*3.2.1 Polypyrroles*

Polypyrroles (**Figure 8**) can be obtained via electrochemical deposition using cyclovoltaic techniques [29]. During the synthesis different solvents could be employed as THF, acetonitrile chloroform, etc. The solvent is important for the morphology of the deposed polymer. Using acetonitrile a product is obtained than from chloroform. The solvent is therefore selected according to the needs of the desired application and in function of the employed oxidating agent.

#### **Figure 8.**

*Polypyrrole in the ideal reduced form (above) is normally obtained as partially oxidized (p-doped) form (below). Charges are compensated by anions trapped in the polymeric material.*

This way and controlling current and cycle number the layer thickness can be determined. Anyhow the polymer obtained this way has some disadvantages that limit its use in photovoltaic devices. First of all doping is only reversible in very thin layers. This limits the available charges as well as the mechanical stability of the polymer film. Second polypyrrols have shown to be very sensitive to excessive oxidation as it leads to chain degradation especially by reactions through positions 3 and 4 of the pyrrole system [30]. Blocking these positions with substituents or forming copolymers with aniline this type of degradation can be prevented leading to electrochemical more stable polymers.

These substituents according to their electron donor or acceptor characteristics will also alter the color of the polymer as they influence the HOMO and LUMO energy levels. There is also an influence due to steric hinderence as this will force the rings to present torsion angles and interrupt the perconjugated π-system. Less conjugation on the other hand will lead to decreased electron mobility and increased electrical resistance. So these effects have to be taken into account when designing a polymer more resistant to oxidative degradation.

#### *3.2.2 Polythiophenes*

Polythiophenes (**Figure 9**) are among the best studied and most promising conducting polymers [31]. Similar to polypyrroles polythiophenes can be obtained by electropolymerization from acetonitrile solutions at potentials between 0 and +1 V. The polymerization process starts with the generation of a cation radical. Due to the high reactivity of this species anhydrous conditions are crucial. The larger attention received by polythiophenes compared to polypyrroles is due to its higher chemical stability. Additionally it can be more easily processed and it is easy to obtain derivatives of the monomer. This allows tuning optical and mechanical properties of the polymeric materials and converts thiophenes in one of the most valuable staring materials for tailor made organic semiconductors.

The influences of the substituents on the properties of the polymer chain depend largely on their positions in the chain. For example, the reduced form of poly-3-methyl-thiophene shows a large variety of colors as small changes in conjugation will lead to large effects on optical- and redox properties and morphologies.

Introducing an electron withdrawing group instead of the methyl moiety will help to stabilize the radical anion when n-doping the polymer material.

#### **Figure 9.**

*Polythiophene in the "normal" reduced form (above) and in the partially oxidized p-doped form presenting two charges in form of a bipolaron (below).*

#### *Polymers in Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.85312*

With copolymers obtained from nitrothiophenes and linked to carbon nanotubes yields of about 2% have been obtained [32]. They have an open circuit potential superior to polythiophenes without nitro group [33]. Other devices from polythiophenes with electron attracting groups equally have shown yields of up to 3% with an open circuit voltage of close to 1 V [34]. This shows that the groups help to stabilize the electron in the LUMO and the band match with the electron acceptor entity increases the yield [35].

Alkoxy groups as donor substituents will increase the electron density in the polymer chain and help to decrease the HOMO-LUMO-band gap leading to a red shift of the corresponding absorption band in the UV-Vis spectrum. Variations in the length of the alkyl chains however have little or no influence on the properties and no changes in conjugation or band gap values have taken place. When formed the alkoxy modified polymers are colorless and do not show absorption in the visible spectrum but turn dark blue on electrochemical reduction.

As an example 3,4-ethylenedioxithiophene (see **Figure 10**) and its polymers have a low oxidation potential, high chemical stability even at elevated temperatures and high conductivity. The copolymer with polystyrenesulfonate is one of the best organic candidates to substitute ITO as electrode material as it is at the same time highly transparent and has a very high ductility. It is already used as antistatic coating [36].
