**3.1 Ethylene (double bond)**

Ethylene (double bond) is a commonly adopted spacer or bridge in conjugated polymers. Common chemical methods to introduce double bond to the polymer include: Wittig-Horner reaction; Wessling sulfonium precursor method (Wessling, 1985); Gilch route (Gilch & Wheelwright, 1966) and palladium catalyzed coupling reactions.

By utilizing Wittig reaction, fully regioregular and regiorandom poly[(2-methoxy-5- ((3',7' dimethyloctyl)oxy)-1,4-phenylenevinylene] (MDMO-PPV, P1 and P2) were synthesized following the route shown in Scheme 1 (Tajima et al., 2008). The device study on these two polymers showed that the regioregular MDMO-PPV-based device had a PCE of 3.1%, which was much higher than 1.7% out of regiorandom MDMO-PPV. The higher crystallinity of the regioregular MDMO-PPV polymer and better mixing morphology with PCBM were ascribed to the improved PCE for regioregular MDMO-PPV. This study highlighted the importance of regioregularity of PPV-based polymer to achieve good solar cell performance.

### **3.2 Acetylene (triple bond)**

Polyacetylene was the first discovered conducting conjugated polymer and inspired a lot of scientific interest in the research of conjugated polymers (Shirakawa et al., 1977). The synthetic chemistry of acetylene-containing polymers has been intensively reviewed by Liu *et al*.(Liu et al., 2009). In polymers designed for solar cell, acetylene is normally introduced to the polymer backbone via Sonogashira cross coupling reaction.

Conjugated Polymers for Organic Solar Cells 459

Benzodifuran moiety was copolymerized with thiophene, electron withdrawing benzothiadiazole and electron donating 9-phenylcarbazole, respectively, to form P3, P4 and P5 as shown in Scheme 2 (H. Li et al., 2010). The ratio of x/y is estimated from the integration of relevant peaks in their NMR spectra. The fraction of benzodifuran is more than 50% due to the self-coupling of diacetylene monomer. All three polymers absorbed beyond 600 nm in the film state and had a LUMO level above -4.0 eV. The high structural order of these three polymers was evidenced by power XRD study, as two reflection peaks, one at 2 = 4.95o–5.5o and the other at 2 = 19.95o – 21.75o, were well observed. The highest

Another category of acetylene containing polymer designed for PSC is -conjugated organoplatinum polyyne polymers (Baek et al., 2008). The platinum-C*sp* bond extends the conjugation of the polymer as a result of the fact that the *d*-orbital of the Pt can overlap with the *p*-orbital of the alkyne. This kind of Pt-C bond can be chemically accessible by a Sonogashira-type dehydrohalogenation between alkyne and platinum chloride precursor. Examples of this type of polymer and their synthetic routes are shown in Scheme 3 (Wong et

In order to tune the energy gap <2.0 eV, internal D-A function was introduced between electron rich Pt-ethyne groups. This effective band gap control strategy rendered P6 UV-vis absorption maximum at 548 nm and an optical band gap of 1.85 eV. This absorption behavior was proved to occur via the charge transfer excited state but not the triplet state of the polymer by photolumiscence lifetime study and PL temperature dependence study. The electrochemical HOMO and LUMO energy level were measured to be -5.37 eV and -3.14 eV, respectively. The best P6/PCBM (1:4, *w/w*) BHJ solar cell gave the open circuit voltage *V*oc= 0.82 V, the short-circuit current density *J*sc=15.43 mA, fill factor *FF*=0.39 and power

For polymers P7-P10 (Wong et al., 2007), the electron withdrawing moiety was replaced by bithiazole heterocycles. Nonyl chains were attached to the bithiazole to assist solvation of the polymer. By extending the conjugation (m: 03) along the polymer backbone, the band gaps of P7-P10 decreased from 2.40 eV to 2.06 eV. The power conversion efficiency (polymer/PCBM=1:4, *w/w*) was found significantly improved from ~0.2% to ~2.5% as the number of thiophene bridge increased from 0 to 3, most likely due to the improved charge

Scheme 2. Synthetic route of acetylene-containing polymers P3, P4 and P5

PCE = 0.59% was obtained based on P3/PCBM (1:4, *w/w*) blend.

al., 2007).

conversion efficiency =4.93%.

carrier mobility of the active layer.

Chart 1. Common monomer building blocks used for construction of solar cell polymers

Scheme 1. synthesis of regioregular and regiorandom MDMO-PPV

Se

N

N N N

<sup>S</sup> <sup>N</sup> <sup>N</sup>

N S S

> N N

R

S S S S

N S N N S N N S N

S S

<sup>N</sup> Se <sup>N</sup>

Si R R

<sup>N</sup> <sup>S</sup> <sup>N</sup> <sup>S</sup>

S

S

NC CN

R R O

Si R R

O

O

O O

N

R

O

N

S S

O R

Si S S

R R

N N S

> S S

S N R

**0**

**1**

**2**

**2'**

**3**

**3'**

**3''**

<sup>N</sup> <sup>S</sup> <sup>N</sup> <sup>N</sup> <sup>O</sup> <sup>N</sup>

N

R R

N R

S

N

R R

S

Scheme 1. synthesis of regioregular and regiorandom MDMO-PPV

R

N

S S S

R

R R

Chart 1. Common monomer building blocks used for construction of solar cell polymers

S

S

S S

Scheme 2. Synthetic route of acetylene-containing polymers P3, P4 and P5

Benzodifuran moiety was copolymerized with thiophene, electron withdrawing benzothiadiazole and electron donating 9-phenylcarbazole, respectively, to form P3, P4 and P5 as shown in Scheme 2 (H. Li et al., 2010). The ratio of x/y is estimated from the integration of relevant peaks in their NMR spectra. The fraction of benzodifuran is more than 50% due to the self-coupling of diacetylene monomer. All three polymers absorbed beyond 600 nm in the film state and had a LUMO level above -4.0 eV. The high structural order of these three polymers was evidenced by power XRD study, as two reflection peaks, one at 2 = 4.95o–5.5o and the other at 2 = 19.95o – 21.75o, were well observed. The highest PCE = 0.59% was obtained based on P3/PCBM (1:4, *w/w*) blend.

Another category of acetylene containing polymer designed for PSC is -conjugated organoplatinum polyyne polymers (Baek et al., 2008). The platinum-C*sp* bond extends the conjugation of the polymer as a result of the fact that the *d*-orbital of the Pt can overlap with the *p*-orbital of the alkyne. This kind of Pt-C bond can be chemically accessible by a Sonogashira-type dehydrohalogenation between alkyne and platinum chloride precursor. Examples of this type of polymer and their synthetic routes are shown in Scheme 3 (Wong et al., 2007).

In order to tune the energy gap <2.0 eV, internal D-A function was introduced between electron rich Pt-ethyne groups. This effective band gap control strategy rendered P6 UV-vis absorption maximum at 548 nm and an optical band gap of 1.85 eV. This absorption behavior was proved to occur via the charge transfer excited state but not the triplet state of the polymer by photolumiscence lifetime study and PL temperature dependence study. The electrochemical HOMO and LUMO energy level were measured to be -5.37 eV and -3.14 eV, respectively. The best P6/PCBM (1:4, *w/w*) BHJ solar cell gave the open circuit voltage *V*oc= 0.82 V, the short-circuit current density *J*sc=15.43 mA, fill factor *FF*=0.39 and power conversion efficiency =4.93%.

For polymers P7-P10 (Wong et al., 2007), the electron withdrawing moiety was replaced by bithiazole heterocycles. Nonyl chains were attached to the bithiazole to assist solvation of the polymer. By extending the conjugation (m: 03) along the polymer backbone, the band gaps of P7-P10 decreased from 2.40 eV to 2.06 eV. The power conversion efficiency (polymer/PCBM=1:4, *w/w*) was found significantly improved from ~0.2% to ~2.5% as the number of thiophene bridge increased from 0 to 3, most likely due to the improved charge carrier mobility of the active layer.

Conjugated Polymers for Organic Solar Cells 461

Buckminsterfullerene, C60, on a picosecond time scale (Sariciftci et al., 1992). This experiment explained one fundamental physical phenomenon present in organic photovoltaic cells and the concept developed by this study significantly inspired later research on organic solar

Another derivative of PPV, poly[(2-methoxy-5-(3',7'-dimethyloctyl)oxy)-1,4-phenylene vinylene] (MDMO-PPV, Chart 2) is also widely studied for solar cells and still being used nowadays. The combination of MDMO-PPV and PCBM is used in BHJ solar cell and efficiency up to 3.1% has been reported (Tajima et al., 2008). However, the relatively low hole mobility of MDMO-PPV (5 x 10-11cm2V-1s-1) (Blom et al., 1997) is reported to limit the charge transport inside the photoactive layer. Most PPV polymers have band gap greater than 2.0 eV and have maximum absorption around 500 nm. Furthermore, PPV materials are not stable in air and vulnerable to oxygen attack. Structural defects generated either during synthesis or by oxidation will substantially degrade the performance of the device. All these

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

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

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.

inductive and/or mesomeric effect from the heteroatom containing substitution.

factors limit the application of PPV polymers in solar cells.

Chart 3. Chemical structures of PEDOT:PSS and P3HT

organic field effect transistors (OFETs) and PSCs.

cells.

**3.4 Thiophene** 

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