= Optical absorption in organic semiconductors
Molecular excitations in <organic semiconductors> often retain strong local character. Weak screening supports <Frenkel excitons> or intermolecular charge-transfer admixtures, so a photon can create a bound neutral excitation rather than free carriers. Transition dipoles, <spin selection rules>, molecular packing and <Franck-Condon factors> set the bright spectral features. Disorder and thermal vibrations broaden them. Electronic overlap can also produce more delocalized excitons; the molecular limit is a useful model, not a definition covering every organic solid.
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