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.