How Do Organic Solar Cells Function? – Part Zero

In a classical inorganic solar cell, pairs of charge carrier – an electron and a hole – are generated by the absorbed sunlight. These two oppositely charged carriers are only weakly Coulomb bound, due to the screening being rather efficient in this material class. The potential drop at the interface between a p- and an n-doped semiconductor layer (the pn junction), leads to their separation and subsequent transport to the respective contacts: a current flows. In organic semiconductors, things are somewhat different.

Here, the screening of opposite charges is much weaker as the dielectric constant is lower. This leads to a much stronger interaction of the photogenerated positive and negative charges. Spring in France Therefore, the primary optical excitation in organic materials is called (singlet) exciton, i.e., a strongly bound electron-hole pair. As this binding is more difficult to be overcome as compared to inorganic systems, the concept of organic solar cells has to be different… which we will come back to later.

Another difference between organic and inorganic solar cells is less principal, but also has significant consequences. About 95% of the silicon solar cells produced every year are made of crystalline silicon, in which the atoms are ordered (almost) perfectly well, and the charges can travel quickly after they have been photogenerated. In contrast, organic semiconductors interesting for electronics applications are rather amorphous, polycrystalline at best. (Now there also exist amorphous silicon as well as organic crystals, but in terms of applications, the statement above describes the usual case.) The advantage with disordered stuff is, and that is also true for inorganics, it is easier to make. Unfortunately, not all is so well: charge transport is more difficult: crystals are like the autobahn for charges, whereas disordered matter are country roads at best. Due to the lack of long-range order, the electrical transport in disordered semiconductors usually takes place by hopping from one localised state to the next, instead of gliding quasi-freely through the carrier band of crystalline semiconductors.

So, up to now we have been talking about two issues making life more difficult for designing an efficient organic solar cell. Positive, however, is the ability to synthesise tailor-made organic substances, which – at least in principle – allow fine tuning of the absorption range, the charge transport properties, and maybe additionally allow for self-assembly… which is almost as good crystallinity;-) Of course, even hard working chemists (and many of them are) have difficulty to judge just from the chemical formula if every aspect of the designed substance is as intended. And that is why chemists and physicists still have to do trial-and-error, in the hope if finding the optimum materials for organic solar cells. One feature which is mostly good is the absorption length. That means that already very thin organic films can absorb all the light shone on them (within their absorption range). We are talking about 100nm or so, 100x thinner than hair. So only very little material is needed… compare this to crystalline silicon, standard wafers being 300 micron thick, i.e., several hairs on top of each other (by diameter, not length;) Considering world domination of photovoltaics (ok, unrealistic), this makes quite a difference in material use. Of course, clever scientists work also on making silicon solar cells much thinner, and there are interesting alternatives such as derivatives of the inorganic compound semiconductor CuInSe2, which is usually polycrystalline and has an absorption length below 1 micron. Not as thin as organics, but certainly not bad! But then, we don’t do inorganic solar cell bashing here;-)

So stay tuned for the next parts, starting with the basic function of how light is converted to current in organic photovoltaics, and the secrets of how to realise a printed multijunction organic solar cell on a flexible substrate!

P.S. If you wish, continue with Part 1 [Update 16.6.2009].

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