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I first thought of this question when I was learning how solar cells work and how ionization happens. The question I have is if atoms can get ionized to net positive charge by removing electrons, then wouldn't the removed electrons just get attracted back due to positive charge of the ion just created? And during ionization what actually happens to the photon that ionizes the electron? Does the photon just disappear while the electron magically gains energy? What determines the direction that photons created during when an electron and an ion join, is heading? Any help with these questions will be much appreciated...Any explanation with math will also be very appreciated.

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Most of these processes are governed by quantum mechanics, and in case of photovoltaic cells require some background in solid state physics and theory of electronic devices. Since the OP suggests no such background, I try to make below a few points on high-school level:

Indeed, the photon is absorbed by an atom, that is the photon disappears and its energy is transferred to the electron.For this reason electron cannot just return to its bound state - it needs to lose this energy, by emitting photons or phonons or by colliding with other electrons (Auger process).

In photovoltaic cells there is usually high bias/voltage applied, that ensures that the electron ejected from an atom does not hover around the positive ion, but is swept away, contributing to the generation of the current - this is actually how the photovoltaic cells function.

Moreover, semiconductors cannot be really described in terms of separate atoms, so it is more appropriate to talk about an electron ejected from the valence band to the conduction band, and the hole left in the valence band. Holes are also mobile charge carriers and are also swept by the electric field.

The process when electron falls back into the valence band and eliminates a hole is called *recombination. The photon emitted can go in nearly arbitrary direction, although there are some limitations and selection rules imposed by the interaction governing this transition.

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  • $\begingroup$ "In photovoltaic cells there is usually high bias/voltage applied" ...... never heard of that. $\endgroup$ Jun 30, 2022 at 13:38
  • $\begingroup$ @PhysicsDave Here is a paragraph that reproduces what I say in my answer $\endgroup$
    – Roger V.
    Jun 30, 2022 at 13:44
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    $\begingroup$ If you re referring to the "equivalent circuit" diagram that is not a diagram of how a solar cell is used .... it is a diagram of convenience for how electrical engineers represent a solar cell in a circuit so they can model it. A solar cell never has a voltage applied to it in practice ... $\endgroup$ Jun 30, 2022 at 13:52
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    $\begingroup$ @alienare4422 The rabbit hole here is quite deep ツ. en.wikipedia.org/wiki/Exciton en.wikipedia.org/wiki/Direct_and_indirect_band_gaps $\endgroup$
    – John Doty
    Jun 30, 2022 at 17:26
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    $\begingroup$ @RogerVadim No condescension intended .... however using the word applied is confusing. I think it would be better said as "there is an internal voltage bias " ..... which is chemical per John Doty. $\endgroup$ Jun 30, 2022 at 19:58
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Why doesn't when an electron gets knocked out of an atom, the electron get attracted back to the atom and reunite?

Electrons and atoms are in the quantum mechanical range, described by wavefunctions that give the probability of a phenomenon happening.

For an electron to get "knocked out" of a single atom, there must be enough energy in an interaction to cover the energy level on which the electron exists. If it is just exact then the probability of reuniting is large due to the attraction of negative to positive. (see for the hydrogen atom) If the energy of the interaction is much higher than the ionization energy, momentum and energy conservation assure that the atom and electron will separate to a large distance where the quantum mechanical probability of attractive interaction is very small, so there will be an electron and an ion, in a gas, for example.

Within a solid the mathematics is complicated but the same holds. If the electron is energetic enough the probability of reuniting is very small, and if a potential is imposed the negative electrons and positive ions will attracted collectively to opposite directions creating a current. For solar cells it is complicated as described here.

And during ionization what actually happens to the photon that ionizes the electron?Does the photon just disappear while the electron magically gains energy?

It is not the electron that is ionized, it is the whole atom that interacts with the photon and becoems an ion by the expulsion of an electron. The photon energy and momentum is absorbed by the whole atom ( or lattice in a solid). That is what is observed and has been successfully fitted with quantum mechanical theories.

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The electron is initially/always attracted to the nucleus but certainly can not combine with the nucleus (QM reasons and many answers on this site), also the electron has energy!

When the electron is more excited (more energy) it is still attracted to the positive charge .... but it can still never combine with the nucleus. The higher energy electron however now has more choices .... if a circuit is hooked up to the solar cell it can dissipate its energy and then return back to the solar cell closer to the nucleus as it now has lower energy.

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  • $\begingroup$ thank you....but one question- What type of energy does the electron gain and is there an equation describing how electrostatic forces work on particles with more energy?... particularly an electron thats been excited? $\endgroup$ Jun 30, 2022 at 16:00
  • $\begingroup$ It's all quantum mechanics (QM).... we say electrons have orbitals (like in Hydrogen, orbitals are energy levels) but in many materials the orbitals become stretched and they are referred to as levels. Certainly the electron gets increased potential energy .... like a mass at a higher height. In QM the electrons have sinusoidal or wave properties and with waves you get quantization .... i.e. only certain discrete energy levels are allowed like 1ev or 2 ev not 1.1 or 1.2 etc. Many measurements are made on materials and the voltage/energy is known .... $\endgroup$ Jun 30, 2022 at 20:05

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