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Organic Solar Cells

Bulk-hetero Junction - Click to enlarge
The absorber layer of an organic solar cell consists of two materials, acceptor and donor. Small domain sizes of this blend account for an efficient charge carrier dissoziation and hence a high photocurrent yield.

Organic solar cell research

Organic solar cells have been attracting much attention since their record power conversion efficiencies reached values beyond 10%, since this makes it comparable to amorphous silicon. Furthermore, manifold opportunities in design and system integration due to color-tunability, mechanical flexibility, semi-transparancy and potentially low production costs increase their potential for commercial utilization. All these results originate from intensive research by physicists, chemists and engineers during the last decade - in companies as well as in universities from all over the world.

Nevertheless, besides obtaining high efficiencies in light harvesting some other issues still exist: Demonstrating appropriate lifetime stabilities and routes to transfer laboratory processes to a competative production are part of several governmentally funded projects. One of the key points of this transfer process into large-scale production is the establishing of non-toxic fabrication methods. This includes the  polymer synthesis as well as the layer deposition from non-halogenated solvents.


Basic architecture and working principle

The very basic architecture of an organic solar cell consists of two electrodes, at least one transparent, and the absorber layer. The incident photons will be absorbed in the active layer and generate electrone-hole-pairs which form excitons due to their coulomb attraction. To overcome this bond in order to extract free charge carriers at the electrodes a certain amount of energy is neccessary. It can be provided by material interfaces exhibiting a distinctive energy difference between their charge carrier transport levels. This holds for acceptor-donor-interfaces. Since the diffusion length of the formed excitons is limited to some nanometers a dense network of these interfaces is preferable. The common approach is to mix both donor and acceptor in one solution and to deposit them in one step. This self-organized single layer is called bulk-hetero junction. After excitation and dissociation of the charge carriers they drift to the corresponding electrodes driven by the internal electrical field induced by a difference in work-function. By contacting the electrodes these carriers can be extracted and exploited for various applications.