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On page 2 of this paper: "When the incident photon hits electrons at the ground state, inorganic semiconductors generate free carriers. However, in organic semiconductors, excited electrons slightly relax and then form an exciton, a bounded electron and hole pair."

Why is this only applicable to organic semiconductors?

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For most semiconductor, e.g. GaAs, a exciton has binding energy about 6 meV ($0.006 eV$) and with a binding length about $200 \dot{A}$. That is a very loosely binded state, can be broken into free electron and hole, easier to propagate out of the surface region.

On the contrary, an organic exciton is usually located within the molecule and has a binding energy in order of 1 eV. These make the exciton hard to break, and slow in propagate out of the surface region.

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