**2. Fluorene-based conjugated polymers**

Fluorene (FL) and its derivatives have been extensively investigated for their application in light-emitting diodes due to its rigid planar molecular structure, excellent hole-transporting properties, good solubility, and exceptional chemical stability.

Chart 1. Flourene based narrow band gap polymers.

Along with their low-lying HOMO levels, polyfluorenes (PFs) are expected to achieve higher *Voc* and *Jsc* in their PSC device, which makes fluorene unit a promising electron-

charge separation and balanced bipolar transport throughout its whole volume. Remarkably, the power conversion efficiency (PCE, defined as the maximum power produced by a photovoltaic cell divided by the power of incident light) of the OSCs has been pushed to more than 7% from 0.1% after a decade's intensive interdisciplinary research. The current workhorse materials employed for PSCs are regioregular poly(3 hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). This material combination has given the highest reported PCE values of 4%~5% (G. Li, 2005). Theoretically, the PCE of polymer solar cells can be further improved (ca 10%) (Scharber et al., 2006) by implementing new materials (Cheng, 2009; Peet, 2009; Tang, 2010) and exploring new device architecture (Dennler, 2008; Ameri, 2009; Dennler, 2009) after addressing several fundamental issues such as bandgap, interfaces and charge transfer (Li,

In this account, we will update the recent 4 years progress in pursuit of high performance BHJ OSCs with newly developed conjugated polymers, especially narrow bandgap polymers from a viewpoint of material chemists. The correlation of polymer chemical structures with their properties including absorption spectra, band gap, energy levels, mobilities, and photovoltaic performance will be elaborated. The analysis of structureproperty relationship will provide insight in rational design of polymer structures and

Fluorene (FL) and its derivatives have been extensively investigated for their application in light-emitting diodes due to its rigid planar molecular structure, excellent hole-transporting

Along with their low-lying HOMO levels, polyfluorenes (PFs) are expected to achieve higher *Voc* and *Jsc* in their PSC device, which makes fluorene unit a promising electron-

2005; Chen, 2008; Cheng, 2009).

reasonable evaluation of their photovoltaic performance.

properties, good solubility, and exceptional chemical stability.

**2. Fluorene-based conjugated polymers** 

Chart 1. Flourene based narrow band gap polymers.

donating moiety in D-A narrow band gap polymers' design. Besides, feasible dialkylation at 9-position and selective bromination at the 2,7-positions of fluorene allow versatile molecular manipulation to achieve good solubility and extended conjugation *via* typical Suzuki or Stille cross-coupling reactions. By using 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) as electron accepting unit and didecylated FL as donating unit, Slooff (Slooff et al., 2007) developed **P1** (**P1-12** structure in Chart 1) with extended absorption spectrum ranging from 300 to 800nm. Spin-coated from chloroform solution, the device ITO/PEDOT:PSS/**P1**:PCBM(1:4, w/w)/LiF/Al harvested a extremely high PCE of 4.2% (Table 1). An external quantum efficiency (EQE) of 66% was achieved in the active layer with a film thickness up to 140 nm, and further increasing the film thickness did not increase the efficiency due to limitations in charge generation or collection. For 4.2% PCE device, a maximum EQE of about 75% was calculated, indicating efficient charge collection.

By using quinoxaline as electron accepting unit, **P2** was synthesized with an *Eg* of 1.95eV (Kitazawa et al., 2009). The device performance is dependent upon the ratio of chloroform(CF)/chlorobenzene(CB) in co-solvent for blend film preparation and a maximal *Jsc* is achieved with CF/CB (2:3 v/v) co-solvent. The optimized device showed 5.5% PCE by inserting 0.1nm LiF layer between BHJ active layer and Al cathode with the structure ITO/PEDOT:PSS/**P2**:PC71BM/LiF/Al. Similarly structured **P3** achieved 3.7% PCE by blending with PC71BM (1:3 w/w) (Gadisa et al., 2007).


λmaxabs: maximum absorption peak in lm; *Eg*: optical band gap; μh: hole mobility; *Jsc*: short-circuit current density; *Voc*: open-circuit voltage; FF: ll factor; PCE: power conversion efciency; apolymer:PC71BM; bpolymer:PCBM in weight ratio.

Table 1. The optical, electrochemical, hole mobility, and PSC characteristics of **P1-12**

Different from the common linear D-A alternating polymer design, Jen and his coworkers designed a series of novel two-dimensional narrow band gap polymers, whose backbone adopts high hole transporting fluorene-triarylamine copolymer (PFM) and is grafted with malononitrile (**P4**) and diethylthiobarbituric acid (**P5**) through a styrylthiophene π-bridge (Huang et al., 2009). Both of them show two obvious absorption peaks, where the first absorption peaks at ~385 nm are corresponding to the π-π \* transition of their conjugated main chains and the others are corresponding to the strong ICT characters of their side chains. Two polymers show narrowed down *Eg* (<2eV) and present similar HOMO energy

Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development 437

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

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

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*.
