# The theoretical maximum efficiency of silicon solar cells is about 29%. Why?

The theoretical maximum efficiency of silicon solar cells is about 29%. How is this theoretical maximum efficiency derived? How can I use the working principles of a silicon solar cell to explain this?

ZapperZ
Staff Emeritus
It would help if you give an exact reference on where you read about such a thing. This is a practice that we try to make into a SOP in this forum when asking something like this.

Zz.

The theoretical maximum efficiency of silicon solar cells is about 29%. How is this theoretical maximum efficiency derived? How can I use the working principles of a silicon solar cell to explain this?

Yes the 29% figure is well known (see Wikipedia). The principle used to come up with solar cell efficiency is to simply consider the amount of total energy in a beam of sunlight and then compare that to the energy converted to electricity by a given solar cell from that beam.

The "theoretical maximum" comes from the fact that sunlight is made up of a diverse bunch of wavelengths. Much energy is in the infrared. Silcon on the other hand has a problem in that for wavelengths longer than just be low the visible spectrum (near infrared) it becomes transparent. Hence all that energy at those wavelengths goes right through unconverted. A second problem occurs in the ultraviolet. Silicon strongly absorbs that. So strong in fact that just an extremely thin layer takes it all. This sets an upper limit because that light gets absorbed before it can reach down into the cell to the level where energy conversion is taking place. Thus comparing the band of wavelengths that silicon can trap and convert to electricity to all the wavelengths in sunlight you end up with about 29% if the cell were converting nearly all the energy that lie in that band. Current silicon cells do quite well with commercial ones getting over 20%. OK?