What is the reason for using insulators instead of conductors to limit surface recombination? Why should the excited electrons be forced to move through the emitter layer towards the Al contacts?
Dear Abbas
This is a good question.
The semicondcutor surface especially silicon contains active bonds that may act as mobile carrier trapping and recombination centers. So a semicondcutor surface is characterized by a surface recombination velocity vs. For active surfaces this velocity can be as high as 10 ^7 cm/s. At this high surface recombination velocity any excess of electrons and holes near this surface will recombine at these recombination centers.
On the other side it is so that any volume recombination or surface recombination of the photogenerated carriers in the solar cells will lead to a reduction in the photoconversion efficiency of the solar cells.
So, one has to reduce the recombination loss in the volume and at the surface.
The reduction of the surface recombination rate is accomplished by reducing the recombination centers at the surface by PASSIVATION.
This is done by saturating the active bonds at the surface of the semicondcutor by letting the atoms at the surface recombine atoms or molecules to terminate these bonds and therefore they will be no longer available for recombination.
There are specific materials which can do that such as hydrogen and oxygen or
some oxides such as SiO2 and Al2O3. You see that all these materials are insulating material to fix the dangling bonds. As metal tend to donate electrons easily they will build a conducting layer at the surface than may short circuit the solar cell. Also metal semicondcutor contacts can have very large recombination rate if they act ohmic.
So, from the functionality point of view metals are not suitable for passivization of dangling bonds at the surface.
Best wishes
Dear Abbas
This is a good question.
The semicondcutor surface especially silicon contains active bonds that may act as mobile carrier trapping and recombination centers. So a semicondcutor surface is characterized by a surface recombination velocity vs. For active surfaces this velocity can be as high as 10 ^7 cm/s. At this high surface recombination velocity any excess of electrons and holes near this surface will recombine at these recombination centers.
On the other side it is so that any volume recombination or surface recombination of the photogenerated carriers in the solar cells will lead to a reduction in the photoconversion efficiency of the solar cells.
So, one has to reduce the recombination loss in the volume and at the surface.
The reduction of the surface recombination rate is accomplished by reducing the recombination centers at the surface by PASSIVATION.
This is done by saturating the active bonds at the surface of the semicondcutor by letting the atoms at the surface recombine atoms or molecules to terminate these bonds and therefore they will be no longer available for recombination.
There are specific materials which can do that such as hydrogen and oxygen or
some oxides such as SiO2 and Al2O3. You see that all these materials are insulating material to fix the dangling bonds. As metal tend to donate electrons easily they will build a conducting layer at the surface than may short circuit the solar cell. Also metal semicondcutor contacts can have very large recombination rate if they act ohmic.
So, from the functionality point of view metals are not suitable for passivization of dangling bonds at the surface.
Best wishes
Dear professor Zekry,
Thank you for this clear and complete explanation!
Hello All, Prof.'s Zekry answer is very clear and satisfactory !, Thank you Prof. Abdelhalim abdelnaby Zekry for you always kind invested time- and clear explanations, as always ...
Kind Regards all !
:)
- Badges
- Science topic