Organic solar cells

Now that solar panels have more than proven their merit, the race is on to enhance or replace the current concept of solar cells with something more (cost-)efficient and even more friendly to the environment. The main problem with the current concept, is that the refinement of solar cell materials (mainly high-grade silicon) is an expensive, energy-inefficient and downright environmentally unfriendly process. Organic solar cells are a relatively recent technology that might well be the next step in the evolution of solar cell technology as a whole. Even though the technology is not quite ready yet, recent scientific breakthroughs have certainly openend up the future to this new and exciting form of solar cell technology. Organics are not only cheap, they are also highly renewable and almost completely emission-free.

How organic solar cells work

Solar cells on an organic basis work on a principle that is slightly different from that of normal solar cells (how solar cells work). The light that is absorbed produces so-called excitons, electrons that are promoted from the valence to conduction band, yet are still electrostatically bound to the hole they occupied before. Since the electrons have no free movement, current can not flow in the material without taking additional measures. 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.

Advantanges and disadvantages

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. Organics are also very flexible and can be applied to almost any surface, such as thin platic film or even layers of paint. 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. Finally, organics are also a factor 10-20 times cheaper than the silicon used in present-day normal solar cells.

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. 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. The thinner the organic cell, the greater the chance that the exciton is able to reach the interface across which it is dissociated (see ‘how organic solar cells work’). The second problem is the inherent sensitivity of organics to ultraviolet (UV) radiation; without a protective UV-film, the organic layer quickly breaks down. Finally, organics are also highly sensitive to oxidation, for which a good protective coating is yet to be invented.

The future of organic solar cells

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: 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 is 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.