**2.4 Favorable blend morphology with fullerene derivatives**

The idea that morphology of the photoactive layer can greatly influence the device performance has been widely accepted and verified by literature reports (Arias, 2002; Peet et al., 2007). However, it is still a 'state-of-art' to control the morphology of specific polymer/PCBM blend. Even though several techniques (Shaheen et al., 2001) have been reported to effectively optimize the morphology of the active layer, precise prediction on the morphology can hardly been done. It is more based on trial-and-error philosophy and theory to explain the structure-morphology relationship is still in infancy. Nevertheless, several reliable and efficient methods have been developed in laboratories to improve the morphology as well as the performance of the solar cell devices.

The first strategy is to control the solvent evaporation process by altering the choice of solvent, concentration of the solution and the spinning rate (Zhang et al., 2006). The slow evaporation process assists in self-organization of the polymer chains into a more ordered structure, which results in an enhanced conjugation length and a bathochromic shift of the absorption spectrum to longer wavelength region. It is reported (Peet. et al., 2007) that chlorobenzene is superior to toluene or xylene as the solvent to dissolve polymer/PCBM blend during the film casting process. The PCBM molecule has a better solubility in chlorobenzene and therefore the tendency of PCBM molecule to form clusters is suppressed in chlorobenzene. The undesired clustering of PCBM molecules will decrease the charge carrier mobility of electrons because of the large hopping boundary between segregated grains.

The second strategy is to apply thermal annealing after film casting process. This processing technique is also widely used for organic field effect transistor materials. The choice of annealing temperature and duration is essential to control the morphology. At controlled annealing condition, the polymer and PCBM in the blend network tend to diffuse and form better mixed network favorable for charge separation and diffusion in the photoactive layer (Hoppe & Sariciftci, 2006).

### **2.5 Stability**

The air stability of the solar cell device, as it is important for the commercialization, has attracted more and more attention from many research groups worldwide. Even though

Conjugated Polymers for Organic Solar Cells 457

level due to the compromise of stability, band gap and open circuit voltage. The ideal polymer LUMO level should range from -3.7 eV to -4.0 eV against vacuum level to assist

Polymer prepared for solar cell application should possess reasonable solubility so that it can be analyzed by solution based characterization methods such as NMR spectroscopy. Meanwhile, polymer with poor solubility will be found inappropriate for solution processing and device performance is normally low due to unfavorable microscopic morphology of the thin film formed by spin coating. Aliphatic chains attached to the polymer backbone are essential to ensure solubility of the polymer. However, it should be noted that aliphatic chains, being electronically inactive, will dilute the conjugated part of

Some rules of thumb regarding the use of alkyl chains include that: 1) longer chain is better than shorter chain to solubilize polymer; 2) branched chain is better than linear chain to solubilize polymer; and 3) the more rigid or planar the polymer backbone is, the more or

Common monomer building blocks to construct conjugated polymer for solar cells have been summarized in Chart 1. They are categorized by number of rings and way of linkage. Due to the space limit, we will only discuss monomers that are commonly encountered in the literature and the property of their representative polymers. Some important building blocks, even though not commonly used for PSC polymer, are also included for comparison.

Ethylene (double bond) is a commonly adopted spacer or bridge in conjugated polymers. Common chemical methods to introduce double bond to the polymer include: Wittig-Horner reaction; Wessling sulfonium precursor method (Wessling, 1985); Gilch route (Gilch

By utilizing Wittig reaction, fully regioregular and regiorandom poly[(2-methoxy-5- ((3',7' dimethyloctyl)oxy)-1,4-phenylenevinylene] (MDMO-PPV, P1 and P2) were synthesized following the route shown in Scheme 1 (Tajima et al., 2008). The device study on these two polymers showed that the regioregular MDMO-PPV-based device had a PCE of 3.1%, which was much higher than 1.7% out of regiorandom MDMO-PPV. The higher crystallinity of the regioregular MDMO-PPV polymer and better mixing morphology with PCBM were ascribed to the improved PCE for regioregular MDMO-PPV. This study highlighted the importance of regioregularity of PPV-based polymer to achieve good solar cell performance.

Polyacetylene was the first discovered conducting conjugated polymer and inspired a lot of scientific interest in the research of conjugated polymers (Shirakawa et al., 1977). The synthetic chemistry of acetylene-containing polymers has been intensively reviewed by Liu *et al*.(Liu et al., 2009). In polymers designed for solar cell, acetylene is normally introduced to

electron injection from polymer to acceptor.

longer alkyl chains are needed.

**3.1 Ethylene (double bond)** 

**3.2 Acetylene (triple bond)** 

the polymer and hence, reduce the effective mass of the polymer.

**3. Common building blocks for BHJ solar cell polymers** 

& Wheelwright, 1966) and palladium catalyzed coupling reactions.

the polymer backbone via Sonogashira cross coupling reaction.

**2.7 Solubility** 

industry pays more attention to the cost rather than the durability of the solar cell device, a shelf lifetime of several years as well as a reasonably long operation lifetime are requested to compete with Si-based solar cells. The air instability of solar cell devices is mainly caused by polymer degradation in air, oxidation on low work function electrode, and the degradation of the morphology of the photoactive layer.

For a conjugated polymer to achieve such a long lasting lifetime, it should have intrinsic stability towards oxygen oxidation which requires the HOMO energy level below the air oxidation threshold (*ca.* -5.27 eV) (de Leeuw et al., 1997). Device engineering can also provide the extrinsic stability by sophisticated protection of the conjugated polymer from air and humidity.

### **2.6 Desired HOMO/LUMO energy level**

The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of the polymer should be carefully tuned for several considerations. First of all, the HOMO energy level of a material, which describes the accessibility of the material molecule to be oxidized, reflects the air stability of the material. The oxidation threshold of air is -5.2 eV ~-5.3 eV against vacuum level. Therefore, the HOMO level cannot be more positive than this value to provide the air stability to the polymer. Secondly, the maximum open circuit voltage (*V*oc) is correlated to the difference between the LUMO energy level of PCBM and the polymer's HOMO energy level based on experimental evidence (Brabec et al., 2001; Scharber et al., 2006). Therefore, in order to achieve high *V*oc in the device, HOMO level should be reasonably low.

To ensure efficient electron transfer from the polymer donor to the PCBM acceptor in the BHJ blend, the LUMO energy level of the polymer material must be positioned above the LUMO energy level of the acceptor by at least 0.2-0.3 eV. Based on these factors, as shown in Fig 2, the ideal polymer HOMO level should range from -5.2 eV to -5.8 eV against vacuum level due to the compromise of stability, band gap and open circuit voltage. The ideal polymer LUMO level should range from -3.7 eV to -4.0 eV against vacuum level to assist electron injection from polymer to acceptor.
