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Name: Christopher 
Status: student 
Grade: 9-12 
Location: MO 



Question:
Dear Scientist, I am a senior at Cass Midway High School in Cleveland, Missouri. Our school is very small and when I was presented with the task of conducting a science project relating to energy I have found it difficult to gather information since we do not have any teachers that are knowledgeable enough in the details of energy. I need to know more about photovoltaic cells. I would like to understand photovoltaic cells better, how to make one, how to increase efficiency of a photovoltaic cell and how to use the energy generated by a photovoltaic cell.


Replies:
Hi Christopher,

Let me begin by addressing one of your last points, where you asked how you could make a photovoltaic ("PV") cell. The manufacturing process to make a PV cell is breathtakingly expensive and requires toxic materials you will not have access to. Most photocells start with a thin wafer of either single-crystal silicon, or a deposited layer of polysilicon. Neither of these basic materials are available to you or I. This is just the starting point; from there you need tens of millions of dollars of equipment to form the P-N junctions that are at the heart of operation of any PV cell. In essence, a PV cell is a type of diode (hence the "P-N" junction that all diodes have) that generates electrical current when exposed to light.

By the way, as a demonstration of this, consider the ordinary Light Emitting Diode (LED). As you know, if you feed current to an LED, it produces light. The reverse is also true. Shine light on an LED, and it will generate a miniscule amount of electrical current!

The 50-year race to improve the efficiency of PV cells started with the simple first-generation silicon cell that had an efficiency of around 6%, and has resulted in modern PV cells that have very complex internal structures with an efficiency of around 40%. The story behind how the efficiency has increased, is extremely complex and highly technical, but you can get an excellent general summary here: http://en.wikipedia.org/wiki/Solar_cell

Note that efficiency is not the real problem here. Cost is. Cost can be lowered by increasing the PV cell's efficiency (thus delivering more power from the same PV cell), but there are other ways to do this too. The single crystal silicon wafers mentioned above are an extremely expensive starting point. A long silicon ingot is "grown" slowly in a furnace from a small silicon crystal, in a process that takes weeks. This ingot is then sawn into thin wafers. One alternative process is to use polysilicon (multicrystalline silicon) that is deposited (often using vapor deposition) in a very thin layer onto a cheap substrate. This results in lower efficiency, but it so much cheaper that the Cost-per-Watt is often lower, even though more polycrystalline PV cells are needed for the same power output.

An new, radical, and promising development is organic PV cells, but so far this is still in its infancy.

Finally, your question about how to use the power generated. PV cells are a very inconvenient source of power. A monocrystalline PV cell (made from a single crystal silicon wafer) has an output voltage of only about 0.6 Volts at room temperature, and with no load. But this voltage falls significantly when the cell is heated by the sun's energy. Much more troublesome, is that output voltage also falls dramatically as increasing current being delivered. The more current is drawn from the cell, the lower its output voltage falls. This is in contrast to an AC plug in your home (for example) that maintains a nearly constant 120 Volt output, whether you plug in a small load like a radio, or a heavy load like a toaster.

In order to be useful, many PV cells are usually connected in series to increase the output voltage, and many of these series "stacks" are connected in parallel to increase the output current. The output power from this array is then fed to a device called an inverter, which is capable of accepting the varying output voltage that this array generates under varying load conditions, and converts this varying, relatively low DC voltage into a constant voltage that can be used by existing electrical equipment, such as a constant 12 Volts DC, or 120 Volts AC. Without this additional power conversion provided by the inverter, the output of a PV cell array is practically useless.

Regards,
Bob Wilson



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