**2.3. New synthetic methodology**

a significant role in determining the overall efficiency of the cell. These factors will be discussed

**Sheme 1.** Synthetic routes of 4,7-dibromobenzo[c][1,2,5]thiadiazole and alkylated 4,8-dibromo-[1,2,5]thiadiazole[3,4-

n

S

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

Ge

R1 R1

S

**PDTG-TPD** PCE = 7.3-8.5%

S

C6H13

C6H13

n

S

C6H13

**PIDTT-DFBT** PCE = 7.4%

S

C6H13

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

F F n

S

n

N S N Br Br

> N S N Br Br

R-Br

HN N N

S S

**PDTP-DFBT** PCE = 7.9%

S

n

OR1

S

C6H13

n

**P3HT** PCE = 7.4%

OR1

O R R

S

**PBDTTT-CF** PCE = 7.7%

S

S

O R2

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

F F

N S N Br Br N N N R

n

<sup>N</sup> <sup>O</sup> <sup>O</sup> R2

S

R1

C6H13

S

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

**PBDT-DTNT** PCE = 8.4%

S

in the following text.

f]benzotriazole.

N S N Br Br

364 Solar Cells - New Approaches and Reviews

fuming HNO3

S

**PTB7** PCE = 7.4-9.2%

S

**PBDTTPD** PCE = 7.1-8.5%

S

R2

R1

S

OR1

OR1

S

F

S

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

F

S COOR S

<sup>S</sup> <sup>n</sup>

S S

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

**Figure 4.** Chemical structures of polymers that exhibit PCE > 7%.

F

S

S

R2

R1

R2

R1

**BFS4** PCE = 7.4%

N O O R2

n

S S S

**PBDTT-C-T** PCE = 7.6-8.8%

S

S S

n

S

C6H13

R1

S

R2

C10H21 C10H21

S R1

H2N NH2 N

Fe HNO3

N S N Br Br O2N NO2

S N SOCl2 Br2, HBr

> N S N Br Br H2N NH2

NaNO2 AcOH

S

O R2

S

S C10H21 C10H21

OR

OR

Currently, the D-A type polymers are typically synthesized via palladium catalyzed cross coupling reactions such as Stille coupling [88] and Suzuki coupling reactions [89]. Stille coupling involves C-C bond formation between trialkylstannyl species and aromatic halide species and has been routinely used for the preparation of a large number of high performance polymers. However, the high toxicity of the tin reagent and the associated environmental issue of the generated tin wastes inhibit its wide industrial applications. Recently, a new polymerization method involving direct heteroarylation polymerization (DHAP) between aryl C-H bond and aromatic halides has been developed as a promis‐ ing greener alternative of Stille coupling for the preparation of conjugated polymers (Scheme 2). Berrouard et al. [90] has demonstrated that the DHAP reaction between 5-alkyl[3,4 c]thienopyrrole-4,6-dione and 5,5'-dibromo-4,4'-dioctyl-2,2'-bithiophene is as efficient as the corresponding Stille approach. As in this direct coupling reaction no organo-tin or organo boron reagents are needed, it shortens the synthesis of final polymer by at least two steps. This strategy has been successfully implemented for the synthesis of OPV polymers [91,92], OFET polymers [92] and EC polymers [93] with reasonable molecular weight and polydis‐ persity after judicious optimization of the coupling condition. Nevertheless, as this polymerization technique is still in its infancy, the reaction is still difficult to control for some substrates and the final polymer might be branched due to unselective C-H activa‐ tion in the substrate [94,95]. The reaction conditions of the reaction including the catalyst, ligand, base, additive, solvent, temperature and duration have to be carefully controlled and optimized in order to achieve the highest molecular weight.

**Sheme 2.** Synthetic approaches of direct heteroarylation polymerization (DHAP) and conventional Stille coupling reac‐ tion.

### **2.4. Molecular weight and purity of the polymer**

The molecular weight and the purity of the polymers are issues beyond the molecular architecture of the semiconducting polymers. But both factors have been demonstrated as essential parameters to ensure the good performance of the prepared polymers within the device. A high molecular weight increases the regularity of thin film and in many cases induces enhanced charge carrier transport in the transistor device [96,97] and power conversion efficiency in the BHJ solar cell device [98]. For instance, **P1** (Figure 5) [99,100] with a low molecular weight (Mn < 10 kg mol-1) exhibits a charge carrier mobility of *µ*=5.2 × 10-5 cm2 V-1 s-1 and power conversion efficiency of η=2.7% with *J*sc=4.2 mA cm-2, *V*oc=0.64 V, and FF=0.35. For **P1** with high molecular weight (Mn > 34 kg mol-1), it exhibits an enhanced mobility of *µ*=3.6 × 10-2 cm2 V-1 s-1 and power conversion efficiency of η=5.9% with *J*sc=17.3 mA cm-2, *V*oc=0.57 V, and FF=0.61. Similar phenomenon is also observed for **P2** [98]. **P2** with a low molecular weight (Mn ~ 46 kg mol-1) exhibits an ambipolar behavior with *µ*h=2 × 10-3 cm2 V-1 s-1 and *µ*e=5.2 × 10-5 cm2 V-1 s-1 and a PCE η=5.48% with *J*sc=12.1 mA cm-2, *V*oc=0.90 V, and FF=0.50. For **P2** with high molecular weight (Mn ~ 61.8 kg mol-1), the mobility increases to *µ*h=0.15 cm2 V-1 s-1 and *µ*e=0.064 cm2 V-1 s-1 and an enhanced PCE η=6.79% with *J*sc=13.7 mA cm-2, *V*oc=0.89 V, and FF=0.56. The improved mobility for high molecular weight samples is ascribed to improved π-π stacking, thin-film formation properties and increased inter-chain interactions. The increased *J*sc and fill factor are mainly because of the improved hole mobility of the polymer, which facilitates the charge collection and inhibit charge recombination in the blend.

The purity [101-103] and the end group effect [104-106] on the performance of transistor materials and OPV materials have also been investigated. However, as the exact determination of "contaminant" or "purity level" of a given material, especially for polymers, is very difficult to achieve, the attempts to correlate the performance of an "impure" material to the existence of some extrinsic impurity would be questionable. Even though the end capping strategy has been found efficient to improve the performance of the polymer [104-106], it is still not commonly adopted by research groups, even not routinely used by the groups who claimed the positive effect. Questions such as how the end group influences the performance of the polymer, what kinds of impurities are detrimental to the performance and what kinds of impurities serve as friendly dopants still remain unaddressed. More research effort, for example, intentional doping [107,108], is in need to solve the impurity issue of organic semiconductors in both the theoretical aspect and the practical aspect. But it is commonly believed that tedious and labor-intensive purification processes, such as Soxhlet extraction and silica gel column chromatography is always necessary to ensure sufficient purity of the sample for characterization.

**Figure 5.** Chemical structures of **P1** and **P2**.
