When looking into organic molecules such as dyes or donor-acceptor pairs for OPV applications one often comes across the term "..organic molecules are strongly absorbing materials...". This raises two questions for me:
a) why are they so strongly absorbing ? Is it the mere presence of pi- orbitals and the hybridised orbitals, or is there more to it?
b) qualitatively and quantitatively I was wondering what the word "strong" is supposed to mean. compared to what? I get that the extinction coefficient is typically in dimensions of "per mole and cm". Is it easily compared to a mole of an inorganic material? Can one really make such statements, the density of inorganic crystals is probably higher in most cases, thus they loose with respect to the 1/[M] renormalisation. But is that a fair comparison?
Thank you Peter for that quick answer. Indeed I am aware of the quantum mechanics behind absorption, that's why I am asking. Of course it does make sense to take the concentration of a dispersion of molecules into account, and therefore the molar extinction coefficient. Still I have a problem with the term "strong". It's probably good to know that I have been working with chalcogenide nanporaticles in the past, which often have molar extinction coefficients (that term I always found peculiar for particles, and it's often been confusing, because many people speak of moles of particles while others mean "molecular" moles, but maybe that's just me) orders of magnitudes higher than 105. This is why I wanted to relate the organics to inorganics, because at present, I have a better feeling for the numbers for inorganic materials. Maybe such a comparison is bound to fail, I don't know. Regarding the absorption cross section: this would be, from my point of view, the most appropriate figure of merit, however here I don't really know any numbers, are organics (let's say rhodamine and silicon for simplicity) still good or have a big absorption cross section? A fast google search didn't really answer that question.
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