**3. Operational principles of organic photovoltaic cells (OPVCs)**

OPVCs use the internal photoelectrical effect that liberates electrons and is used to transform light into electrical energy. The energy generation depends on the

absorbed light. The higher the absorption rate the more electrical energy is produced [17] in form of excitons. This makes is clear that the conversion rate is linked to the absorption coefficient of the substrate in the UV-Vis spectrum. The maximum is also indicative for the band gap. The higher the wavelength of the absorption maximum the lower is the band gap and the higher can be the theoretical photovoltaics yield. On the other hand too low band gaps favor the recombination of the electrical charges as the substrate becomes a conductor and inhibit the proper work of the cell. Therefore values of 1–2 eV are desirable what would correspond to an absorption maximum at about 1200–600 nm [18]. Absorbed energy is used to promote an electron from the HOMO to the LUMO. In a first moment the electron stays close to the positive charge produced in the same process (see **Figure 5**). The electron-hole pair is known as "exciton." It has to be taken to the polymer—electron acceptor interface where it is separated and the electron is guided to the electrode. An exciton blocker layer (EBL) between the acceptor layer and the electrode inhibits that positive charges migrate to the electrode as this would result in direct charge recombination and no electrical current could be obtained from the device.

The distance an exciton could migrate in a layer is of the order of several 100 nm [19, 20]. The polymer donor layer should therefore have a thickness that is inferior to the migration distance to avoid that excitons recombine on their way to the acceptor-layer. Better conductivity of the polymer allows building thicker layers as it increases the maximum migration distance. Thicker polymer layers are desirable as they lead to higher absorption rates. The same accounts for the acceptor layer.

Typical dimensions found in cells using phtalocyanine-copper as donor and fullerene as acceptor, are some 20 nm for the fullerene and some 40 nm for the phtalocyanine layer for larges possible negative charge transfer.

For charge transfer also a high contact surface is desirable. Double layer and interpenetrated layer devices have been developed. Interpenetrated devices present at least theoretically higher conversion rates due to the large contact surface. Anyhow their construction is difficult and often cannot be reproduced easily.

Double layer devices on the other hand are easily obtained and their structure can be controlled with little effort but have small contact surfaces and herewith lower yields. At least for comparison reasons, the benefits of the high reproducibility compensate the lower yields as they permit detecting promising materials with more accuracy. For real world applications the thus found substrate can later still be used to produce interpenetrated devices.

The construction of a double layer device starts with a transparent and conducting substrate. Normally this is glass or poly ethylene terephthalate (PET) with an

#### **Figure 5.**

*Excitons can travel along the organic semiconducting material and are separated to electrons and holes on the exciton blocking layer. Long diffusion length favors recombination and efficiency loss [23].*

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

indium-tin-oxide (ITO) layer as transparent conductor. On this generally commercially available substrate an anode buffer layer (ABL) of some 20 nm is deposed. In our work we use molybdenum-(VI)-oxide and copper-(I)-iodide. In spite of their high band gap (>3 eV) characteristic for electrical insulators they help to extract cations as ITO has a high barrier to hole collection. The ABL reduces the barrier matching the energies of the involved bands. In some cases and depending on the ABL substrate it also induces higher crystallinity and improved morphology in the following polymer layer [21]. In our work with polyaniline and polyaminothiophene we observed an increase of the photoelectrical yield of some 10 times due to the proper design of the ABL using MoO3/CuI. An explication could be the interaction of copper ions with the nitrogen atoms present in the polymer. Optimization of these interactions forces the polymer chains in a parallel direction to the polymer-ABL-interface and herewith leads to an increased order close to the interface. A prove for higher degree of orientation is given by X-ray diffraction where polymer deposits on CuI layers show more and sharper peaks than polymers grown on other substrates. The higher crystalline degree lowers the electrical resistance and this leads to the observed yield improvements. Further investigation shall show if other elements known to show high tendency to form interactions with nitrogen as cobalt, nickel or zinc have similar or even better effects on the polymer growth. After the polymer is deposed as electron donor it follows an electron acceptor layer, normally 40 nm of fullerene for best electron mobility [22].

It follows a thin layer of bathocuproine (BCP). As in the case of the ABL it is a substrate with a relatively high band gap (about 3.5 eV). Its purpose is to avoid the pass of the positive holes to the cathode and herewith recombination of holes with electrons. Therefore the layer is also known as exciton blocking layer (EBL). On the other hand the insulating properties of the material that are necessary for the exciton blocking will decrease the yield of the solar cell. Thus it is necessary to find a balance that optimizes both effects looking for an improved layer thickness. In most applications this is about 4–5 nm.

Finally on the EBL follows aluminum as cathode and selenium as protecting material that prevents oxidation of the organic molecules involved in the sandwich structure.

As shown above buffer—and blocking layers play an important role in the optimization strategy of organic photovoltaic devices and have to be treated along with the organic substrate used as electron donor. The combined tuning of all parameters has allowed designing photovoltaic cells that reach already 13% [24].

When looking for improved electron donor materials at least two important features should be taken into account:


Additional optimization can be done in all other involved layers. Thus molecules with higher electron acceptor capacities and better stability than fullerene are investigated [25].

Apart of this more chemical part also the electrical behavior of the solar cell is important. Generally it can be seen as a serial circuit. All layers act in this case as

serial resistors (RS). The electrical resistance especially of the donor—and acceptor layers will diminish the current density (Jsc) in the solar cell. Additionally due to the cell construction and its electrical contacts some parallel resistances (Rsh) can be observed. The resistances increment also when quenching (coQ ) occurs as it lowers the quantity of charges present in the valence- and conduction band.

There also exists a phenomenon known as "dark current." It consists in the flow of electrons that have already passed to the fullerenes as electron acceptors back to the HOMO of the electron donor. This effect is increased by the polarization observed in the working solar cell as here the negative charge produced in the cathode attracts the positive charges still present in the donor material and increases its density close to the electrode. The dark current could be seen as an "indirect quenching" and also lowers the obtained electrical current. To diminish the dark current, an electron blocking layer (EBL) is applied with an Eg sufficiently high as to prevent the migration of positive charges to the cathode.

As one can deduct from the previous chapters after the exciton formation charge separation is a key step of the whole photo-electrical process. Whatever substrate is used as donor- and acceptor, ionization potential (HOMO) and electron affinity (LUMO) have to match in the way that it permits the exciton dissociation. The electron affinity of the donor has to be less than that of the acceptor and the ionization potential of the acceptor has to be considerably higher than that of the donor molecule. Otherwise no exciton separation corresponding to the crossing of electrons from the LUMO of the donor to the LUMO of the acceptor will occur [26]. The whole construct can be seen in **Figure 6**.

The already mentioned need for high absorption rates makes it clear that the design of solar cell materials and the design of dyes have to be intimately related. In fact a part of the here mentioned bulk layer solar cells there exist other designs as the "dye-sensitized solar cell" (or Grätzel-cell) where the relation becomes even more evident [27]. One of the best known dye-precursor is aniline. Already its discoverer Wöhler mentioned that wood treated by aniline turned to several colors of a while. Anyhow it was Perkin some years later who patented the first synthetic dye also based on aniline—the mauveine [28]. Since then a whole family of colored compounds known as "aniline dyes" has been synthesized. It is also known that aniline can be polymerized under oxidating conditions forming a green semiconductive polymer known as "emeraldine" due to its intense green color. Further oxidation would lead to blue-violet pernigraniline. The difference is the presence of varying quantities of hydrogen on the nitrogen atoms. Higher hydrogen-content is related to a reduced leuco-form with only amino groups while oxidation and hydrogen abstraction leads to a large proportion of quinoid structures that allow better perconjugation and herewith lowers the energy of the LUMO-orbitals. The result is a semiconducting organic material with low band gap and high absorption coefficient—perfect for applications in organic solar cells.

#### **Figure 6.**

*Typical disposition of the elements that belong to a bulk heterojunction solar cell made of organic polymers as electron donor material.*

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

Another substructure known to lead to good properties as organic semiconducting material is thiophene. The thienyl-system is in many aspects equivalent to a benzene moiety buy has an even higher "aromatic" character. Additionally due to the presence of the sulfur heteroatom it undergoes easier oxidation reactions. For this reason we became interested in aniline-analog polymers based on thiophene systems. The natural precursors in this case are 2- and 3-aminothiophenes. Unlike aniline in an amino substituted thienyl system all carbons are different leading to a larger amount of possible isomers formed in the polymerization reaction that is formally an additional substitution.
