**3.4 Thiophene**

460 Solar Cells – New Aspects and Solutions

P(C4H9) Pt <sup>3</sup> Cl

<sup>m</sup> <sup>m</sup> <sup>m</sup> <sup>m</sup>

Benzene ring is the most fundamental building block for polymer solar cell materials. A lot of chemistry and reaction carried out in this research area are rooted back to the reactivity of benzene ring. Benzene can be polymerized by direct linkage at the 1,4-position to form poly(*para*-phenylene) (Chart 2). Poly(*para*-phenylene) without any substituents has a linear rod-like structure and poor solubility in common organic solvents which limits its application as organic electronics. One strategy to increase the solubility is to introduce alkyl or alkoxyl chain on the backbone. However, the planarity of the poly(*para*-phenylene) will be disturbed due to the steric effect of the R group attached (Chart 2, P12) and therefore the effective conjugation between adjacent benzene rings will be sacrificed. To address this issue, bridges can be introduced between benzene rings, e.g., double bond in

PPV and its derivatives have been intensively studied in organic electronics research for OLED and PSC materials due to their excellent conducting and photoluminescent properties (Burroughes et al., 1990). Poly[2-methoxy-5-((2'-ethylhexyl)oxy)-1,4- phenylenevinylene] (MEH-PPV, P14) was utilized to fabricate bilayer solar cell with C60 in the early days and it was reported that photoinduced electron transferred from electron donating MEH-PPV onto

N S N

> Pt Bu3P Bu3P <sup>n</sup>

> > Pt Bu3P Bu3P n

S S

S N

**P7** (m=0), **P8** (m=1),**P9** (m=2), **P10** (m=3)

C9H19

S

N S

C9H19

**P6**

S

Cl CuI, NEt3

Cl

Cl CuI, NEt3

(C4H9)3P

<sup>S</sup> P(C4H9) Pt <sup>3</sup>

(C4H9)3P

Scheme 3. Synthesis of organoplatinum polyyne polymers: P6, P7-P10

N S N

S S

N S

poly(phenylvinylene) (PPV)(Chart 2, P13).

Chart 2. Structures of polyphenylene and its derivatives

C9H19

S N

C9H19

**3.3 Phenylene (benzene)** 

S

Thiophene has become one of the most commonly used building blocks in organic electronics due to its excellent optical and electrical properties as well as exceptional thermal and chemical stability (Fichou, 1999). Its homopolymer, polythiophene (PT), was first reported in 1980s as a 1D-linear conjugated system (Yamamoto et al., 1980; Lin & Dudek, 1980). Substitution by solubilizing moieties is adopted to increase the solubility of polythiophenes. The band gap of the polythiophene can also be tuned at the same time by inductive and/or mesomeric effect from the heteroatom containing substitution.

Chart 3. Chemical structures of PEDOT:PSS and P3HT

Two frequently encountered thiophene-based conjugated polymers in literature are poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS, Chart 3) in conducting and hole transport layers for organic light emitting diodes (OLEDs) and PSCs and regioregular poly(3-hexylthiophene) (P3HT, Chart 3) as a hole transporting material in organic field effect transistors (OFETs) and PSCs.

As in PPV polymer, regioregularity is essential for the thiophene unit to conjugate effectively on the same plane since in regioregular form, steric consequence of the substitution is minimized, resulting in longer effective conjugation length and a lower band gap. As shown in Chart 4, three different regioisomers, head-to-head (HH), head-to-tail (HT) and tail-to-tail (TT) can be formed when two 3-alkyl thiophene units are linked via 2,5 position. Presence of HH and TT linkage in polythiophene will cause plane bending and generate structural disorder, which consequently weaken the intermolecular interaction.

Conjugated Polymers for Organic Solar Cells 463

2,1,3-benzothiadiazole (BT, Chart 6) is an electron deficient heterocycle that has been incorporated with electron donating species to construct low band gap polymer donor for BHJ polymer solar cell. The electron withdrawing ability of BT can be further increased by replacing one carbon atom with *sp*2-hybridized N atom (Chart 6). The sulfur atom in the BT

A variety of low band gap polymers containing BT have been synthesized and tested for PSC performance (P19-P23) (Zhang et al., 2006; E. Zhou et al., 2008; Svensson et al., 2003; Slooff et al., 2007; Q. Zhou et al., 2004; Blouin et al., 2007). The electron donating moiety in the low band gap polymer varies from carbazole, fluorene, dibenzosilole, bridged thiophene-phenylene-thiophene, and dithienopyrrole. This type of polymer has a band gap <2.0 eV and gives moderate to good PCE value ranging from ~1% to ~5%, rendering a

unit can also be replaced to selenium, by doing so the band gap is further decreased.

**3.6 Benzothiadiazole/ Aza-benzothiadiazole/ Se-benzothiadiazole** 

Chart 6. Structure of BT, aza-BT and Se-BT

**3.7 Isothianaphthene/ Thienopyrazine** 

Scheme 4. Synthesis of poly (isothianaphthene)

promising direction for the research of PSC donor material.

Chart 7. Benzothiadiazole containing low band gap polymers P19-P23

which is about half that of polythiophene (~2.0 eV) (Kobayashi et al., 1984).

The first example of poly(isothianaphthene) is reported by Wudl *et al.* in 1984 (Wudl et al., 1984) as shown in Scheme 4. Poly(isothianaphthene) has a greater tendency to adopt the quinoid structure due to the stabilization of the benzenoid ring formation (Chart 8). The quinoid structure adoption reduces the band gap of poly(isothianaphthene) to *ca.* 1.0 eV,

Chart 4. 3-substituted thiophene dimer isomers, regioregular P3HT and regioirregular P3HT

Regioregular P3HT was first synthesized by McCullough's group via a Grignard metathesis method (McCullough & Lowe, 1992, 1993). Polymerization with a Ni(0) catalyst yielded a highly regioregular (>99% HT) PT polymer (Mn=20000-35000, PDI=1.2-1.4). The mechanism of this Ni coupling reaction was proposed and justified to be a 'living' chain growth mechanism (Miyakoshi et al., 2005). Regioregular P3HT has been treated as a standard polymer for solar cell devices and commonly used for device testing and comparison. It represents the 'state of art' in the field of PSCs and efficiency up to ~5% has been reported based on P3HT/PCBM device (Ma et al., 2005).

### **3.5 Silole**

Siloles or silacyclopentadienes, are a group of five-membered silacyclics with 4 accessible substitution positions on the butadiene and another 2 positions on the silicon atom. Compared with many other 5-membered heterocyclopentadiene, such as thiophene, furan or pyrrole, the silole (silacyclopentadiene) ring has the smallest HOMO-LUMO band gap and the lowest lying LUMO level due to the \* to \* conjugation arising from interaction between the \* orbital of two exocyclic bonds on silicon and the \* orbital of the butadiene moiety. The small band gap and lowest LUMO energy level render silole outstanding optoelectronic properties such as high PL efficiency and excellent electron mobility (Chen & Cao, 2007).

Random and alternating silole-containing copolymers P18 (Chart 5) (F. Wang et al., 2005) were synthesized via Suzuki coupling reaction from fluorene and 2,5-dithienyl-silole. The band gap of this series of polymer could be tuned from 2.95 eV to 2.08 eV by varying the silole content from 1% to 50% in the polyfluorene chain. The decrease of the band gap was found mainly due to the decrease of LUMO energy level while the HOMO of this series of polymer remained unchanged at ~ -5.7 eV. For the alternating copolymer, field effect charge mobility was measured to be 4.5x10-5 cm2V-1s-1 and the best PCE value was reported to be 2.01% using a P18(m=1)/PCBM (1:4, *w/w*) blend as active layer.

Chart 5. Chemical structure of silole containing polymer P18

Chart 4. 3-substituted thiophene dimer isomers, regioregular P3HT and regioirregular P3HT Regioregular P3HT was first synthesized by McCullough's group via a Grignard metathesis method (McCullough & Lowe, 1992, 1993). Polymerization with a Ni(0) catalyst yielded a highly regioregular (>99% HT) PT polymer (Mn=20000-35000, PDI=1.2-1.4). The mechanism of this Ni coupling reaction was proposed and justified to be a 'living' chain growth mechanism (Miyakoshi et al., 2005). Regioregular P3HT has been treated as a standard polymer for solar cell devices and commonly used for device testing and comparison. It represents the 'state of art' in the field of PSCs and efficiency up to ~5% has been reported

Siloles or silacyclopentadienes, are a group of five-membered silacyclics with 4 accessible substitution positions on the butadiene and another 2 positions on the silicon atom. Compared with many other 5-membered heterocyclopentadiene, such as thiophene, furan or pyrrole, the silole (silacyclopentadiene) ring has the smallest HOMO-LUMO band gap and the lowest lying LUMO level due to the \* to \* conjugation arising from interaction between the \* orbital of two exocyclic bonds on silicon and the \* orbital of the butadiene moiety. The small band gap and lowest LUMO energy level render silole outstanding optoelectronic properties such as high PL efficiency and excellent electron mobility (Chen &

Random and alternating silole-containing copolymers P18 (Chart 5) (F. Wang et al., 2005) were synthesized via Suzuki coupling reaction from fluorene and 2,5-dithienyl-silole. The band gap of this series of polymer could be tuned from 2.95 eV to 2.08 eV by varying the silole content from 1% to 50% in the polyfluorene chain. The decrease of the band gap was found mainly due to the decrease of LUMO energy level while the HOMO of this series of polymer remained unchanged at ~ -5.7 eV. For the alternating copolymer, field effect charge mobility was measured to be 4.5x10-5 cm2V-1s-1 and the best PCE value was reported to be

2.01% using a P18(m=1)/PCBM (1:4, *w/w*) blend as active layer.

Chart 5. Chemical structure of silole containing polymer P18

based on P3HT/PCBM device (Ma et al., 2005).

**3.5 Silole** 

Cao, 2007).

### **3.6 Benzothiadiazole/ Aza-benzothiadiazole/ Se-benzothiadiazole**

2,1,3-benzothiadiazole (BT, Chart 6) is an electron deficient heterocycle that has been incorporated with electron donating species to construct low band gap polymer donor for BHJ polymer solar cell. The electron withdrawing ability of BT can be further increased by replacing one carbon atom with *sp*2-hybridized N atom (Chart 6). The sulfur atom in the BT unit can also be replaced to selenium, by doing so the band gap is further decreased.

Chart 6. Structure of BT, aza-BT and Se-BT

A variety of low band gap polymers containing BT have been synthesized and tested for PSC performance (P19-P23) (Zhang et al., 2006; E. Zhou et al., 2008; Svensson et al., 2003; Slooff et al., 2007; Q. Zhou et al., 2004; Blouin et al., 2007). The electron donating moiety in the low band gap polymer varies from carbazole, fluorene, dibenzosilole, bridged thiophene-phenylene-thiophene, and dithienopyrrole. This type of polymer has a band gap <2.0 eV and gives moderate to good PCE value ranging from ~1% to ~5%, rendering a promising direction for the research of PSC donor material.

Chart 7. Benzothiadiazole containing low band gap polymers P19-P23

### **3.7 Isothianaphthene/ Thienopyrazine**

The first example of poly(isothianaphthene) is reported by Wudl *et al.* in 1984 (Wudl et al., 1984) as shown in Scheme 4. Poly(isothianaphthene) has a greater tendency to adopt the quinoid structure due to the stabilization of the benzenoid ring formation (Chart 8). The quinoid structure adoption reduces the band gap of poly(isothianaphthene) to *ca.* 1.0 eV, which is about half that of polythiophene (~2.0 eV) (Kobayashi et al., 1984).

Scheme 4. Synthesis of poly (isothianaphthene)

Conjugated Polymers for Organic Solar Cells 465

Annulations of two thiophene rings generates 4 isomers (Chart 10), namely, thieno[3,4-b] thiophene, thieno[3,2-b]thiophene, thieno[2,3-b] thiophene and thieno[3,4-c]thiophene. The first three isomers have been synthesized and isolated. Thieno[3,4-c]thiophene is predicted

Chart 10. Isomer structure of thienothiophenes; from left to right: thieno[3,4-b]thiophene,

Thieno[2,3-b] thiophene, thieno[3,2-b]thiophene and thieno[3,4-b]thiophene are useful building blocks in preparing conjugated polymer for organic electronics applications due to their planarity and electron richness. Compared with thiophene, thienothiophene has a larger -conjugated system. Therefore, it is introduced to the polymer backbone in the hope that it can facilitate interchain -stacking to increase the structural order and improve the

An efficient polymer donor P27 copolymerized by thieno[3,4-b]thiophene and benzodithiophene has been reported (Liang et al., 2009). Three dodecyl chains were used in each repeating unit to assist solvation of the polymer. BHJ solar cell fabricated using P27/PCBM(1:1 *w/w*) gave an excellent PCE of 4.76%, with *V*oc = 0.58V, *J*sc = 12.5 mA/cm2 and *FF* = 0.654. A further improvement of the PCE to 5.3% was obtained by utilizing PC70BM as the electron acceptor in the active layer. This high PCE value was ascribed to several factors including well tuned band gap (1.6 eV), proper HOMO/LUMO energy levels, balanced carrier mobility (P27 h=4.5x10-4cm2V-1s-1), favorable morphology of the active layer and thieno-thiophene's ability of stabilizing the quinoid structure along the polymer backbone to

Chart 11. Chemical structures of thienothiophene containing polymer P27, P28 and P29

Liquid-crystalline semiconductor polymers P28 and P29 were prepared by copolymerization of thienothiophene and thieno[2,3-b]thiophene, respectively, with 4,4'-dialkylbithiophene unit (McCulloch et al., 2006; Heeney et al., 2005). P28 had good field effect charge mobility of h = 0.15 cm2V-1s-1. However, its relatively large band gap (absorption maximum max = 470 nm) limited its application as efficient solar cell material. For P29, the absorption maximum was red shifted to 547 nm and the field effect charge mobility was increased to

**3.8 Thieno[3,4-b]thiophene / Thieno[3,2-b]thiophene/ Thieno[2,3-b]thiophene** 

to be kinetically unstable and not isolated yet (Rutherford et al., 1992).

thieno[3,2-b]thiophene, thieno[2,3-b]thiophene and thieno[3,4-c]thiophene

charge carrier mobility.

enhance the planarity of the polymer.

Poly(thianaphthene) adopts a non-planar conjugation due to the steric hinderance present between benzo-H and thiophene-S atoms as shown in Chart 8. To increase the effective conjugation, one C-H can be replaced by *sp*2-hybridized nitrogen to release the steric strain. As evidenced by X-ray structure analysis of 2,5-di(2-thienyl) pyridino[c]thiophene (Chart 8) (Ferraris et al., 1994), the torsional angle between the pyridinothiophene moiety and the thiophene unit on the N side is only 3.5o, while it is 39o on the other side. To further release the steric strain, the CH groups on both sides of the isothianaphthene can be replaced by N atom. Due to its effective conjugation and electron withdrawing nature, thieno[3,4 b]pyrazine is proposed as another type of electron withdrawing building block for the construction of low band gap polymers. In necessity of solubility, substituted thieno[3,4 b]pyrazines can be synthesized by condensation of 3,4-diaminothiophene with substituted 1,2-diones.

Chart 8. Resonance structure of poly(isothianaphthene) and demonstration of steric strains

The thienopyrazine unit is commonly linked to two thiophene rings at each side and coupled with electron donating moiety, such as fluorene to construct low band gap polymer (P24, P25, and P26) (Zhang et al., 2005, 2006; Mammo et al., 2007). P24 was reported to suffer from low solubility and low molecular weight. The polymerization yield was only 5% owing to the poor solubility in chloroform. The best PCE based on P24/PCBM(1:6, *w/w*) was obtained as = 0.96%. To address the solubility issue, alkyl and alkoxyl chains were attached to the thienopyrazine moiety and another two low band gap polymers P25 and P26 were synthesized by copolymerization between thienopyrazine and fluorene. The addition of side chains did not change the band gap and HOMO/LUMO energy level as evidenced from absorption spectra and cyclic voltammetry measurement. These two polymers had almost identical absorption and HOMO/LUMO values. The best PCE value obtained for P25 was 1.4% while for P26 the optimal PCE value was 2.2%.

Chart 9. Chemical structures of thienopyrazine containing polymers P24, P25 and P26

Poly(thianaphthene) adopts a non-planar conjugation due to the steric hinderance present between benzo-H and thiophene-S atoms as shown in Chart 8. To increase the effective conjugation, one C-H can be replaced by *sp*2-hybridized nitrogen to release the steric strain. As evidenced by X-ray structure analysis of 2,5-di(2-thienyl) pyridino[c]thiophene (Chart 8) (Ferraris et al., 1994), the torsional angle between the pyridinothiophene moiety and the thiophene unit on the N side is only 3.5o, while it is 39o on the other side. To further release the steric strain, the CH groups on both sides of the isothianaphthene can be replaced by N atom. Due to its effective conjugation and electron withdrawing nature, thieno[3,4 b]pyrazine is proposed as another type of electron withdrawing building block for the construction of low band gap polymers. In necessity of solubility, substituted thieno[3,4 b]pyrazines can be synthesized by condensation of 3,4-diaminothiophene with substituted

Chart 8. Resonance structure of poly(isothianaphthene) and demonstration of steric strains The thienopyrazine unit is commonly linked to two thiophene rings at each side and coupled with electron donating moiety, such as fluorene to construct low band gap polymer (P24, P25, and P26) (Zhang et al., 2005, 2006; Mammo et al., 2007). P24 was reported to suffer from low solubility and low molecular weight. The polymerization yield was only 5% owing to the poor solubility in chloroform. The best PCE based on P24/PCBM(1:6, *w/w*) was obtained as = 0.96%. To address the solubility issue, alkyl and alkoxyl chains were attached to the thienopyrazine moiety and another two low band gap polymers P25 and P26 were synthesized by copolymerization between thienopyrazine and fluorene. The addition of side chains did not change the band gap and HOMO/LUMO energy level as evidenced from absorption spectra and cyclic voltammetry measurement. These two polymers had almost identical absorption and HOMO/LUMO values. The best PCE value obtained for P25 was

Chart 9. Chemical structures of thienopyrazine containing polymers P24, P25 and P26

1.4% while for P26 the optimal PCE value was 2.2%.

1,2-diones.

### **3.8 Thieno[3,4-b]thiophene / Thieno[3,2-b]thiophene/ Thieno[2,3-b]thiophene**

Annulations of two thiophene rings generates 4 isomers (Chart 10), namely, thieno[3,4-b] thiophene, thieno[3,2-b]thiophene, thieno[2,3-b] thiophene and thieno[3,4-c]thiophene. The first three isomers have been synthesized and isolated. Thieno[3,4-c]thiophene is predicted to be kinetically unstable and not isolated yet (Rutherford et al., 1992).

Chart 10. Isomer structure of thienothiophenes; from left to right: thieno[3,4-b]thiophene, thieno[3,2-b]thiophene, thieno[2,3-b]thiophene and thieno[3,4-c]thiophene

Thieno[2,3-b] thiophene, thieno[3,2-b]thiophene and thieno[3,4-b]thiophene are useful building blocks in preparing conjugated polymer for organic electronics applications due to their planarity and electron richness. Compared with thiophene, thienothiophene has a larger -conjugated system. Therefore, it is introduced to the polymer backbone in the hope that it can facilitate interchain -stacking to increase the structural order and improve the charge carrier mobility.

An efficient polymer donor P27 copolymerized by thieno[3,4-b]thiophene and benzodithiophene has been reported (Liang et al., 2009). Three dodecyl chains were used in each repeating unit to assist solvation of the polymer. BHJ solar cell fabricated using P27/PCBM(1:1 *w/w*) gave an excellent PCE of 4.76%, with *V*oc = 0.58V, *J*sc = 12.5 mA/cm2 and *FF* = 0.654. A further improvement of the PCE to 5.3% was obtained by utilizing PC70BM as the electron acceptor in the active layer. This high PCE value was ascribed to several factors including well tuned band gap (1.6 eV), proper HOMO/LUMO energy levels, balanced carrier mobility (P27 h=4.5x10-4cm2V-1s-1), favorable morphology of the active layer and thieno-thiophene's ability of stabilizing the quinoid structure along the polymer backbone to enhance the planarity of the polymer.

Chart 11. Chemical structures of thienothiophene containing polymer P27, P28 and P29

Liquid-crystalline semiconductor polymers P28 and P29 were prepared by copolymerization of thienothiophene and thieno[2,3-b]thiophene, respectively, with 4,4'-dialkylbithiophene unit (McCulloch et al., 2006; Heeney et al., 2005). P28 had good field effect charge mobility of h = 0.15 cm2V-1s-1. However, its relatively large band gap (absorption maximum max = 470 nm) limited its application as efficient solar cell material. For P29, the absorption maximum was red shifted to 547 nm and the field effect charge mobility was increased to

Conjugated Polymers for Organic Solar Cells 467

solar cell. Therefore, fluorene is normally copolymerized with electron withdrawing moieties to construct polymers with band gap <2.0 eV so as to extend sunlight harvesting to longer wavelength. Although some solar cell polymers have been prepared by copolymerization of fluorene and electron-rich moieties, such as thienothiophene and pentacene, their absorption behaviors and wide band gaps are found to account for the moderate to poor performance (Schulz et al., 2009; Okamoto et al., 2008). Palladium catalyzed cross coupling reaction is normally adopted for the polymerization due to the ease of halogenation at the 2,7-position of fluorene unit. Alkynation at the 9-position of the fluorene assists solvation for the D-A type polymer, whereas necessary, alkynation on the electron deficient counterpart is also required. Fluorene copolymers prepared from electron

By replacing two benzene rings of fluorene with thiophene, cylcopenta[2,1-b:3,4-b'] dithiophene can be obtained as another novel building block to construct D-A type low band gap polymer. Alkynation at the bridge *sp3*-carbon renders solubility for the polymer. Cyclopenta[2,1-b:3,4-b']dithiophene based polymer P33 has been synthesized with a low band gap of *ca.* 1.4 eV (Mühlbacher et al., 2006). It was utilized by Kim *et al.* (Kim et al., 2007) to fabricate an efficient tandem solar cell. This brilliant and excellent work addressed the issue that while most low band gap polymers absorb at wavelength longer than 700 nm, there is a hollow at shorter wavelength and the lack of sufficient absorption at the hollow will drag the power conversion efficiency. In this case, P33 had an absorption maximum at *ca.* 800nm and a hollow at *ca.* 450nm. Kim et al. fabricated a tandem BHJ solar cell by utilizing P3HT ( max =~ 550nm) to absorb at the hollow of P33 and low band gap polymer P33 to absorb light at the NIR region. Tandem solar cell device (Al/TiOx/P3HT:PC70BM/PEDOT:PSS/TiOx/P33:PCBM/PEDOT:PSS/ITO/glass) based on P3HT and P33 gave a typical performance parameter of *J*sc = 7.8 mA/cm2, *V*oc = 1.24 V,

Silafluorene and dithieno[3,2-b:2',3'-d]silole are two interesting electron rich moieties that are structurally analogous to fluorene. Low band gap polymer P34 was synthesized by copolymerization of 2,7-silafluorene and dithienyl-benzothiadiazole (E. Wang et al., 2008). Field effect charge mobility of P34 was found to be ~1x10-3 cm2V-1s-1. High efficiency up to 5.4% with *V*oc = 0.9 V, *J*sc = 9.5 mA/cm2, *FF* = 0.51 was obtained by using P34/PCBM(1:2 *w/w*) as active layer. Polymer P35 was synthesized by Stille coupling between dithieno[3,2 b:2',3'-d]silole and benzothiadiazole (Hou et al., 2008). The optical band gap of P35 was found to be 1.45 eV, which was similar to that of P33. Hole transport mobility of the polymer was determined to be 3 x 10-3 cm2V-1s-1, about 3 times higher than that of P33. The best device based on P35 gave a PCE of 5.1% with *J*sc = 12.7 mA/cm2, *V*oc=0.68 V and *FF* = 0.55.

deficient benzothiadiazole and thienopyrazine have been discussed previously.

*FF* =0.67 and PCE= 6.5%, which was among the highest values reported.

Chart 13. Structures of low band gap polymers P33, P34 and P35

h = ~0.7 cm2V-1s-1. The improved mobility was suggested due to the improved control of crystallization. The PSC device fabricated from P29/PC70BM(1:4 *w/w*) blend achieved an optimized PCE = 2.3% in nitrogen atmosphere. The high lying HOMO energy level (-5.1eV) of P29, which is above the air oxidation threshold (-5.2 eV), makes the polymer relatively unstable in air.
