**2.5. Acceptor**

S

**DHAP**

**Stille**

tion.

cm2

cm2

+

+

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

S

S

C8H17

(Mn ~ 46 kg mol-1) exhibits an ambipolar behavior with *µ*h=2 × 10-3 cm2

charge collection and inhibit charge recombination in the blend.

Me3Sn

C8H17

Br

S

S

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

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

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

 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

 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

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

C8H17

Br

C8H17

SnMe3

S

S

C8H17

S

C8H17

n

V-1 s-1

V-1 s-1 and *µ*e=5.2 × 10-5

<sup>N</sup> <sup>O</sup> <sup>O</sup> R

S

Br Br

<sup>N</sup> <sup>O</sup> <sup>O</sup> R

<sup>N</sup> <sup>O</sup> <sup>O</sup> R

366 Solar Cells - New Approaches and Reviews

The other important active species in the BHJ blend is the acceptor. The benchmark acceptors are fullerene based derivatives, mainly [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) (Figure 6) [109,110]. The key features of these sphere-shaped acceptors are their low internal reorganization energy, high polariza‐ bility, relatively high dielectric constant, favorable LUMO energy level, reversible redox properties, good electron transport properties and anisotropic charge transport behavior [109,110]. The superior performance of these two acceptors in the BHJ devices renders them as the first choice for most of the newly developed donor materials. Whereas PC61BM absorbs minimal amount of light in the visible region, PC71BM is strongly blue and green light absorbing acceptor and is more useful when the absorption of it is complementary to that of the donor so that more sunlight can be captured [111]. Nevertheless, some drawbacks of these fullerene derivatives would hamper their wide application in the industrial production. One is the high production energy cost of these fullerene based acceptors. For PC71BM, the pro‐ duction energy is approximately 90 GJ kg-1 [112]. For comparison, the production energy of P3HT is only about 1.9 GJ kg-1[113]. The other concern is the relative high price of these fullerene derivatives. A recent analysis by Lewis and Nocera [114] indicates that the OPV system should cost no more than \$10 per m2 to compete with fossil fuels for energy production. The cost of PCBM at roughly \$500-1000 per m2 [42] makes the BHJ based PV technology with PCBM problematic for commercialization. Use of technical grade PCBM (~80% PC61BM and 20% PC71BM) [115] might help relieve the stress but is far away from the desired price range. In fact, various types of small molecule based [116] or polymer based [117] acceptors have been tested to replace fullerene derivatives in OPV cells. But yet the efficiency of these acceptors still cannot surpass that of fullerene based derivatives.

**Figure 6.** Chemical structures of PC61BM and PC71BM.
