Question

[HemaGh]

Hey all ... I know this forum is about Solar panels .. but I have some questions about solar cell technology, especially the manufacturing process. not so many sources or explanations. So I was wondering if there was anyone who has an insight about that ?

[inetdog]

Hey all ... I know this forum is about Solar panels .. but I have some questions about solar cell technology, especially the manufacturing process. not so many sources or explanations. So I was wondering if there was anyone who has an insight about that ?

Hi and welcome to Solar Panel Talk!

Probably the best way for you to start would be to try asking some specific questions and see what happens.
If you really mean that you want to learn the entire manufacturing process, some studies in solid state physics and materials science would give you a good starting point.
It is complicated.....

[HemaGh]

I have read the process .. but as all processes go there are some steps that are not given any physical or chemical reason ..

to be more specific. When obtaining silicon they do that in 2 steps. the first one gives out silicon with 99% purity. which is not bad but not suitable for solar cells.
why is that ? Is it to achieve maximum efficiency? because later in the process you have to add impurities. different impurities, sure ... but a 1% impurity can't be that crucial .

[inetdog]

I have read the process .. but as all processes go there are some steps that are not given any physical or chemical reason ..

to be more specific. When obtaining silicon they do that in 2 steps. the first one gives out silicon with 99% purity. which is not bad but not suitable for solar cells.
why is that ? Is it to achieve maximum efficiency? because later in the process you have to add impurities. different impurities, sure ... but a 1% impurity can't be that crucial .

Here is where you need to understand solid state physics.
An impurity level of even .001% can affect the behavior of a semiconductor material. It all has to do with free electrons in the conduction band and the level of the Fermi sea.

[HemaGh]

So the probability of allowed states changes a bit and the number of charge carriers is disturbed... is that enough to render it useless ?

[pleppik]

So the probability of allowed states changes a bit and the number of charge carriers is disturbed... is that enough to render it useless ?

Yes.

Semiconductors are weird things because their properties are determined by quantum mechanical effects and not the kinds of physics most people experience in everyday life.

One very rough way to think about it which is sort of right is that 100% pure silicon is an insulator but not a very good one. Even a small amount of impurity (less than 1%) is enough to make it into a conductor. Commercially useful semiconductors have just enough impurity so they're balanced on the knife edge between being an insulator and a conductor--and that's what makes them useful.

A physical analogy which is completely wrong but gives you the right idea is to think of one of those inflatable pools.inflatable-pool.jpg

As long as the vinyl skin of the pool has zero defects, no water will leak out. But if there's even a single defect you will get a little jet of water. One or two jets of water could be a fun addition to backyard water-play, but get too many holes and the pool quickly becomes useless. Even if 99.9% of the vinyl is still intact, the pool won't hold water.

[HemaGh]

Yes.

Semiconductors are weird things because their properties are determined by quantum mechanical effects and not the kinds of physics most people experience in everyday life.

One very rough way to think about it which is sort of right is that 100% pure silicon is an insulator but not a very good one. Even a small amount of impurity (less than 1%) is enough to make it into a conductor. Commercially useful semiconductors have just enough impurity so they're balanced on the knife edge between being an insulator and a conductor--and that's what makes them useful.

ok.. so a semi-conductor with 1% impurities is not commercially useful. but does it have photo-voltaic properties ? it doesn't have to be high or efficient, just existing.
and do you know a source where the physical reason behind that is explained ? I am reading Solar Cells by Martin A. Green and he just skipped that part when he talked about purifying.

[Mike90250]

Commercial semiconductor grade silicone is usually 99.99% pure. The purer it is, the better the final product. Additives (dopant) are usually carefully applied in a thermal reactor, heating the sliced wafers to several thousand degrees, and introducing a tiny whiff of some gas that has the right atoms. The dwell time depends on how deep they want the alien atoms to migrate into the silicon, and then the gas is evacuated and the wafers annealed.

http://www.halbleiter.org/en/waferfabrication/doping/

http://en.wikipedia.org/wiki/Doping_%28semiconductor%29

This is not likely to be accomplished in a backyard or kitchen. We would spend weeks baking out ovens to get them atomically clean before we can process a batch of wafers
Doping-silicon-wafers.jpg

[pleppik]

ok.. so a semi-conductor with 1% impurities is not commercially useful. but does it have photo-voltaic properties ? it doesn't have to be high or efficient, just existing.
and do you know a source where the physical reason behind that is explained ? I am reading Solar Cells by Martin A. Green and he just skipped that part when he talked about purifying.

Any material has some level of photovoltaic property, in that it can absorb photons of a certain energy and elevate electrons to a higher energy state. The question is whether the material will absorb light of an abundant wavelength (usually meaning visible light or near-infrared) and capture the energy of the electrons in a useful form.

You could get a chunk of granite to release high energy electrons by bombarding it with hard X-rays, but we (fortunately) don't get too many hard X-rays here on Earth, and the electrons won't be released in a way that's easy to capture or use.

The Wikipedia article on semiconductors has a reasonable introduction to semiconductor physics, but it's not going to make much sense if you're not already familiar with quantum statistics. I don't know of a good explanation of semiconductor physics for the lay person.

A semiconductor with too much impurity (or the wrong kind) will not work as a PV device. It will absorb light just fine, but the electrons which absorb the photons will simply pass their extra energy to all the other electrons in the material, generating heat instead of electric power.

The magic of a PV cell is that it has a very particular arrangement of slightly different semiconductor materials such that when an electron absorbs a photon of light, the electron can get "stuck" with that extra energy instead of immediately passing it on to all the other electrons in the material in the form of heat. These higher energy electrons can then be harvested for electric power. The extra energy each electron carries we call "voltage" and the number of electrons per second we can harvest is called "current."

But in order to get those electrons stuck with the extra energy, you need to have extremely precise amounts of just the right kinds of impurities and in exactly the right place in the cell. You can't just take a blob of silicon and expect something interesting to happen when you shine light on it.

[bcroe]

When obtaining silicon they do that in 2 steps. the first one gives out silicon with 99% purity. which is not bad but not suitable for solar cells.
why is that ? Is it to achieve maximum efficiency? because later in the process you have to add impurities. different impurities, sure ... but a 1% impurity can't be that crucial .

The idea than 1% doesn't matter is just completely wrong. There are plenty of
industrial processes beside silicon, where far higher purity is needed. Bruce Roe

[PNjunction]

You may be interested in visiting pveducation.org and reading through some of the sections.

There you will find many of the more minute details of solar panel manufacturing and the common processes most manufacturers try to meet.

[russ]

Sounds like a business major from an Indian Institute of Management (IIM). They are always coming up with ways to cut corners - 99.99% failure rate however.

Do not post in large font - next time I delete the post.