P-N Junctions in Solar Cells (1 Viewer)

[nightweaver066]

Why is it that light has to hit the n-type semiconductor?
Wouldn't that just promote electrons from the donor level to the conduction band so that there's a free electron but no hole?

I thought light has to hit the depletion layer to promote an electron from the valence band to the conduction band, producing an electron-hole pair which is forced in opposite directions due to the electric field allowing for the flow of current.

This has had me really confused over the past few weeks....

 

[nightweaver066]

^Bump

 

[nightweaver066]

Bump

 

[Rathaen]

If I still remember it correctly, the depletion zone in a PN junction forms when holes from the P-type side and free electrons from the N-type side move across the boundary (being attracted by the opposite charge) and cancel each other out. As the electrons move, they leave behind positively charged ions on the N type side, and the holes likewise leave behind negatively charged ions on the P type side of the boundary, which sets up an electric field pointing from the N type side to the P type side.

As more and more electrons cancel out with holes, these charged regions increase in size and hence the electric field also increases in strength up until it is stronger than the attraction between the holes and the electrons, which prevents movement of charge-carriers from the P type or N type sides across the depletion zone.

When a photon strikes the N type layer, it can excite an electron from the valence band into the conduction band (thus allowing the electron to move freely around the semiconductor). In a solar cell a conductive grid on top of the N type layer is connected to a conductive panel under the P type layer, forming a circuit. The electron cannot move through the depletion zone to the P type layer because of the aforementioned electric field, so the only direction it can travel is around the circuit into the P type layer where it fills a hole.

The moving electron is actually part of an electron hole pair. The hole is just one of the holes in the P-type side, which becomes occupied by the free electron and "travels" over to the N-type side.