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Basics of OSC

Organic solar cells are made of thin layers of organic materials with thickness in the 100 nanometer range. They were first introduced by the research group of Dr. Ching Tang at Kodak Research Laboratories in 1986. The motivation for using organic dyes is to replace the expensive silicon in conventional photovoltaics and to apply simple production techniques. Additionally, organic solar cells can be prepared on plastic foil and so they are ideal candidates for flexible and portable systems.

Organic solar cells basically comprise the following layers: first electrode, electron transport layer, photoactive layer, hole transport layer, and second electrode. In general, a solar cell absorbs light, separates the created electrons and holes from each other and delivers electrical power at the contacts. The fundamental difference between the working principles of organic and inorganic solar cells is the direct generation of free charge carries in the inorganic solar cells. In organic materials the light absorption is followed by the creation of excitons with a typical binding energy (due to coulomb-interaction) of 0.3-0.5 eV.

Since the necessary electric field (> 106 V/cm) to overcome this binding energy is not available in an organic solar cell, the excitons are usually separated a the interface between two different organic layers (heterojunction). The energy alignment of these two materials has to be optimised, so that on the one hand the excitons are efficiently separated, but on the other hand no energy might be lost in this process.

Today three different types of organic solar cells are known: the organic semiconducting material can either be comprised of so-called small molecules (SM solar cells) or polymers (polymer solar cells). The third type of organic solar cells is called dye-sensitised solar cell (or Grätzel cell) and contains a highly porous layer of titanium dioxide as electron transport layer on which dye molecules are adsorbed. Small molecule solar cells are processed in vacuum by physical vapour deposition, whereas polymer solar cells are processed by spin-coating or ink-jet printing (vacuum deposition is still necessary for metal deposition). Grätzel cells are typically processed by screen-printing of the titanium dioxide with subsequent sintering and dying. The OSOL group at the IAPP concentrates on small molecule solar cells.