**3.9 Diketopyrrolopyrrole (DPP)**

The DPP moiety has been utilized for construction of low band gap polymer for BHJ solar cells due to its electron deficient nature, planarity of the core and ability to accept H-bonding. D-A type low band gap polymers based on DDP have been synthesized by varying the electron donating part of the polymer (P30, P31, P32) (Wienk et al., 2008; Bronstein et al., 2011).

By combining electron rich quarterthiophene with electron deficient DDP unit, a low band gap polymer (1.4 eV in film state) P30 was obtained. P30 showed a good solubility in chloroform and tended to aggregate in dichlorobenzene (DCB). Device based on P30/PC60BM (1:2, *w/w*) BHJ thin film prepared from solution in CHCl3/DCB (4:1 *v/v*) gave a PCE of 3.2%. By utilizing PC70BM as the acceptor in the active layer, an improved PCE of 4.0% was achieved under the same condition.

Chart 12. Chemical structures of DPP-containing polymers P30, P31 and P32

By replacing the thiophene unit with larger thieno[2,3-b] thiophene, P31 and P32 were prepared. Long branched chains have been incorporated at the DDP unit to assist solvation. Both polymers had band gap of ~1.4 eV and absorbed beyond 800 nm in the film state. Ambipolar charge transport behavior was found for both of the polymers. P31 had a high hole mobility of 0.04 cm2V-1s-1 and good PCE of 3.0% based on P31/PC70BM (1:2 *w/w*) thin film prepared from CHCl3/DCB (4:1 *v/v*) solution. By modifying the backbone with one more thiophene unit introduced to the repeating unit, P32 showed an even higher hole mobility of *ca.* 2 cm2V-1s-1 and the BHJ solar cell device fabricated under the same condition as that of P31 showed an improved efficiency up to 5.4%.

### **3.10 Fluorene/ cylcopenta[2,1-b:3,4-b'] dithiophene/ silafluorene/ dithieno[3,2-b:2',3'-d] silole**

Fluorene based polymers have been widely explored as organic electronic material in the field of OLED, OFET and PSC due to their high photoluminescence quantum yield, high thermal and chemical stability, good film-forming properties and good charge transport properties. Polyfluorene, however, has a band gap of ~3.0eV, which limits its application in

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

The DPP moiety has been utilized for construction of low band gap polymer for BHJ solar cells due to its electron deficient nature, planarity of the core and ability to accept H-bonding. D-A type low band gap polymers based on DDP have been synthesized by varying the electron donating part of the polymer (P30, P31, P32) (Wienk et al., 2008;

By combining electron rich quarterthiophene with electron deficient DDP unit, a low band gap polymer (1.4 eV in film state) P30 was obtained. P30 showed a good solubility in chloroform and tended to aggregate in dichlorobenzene (DCB). Device based on P30/PC60BM (1:2, *w/w*) BHJ thin film prepared from solution in CHCl3/DCB (4:1 *v/v*) gave a PCE of 3.2%. By utilizing PC70BM as the acceptor in the active layer, an improved PCE of

Chart 12. Chemical structures of DPP-containing polymers P30, P31 and P32

as that of P31 showed an improved efficiency up to 5.4%.

**dithieno[3,2-b:2',3'-d] silole** 

**3.10 Fluorene/ cylcopenta[2,1-b:3,4-b'] dithiophene/ silafluorene/** 

By replacing the thiophene unit with larger thieno[2,3-b] thiophene, P31 and P32 were prepared. Long branched chains have been incorporated at the DDP unit to assist solvation. Both polymers had band gap of ~1.4 eV and absorbed beyond 800 nm in the film state. Ambipolar charge transport behavior was found for both of the polymers. P31 had a high hole mobility of 0.04 cm2V-1s-1 and good PCE of 3.0% based on P31/PC70BM (1:2 *w/w*) thin film prepared from CHCl3/DCB (4:1 *v/v*) solution. By modifying the backbone with one more thiophene unit introduced to the repeating unit, P32 showed an even higher hole mobility of *ca.* 2 cm2V-1s-1 and the BHJ solar cell device fabricated under the same condition

Fluorene based polymers have been widely explored as organic electronic material in the field of OLED, OFET and PSC due to their high photoluminescence quantum yield, high thermal and chemical stability, good film-forming properties and good charge transport properties. Polyfluorene, however, has a band gap of ~3.0eV, which limits its application in

relatively unstable in air.

Bronstein et al., 2011).

**3.9 Diketopyrrolopyrrole (DPP)** 

4.0% was achieved under the same condition.

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 deficient benzothiadiazole and thienopyrazine have been discussed previously.

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

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.

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

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