Şükran GÜR Yelda ÇİFLİK

 Organic photovoltaic cells convert solar into electric energy is probably the most interesting research challenge nowadays.  A good efficiency of these devices has been obtained by using organic semiconductor materials.  Organic materials are abundant and easily handling  Unfortunately OPV cells efficiency is significantly lower than that of inorganic-based devices, representing a big point of weakness at the present.  OPV cells are very susceptible to oxygen and water.

How organic solar cells work? Semiconducting conjugated polymers are the organic materials used in OPV cells, since they possess the base property required to activate the fundamental mechanisms to transform the radiative energy of light into an electric current. When a donor (D) and an acceptor (A) material are being in contact, the result is the so-called heterojunction and this is the basis for the operation of organic solar cells.

 The creation of an exciton after the absorption of a photon is the first step. The exciton diffuses inside the material to reach the donor-acceptor interface where it will be separated.  Clearly the morphology of the D/A boundary also is of great importance. third step is the exciton split-up into free charges.  The last step, that is the transport of the free charges through the sample and their collection at the electrodesis the final step and it can be considered as central in a organic photovoltaic device.

Figure 2: Examples of materials used as donors (a) and acceptors (b).

 These measures are taken in the form of introducing a layer interface across which the potential of the electrons increases.  When an exciton crosses this boundary layer, it breaks down into an electron on one side and a hole on the other. The resulting charge imbalance causes an electric field to form at the boundary layer, pushing electrons to one side and holes to the other.  This results in clear charge separation and a difference in potential (a voltage) between the two extremities of the cell. By connecting the two sides of the cell, the electrons flow from one side to the other; a current flows across the connecting wire.

 Single-layer cells are the simplest, comprised of one organic PV material sandwiched between to metallic conductors. They are also the least advantageous, with very low efficiency and a conspicuous inability to create an electric field powerful enough to move excited electrons through the solar cell. In response, researchers devised the...

 Multi-layer organic solar cell, which contains two different layers of organic PV material carefully chosen to maximize the electrostatic forces created between the two. This breaks up electrons more efficiently, creating a better working but still problematic solar cell. Therefore...

 The dispersed hetorojunction photovoltaic organic solar cell was developed. In this type those two layers, one an electron donor, one an electron acceptor, are mixed together to form a polymer. This enables a more efficient solar cell than its predecessors, although these solar cells still fall prey to the common problems with organic photovoltaics described above.

The idea of organic solar cells was first put forward in the seventies, when it was discovered that the electrical conductivity of certain organic polymers greatly increased after contaminating their molecular structure with other chemicals.  Organics are a highly diverse kind of materials; their complex molecular structure allows many modifications to increase a specific material’s viability for a certain task.  There are problems though, the first one being efficiency. Solar cells on an organic basis have a conversion efficiency of only 3-5%, three to five times lower than the 15% casually reached by crystalline silicon solar panels.  Organics are also very flexible and can be applied to almost any surface, such as thin platic film or even layers of paint.  The second problem is the inherent sensitivity of organics to ultraviolet (UV) radiation; without a protective UV-film, the organic layer quickly breaks down.

 Another advantage inherent to organic materials is that its almost non-reflective. This means organic solar cells are less sensitive to less-then-ideal light conditions.  A large part of this limited eficiency is due to the principle on which organic solar cells operate. Excitons can only travel 3 to 10 nanometers before the electron drops back into its hole, effectively canceling itself out.  Organics are also a factor times cheaper than the silicon used in present-day normal solar cells. Solar cells on an organic basis have a conversion efficiency of only 3-5%, three to five times lower than the 15% casually reached by crystalline  Organics are also highly sensitive to oxidation, for which a good protective coating is yet to be invented.

 Much research has recently gone into improving the lifetime of organic solar cells.  And even though many improvements have been made, lots of work needs to be done before the technology becomes commercially viable.  One thing is however certain: is organic solar cells have the potential to revolutionize the way we see solar power. Part of this revolution is due to the enormous flexbility of organic material: it not unthinkable that they might one day even be applied as a special solar paint!

 Imagine a house that is entirely covered in solar paint, providing it with all the electricity it could possible need.  Think about clothing that recharges your cell-phone, an electric car which charges its own battery or a tent which powers the cooking gear on a camping site.  As you can see, the possiblities are many and exciting. We’ll just have to see if and when the organic revolution comes.

THANK YOU…