**3. Carbazole-based conjugated polymers**

Structurally analogous to FL, carbazole has been also known as 9-azafluorene. The central fused pyrrole ring makes tricyclic carbazole fully aromatic and electron-rich with its donating nitrogen. The alkyl chains introduced into nitrogen of carbazole improve the solubility. Carbazole derivatives have been widely used as electron-donor materials due to their excellent thermal and photochemical stabilities as well as relatively high charge mobility and good solubility with 9-position alkylation. When two carbazoles fused together

level as that of PFM, while these two polymers exhibit much lowered LUMO levels (-3.43 and -3.50eV for **P4** and **P5**, respectively) than PFM,. The devices with structure of ITO/PEDOT:PSS(40nm)/**P4-5**:PC71BM(1:4 w/w, 85nm)/Ca(10nm)/Al(100nm) exhibit a PCE of 4.74% and 4.37% for **P4** and **P5**, respectively (refer to Table 1). Determined by using space-charge-limited-current (SCLC) method, the hole mobility of these two polymer/P71BM blend films was found to be 5.27×10-4cm2V-1s-1 for **P4** and 1.16×10-3cm2V-1s-1 for **P5**, respectively, which is even higher than that of P3HT with similar device configuration (G. Li, 2005). These two-dimensional polymers are thought to possess better isotropic charge transport ability than linear polymers, which is beneficial for PSC applications. By replacing FL in **P4** and **P5** with silafluorene (2,7-dibenzosilole) unit, similarly structured two-dimensional conjugated polymers **P6** and **P7** were developed (Duan et al., 2010) developed. With an *Eg* of 1.83 and 1.74eV, **P6** and **P7** OSCs harvest high PCE as 2.5% and 3.51%, respectively, with device configuration ITO/PEDOT:PSS/**P6- 7**:PC71BM/Ba/Al. It should be noted that the silofluorene polymers display lower hole

By alternating silafluorene and DTBT units, copolymer **P8** was synthesized (Boudreault et al., 2007a). With an *Eg* of 1.82eV, **P8** films presented an absorption spectrum blend covered the range from 350 to 750nm. **P8** device displayed a PCE of 1.6% under AM 1.5 (90 mW/cm2) illumination with the structure ITO/PEDOT:PSS/**P8**:PCBM(1:4)/Al. By using similar polymer, **P8**:PCBM (1:2 w/w) blend film delivered as high as PCE of 5.4% under AM 1.5 (80 mW/cm2) (Zhou et al., 2004). By using DTBT as electron accepting unit, Leclerc (Allard et al., 2010) developed germafluorene based polymer **P9** whose device displayed a good PCE of 2.8% with the structure of ITO/PEDOT-PSS/**P9**:PC71BM/LiF(20nm)/Al. In order to extend π-conjugation of polymer backbone, the ladder-type oligo-*p*-phenylenes (indenofluorene) consisting of several "linearly overlapping" fluorene was developed. Compared to FL unit, indenofluorene presents a broader more intense absorption band. Solubilizing alkyl chains can be easily introduced into this unique molecular backbone, which may provide a better solution processability of the polymers. Zheng (Zheng et al., 2010) reported three alternating D-A copolymers (**P10-12**) combining indenofluorene as the donor and DTBT or 5,8-dithien-2-yl-2,3-diphenyl quinoxaline (DTQX) as the acceptor unit. By spin-coating from chlorobenzene (CB):o-dichlorobenzene(DCB) (4:1 v/v) co-solvent, **P10** with decylated indenofluorene presents good solubility and its device achieved 2.44% PCE with the configuration ITO/PEDOT:PSS/**P10**:PCBM/Cs2CO3/Al (Veldman et al., 2008). In comparison with **P10**, **P12** with one more phenylene group in indenofluorene unit presents greatly improved device PCE (3.67%) with the same configuration, due to great improvement of *Jsc*. By blending with PC71BM in 1:4 weight ratio, **P18** OSC showed significantly improved PCE (4.5%). The DTQX-containing **P11** presents a good PCE of

2.32%, which was attributed to broad absorption spectra and high *Jsc*.

Structurally analogous to FL, carbazole has been also known as 9-azafluorene. The central fused pyrrole ring makes tricyclic carbazole fully aromatic and electron-rich with its donating nitrogen. The alkyl chains introduced into nitrogen of carbazole improve the solubility. Carbazole derivatives have been widely used as electron-donor materials due to their excellent thermal and photochemical stabilities as well as relatively high charge mobility and good solubility with 9-position alkylation. When two carbazoles fused together

**3. Carbazole-based conjugated polymers** 

transport ability than FL-based **P4** and **P5**.

to form indolo[3,2-*b*]carbazole (IC) unit, it exhibits even stronger electron-donating properties, higher hole mobility and better stability than carbazole (Boudreault, 2007b; Li, 2006). With bulky heptadecanyl modified carbazole to improve solubility, Leclerc (Blouin et al., 2007a) developed DTBT-based conjugated poly(N-alkyl-2,7-carbazole) (PC) **P13** (**P13-26** structure in Chart 2), which exhibits excellent thermal stability, relatively high molecular weight and good solubility. Spin-coated from CF solution, **P13**:PCBM(1:4 w/w) OSC delivered a high PCE of 3.6% (Table 2). Such high PCE and excellent stability show its potential application in OSC. By using longer alkyl chain, Hashimoto (Zou et al., 2010) reported a similar PC **P14**, whose device based on **P14**:PCBM (1:3w/w) showed 3.05% PCE, with all photovoltaic parameters similar to those of **P13**. By using 4,7-dithien-2-yl-2,1,3 benzothiadiazoxaline (DTBX) as accepting unit, Leclerc (Blouin et al., 2007b) synthesized **P15**. Due to its symmetrical backbone, **P15** showed good structural organization, which leads to good hole mobility and thus resultant improvement of *Jsc* and FF for its OSC. **P15** based OSC achieved a good PCE of 2.4%.

Chart 2. Carbazole based narrow band gap polymers

In order to improve the solubility and close packing of polymer backbone, Zhang (Qin et al., 2009) designed **P16** with planar polymer conformation by introducing two octyloxy chains onto benzothiazole (BT) ring and an octyl chain onto carbazole ring. **P16** showed good solubility at elevated temperature and an *Eg* of 1.95eV. Spin-coated from DCB mixture solutions with 2.5% 1,8-diiodooctane (DIO), **P16**:PC71BM device showed extremely high

Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development 439

conjugated structure. Hashimoto (Zou et al., 2009) developed narrow band gap (1.90eV) PIC **P24** by alternating IC donating unit and DTBT accepting unit. The optimized **P24** device ITO/PEDOT:PSS/**P24**:PCBM1:3(w/w)/Al demonstrated a PCE of 2.07%. The study reveals that the formation of an interpenetrated network of donors and acceptors inside active layer is critical for obtaining high-performance OSCs. Such interpenetrated networks can be built by tuning device processing parameters such as the ratio of polymer to PCBM in the blend and post-annealing treatment at suitable temperatures. Besides, a simple and effective method to improve the photovoltaic performance of OSCs is to increase the number of unsubstituted thienyl units in D–A copolymers consisting of DTBT as acceptor. The *Voc* of **P24** is similar with those of some DTBT based narrow band gap polymers with donating units like FL (1.0eV) (Slooff et al., 2007) dibenzosilole (0.9–0.97eV) (Boudreault et al., 2007) or carbazole (0.88 eV) (Blouin et al., 2007) but higher than those of D–A derivatives with donating units such as cyclopenta[2,1-b:3,4-b']dithiophene (0.6eV), dithienosilole (0.44eV) (Liao et al., 2007) or dithieno[3,2-b:2',3'-d] pyrrole(0.52eV) (Zhou et al., 2008). By adopting DTTP as accepting unit, the same group developed **P25**, with an *Eg* of 1.91 Hashimoto (Zhou et al., 2010a). Spin-coating from CF solution, **P25**:PCBM (1:4, w/w) based OSC contributed a

PCE of 1.56% and was further improved to 2.54% by using DCB instead of CF.

ITO/PEDOT:PSS/**P26**:PCBM(1:2w/w)/LiF/Al achieved a high PCE of 3.6%.

PCs devices generally display low *Jsc* and FF, which was similar to PFs.

**4. Thiophene-based conjugated polymers** 

thermal and chemical stability (Roncali, 1992).

Chart 3. Thiophene-based narrow band gap polymers

By using hexylthiophene moiety in DTBT accepting unit and shorter alkyl chains in IC unit, Movileanu (Lu et al., 2008) developed **P24** analogous PIC **P26**. Long solubilizing side-chains generally resulted in lower charge mobility, balance of the alkyl chains in conjugated polymers was important for charge mobility. **P26** film showed pronounced peak broadening and significant red shift (100nm) in absorption spectra than its solution. **P26** device

As discussed above, both PC and PF exhibit relatively low-lying HOMO level and thus high *Voc* for OSCs via copolymerizing with accepting unit including DTBT, DTTP and DTQX. In contrast with other accepting units, DTBT-containing polymers may maintain deep HOMO and narrow the band gap through reducing LUMO, which results in a high *Voc*. However,

Polythiophenes (PTs) are considered as the most important conjugated polymers for a broad spectrum of optoelectronic applications such as light-emitting diodes, field-effect transistors, and OSCs due to their excellent optical and electrical properties as well as exceptional

PCE of 5.4% with the structure ITO/PEDOT:PSS/**P16**:PC71BM(1:2.5 w/w)/LiF/Al. By replacing the BT unit with benzotriazole, Cao (Zhang et al., 2010) developed PC **P17**, with an *Eg* of 2.18eV, which was broader than that of P3HT. The optimized device of **P17** achieved improved PCE from 1.51% to 2.75% by sandwiching a thin layer of PFN between Al cathode and active layer.

On the basis of successful design of two-dimensional PF **P4** and **P5**, Cao [Duan et al., 2010b] developed a series of carbazole-based two-dimensional polymers **P18-23**, with their *Eg* ranging from 1.7 to 1.9eV. **P18-23** present similar HOMO energy levels as the conjugated backbone while their LUMO levels were mainly determined by narrow band gap sidechains. Among them, **P22** and **P23** exhibited stronger absorption intensity and broader absorption spectra. The photovoltaic performance of **P22** and **P23** devices indicates the 2 ethylhexyl chains had little influence on the *Voc* of OSC while the bulky heptadecanyl chains on carbazole led to a significant decrease in *Jsc* and FF. In comparison with that of **P4** and **P5**, **P18** devices exhibited excellent PCE of 4.16%. **P18-23** OSCs possessed high *Voc* (>0.8eV) values, which was relevant with their relatively low HOMO levels. As known, *Voc* is related to the offset between the HOMO levels of donor materials and the LUMO levels of acceptor materials. The alkyl chains on carbazole unit had important influence on the holetransporting ability of the resultant polymers: **P18** and **P19** with short side-chains demonstrated hole mobility high as 2.4×10-4 and 1.1×10-3cm2V-1s-1, respectively, which are much higher than those of **P20**-**P23** with bulkier alkyl side-chains. The bulky alkyl chains possibly are thought to result in imbalanced electron transporting and consequently decreased FF of polymer.


Definitions for all parameters are the same as those in Table 1.

Table 2. The optical, electrochemical, hole mobility, and PSC characteristics of **P13-26**

It is widely accepted that the formation of strong intermolecular π-π stacking is necessary to achieve high charge mobility in OSC (Payne, 2004; Mcculloch, 2006). Rigid coplanar fused aromatic rings can efficiently enhance this interaction and hence improve charge mobilities. Indolo[3,2-*b*]carbazole (IC) has large-size fused aromatic ring, making closer packing of

PCE of 5.4% with the structure ITO/PEDOT:PSS/**P16**:PC71BM(1:2.5 w/w)/LiF/Al. By replacing the BT unit with benzotriazole, Cao (Zhang et al., 2010) developed PC **P17**, with an *Eg* of 2.18eV, which was broader than that of P3HT. The optimized device of **P17** achieved improved PCE from 1.51% to 2.75% by sandwiching a thin layer of PFN between Al cathode

On the basis of successful design of two-dimensional PF **P4** and **P5**, Cao [Duan et al., 2010b] developed a series of carbazole-based two-dimensional polymers **P18-23**, with their *Eg* ranging from 1.7 to 1.9eV. **P18-23** present similar HOMO energy levels as the conjugated backbone while their LUMO levels were mainly determined by narrow band gap sidechains. Among them, **P22** and **P23** exhibited stronger absorption intensity and broader absorption spectra. The photovoltaic performance of **P22** and **P23** devices indicates the 2 ethylhexyl chains had little influence on the *Voc* of OSC while the bulky heptadecanyl chains on carbazole led to a significant decrease in *Jsc* and FF. In comparison with that of **P4** and **P5**, **P18** devices exhibited excellent PCE of 4.16%. **P18-23** OSCs possessed high *Voc* (>0.8eV) values, which was relevant with their relatively low HOMO levels. As known, *Voc* is related to the offset between the HOMO levels of donor materials and the LUMO levels of acceptor materials. The alkyl chains on carbazole unit had important influence on the holetransporting ability of the resultant polymers: **P18** and **P19** with short side-chains demonstrated hole mobility high as 2.4×10-4 and 1.1×10-3cm2V-1s-1, respectively, which are much higher than those of **P20**-**P23** with bulkier alkyl side-chains. The bulky alkyl chains possibly are thought to result in imbalanced electron transporting and consequently

and active layer.

decreased FF of polymer.

nm

*Eg* eV

μh cm2 V-1s-1

Definitions for all parameters are the same as those in Table 1.

HOMO /LUMO, eV

Table 2. The optical, electrochemical, hole mobility, and PSC characteristics of **P13-26**

It is widely accepted that the formation of strong intermolecular π-π stacking is necessary to achieve high charge mobility in OSC (Payne, 2004; Mcculloch, 2006). Rigid coplanar fused aromatic rings can efficiently enhance this interaction and hence improve charge mobilities. Indolo[3,2-*b*]carbazole (IC) has large-size fused aromatic ring, making closer packing of

**P13** 576 1.87 - -5.50/-3.60 1:4 6.92 0.89 0.63 3.6 **P14** 551 1.72 - -5.48/-3.46 1:3 7.66 0.80 0.50 3.05 **P15** - 1.87 1×10-4 -5.47/-3.65 1:4 3.70 0.96 0.60 2.4 **P16** 579 1.95 1×10-4 -5.21/-3.35 1:2.5a 9.8 0.81 0.69 5.4 **P17** 523 2.18 - -5.54/-3.36 1:2 4.68 0.9 0.65 2.75 **P18** 548 1.83 2.4×10-4 -5.23/-3.40 1:4a 8.94 0.91 0.51 4.16 **P19** 586 1.74 1.1×10-3 -5.25/-3.51 1:4a 8.18 0.92 0.47 3.52 **P20** 545 1.88 5.3×10-5 -5.29/-3.41 1:4a 7.51 0.91 0.44 2.91 **P21** 575 1.77 1.5×10-4 -5.30/-3.53 1:4a 6.23 0.89 0.38 2.09 **P22** 546 1.83 8.8×10-5 -5.20/-3.37 1:4a 7.18 0.83 0.39 2.34 **P23** 598 1.74 1.6×10-4 -5.24/-3.50 1:4a 6.53 0.84 0.40 2.19 **P24** 551 1.85 - -5.43/-3.62 1:3 4.83 0.90 0.48 2.07 **P25** 527 1.91 - -5.53/-3.54 1:4a 7.38 0.80 0.43 2.54 **P26** - - - -5.17/3.15 1:2 9.17 0.69 0.57 3.6

Polymer: PCBM

*Jsc* mA/cm2 *Voc*

<sup>V</sup>FF PCE

Polymer λmaxabs

conjugated structure. Hashimoto (Zou et al., 2009) developed narrow band gap (1.90eV) PIC **P24** by alternating IC donating unit and DTBT accepting unit. The optimized **P24** device ITO/PEDOT:PSS/**P24**:PCBM1:3(w/w)/Al demonstrated a PCE of 2.07%. The study reveals that the formation of an interpenetrated network of donors and acceptors inside active layer is critical for obtaining high-performance OSCs. Such interpenetrated networks can be built by tuning device processing parameters such as the ratio of polymer to PCBM in the blend and post-annealing treatment at suitable temperatures. Besides, a simple and effective method to improve the photovoltaic performance of OSCs is to increase the number of unsubstituted thienyl units in D–A copolymers consisting of DTBT as acceptor. The *Voc* of **P24** is similar with those of some DTBT based narrow band gap polymers with donating units like FL (1.0eV) (Slooff et al., 2007) dibenzosilole (0.9–0.97eV) (Boudreault et al., 2007) or carbazole (0.88 eV) (Blouin et al., 2007) but higher than those of D–A derivatives with donating units such as cyclopenta[2,1-b:3,4-b']dithiophene (0.6eV), dithienosilole (0.44eV) (Liao et al., 2007) or dithieno[3,2-b:2',3'-d] pyrrole(0.52eV) (Zhou et al., 2008). By adopting DTTP as accepting unit, the same group developed **P25**, with an *Eg* of 1.91 Hashimoto (Zhou et al., 2010a). Spin-coating from CF solution, **P25**:PCBM (1:4, w/w) based OSC contributed a PCE of 1.56% and was further improved to 2.54% by using DCB instead of CF.

By using hexylthiophene moiety in DTBT accepting unit and shorter alkyl chains in IC unit, Movileanu (Lu et al., 2008) developed **P24** analogous PIC **P26**. Long solubilizing side-chains generally resulted in lower charge mobility, balance of the alkyl chains in conjugated polymers was important for charge mobility. **P26** film showed pronounced peak broadening and significant red shift (100nm) in absorption spectra than its solution. **P26** device ITO/PEDOT:PSS/**P26**:PCBM(1:2w/w)/LiF/Al achieved a high PCE of 3.6%.

As discussed above, both PC and PF exhibit relatively low-lying HOMO level and thus high *Voc* for OSCs via copolymerizing with accepting unit including DTBT, DTTP and DTQX. In contrast with other accepting units, DTBT-containing polymers may maintain deep HOMO and narrow the band gap through reducing LUMO, which results in a high *Voc*. However, PCs devices generally display low *Jsc* and FF, which was similar to PFs.

### **4. Thiophene-based conjugated polymers**

Polythiophenes (PTs) are considered as the most important conjugated polymers for a broad spectrum of optoelectronic applications such as light-emitting diodes, field-effect transistors, and OSCs due to their excellent optical and electrical properties as well as exceptional thermal and chemical stability (Roncali, 1992).

Chart 3. Thiophene-based narrow band gap polymers

Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development 441

interactions and reduction of *Eg* as well as relatively low HOMO level due to its symmetric, rigidly fused, coplanar structure (Zou et al., 2010). Controlling the active layer thickness of device ITO/PEDOT:PSS/**P31**:PCBM(1:2 w/w)/Al between 90 and 100nm, the best device presented 4.7% PCE, with relatively high *Voc* (0.95V) attributed to its deep HOMO level. By alternating cyclopentadithiophene (CPDT) and BT unit, Mühlbacher (Mühlbacher et al., 2006) developed **P32**, which exhibited an *Eg* of 1.40eV and an absorption spectrum ranged from 300 to 850nm. The device based on **P32**:PCBM obtained a PCE of 3.2%. By incorporating CPDT with bithiazole unit, Lin (K. Li et al., 2009) developed polymer **P33**, with the best device contributed a PCE of 3.04% with the configuration of

Recently, dithieno-[3,2-b:2',3'-d]silole-containing polymers have attracted considerable attention due to their potential applications in the field of optoelectronic devices. Many silole-containing polymers have been proven to achieve high *Voc* by reducing the LUMO levels of polymers (Lu et al., 2008). It is feasible to realize a low-lying HOMO energy level and high *Voc* simultaneously under the condition of ensuring a sufficient energy level offset (0.3-0.5V) between the LUMO level of silole-containing donor and the LUMO of PCBM. Furthermore, silole-containing polymers applied in OSC have also proved to be effective in improving hole mobility and rendering higher efficiencies compared to CPDT (Wang, 2008; Hou, 2008). By alternating dithieno-[3,2-*b*:2',3'-*d*]silole and BT unit, Yang (Hou et al., 2008) developed **P34**, with an *Eg* low 1.45eV. P40 has a broader absorption spectrum ranging from 350-800nm. Its device ITO/PEDOT/PSS/**P34**:PC70BM/Ca/Al achieved an average PCE of 4.7 % for 100 devices, with the OSC showing a PCE of 5.1%. The same group reported DTBT-based silole-containing polymers **P35** (Huo et al., 2009). The device presented 3.43% PCE with the structure of ITO/PEDOT:PSS/**P35**:PCBM/Ca/Al. By alternating dithieno-[3,2 *b*:2',3'-*d*]silole and dithiazole unit, Li (Zhang et al., 2010) designed **P36**, whose device

Recently, the polymers containing benzo[1,2-*b*:4,5-*b*']dithiophene (BDT) have been successfully applied to field-effect transistor (FET) and PSC devices (Huo, 2008; Pan, 2007; Chen, 2009) with a high mobility of 0.15-0.25cm2V-1s-1, owing to the large and planar conjugated structure promoting π-π stacking and thus benefiting charge transport. By incorporating alkoxyl BDT with thieno[3,4-*b*]thiophene, **P37** (**P37-50** structure in Chart 4) was developed, with an *Eg* 1.62eV (Liang et al., 2009a). Thieno[3,4-*b*]thiophene repeating units contributed to the stability of the quinoidal structure of the backbone narrowing the energy gap of resulting polymer (Sotzing & Lee, 2002). Thieno[3,4-*b*]thiophene with ester substituents in **P37** contributed both good solubility and oxidative stability. The absorption of **P37** coincided with the corresponding maximum photon flux region in solar spectrum. **P37** OSC with the structure of ITO/PEDOT:PSS/**P37**:PCBM(1:1 w/w)/Ca/Al achieved a PCE of 4.76% (Table 4). **P37**:PC71BM presented an improved PCE of 5.30% with the same device structure. The absorption spectra of two devices almost covered the whole visible range. Furthermore, high EQEs were obtained for both **P37**:PCBM and **P37**:PC71BM OSCs. For the former, the maximum EQE was over 60% at 650 nm and was 50% in the range of 550-750 nm; the latter exhibited a higher EQE, almost all over 60% in the range from 400 to

By changing side-chain substituents, **P37** analogous polymers **P38-41** were developed (Liang et al., 2009b). All polymers showed very close between 2.6×10-4 and 4.7×10-4cm2V-1s-1 and

similar absorption spectra, covering the whole visible region.

(ITO/PEDOT:PSS/**P36**:PC71BM(1:2 w/w)/Al) achieved a PCE of 2.86%.

ITO/PEDOT:PSS/**P33**:PCBM(1:2 w/w)/Ca/Al.

750 nm.

By alternating quaterthiophene and diketopyrrolopyrrole (DPP) units, Janssen (Wienk et al., 2008) developed PT **P27** (**P27-36** structure in Chart 3), with an *Eg* of 1.4eV (Table 3). The absorption spectrum of **P27** in DCB showed stronger red-shift than its solution in CF. Fast evaporation of CF did not allow **P27** to crystallize, which results in poor morphology when spin-coating **P27:**PCBM blend film from CF solution. Certain degree of crystallization could be achieved by heating the film at 130oC. Spin-coated from CF:DCB (4:1 v/v) mixture solution, **P27**:PCBM (1:2 w/w) optimized OSC reached highest PCE of 4%, while OSC achieving only 1.1% PCE using CF as blending solvent and good PCE of 3.2% when prepared from DCB.

Furans have been used as an alternative to thiophenes in organic dyes for dye-sensitized solar cells and have shown very similar optical and electronic properties (R. Li et al., 2009). The first furan-containing DPP-based low band gap polymers **P28**-**29** were developed (Claire et al., 2010). By spin-coating from CB solution with 1vol% high-boiling-point 1 chloronaphthalene (CN), **P28** device achieved slightly improved PCE from 3.4% to 3.8% with the structure ITO/PEDOT:PSS/**P28**:PCBM(1:3w/w)/LiF/Al, due to optimized blend morphology. The best **P28**:PC71BM (1:3w/w) OSC contributed a PCE of 5.0% with the addition of 9% CN in CB. The device with blend **P29**:PC71BM (1:3 w/w) a PCE of 4.1% with 5% CN addition in CB. The addition of CN led to much finer phase separation between polymer and fullerene to form a fiber-like inter-penetrating morphology at the length scale of 20 nm, which is close to the ideal domain size, assuming an exciton diffusion length of 5- 10 nm (Markov et al., 2005; Scully et al., 2006; Shaw et al., 2008). By alternating bithiophene donating unit and DPP accepting unit, Janssen (Johan et al., 2009) developed **P30**, with an *Eg* of 1.3eV. Terthiophene induced additional planarity, which enhanced packing and charge carrier mobility, as **P30**-based transistors exhibited nearly balanced hole and electron mobilities of 0.04 and 0.01cm2V-1s-1, respectively. The optimized device of **P30**:PCBM (1:2w/w) presented 3.8% PCE by spin-coating from CB solution with a small amount of DIO. The corresponding optimized device of **P30**:PC71BM achieved a PCE of 4.7% by adding 100mg/mL DIO in CF solution, due to low solubility of PC71BM in CF.


Definitions for all parameters are the same as those in Table 1.

Table 3. The optical, electrochemical, hole mobility, and PSC characteristics of **P27-36**

By alternating bithiophene donating unit and thieno[3,4-*c*]pyrrole-4,6-dione (TPD) accepting unit, Wei (Yuan et al., 2010) prepared **P31**, with an *Eg* of 1.82eV. TPD presented strong electron-withdrawing properties, which were attributed to improvement of intramolecular

By alternating quaterthiophene and diketopyrrolopyrrole (DPP) units, Janssen (Wienk et al., 2008) developed PT **P27** (**P27-36** structure in Chart 3), with an *Eg* of 1.4eV (Table 3). The absorption spectrum of **P27** in DCB showed stronger red-shift than its solution in CF. Fast evaporation of CF did not allow **P27** to crystallize, which results in poor morphology when spin-coating **P27:**PCBM blend film from CF solution. Certain degree of crystallization could be achieved by heating the film at 130oC. Spin-coated from CF:DCB (4:1 v/v) mixture solution, **P27**:PCBM (1:2 w/w) optimized OSC reached highest PCE of 4%, while OSC achieving only 1.1% PCE using CF as blending solvent and good PCE of 3.2% when

Furans have been used as an alternative to thiophenes in organic dyes for dye-sensitized solar cells and have shown very similar optical and electronic properties (R. Li et al., 2009). The first furan-containing DPP-based low band gap polymers **P28**-**29** were developed (Claire et al., 2010). By spin-coating from CB solution with 1vol% high-boiling-point 1 chloronaphthalene (CN), **P28** device achieved slightly improved PCE from 3.4% to 3.8% with the structure ITO/PEDOT:PSS/**P28**:PCBM(1:3w/w)/LiF/Al, due to optimized blend morphology. The best **P28**:PC71BM (1:3w/w) OSC contributed a PCE of 5.0% with the addition of 9% CN in CB. The device with blend **P29**:PC71BM (1:3 w/w) a PCE of 4.1% with 5% CN addition in CB. The addition of CN led to much finer phase separation between polymer and fullerene to form a fiber-like inter-penetrating morphology at the length scale of 20 nm, which is close to the ideal domain size, assuming an exciton diffusion length of 5- 10 nm (Markov et al., 2005; Scully et al., 2006; Shaw et al., 2008). By alternating bithiophene donating unit and DPP accepting unit, Janssen (Johan et al., 2009) developed **P30**, with an *Eg* of 1.3eV. Terthiophene induced additional planarity, which enhanced packing and charge carrier mobility, as **P30**-based transistors exhibited nearly balanced hole and electron mobilities of 0.04 and 0.01cm2V-1s-1, respectively. The optimized device of **P30**:PCBM (1:2w/w) presented 3.8% PCE by spin-coating from CB solution with a small amount of DIO. The corresponding optimized device of **P30**:PC71BM achieved a PCE of 4.7% by

adding 100mg/mL DIO in CF solution, due to low solubility of PC71BM in CF.

HOMO /LUMO, eV

**P27** 650 1.4 - - 1:2a 11.5 0.61 0.58 4.0 **P28** 789 1.41 - -5.40/-3.80 1:3a 11.2 0.74 0.60 5.0 **P29** 767 1.35 - -5.50/-3.80 1:3a 9.1 0.73 0.58 4.1 **P30** - 1.36 0.04 -5.17/-3.61 1:2a 11.8 0.65 0.60 4.7 **P31** 572 1.82 1.0×10-4 -5.56/-3.10 1:1.5 8.02 0.95 0.62 4.7 **P32** 775 1.40 - -5.30/-3.57 1:3 11.0 0.70 0.47 3.2 **P33** 560 1.84 - -5.07/-3.55 1:1 7.70 0.68 0.53 2.79 **P34** - 1.45 3×10-3 -5.05/-3.27 1:1a 12.7 0.68 0.55 5.1 **P35** 652 1.53 3.0×10-6 -4.99/-3.17 1:1a 9.76 0.60 0.50 2.95 **P36** 558 1.85 3.1×10-4 -5.18/-3.09 1:1a 7.85 0.68 0.54 2.86

Table 3. The optical, electrochemical, hole mobility, and PSC characteristics of **P27-36**

By alternating bithiophene donating unit and thieno[3,4-*c*]pyrrole-4,6-dione (TPD) accepting unit, Wei (Yuan et al., 2010) prepared **P31**, with an *Eg* of 1.82eV. TPD presented strong electron-withdrawing properties, which were attributed to improvement of intramolecular

Polymer: PCBM

*Jsc* mA/cm2 *Voc*

<sup>V</sup>FF PCE

prepared from DCB.

Polymer λmaxabs

nm

*Eg* eV

μh cm2 V-1s-1

Definitions for all parameters are the same as those in Table 1.

interactions and reduction of *Eg* as well as relatively low HOMO level due to its symmetric, rigidly fused, coplanar structure (Zou et al., 2010). Controlling the active layer thickness of device ITO/PEDOT:PSS/**P31**:PCBM(1:2 w/w)/Al between 90 and 100nm, the best device presented 4.7% PCE, with relatively high *Voc* (0.95V) attributed to its deep HOMO level.

By alternating cyclopentadithiophene (CPDT) and BT unit, Mühlbacher (Mühlbacher et al., 2006) developed **P32**, which exhibited an *Eg* of 1.40eV and an absorption spectrum ranged from 300 to 850nm. The device based on **P32**:PCBM obtained a PCE of 3.2%. By incorporating CPDT with bithiazole unit, Lin (K. Li et al., 2009) developed polymer **P33**, with the best device contributed a PCE of 3.04% with the configuration of ITO/PEDOT:PSS/**P33**:PCBM(1:2 w/w)/Ca/Al.

Recently, dithieno-[3,2-b:2',3'-d]silole-containing polymers have attracted considerable attention due to their potential applications in the field of optoelectronic devices. Many silole-containing polymers have been proven to achieve high *Voc* by reducing the LUMO levels of polymers (Lu et al., 2008). It is feasible to realize a low-lying HOMO energy level and high *Voc* simultaneously under the condition of ensuring a sufficient energy level offset (0.3-0.5V) between the LUMO level of silole-containing donor and the LUMO of PCBM. Furthermore, silole-containing polymers applied in OSC have also proved to be effective in improving hole mobility and rendering higher efficiencies compared to CPDT (Wang, 2008; Hou, 2008). By alternating dithieno-[3,2-*b*:2',3'-*d*]silole and BT unit, Yang (Hou et al., 2008) developed **P34**, with an *Eg* low 1.45eV. P40 has a broader absorption spectrum ranging from 350-800nm. Its device ITO/PEDOT/PSS/**P34**:PC70BM/Ca/Al achieved an average PCE of 4.7 % for 100 devices, with the OSC showing a PCE of 5.1%. The same group reported DTBT-based silole-containing polymers **P35** (Huo et al., 2009). The device presented 3.43% PCE with the structure of ITO/PEDOT:PSS/**P35**:PCBM/Ca/Al. By alternating dithieno-[3,2 *b*:2',3'-*d*]silole and dithiazole unit, Li (Zhang et al., 2010) designed **P36**, whose device (ITO/PEDOT:PSS/**P36**:PC71BM(1:2 w/w)/Al) achieved a PCE of 2.86%.

Recently, the polymers containing benzo[1,2-*b*:4,5-*b*']dithiophene (BDT) have been successfully applied to field-effect transistor (FET) and PSC devices (Huo, 2008; Pan, 2007; Chen, 2009) with a high mobility of 0.15-0.25cm2V-1s-1, owing to the large and planar conjugated structure promoting π-π stacking and thus benefiting charge transport. By incorporating alkoxyl BDT with thieno[3,4-*b*]thiophene, **P37** (**P37-50** structure in Chart 4) was developed, with an *Eg* 1.62eV (Liang et al., 2009a). Thieno[3,4-*b*]thiophene repeating units contributed to the stability of the quinoidal structure of the backbone narrowing the energy gap of resulting polymer (Sotzing & Lee, 2002). Thieno[3,4-*b*]thiophene with ester substituents in **P37** contributed both good solubility and oxidative stability. The absorption of **P37** coincided with the corresponding maximum photon flux region in solar spectrum. **P37** OSC with the structure of ITO/PEDOT:PSS/**P37**:PCBM(1:1 w/w)/Ca/Al achieved a PCE of 4.76% (Table 4). **P37**:PC71BM presented an improved PCE of 5.30% with the same device structure. The absorption spectra of two devices almost covered the whole visible range. Furthermore, high EQEs were obtained for both **P37**:PCBM and **P37**:PC71BM OSCs. For the former, the maximum EQE was over 60% at 650 nm and was 50% in the range of 550-750 nm; the latter exhibited a higher EQE, almost all over 60% in the range from 400 to 750 nm.

By changing side-chain substituents, **P37** analogous polymers **P38-41** were developed (Liang et al., 2009b). All polymers showed very close between 2.6×10-4 and 4.7×10-4cm2V-1s-1 and similar absorption spectra, covering the whole visible region.

Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development 443

BDT-containing polymers can finely tune the HOMO level and efficiently narrow the band gap (Pan, 2007; Liang, 2008; Liang, 2009). Besides, alkoxy groups have been proved to be beneficial to reduce the HOMO level of polymer due to its stronger electron-donating property (Liang et al., 2009b). By removal of the oxygen atom on ester group in thieno[3,4-*b*]thiophene unit, ketone-substituted polymer **P43** (Chen et al. 2009) was developed to further reduce the

By alternating BDT and TPD, Leclerc (Zou et al, 2010) presented polymer **P44**, with an *Eg* of 1.8eV and HOMO/LUMO energy levels -5.56eV/-3.75eV, respectively. The device ITO/PEDOT:PSS/**P53**:PC71BM/LiF/Al showed a high PCE of 5.5%. By incorporating BDT with DTBT, You (Price et al., 2010) synthesized **P45-46**. Compared with **P45**, **P46** exhibited higher molecular weight while lower hole mobility than other BDT-containing **P37-43**. **P45- 46** device showed a PCE of 3.85% and 4.31%, respectively. By introducing thiopene moiety onto BDT, Yang (Huo et al., 2010) synthesized **P45** analogous polymer **P47**, with an *Eg* of 1.75eV. Spin-coated from DCB, **P47** device (ITO/PEDOT:PSS/**P47**:PC71BM(1:2

HOMO level. The best OSC based on **P43**:PC71BM showed a high PCE of 6.58%.

w/w)/Ca/Al) contributed the optimized PCE of 5.66% without annealing treatment. By changing the alky substituent in TPD unit, Frechet (Piliego et al., 2010) synthesized **P44** analogue polymers **P48-50**, with *Eg* ranging from 1.70 to 1.75 eV and HOMO levels lowered down to -5.4~ -5.57 eV. Spin-coated from CB, **P48-50**:PCBM devices achieved a PCE of 2.8%, 3.9%, 6.4%, respectively. Clearly, the device performance increased as the branch length was decreased in **P48-50**. By spin-coating from CB solution with a small amount of DIO, **P48** and **P49** OSCs achieved improved PCE of 4.0% and 5.7%, respectively (Peet et al., 2008). However, for **P50**, the addition of DIO led to only slight improvements, which was attributed to a high level of order of **P50** in the blend without DIO. It was concluded that alkyl-substituted groups had much impact on the PCE of **P48-50**. The use of DIO is effective in promoting the packing of the polymer by preventing excessive crystallization of PCBM. Dithieno[3,2-*b*:2',3'-*d*]pyrrole (DTP) has good planarity and stronger electron-donating ability of nitrogen atoms. Geng (Yue et al. 2009) synthesized BT-based P(DTP)s **P51-52** (Chart 5), with an *Eg* ~1.43eV. Owing to different alkyl chain, **P52** exhibited stronger absorption in the range of 600-900nm compared to **P51**. **P51-52** devices showed a PCE of 2.06% and 2.8%, respectively with the structure ITO/PEDOT:PSS/**P51-52**:PCBM(1:3 w/w)/LiF/Al, at the optimized blend film thickness of 100 nm for **P51** and 90 nm for **P52**. The device of **P51**-52 delivered a *Voc* in the range of 0.4-0.54V, leading to relatively low PCE. By adopting DTP and DTBT units as the donor and acceptor segments, Hashimoto (Zhou et al., 2008) synthesized **P53** with an *Eg* of 1.46eV, which was the lowest compared with other D-A copolymers based on DTBT acceptor segment with other donor segments such as FL, silafluorene, carbazole, DTS, and CPDT (Slooff, 2007; Blouin, 2007; Moulle, 2008). **P53** device exhibited a PCE of 2.18% and a high *Jsc* of 9.47mA/cm2, which was attributed to its broad absorption spectrum. However, **P53** device presented a *Voc* of 0.52V, which is lower than that of the devices based on DTBT-containing D-A copolymers with FL (1V), silafluorene

3,6-Dithien-2-yl-2,5-dialkylpyrrolo[3,4-*c*]pyrrole-1,4-dione (DTDPP) emerged as a promising electron acceptor in the design of photovoltaic polymers (Wienk et al., 2008). With DTDPP as accepting unit, Hashimoto (Zhou et al., 2010b) presented **P54**, with an *Eg* lowed down to 1.13eV. **P54** presented a hole mobility of 0.05cm2V-1s-1, 1-2 orders of magnitude higher than that of other D-A polymers. Spin-coated with mixing solution of CF and DCB (4:1 v/v), **P54** device with structure of ITO/PEDOT:PSS/**P54**:PCBM(or PC71BM)/LiF/Al showed a PCE of 2.34% with PCBM and 2.71% with PC71BM as acceptor in the blend films. The PCE of **P54**

(0.9V) or carbazole (0.9V) as donating unit.

Chart 4. BDT based narrow band gap polymers.

The substitution of octyloxy to octyl group lowered the HOMO energy level of **P38** from - 4.94 to -5.04 eV (**P39)**. With electron-withdrawing fluorine in polymer backbone, **P40** had significantly lower HOMO level in comparison to **P41**. The devices based on ITO/PEDOT: PSS/**P38-41**:PCBM (1:1 w/w)/Ca/Al exhibited a PCE of 5.10%, 5.53%, 3.20% and 3.02%,respectively, with active layer spin-coated from DCB solution. By using DCB/DIO (97:3 v/v) mixture solvent, PCE of **P39-P41** can be dramatically improved to 5.85%, 5.90% and 4.10%, respectively. By replacing octyl group in **P40** into 2-ethylhexyl, **P52** was further developed (Liang et al., 2010). Spin-coated from DCB solution, **P51**:PC71BM harvested a PCE of 5.74%, 6.22% and 5.58%, respectively at their D/A weight ratio as 1:1, 1:1.5 and 1:2. The best device contributes PCE as high as 7.4% with the mixture solvent of CB/DIO(97:3 v/v), where uniform morphology of blend film was prepared.


Definitions for all parameters are the same as those in Table 1.

Table 4. The optical, electrochemical, hole mobility, and PSC characteristics of **P37-50**

The substitution of octyloxy to octyl group lowered the HOMO energy level of **P38** from - 4.94 to -5.04 eV (**P39)**. With electron-withdrawing fluorine in polymer backbone, **P40** had significantly lower HOMO level in comparison to **P41**. The devices based on ITO/PEDOT: PSS/**P38-41**:PCBM (1:1 w/w)/Ca/Al exhibited a PCE of 5.10%, 5.53%, 3.20% and 3.02%,respectively, with active layer spin-coated from DCB solution. By using DCB/DIO (97:3 v/v) mixture solvent, PCE of **P39-P41** can be dramatically improved to 5.85%, 5.90% and 4.10%, respectively. By replacing octyl group in **P40** into 2-ethylhexyl, **P52** was further developed (Liang et al., 2010). Spin-coated from DCB solution, **P51**:PC71BM harvested a PCE of 5.74%, 6.22% and 5.58%, respectively at their D/A weight ratio as 1:1, 1:1.5 and 1:2. The best device contributes PCE as high as 7.4% with the mixture solvent of CB/DIO(97:3

> HOMO/ LUMO, eV

Table 4. The optical, electrochemical, hole mobility, and PSC characteristics of **P37-50**

**P37** 687 1.62 4.5×10-4 -4.90/-3.20 1:1.2a 15.0 0.56 0.63 5.30 **P38** 683 1.59 4.0×10-4 -4.94/-3.22 1:1 12.8 0.60 0.66 5.10 **P39** 682 1.60 7.1×10-4 -5.04/-3.29 1:1 13.9 0.72 0.58 5.85 **P40** 682 1.63 7.7×10-4 -5.12/-3.31 1:1 13.0 0.74 0.61 5.90 **P41** 677 1.62 4.0×10-4 -5.01/-3.24 1:1 10.7 0.66 0.58 4.10 **P42** 675 1.61 5.8×10-4 -5.15/-3.31 1:1.5a 14.5 0.74 0.69 7.4 **P43** 675 1.61 2×10-4 -5.12/-3.55 1:1.5a 14.7 0.7 0.64 6.58 **P44** 610 1.80 - -5.56/-3.75 1:2a 9.81 0.85 0.66 5.50 **P45** 595 1.70 1.6×10-5 -5.26/-2.96 1:1 9.70 0.81 0.55 4.31 **P46** 644 1.70 3.8×10-5 -5.33/-3.17 1:1 7.79 0.83 0.60 3.85 **P47** 596 1.75 - -5.31/-3.44 1:2a 10.7 0.92 0.58 5.66 **P48** 608 1.75 - -5.48/- 1:2 8.1 0.87 0.56 4.0 **P49** 616 1.70 - -5.57/- 1:1.5 9.7 0.81 0.67 5.7 **P50** 627 1.73 - -5.40/- 1:1.5 11.5 0.85 0.68 6.8

Polymer: PCBM

*Jsc* mA/cm2 *Voc*

<sup>V</sup>FF PCE

Chart 4. BDT based narrow band gap polymers.

Polymer λmaxabs

nm

*Eg* eV

v/v), where uniform morphology of blend film was prepared.

μh cm2 V-1s-1

Definitions for all parameters are the same as those in Table 1.

BDT-containing polymers can finely tune the HOMO level and efficiently narrow the band gap (Pan, 2007; Liang, 2008; Liang, 2009). Besides, alkoxy groups have been proved to be beneficial to reduce the HOMO level of polymer due to its stronger electron-donating property (Liang et al., 2009b). By removal of the oxygen atom on ester group in thieno[3,4-*b*]thiophene unit, ketone-substituted polymer **P43** (Chen et al. 2009) was developed to further reduce the HOMO level. The best OSC based on **P43**:PC71BM showed a high PCE of 6.58%.

By alternating BDT and TPD, Leclerc (Zou et al, 2010) presented polymer **P44**, with an *Eg* of 1.8eV and HOMO/LUMO energy levels -5.56eV/-3.75eV, respectively. The device ITO/PEDOT:PSS/**P53**:PC71BM/LiF/Al showed a high PCE of 5.5%. By incorporating BDT with DTBT, You (Price et al., 2010) synthesized **P45-46**. Compared with **P45**, **P46** exhibited higher molecular weight while lower hole mobility than other BDT-containing **P37-43**. **P45- 46** device showed a PCE of 3.85% and 4.31%, respectively. By introducing thiopene moiety onto BDT, Yang (Huo et al., 2010) synthesized **P45** analogous polymer **P47**, with an *Eg* of 1.75eV. Spin-coated from DCB, **P47** device (ITO/PEDOT:PSS/**P47**:PC71BM(1:2 w/w)/Ca/Al) contributed the optimized PCE of 5.66% without annealing treatment.

By changing the alky substituent in TPD unit, Frechet (Piliego et al., 2010) synthesized **P44** analogue polymers **P48-50**, with *Eg* ranging from 1.70 to 1.75 eV and HOMO levels lowered down to -5.4~ -5.57 eV. Spin-coated from CB, **P48-50**:PCBM devices achieved a PCE of 2.8%, 3.9%, 6.4%, respectively. Clearly, the device performance increased as the branch length was decreased in **P48-50**. By spin-coating from CB solution with a small amount of DIO, **P48** and **P49** OSCs achieved improved PCE of 4.0% and 5.7%, respectively (Peet et al., 2008). However, for **P50**, the addition of DIO led to only slight improvements, which was attributed to a high level of order of **P50** in the blend without DIO. It was concluded that alkyl-substituted groups had much impact on the PCE of **P48-50**. The use of DIO is effective in promoting the packing of the polymer by preventing excessive crystallization of PCBM.

Dithieno[3,2-*b*:2',3'-*d*]pyrrole (DTP) has good planarity and stronger electron-donating ability of nitrogen atoms. Geng (Yue et al. 2009) synthesized BT-based P(DTP)s **P51-52** (Chart 5), with an *Eg* ~1.43eV. Owing to different alkyl chain, **P52** exhibited stronger absorption in the range of 600-900nm compared to **P51**. **P51-52** devices showed a PCE of 2.06% and 2.8%, respectively with the structure ITO/PEDOT:PSS/**P51-52**:PCBM(1:3 w/w)/LiF/Al, at the optimized blend film thickness of 100 nm for **P51** and 90 nm for **P52**. The device of **P51**-52 delivered a *Voc* in the range of 0.4-0.54V, leading to relatively low PCE. By adopting DTP and DTBT units as the donor and acceptor segments, Hashimoto (Zhou et al., 2008) synthesized **P53** with an *Eg* of 1.46eV, which was the lowest compared with other D-A copolymers based on DTBT acceptor segment with other donor segments such as FL, silafluorene, carbazole, DTS, and CPDT (Slooff, 2007; Blouin, 2007; Moulle, 2008). **P53** device exhibited a PCE of 2.18% and a high *Jsc* of 9.47mA/cm2, which was attributed to its broad absorption spectrum. However, **P53** device presented a *Voc* of 0.52V, which is lower than that of the devices based on DTBT-containing D-A copolymers with FL (1V), silafluorene (0.9V) or carbazole (0.9V) as donating unit.

3,6-Dithien-2-yl-2,5-dialkylpyrrolo[3,4-*c*]pyrrole-1,4-dione (DTDPP) emerged as a promising electron acceptor in the design of photovoltaic polymers (Wienk et al., 2008). With DTDPP as accepting unit, Hashimoto (Zhou et al., 2010b) presented **P54**, with an *Eg* lowed down to 1.13eV. **P54** presented a hole mobility of 0.05cm2V-1s-1, 1-2 orders of magnitude higher than that of other D-A polymers. Spin-coated with mixing solution of CF and DCB (4:1 v/v), **P54** device with structure of ITO/PEDOT:PSS/**P54**:PCBM(or PC71BM)/LiF/Al showed a PCE of 2.34% with PCBM and 2.71% with PC71BM as acceptor in the blend films. The PCE of **P54**

Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development 445

**P51** 764 1.42 - -4.86/-3.07 1:3 11.1 0.43 0.43 2.06 **P52** 764 1.43 - -4.81/-3.08 1:3 11.9 0.54 0.44 2.80 **P53** 697 1.46 - -5.00/-3.43 1:1 9.47 0.52 0.44 2.18 **P54** 790 1.13 0.05 -4.90/-3.63 1:2a 14.87 0.38 0.48 2.71 **P55** 547 - - - 1:1 10.93 0.59 0.46 3.00 **P56** 548 - - - 1:1 10.67 0.69 0.46 3.36 **P57** 580 1.53 - -5.36/-3.42 1:1 14.16 0.71 0.62 6.20 **P58** 650 1.56 - -5.50/-3.44 1:1 13.49 0.75 0.55 5.57 **P59** 675 1.51 - -5.47/-3.44 1:1 12.78 0.85 0.58 6.32 **P60** 681 1.61 1.7×10-5 -5.34/-3.29 1:0.8 14.20 0.67 0.54 5.10 **P61** 647 1.70 7.2×10-6 -5.46/-3.28 1:1.2 11.38 0.83 0.46 4.3 **P62** 928 1.34 - -5.21/-3.63 1:2a 10.1 0.68 0.63 4.31 **P63** 520 1.76 7.0×10-4 -5.46/-3.56 1:3a 8.7 0.84 0.53 3.9 **P64** 590 1.70 3.4×10-3 -5.43/-3.66 1:3a 10.1 0.80 0.53 4.3

Polymer: PCBM

*Jsc* mA/cm2 *Voc*

<sup>V</sup>FF PCE

HOMO/ LUMO, eV

Table 5. The optical, electrochemical, hole mobility, and PSC characteristics of **P51-64**

By replacing BT with a new stronger acceptor, thiadiazolo[3,4-*c*]pyridine, You (H. Zhou, 2010d) developed **P57-59** by adopting NDT, QDT or BnDT as donating unit. **P57-59** exhibit an *Eg* of 1.53eV, 1.56eV and 1.51eV, respectively, which is noticeably reduced (ca.0.09– 0.19eV) compared with the band gaps of their BT counterparts (Price, 2010; H. Zhou, 2010c). The device with ITO/PEDOT:PSS/**P57-59**:PCBM/Ca/Al achieved a PCE of 6.20% for **P57**, 5.57% for **P58**, and 6.32%% for **P59**.By alternating DTBT acceptor with NDT or QDT weak donor, You (H. Zhou, 2010e) developed **P60-61**, with an *Eg* of **P60-61** of 1.61eV and 1.70eV, respectively. The devices based on **P60-61**:PCBM achieved a PCE of 5.1% and 4.3%, respectively. The donor unit with electron-withdrawing atoms was demonstrated to lead to a lower HOMO level while have little impact on the LUMO level. You thought that future research should be focused on employment of even weaker donors and stronger acceptors via innovative structural modification in order to concurrently achieve a higher *Voc* and a higher *Jsc*. By alternating DPP and benzo[1,2-*b*:4,5-*b*']dithiophene, Yang (Huo et al., 2009) synthesized **P62**. Its device with the structure of ITO/PEDOT:PSS/**P62**:PC71BM(1:2 w/w)/Ca/Al achieved improved PCE from 4.31% to 4.45% after device annealing under 110oC for 30 min. **P62** was expected to a very promising candidate for highly efficient OSCs

Ladder-type coplanar thiophene-phenylene-thiophene (TPT) structure tends to lead to strong molecular π-π interaction and thus exhibits remarkable hole mobility (Wong et al., 2006). Poly(TPT)s demonstrated an *Eg* ~2.1eV and hole mobility as high as 10-3cm2V-1s-1 (Chan et al., 2008). By alternating TPT donor and DTBT acceptor, Ting (Chen et al., 2008) developed **P63-64**, which exhibited an *Eg* of 1.76eV and 1.70eV, and absorption band in the range of 350-700nm and 350-730nm, respectively. **P64** displayed a hole mobility up to 3.4×10-3cm2V-1s-1. Both of them showed good solubility at room temperature in organic solvents. Using DCB as blending solvent, the devices ITO/PEDOT:PSS/**P63-64**:PCBM(1:3 w/w)/Ca/Al harvested a PCE of 2.0% and 2.5%, respectively, a little lower than that (3.9% PCE) of P3HT:PCBM(1:1 w/w) device under the same conditions. The optimized device

Polymer λmax

nm

*Eg* eV

μh cm2 V-1s-1

Definitions for all parameters are the same as those in Table 1.

owing to relatively low *Eg* and suitable HOMO level.

was lower that of P3HT, which was mainly attributed to the lower *Voc*, resulting from its high-lying HOMO level (-4.90eV). It was expected that the device performance would be improved by further reducing the HOMO and LUMO level.

Chart 5. Fused Thiophene based narrow band gap polymers.

Recently, You (H. Zhou, 2010b) proposed a design strategy for narrow band gap polymers by alternating "weak donor" and "strong acceptor" to approach ideal polymers with both low HOMO energy level and small *Eg* in order to achieve both high *Voc* and *Jsc* (H. Zhou, 2010e). By alternating DTBT or thiadiazolo[3,4-*c*]pyridine as strong acceptor,with naphtho[2,1-*b*:3,4-*b*']-dithiophene (NDT), dithieno[3,2-f:2',3'-*h*]quinoxaline (QDT) or BDT as weak donor, polymers **P55-61** were developed (Yang, 2010; H. Zhou, 2009; H. Zhou, 2010b, 2010c). For NDT-containing **P55-56**, a 4-(2-ethylhexyl)thiophene unit in DTBT was used to reduce the steric hindrance of polymer backbone and hence to achieve near identical *Eg* and energy levels (H. Zhou, 2010a). It was believed that *Eg* and energy levels of a conjugated polymer were primarily determined by the molecular structure of conjugated backbone, while the solubilizing alkyl chains should have a negligible impact on these properties and hence a little impact on device's *Jsc* and *Voc* . However, Yu's study showed variations of length and shape of alkyl chains in NDT had significant influence on device performance: long and branched side-chains would weaken the intermolecular interaction, benefiting for improvement of *Voc*; while short and straight side-chains would promote the intermolecular interaction, rendering a large *Jsc*. The desirable balance between *Voc* and *Jsc* could be tuned through adopting suitable short and branched side-chains. **P55-56**:PCBM (1:1,w/w) devices showed a best PCE of 3.0% and 3.36%, respectively.


was lower that of P3HT, which was mainly attributed to the lower *Voc*, resulting from its high-lying HOMO level (-4.90eV). It was expected that the device performance would be

Recently, You (H. Zhou, 2010b) proposed a design strategy for narrow band gap polymers by alternating "weak donor" and "strong acceptor" to approach ideal polymers with both low HOMO energy level and small *Eg* in order to achieve both high *Voc* and *Jsc* (H. Zhou, 2010e). By alternating DTBT or thiadiazolo[3,4-*c*]pyridine as strong acceptor,with naphtho[2,1-*b*:3,4-*b*']-dithiophene (NDT), dithieno[3,2-f:2',3'-*h*]quinoxaline (QDT) or BDT as weak donor, polymers **P55-61** were developed (Yang, 2010; H. Zhou, 2009; H. Zhou, 2010b, 2010c). For NDT-containing **P55-56**, a 4-(2-ethylhexyl)thiophene unit in DTBT was used to reduce the steric hindrance of polymer backbone and hence to achieve near identical *Eg* and energy levels (H. Zhou, 2010a). It was believed that *Eg* and energy levels of a conjugated polymer were primarily determined by the molecular structure of conjugated backbone, while the solubilizing alkyl chains should have a negligible impact on these properties and hence a little impact on device's *Jsc* and *Voc* . However, Yu's study showed variations of length and shape of alkyl chains in NDT had significant influence on device performance: long and branched side-chains would weaken the intermolecular interaction, benefiting for improvement of *Voc*; while short and straight side-chains would promote the intermolecular interaction, rendering a large *Jsc*. The desirable balance between *Voc* and *Jsc* could be tuned through adopting suitable short and branched side-chains. **P55-56**:PCBM (1:1,w/w) devices

improved by further reducing the HOMO and LUMO level.

Chart 5. Fused Thiophene based narrow band gap polymers.

showed a best PCE of 3.0% and 3.36%, respectively.


Definitions for all parameters are the same as those in Table 1.

Table 5. The optical, electrochemical, hole mobility, and PSC characteristics of **P51-64**

By replacing BT with a new stronger acceptor, thiadiazolo[3,4-*c*]pyridine, You (H. Zhou, 2010d) developed **P57-59** by adopting NDT, QDT or BnDT as donating unit. **P57-59** exhibit an *Eg* of 1.53eV, 1.56eV and 1.51eV, respectively, which is noticeably reduced (ca.0.09– 0.19eV) compared with the band gaps of their BT counterparts (Price, 2010; H. Zhou, 2010c). The device with ITO/PEDOT:PSS/**P57-59**:PCBM/Ca/Al achieved a PCE of 6.20% for **P57**, 5.57% for **P58**, and 6.32%% for **P59**.By alternating DTBT acceptor with NDT or QDT weak donor, You (H. Zhou, 2010e) developed **P60-61**, with an *Eg* of **P60-61** of 1.61eV and 1.70eV, respectively. The devices based on **P60-61**:PCBM achieved a PCE of 5.1% and 4.3%, respectively. The donor unit with electron-withdrawing atoms was demonstrated to lead to a lower HOMO level while have little impact on the LUMO level. You thought that future research should be focused on employment of even weaker donors and stronger acceptors via innovative structural modification in order to concurrently achieve a higher *Voc* and a higher *Jsc*. By alternating DPP and benzo[1,2-*b*:4,5-*b*']dithiophene, Yang (Huo et al., 2009) synthesized **P62**. Its device with the structure of ITO/PEDOT:PSS/**P62**:PC71BM(1:2 w/w)/Ca/Al achieved improved PCE from 4.31% to 4.45% after device annealing under 110oC for 30 min. **P62** was expected to a very promising candidate for highly efficient OSCs owing to relatively low *Eg* and suitable HOMO level.

Ladder-type coplanar thiophene-phenylene-thiophene (TPT) structure tends to lead to strong molecular π-π interaction and thus exhibits remarkable hole mobility (Wong et al., 2006). Poly(TPT)s demonstrated an *Eg* ~2.1eV and hole mobility as high as 10-3cm2V-1s-1 (Chan et al., 2008). By alternating TPT donor and DTBT acceptor, Ting (Chen et al., 2008) developed **P63-64**, which exhibited an *Eg* of 1.76eV and 1.70eV, and absorption band in the range of 350-700nm and 350-730nm, respectively. **P64** displayed a hole mobility up to 3.4×10-3cm2V-1s-1. Both of them showed good solubility at room temperature in organic solvents. Using DCB as blending solvent, the devices ITO/PEDOT:PSS/**P63-64**:PCBM(1:3 w/w)/Ca/Al harvested a PCE of 2.0% and 2.5%, respectively, a little lower than that (3.9% PCE) of P3HT:PCBM(1:1 w/w) device under the same conditions. The optimized device

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with **P63-64**:PC71BM (1:3 w/w) showed a PCE of 3.9% for **P63** and 4.3% for **P64**, higher than that of the device based on P3HT:PC71BM (1:1 w/w) (3.4%) under the same conditions.
