**2.7 Solubility**

456 Solar Cells – New Aspects and Solutions

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

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

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

Fig. 2. Optimal HOMO/LUMO energy level of optical polymer used in BHJ solar cell with

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

of the morphology of the photoactive layer.

**2.6 Desired HOMO/LUMO energy level** 

level should be reasonably low.

PC60BM as acceptor (Blouin et al., 2008)

and humidity.

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

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 longer alkyl chains are needed.
