Multijunction photovoltaic cells

[candy4me]

Is there a theoretical efficiency limit for multijunction photovoltaic cells, and if so, what is it?

I have read that the record efficiency for a 3-junction solar cell is about 44%. If there is no theoretical efficiency limit to multijunction photovoltaic cells, why are multijunction solar cells still less than 50% efficient? Are there other challenges to reaching higher efficiency solar cells, or perhaps multijunction cells are costly to produce and experiment with?

Thank you for your time.

[kgudger]

Hello and welcome to the forums:

I think that you can find the answers to your question by an online search. One article I found, http://en.wikipedia.org/wiki/Multijunct ... efficiency states the following (which is repeated on other sites):

Traditional single-junction cells have a maximum theoretical efficiency of 34%, a theoretical "infinite-junction" cell would improve this to 87% under highly concentrated sunlight.

In reality, there will be many practical problems with reaching maximum efficiency, such as blackbody radiation, "recombination", not perfect transmission of light by each layer, and the reflection of the incident light.

I think with a little searching you can find out the answers to your questions and come up with a great science experiment (or two)!

Keith

[candy4me]

Thank you for your answer. However, I do have another question. I came across an article on quantum dot photovoltaics and decided to research further. I found many articles describing studies on quantum dot solar cells, but I am wondering how a quantum dot solar cell actually produces electricity? Is it similar to the way electrons are excited in a typical silicon cell?

Also, one of the advantages of quantum dot solar cells is that the size of the quantum dot determines the band gap, meaning you can create a quantum dot solar cell much like a mulitjunction cell. In that case, why are quantum dot solar cells only about 7% efficient, and why is 61% the theoretical effciency limit for a quantum dot solar cell, as opposed to 87% for multijunction?

[deleted-71588]

I am wondering how a quantum dot solar cell actually produces electricity? Is it similar to the way electrons are excited in a typical silicon cell?

Yes, electron energy level is raised to free an electron by photon interaction.

why are quantum dot solar cells only about 7% efficient

Quantum dot solar cells production methods are relatively new and have yet to be optimized so 7% efficient compared to 17% efficiency of bulk material solar cell fabrications is something people are working to improve.

why is 61% the theoretical effciency limit for a quantum dot solar cell, as opposed to 87% for multijunction?

I suspect you are comparing apples to oranges. People are working on figuring out how to make quantum dot fabrications into multijunction quantum dot fabrications.

[candy4me]

Yes, electron energy level is raised to free an electron by photon interaction.

In that case, is the material used to make the quantum dots (e.g. lead sulfide) doped with impurities to create the P/N junction?

[deleted-71588]

In that case, is the material used to make the quantum dots (e.g. lead sulfide) doped with impurities to create the P/N junction?

Different manufacturing processes and different substrates would require different doping agents. People using TiO2 opal substrates are reporting doping with ZnS and flouride ions.
PbS is probably being used by others.