**1. Introduction**

Organic photovoltaic devices are often seen as the future of solar cells [1] as they permit energy conversion with much less consumption of material resources than the conventional silicon based semiconductor cells [2]. Additionally they permit the development of flexible devices while the traditional cells are rigid and fragile [3].

Even though since the discovery of the photoelectrical effect in organic substrates large progresses have been achieved [4] to increase yield and stability the organic devices still are much less efficient and present lower lifetimes than the traditional solar cell form [5]. There have also been progresses in developing different cell types. For practical reasons here we center on heterojunction bulk cells as they provide high reproducibility and easy experimental access to poly-layer devices [6].

The construction of heterojunction cells requires several basic elements apart of the proper semiconducting polymer. The polymer has to be linked to a layer normally fullerene—that accepts the electrons from it. It is also important to count with a "buffer layer" that inhibits the recombination of electrons with the "holes" formed in the same process. Buffer layers are often made of copper-(I)-iodide and molybdenum-(VI) oxide. An appropriate design of this layer can increase the electrical yield in up to 10 times in some cases.

As limits for commercialization 10% of efficiency and 10 years of life expectancy are considered for organic cells. The yield limit already has been reached in some cases due to improvements in the chemical design of donor and acceptor materials [7]. Lifetime is still problematic as degradation of the organic substrate can take place for several reasons as photochemical reactions, interaction with traces of water, encapsulation of the device, thermal instability, etc. Same problems have been found also in other applications and strategies to overcome them are developed [8].
