*3.2.3 Polyanilines*

Like polythiophenes, polyanilines (PANI) can be obtained by electrochemical oxydation via cationic radicals in a complex oxidation and reduction process as it goes along with protonation and deprotonation reactions. In acid media a typical voltamperogram of PANI shows two redox processes that correspond to the conversion of two oxidation stages present in this polymer that go along with electrocromatism. The completely reduced form is colorless and therefore known as leucoemeraldine, from Greek "leucos" = white. In this form only phenylic systems are present that are linked by secondary amino groups.

A partially oxidized form, known as emeraldine due to its green color, is formed by alternating quinoid and phenylic structures with one quinoid structure every four ring systems. The end product of oxidation known as pernigranile, presents only quinoid structures and can be distinguished by its dark blue to black color (see **Figure 11**).

Chemically all polyanilines are basic due to the presence of trivalent nitrogen. Hydrogen of secondary amino groups can also become slightly acidic, especially when the polymer is p-doped and thus positive charges are introduced to the chain.

**Figure 10.** *Motive of poly-3,4-ethylenedioxithiophene.*

Deprotonation will reduce charge density in the polymer and limits its conductivity. Otherwise it is of the same magnitude as that of other polymeric aromatic systems.

## *3.2.4 Aniline copolymers*

In previous works we have synthesized aniline-thiophene and aniline-pyrrole copolymers to improve conductivity, conductivity and optical properties. The obtained materials show—as expected—optical and electrical properties in between the pure polymers obtained from aniline and thiophene respectively. Anyhow solubility in all cases is higher for the copolymers than for the simple separated substrates.

In the UV-Vis spectrum of the aniline-thiophene-copolymer various absorption maxima could be observed: bencenic π-π\* transitions at around 350 nm; π-π\* transitions of the thiophene systems at 450 nm and quinoid transitions at about 700 nm. Additionally in the 650–800 nm range n-π\* transitions could be detected that correspond to the transitions of non-bonding sp2 nitrogen electrons present on quinoid ring systems [37].

When photovoltaic devices were built from this copolymer the ABL material did not have any influence on the light absorption of the polymer neither in presence of MO3 nor CuI with no change in absorption rates and position of the peaks in the UV-Vis spectra. As consequence the band gap also remains unaltered. On the other hand the copolymer had a higher red shift than any of the pure substrates. This means that conjugation rate had increased and the band gap diminished when compared to polyaniline and polythiophene. Herewith the copolymer is a better candidate for photovoltaic devices. Effectively unlike polyaniline it shows photovoltaic yield as electron donor when used with MO3, CuI or mixtures of both as ABL layers with a modest yield of 0.2–0.5%. A possible explication could be the interaction of the polymer with the copper metal centers that only leads to crystallinity close to the

#### **Figure 11.**

*Polyaniline in the completely reduced (leuco) form (above), the partially oxidized (emeraldine) form and the completely oxidized (pernigraniline) form (below).*

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

polymer-buffer layer interface while the rest of the polymer has low crystallinity. The polymer has also only a modest conductivity and a medium light absorption coefficient at about 700 nm. Thus the few polarons generated by light absorption have little mobility and most of them recombine before reaching the next layer [18, 38].

Bernède et al. have shown that MO3 helps to increase extraction work achieving a match for the band gap between ABL and polymer. When poly-aniline-pyrrole is deposed over MO3/CuI as ABL a small red-shift of the absorption maxima in the UV-Vis spectrum is observed compared to other ABL substrates. As in other cases the red-shift in absorption corresponds to a smaller band gap in the electronic characteristics. The photovoltaic behavior is similar to that of aniline-thiophene copolymer on any MO3/CuI combination as ABL substrate.

### *3.2.5 Non polymeric organic and organometalic substrates*

As already mentioned the use of different tampon layers changes the photovoltaic yield and in the right combination it helps to improve it. The effect has been studied by us and Bernède et al. not only with polymers but also with monomeric species. Copper-phtalocyanines behave similar to polymers when deposed on different buffer layers.

To find out if these or other molecules could be used in organic photovoltaic devices their electrochemical properties play a crucial role and herewith the energy of the frontier orbitals. When these molecules are deposed on an electrode by cyclovoltametry the potentials of oxidation and reduction starts can be determined. These values are related to the energies of the frontier orbitals as in oxidation an electron is withdrawn from the HOMO while during reduction an electron is deposed in the LUMO. Orbital energies are proportional to the corresponding redox potentials. Due to the experimental ease these values can be obtained the technique is used by an increasing number of workgroups.

### **3.3 Functional groups modifying electron density**

#### *3.3.1 Electron withdrawing groups*

Electron withdrawing groups lower the energy of the antibonding LUMOs and herewith stabilize charges present in this orbital. For this reason they are used often in the design of material for electron accepting layers. They can also help to improve the electron transition between accepting and donating molecules. This is not only due to the stabilization effect on the LUMO but they also increase the interaction between both molecules. In some cases they can directly bind acceptor and donor units. The most commonly used electron attracting groups are carboxylic acids and their derivatives.

Lately investigations turned to other functional moieties as sulfonates, salicylates pyridine, catechol, etc. that help to fix both parts together [39–41] even so their electron attraction capacity is only moderate. All of these groups have proven useful in the development of devices with higher overall yield. Another new target group is the nitro moiety. It does not only withdraw more electron density, shifts the absorption maximum of the modified substrates to farther red but also has higher stability than the carboxy-derivatives. Its anchoring abilities could be attributed to the formation of π-complexes as these interactions are well known for electron rich aromatic systems and others bearing nitro groups. All these effects even increase when several nitro groups are present. As mentioned earlier this is caused by the fact that the nitro group helps to link to the electron donor and it increases the band gap and with it the open circuit voltage [42, 43].

#### *3.3.2 Electron donating groups*

While electron withdrawing groups stabilize electrons in the LUMO or the corresponding conduction band electron donor groups have the opposite effect and achieve a relative stabilization of holes in the HOMO increasing the energy of this orbital. The observed increase is even higher on the LUMO and as a consequence the gap between conduction and valence band becomes larger. When charges are present in both bands the larger energy difference leads electrically to a higher measurable open circuit cell voltage.

Donor groups also diminish the translation energy for holes making positive charge transport more effective. Unfortunately at the same time electron transport is hindered and slows down. For this reason one has to take care to equilibrate the number of electron donor and electron withdrawing groups to guarantee that both speeds are comparable and holes and electrons reach their respective electrodes at about the same time [44–46].

#### **3.4 Buffer layers**

Buffer layers also known as interface layers are crucial in organic photovoltaic devices to improve the overall yield. The correct selection and deposition of the materials that form this layer are therefore important decisions that could decide if a new approach has success or not. Changing the construction of the buffer layers up to 10-fold increase in the overall electrical yield has been observed in an extreme case [47] due to rise of the open circuit potential, the short circuit current and the form factor.

Many materials that could be used in buffer layers are permeable as well for electrons as for holes. This lowers the yield as it leads to electron-hole recombination on the electrodes. MoO3/CuI-layers however let pass selectively positive charges and improve their pass to the anode. As during this step they are separated from the electrons this prevents recombination and herewith keeps the yield high.

The buffer layer material has also influence on the morphology of the material deposed in the next layer. In chemistry the affinity between copper and nitrogen, sulfur or other heteroatoms with free electron pairs is well known. When these elements are present in the substrate used in the next layer the intermolecular interactions will force the molecules in a defined shape and herewith provoke a determined morphology. Generally it increases the degree of order and crystallinity at least close to the interphase. This helps to increase the charge transport to the anode [47–49]. The effect of incorporation of other elements with modified affinities has still to be studied.

### **4. Conclusions**

Photovoltaic devices nowadays represent a fast growing multi-million dollar market that has the potential to provide energy to humanity without contributing to CO2 emissions. The largest part of it belongs to classical cells based on silicon as inorganic semiconductor. These devices however require a large amount of prime matter that are themselves highly energy consuming in their fabrication. Additionally they are not flexible, a fact that reduces the possible application fields.

A solution can be organic photovoltaic solar cells based on conducting polymers. These devices can be built as thin multi-layer constructions on flexible substrates as plastics or thin glass plates consuming just a little portion of the resources used for silicon devices. Anyhow organic cells still provide much lower conversion yields than the inorganic counterparts.

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

Organic bulk heterojunction cells are large constructs with various stacked layer. In the stack every layer has a dedicated function and the overall yield depends on the combination and cooperation of all parts. Strategies to improve the device will have to include necessarily not only fine tuning of each of the parts but also take into account the interaction of the layers. We present some viable and proven strategies to ameliorate the properties and interactions of several of the involved materials. These strategies include co-polymerization, the selection of appropriate functional groups understanding the influence they have on the polymer and the construction and interaction of the layers in direct contact with the semiconducting polymer.
