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Q & A: What photovoltaic cells are made of

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Most recent answer: 10/22/2007
Q:
What are the different layers involved in making a photovoltaic cell?
- Cody Whitaker (age 16)
Dongola, IL. USA
A:

Hi Colby,

The operation of a solar cell is described briefly in our Q&A about photovoltaic cells. The active layers are the positively and negatively doped silicon layers, charge-collecting layers (a grid of metal wires on the top and a flat metal layer on the bottom), an anti-reflecting layer on top, and a glass window on top. This excellent site at howstuffworks.com describes in detail the operation and construction of practical photovoltaic cells: https://science.howstuffworks.com/environmental/energy/solar-cell.htm

Now it's possible that after checking that Web site, you have noticed something a little fishy. The electron made by the light is supposed to have been pulled to the side with the other electrons by an electric field. Then an electric field is supposed to push it over to the opposite side through the external circuit. (The hole is supposed to just do the opposite, so it's enough just to talk about the electron.) The problem is that's like saying that the electron rolled downhill to go to one side, and then downhill again back to the other side. You can't go downhill on the way to school and on the way home every day. So there must be something else involved.Before the light hits, the electrons are in equilibrium (concentrations not changing) because there are many more of them "downhill" where the field is pulling them than there are uphill. The current of electrons being pulled down the hill is just balanced by a current flowing the other way of electrons picking up enough random thermal energy to flow uphill. The random flow goes almost completely one way because almost all the electrons are on one side.

Now when light hits and makes electron-hole pairs, they do just what the site says- the electron typically rolls down to join the electron crowd, and the hole rolls down to join the holes. This does two things:

1. It reduces the electric field between the two sides.

2. It slightly increases the number of electrons which have a chance to randomly jump away from the electron side. Both effects throw the old equilibrium out of balance, increasing the net current of electrons from the electron-rich side to the hole side. If nothing is hooked up to the photocell, that current will flow right back through the middle of the photocell. But that current is small because it goes through the region where there are few holes or electrons. By hooking up an external circuit, you provide another route for electrons to go over to the hole side, driving your electronics. So long as the light stays on, the device stays out of equilibrium and the current keeps flowing. You can think of this current as going "uphill" as far as the electrical field in the device is concerned, driven by the random thermal jumping of the dense crowd of electrons. In thermodynamics, we would say that the current is energetically uphill, but overall entropically downhill, like a chemical reaction that soaks up heat instead of releasing it.

It sounds complicated, but turns out to have precise descriptions which are not too messy.

Tom & Mike


(published on 10/22/2007)

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