In organic solar cells, the excitons generation by the incident photon is dissociated at the donar-aceptor junction. So the dissociated electrons move to cathode (Al) and holes to anode (ITO). Then, my question is how can an electron move to cathode (negatively charged electrode), even though there is a coloumbic repulsion?
I'm expecting an answer for:
1. Basics of exciton generation,
2. Is exciton generation possible in all semi,metal and insulator material. If no/yes ,Why?
3. From the figure, it is understood that electron moves from higher to lower energy level since lower energy love of the system. However, the hole moves from higher to lower, why it is so?
3) Contrary to what you write, the hole in the final part of the graph seems to be moving from lower to higher energy. You may understand this by considering what happens in terms of "the real objects" (electrons): since there is a hole, an electron can lower its energy by moving from an occupied higher energy state into the hole. As a consequence, the hole is now in a state of higher energy.
In terms of electrons and holes, the energy scale is thus inverted with respect to each other. In the graph, an electron state further down corresponds to a state of lower energy. For a hole, a state further down is actually a state of higher energy.
For 3)
It isnot well understood...could you please /modify rectify the question ?
With respect to Prakash Mahakul's answer: I was astonished to read about the possibility of excitons in semimetals and googled it. I spent very little time on this but found the following:
SEMICONDUCTORS & SEMIMETALS
books.google.de/books?isbn=0080863922
Robert K. Willardson, Albert C. Beer - 1967 - Technology & Engineering
This indicates that bound excitons probably do not exist in heavily doped semiconductors and definitely do not exist as such in semimetals and metals.
This is not a most recent quotation, though. Therefore, dear Prakash Mahakul, if you have evidence or a reference for excitons in semimetals, would you please provide us with that. Many thanks.
Also I was astonished to read your opinion about the electron hole interaction being the larger, the larger the dielectric constant (DK). I would actually say the opposite holds, at least for Wannier type excitons. Also in the usual energy expression for the exciton, you find the DK in the denominator (and squared. I just looked it up in wikipedia, German edition): thus, the larger the DK, the smaller the exciton binding energy. You may classically understand this in the following way: the larger the DK, the more polarizable the material. The more polarizable the material, the better it screens Coulomb interactions and thus effectively reduces them.
Sorry Dear Kal that is actually weaker interaction, But by mistake i have written in totally opposite sense..
m very very sorry for it
Actually due to the higher dielectric constant the interaction is weak and hence they are easily separated and hence exciton formation not possible..
Sorry due to some Mistake in the above answer i have deleted the answer...Here is the rectified one
For 2)
Exciton generation in semimetal and metal not possible due to their higher value of dielectric constant. Due to the higher dielectric constant the electrostatic interaction is very less . Hence they get easily separated out
1. Bound electron-hole pair is known as exciton. Even though they got separated by photon excitation, they have strong tendency to recombine again due to the low dielectric leading to strong attractive force. The binding energy necessary to break their attraction is provided by the junction (SCR).
2. Exciton is not possible in metals and semi metals
Current generated due to illumination is in opposite direction to the forward bias. You are simultaneously decreasing the space charge region by applied bias and measuring the light generated current till the flat band condition is reached (J forward=J light).
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