organic semiconductor

yfir

Hi,

I just read a paper about charge transport in organic semiconductor. There, it was mention that from electrochemical experiments, it is well known that after the removal of one electron from an individual molecule, more energy is required to remove a second electron. I'm still confuse with this concept, can anybody explain me why it is the case?

thanks,
yfir

chill_factor

sure. you try to remove an electron from a molecule; the molecule is neutral. after you remove the electron it is now a positively charged ion. if you try to remove a negative charge from something with positive charge, that's harder than removing it from something that's neutral. there's a quantum mechanical explanation as well but that's less intuitive than the EM one.

yfir

thanks for your reply. I thought the same think too after posted this thread. It must be related to the increase of a net positive charge (from the atomic nucleus contribution), so the coulomb attraction between the next negative charge and positive charges is increasing. The implication is the molecular orbital energy is shifting towards the vacuum energy. The opposite way also happen if we add electron to the bonding orbital. Is that right?

chill_factor

Don't worry about the zero point energy for this. If you add an electron to the bonding orbital, that is also disfavorable compared to the original molecule due to the repulsion of the electrons already in the bonding orbital.

In quantum mechanics too, there's an explanation. An electron bound to a potential source of any sort has 4 properties that can be represented by quantum numbers. They are: the energy level n, the angular momentum l, the z-axis projection of the angular momentum m, and the spin s. That is to say, each orbital whether molecular or atomic described by the first 3 quantum numbers can only hold 2 electrons, since electrons are fermions that must obey the Pauli exclusion principle and cannot have the same state; the 2 electrons have opposite spins.

If you try to add an electron, it can't be added to that orbital, since electrons only have +1/2 or -1/2 spin, so it must go to a higher energy level which is disfavorable. It may still bond, but in most existing molecules, the bonding states have already been filled, so the addition of another electron would force the electron to go to the even higher energy level anti-bonding states, which is unstable.