than other regioisomers. HTpoly(3hexylthiophene) has been the most widely investigated and has resulted in the highest power conversion efficiency ( about 5% PCE) 2. However, solar cell devices similarly made from related regioregular poly(3alkylthiophene)s (P3ATs), such as poly(3butylthiophene) (P3BT), poly(3pentylthiophene), poly(3octylthiophene) (P3OT), and poly(3decylthiophene) (P3DT), have very poor efficiencies (< 1% PCE). Recent efforts in extensive materials processing and device optimization, have pushed the power conversion efficiencies up 3 – 4.2% 3. On the other hand, their copolymers were also reported by several groups. Jenekhe et al. showed that PCE value of poly(3butylthiophene)bpoly(3octylthiophene) is about 3% at 50 mol% of poly(3butylthiophene) block 4. The PCE of another copolymer, poly(3butylthiophene)bpoly(3hexylthiophene), is as high as over 4% at the molar ratio 2:15. Extension of conjugation chain is interesting in the aim lowering the bandgap value and extended wavelength absorption of the solar radiation for polythiophene. Moreover, narrow bandgap and highefficiency materials reported recently is a good selection for improvement of solar cell performance. Therefore, goal in our next researches is discovery of which poly(3substituted)thiophene and copolythiophene materials get excellently electric properties used for solar cells.

A brief description for next researches on Poly[(3-substituted)thiophene]s and

Copolythiophenes.

Student: Nguyen Thanh Danh.

Date: May 7 th , 2013.

1 Objective

The electric properties of poly(3-alkylthiophene)s (P3AT) are much depended on conformation of designed polymers as well as greatly affected by the regioregularity which can be expressed as the percentage of head-to-tail (HT) couplings [1] In fact, HT regioregular was demonstrated that electrical conductivity and PCE value are better than other regioisomers HT-poly(3-hexylthiophene) has been the most widely investigated and has resulted in the highest power conversion efficiency ( about 5% PCE) [2] However, solar cell devices similarly made from related regioregular poly(3-alkylthiophene)s (P3ATs), such

as poly-(3-butylthiophene) (P3BT), poly-(3-pentylthiophene), poly(3-octylthiophene) (P3OT), and poly(3-decylthiophene) (P3DT), have very poor efficiencies (< 1% PCE) Recent efforts in extensive materials processing and device optimization, have pushed the power conversion efficiencies up 3 – 4.2% [3] On the other hand, their copolymers were also reported by several groups Jenekhe et al showed that PCE value of poly(3-butylthiophene)-b-poly(3-octylthiophene) is about 3% at 50 mol% of poly(3-butylthiophene) block [4] The PCE of another copolymer, poly-(3-butylthiophene)-b-poly(3-hexylthiophene),

is as high as over 4% at the molar ratio 2:1[5]

Extension of conjugation chain is interesting in the aim lowering the bandgap value and extended wavelength absorption of the solar radiation for polythiophene Moreover, narrow bandgap and high-efficiency materials reported recently is a good selection for improvement

of solar cell performance Therefore, goal in our next researches is discovery of which poly[(3-substituted)thiophene] and copolythiophene materials get excellently electric properties used for solar cells

2 Research Content

2.1 Synthesis of the monomers and the polymers from commercial starting materials

- Studying on the Grignard-exchanged reaction:

 Using various Grignard reagents (Such as iPrMgCl, MeMgCl, n-BuMgCl, PhMgCl, and so on)

 Exploring reaction time as well as quantitative and kinetic study

- Studying on polymerization reaction:

 Using various Ni catalysts, amount of catalyst, reagent ratio, reaction time and end-capped groups

 Studying on purity of catalyst and rest of the catalyst in the polymer

 Investigating influence of catalyst amount to device performance and physic properties

- Studying on the end group with using various Grignard reagents and using various quenching substances (such as H2O, aq HCl, LiAlH4)

- Controlling MW and regioregularity

2.2 Studying on optical absorption, DSC and PL quantum yield for the polymers

2.3 Preparation of devices and detecting electrical properties of the polymer products and

Investigating PCE value and physic properties of various MW polymer

3 Research Method.

3.1 Synthesis of Poly(3-alkylthiophene)s.

S

R

S

R

R

1 R'MgCl/THF,0 o C

Br 2 /CHCl 3 , NaHCO 3

Or 1.NBS,THF

2 I2/C6H5I(OAc)2

R: CH3, C4H9, C6H13, C8H17 X: Br or I

Scheme 1 Synthesis of P3ATs from 3-alkylthiophenes

GRIM and Yokoyawa method will be employed to prepare P3AT in the next researches Both methods are synthesized from 3-alkylthiophenes as the starting materials Preparation of the monomers was reported much in the recent years [6-9] On the other hand, the monomer materials can either be purchased from chemical vending (X = Br) or be halogenated from 3-alkylthiophene

Size - Code No.

Price (Won)

3-methylthiophene

3-butylthiophene Huicheng (China) 1 kg - 34722-01-5

2,5-dibromo-3-octylthiophene Huicheng (China) 1 kg - 149703-84-4

3.2 Synthesis of poly(3-[alk-1-en-1-yl]thiophene)s (P3AETs).

S

CHO

S

CHO

P(Ph)3Br

R

n-BuLi

1 R'MgCl/THF,0 o C

Br2/CHCl3

R : H, CnH2n+1

Scheme 2 Synthesis of P3AETs from 3-thionylcarboxadehyde.

Combination of thiophene ring and vinyl group in the backbone polymer lowered significantly their bandgap value as shown in literature [10] In the similar aim of approach to materials possessing low bandgap and longer wavelength absorption, our synthesis is focused

on extended conjugation length of polythiophenes by vinyl group at 3-substituted chain These polythiophenes and their copolythiophenes, P3AETs, have not been reported before However, 3-vinylthiophene and similar monomers were prepared in literature [11-13]from commercial materials

In order to synthesize the P3AETs, we could not prepare these monomers [13] like the way of synthesis of poly(3-alkylthiophene)s because the double bond in the substituted chain is not stable with oxidants as NBS, NIS or Br2.Therefore, we cannot brominate or Iodinate directly from 3-alkenylthiophenes Alternatively, he polymers will be synthesized from that 2,5-dibromovinylthiophene derivatives was been prepared according to reference [11] by GRIM

method as depicted in scheme 2

3.3 Synthesis of narrow bandgap and high-efficiency copolythiophenes.

Theoretically, the PCE of polymer solar cells can be improved to over 10 % for single-layer device [14] and 15% for tandem device [15] by implementing new materials, exploring new device architecture and optimizing device processing approaches In fact, to my best

knowledge, the currently champion materials are as showed in table 1 The PCE of

PCPDTBT could be achieved 9% for tandem solar cells [16] and the PCE of PTB7 about 8% for single cells Therefore, the next researches should be focused on preparing the materials and improving their PCE value of solar cell device The strategy of the polymer synthesis is

showed in below schemes In the scheme 8, the document was collected from Dr.Zhang’s

synthesis on the monomers of PCDTBT in 2009

Polymer M n (kDa)

(PDI)

E g

(eV)

HOMO/

LUMO(eV)

Polymer:

PCBM

J sc

(mA/

cm 2 )

V oc

(V)

(single Cell)

PCE(%) (Tandem Cell)

Ref.

Table 1 The physic data for high efficiency polymers .

S

S ROOC

S

S OR

OR F

*

n

R: 2-ethylhexyl

N S N

N

S

N S N

S

C8H17 C8H17

n

Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole]

(PCDTBT)

thieno[3,4-b]-thiophene/benzodithiophene

(PTB7)

Poly(4,4-dialkyl-cyclopenta[2,1-b:3,4-b′]-dithiophene-alt-2,1,3-benzothiadiazole)

(PCPDTBT)

Fig 1 High-efficiency Polymers

3.3.1 Synthesis of PCPDTBT

N

S N n +

Pd(OAc)2 Phosphine

K2CO3 DMAc

PCPDTBT

CPDT

4,7-Dibromo-2,1,3-benzothiadiazole

Mn~ 40.000 Yield = 70%

Scheme 3 Synthesis of PCPDTBT [19]

CPDT

R R

O

NH 2 NH 2 /KOH 65%

RBr/KOH/KI/DMSO 80%

4H-Cyclopenta-[2,1-b:3,4-b']dithiophene

cyclopenta [2,1-b;3,4-b']dithiophen-4-one

R = alkyl

Scheme 4 Synthesis of the monomer [20]

3.3.2 Synthesis of PTB7.

S

S

Br Br

F

COOR

S

S OR

OR

S

S ROOC

S

S OR

OR F

*

n

DMF/Toluene

R: 2-ethylhexyl

PBT7

Scheme 5 Synthesis of PTB7 [21].

S

S

S HOOC

F

S

S ROOC

F

S

S ROOC

S

S F COOR

S

S Br Br

F COOR

1 BuLi

2 PhSO2NF

ROH/DCC DMAP

McPBA

EtOAc

DMF

S

S OR

OR

S

S O

O

S

S OR

OR

1 Zn, NaOH, EtOH

2 H3C S

O

O OR

1 BuLi

2 SnMe3Cl

Scheme 6 Synthesis of the monomers [21].

3.3.3 Synthesis of PCDTBT (Dr Zhang performed in 2009).

N

C 8 H 17 C 8 H 17

O

O O

S

Br Br

N

S N

N

S

N S N

S

C 8 H 17 C 8 H 17

n

+

Pd(OAc)2/P(C6H11)3

70%

24

27

PCDTBT

Scheme 7 Synthesis of PCDTBT [22]

N

C 8 H 17 C 8 H 17

O

O O

O N

N

C8H17 C8H17

C8H17 C8H17 OTs

O B O O

THF, -78 o C DMSO/KOH

S

N

Br

Br

N

S N

S

S

N

S N

S

S Br

Br S

B HO HO

Pd(OAc)2/PPh3 THF/Na2CO3

NBS o-Dichlorobenzene

Scheme 8 Synthesis of the monomers [23]

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[23] From Dr Zhang’s data (2009).