# Better Solar Cells

1. Jan 16, 2005

### Jake

Ok so right now the best solar cells get only about 20-30% of the energy available to it from the sun. This is because for each photon that knocks off an electron, any excess energy not needed to knock off the electron in the silicon is wasted.

And I read in howstuffworks.com that really good solar cells do use multi-layer cells which absorve the light differently at different layers, and thus make efficient use of the light at each layer.

So my question is, what's stopping the creation of a cell that has like lets say, 30 layers, each for a specific level of photon energy, thus extracting a much higher amount of energy from the sun?

I also read an article in Discover/SCIAM I forget which which talked about how silicon, when made right, has the property to absorb light differently at different depths of the same piece of silicon. Now this article was how it could be used for better digital cameras, but could it be used for solar cells? Each layer absorbing photons with different energy levels.

Thanks!

2. Jan 16, 2005

### DaVinci

The lack of money due to the oil industry not wanting solar power to happen as their empire will collapse overnight?

:rofl:

3. Jan 17, 2005

Could you show the math for this?

4. Jan 17, 2005

### Staff: Mentor

Jake, one problem is absorption - not all the excess light will be transmitted to the next layer. Another is that if 30% is converted to electricity, there isn't as much light available for he second layer to absorb. So the law of diminishing returns applies (in 2 ways, actually).
Say 30% is converted to electricity, 70% transmitted by each layer: in the second layer, you'd convert 30% of that remaining 70% or 21% of the total light. Third layer is 30% of the remaining 51% or 15% of the total light. So the 3rd layer only collects half as much light as the first layer - and probably costs just as much to manufacture. So it would actually increase the cost per kW of the cell.

Last edited: Jan 17, 2005
5. Jan 17, 2005

### DaVinci

This is a pretty good explanation of the whole PV effect: http://www.renewableenergyaccess.com/rea/tech/solarenergy;jsessionid=abtm-DyFL_F_ [Broken]

The US Department of Energy has more funding for other energy research besides Solar power. I tried to find the numbers on the internet (as they must be available) but have been unable to locate them in my 10 minute search. My comment stems from a discussion I had with a distant family member that works for the DoE. But that is not enough information to 'prove' my case and I cannot find the numbers on the web at this time.

Last edited by a moderator: May 1, 2017
6. Jan 17, 2005

### Jake

4 layers then could absorb about 70% of the energy, and then some lose to electrical resistence and lost space for the metal conductors. So say at best youd get 60% out of four layers....an incredible amount by today's standards.

Are you saying this is actually possible? As in, we have the technology today? As far as cost, its actually a lot cheaper if you think about it...Oil will run out, but solar won't, no matter how much it costs to make a cell. And it wont destroy the enviroment. And given enough demand, I'm sure mass-production could lower costs.

So what, other than costs, is stopping someone from making 3-4-or 5 layer solar cells, be interested to know, thanks!

7. Jan 17, 2005

### Jake

Another question

What is it about a photon that causes it too have too much or to little energy for a precise one-electron knock off? Is it the photons frequency? (IE color) Or is it some other aspect?

8. Jan 17, 2005

Oil can be manufactured with the help of nuclear energy, and there remain thousands of years worth of fissile uranium in the world's oceans.

The lifecycle of a solar cell has environmental impacts. Solar energy also, since it is so diffuse, tends to be more dangerous for maintenance crews than nuclear.

Ditto for nuclear power plants and their accompanying oil-production facilities. No one that I am aware of who is knowledgable about grid solar pretends that implementing it might involve PV cells. PV cells are used to power satellites and remote radio repeater towers. For grid power production, thermal solar (powering turbines) is envisioned.

This is a solar-trough concentrator:
http://happy.emu.id.au/neilp/solar/solartrough.htm

It heats up oil which can then be used to power rankine-cycle (steam) turbines.

Last edited: Jan 17, 2005
9. Jan 17, 2005

### Jake

Please, they were just passing comments, I see no validity in your logic, but let's not go off topic...

Last edited: Jan 17, 2005
10. Jan 17, 2005

### Staff: Mentor

That was an example (and a generous one, at that), I don't actually know what they are capable of today - probably not.
Well, cost ($/kW) is everything here. Whether you use 10% or 20% efficient solar panels, the payback for setting up an array on your roof is about the same - about 20 years, with generous assumptions. Currently, that fact alone is what keeps people from doing it. Presumably, as technology improves, costs will drop, but higher efficiency cells will always be more expensive than low efficiency ones - so efficiency is always going to be secondary to$/kW.

Also, you must remember that nothing is maintenance free - not even solar cells. They also don't last forever. So its not like you can set up a plant today and forget about it.

Regarding the environment, you'd be surprised what environmentalists will oppose. Windmills are clean to, but....

You are right, however, that mass production would lower costs. Economically, though, they need to come down by about a factor of 4 before they begin to become viable. Most individuals and business still have a much shorter time horizon than a 5 year payback though. I've seen businesses balk at a 5 month payback.

11. Jan 18, 2005

Well when oil is running low and costs $15 a gallon and we start choking on the air we breath, it might seem rather a good deal to them 12. Jan 18, 2005 ### willib i believe if the solar cells were ecthed properly they could yeald a lot more power , say they were ecthed in a W shape or something similiar , wouldnt that give more surface area? what about wind energy http://www.otherpower.com/ 13. Jan 18, 2005 ### Chronos Last edited by a moderator: Apr 21, 2017 14. Jan 18, 2005 ### Jake Energy Towers are also very promissing, an indirect solar power method, gathers the energy from the air itself. 15. Jan 18, 2005 ### hitssquad Last edited by a moderator: Apr 21, 2017 16. Jan 18, 2005 ### russ_watters ### Staff: Mentor Specific surface area of the cell itself is largely irrelevant: if you angle-in some cells to increase the surface area, you don't increase the amount of light that hits it. Yeah - energy break-even is a pretty much useless measure. It may actually be designed to fool people into thinking its the same thing as a financial payback period. I can't stress enough that barring a goverment mandated switch from coal and oil (not happening in our lifetime, even with coal), economics will continue to drive the issue. Scarcity will drive up the price of oil, but I doubt its going to$15 a gallon (today's dollars) in my lifetime. People have been crying wolf on scarcity for decades and we're still not seeing it. Government regulations on emissions and taxes will also drive up prices, but mabe 50%-100% tops.

But like I said before - photocells need to drop in price by a factor of 4 before they can even be worth considering. And they aren't moving down that fast.

RE: "Energy Towers" - if you mean solar towers, we had a thread on that a while ago and I poked into it a little bit. Its a sound concept - it would work - trouble is, they require some engineering like we've never seen. If you notice, every big skyscraper is 10, 20, or 50 feet higher than the last: this is supposed to be 1km high - around twice the height of any existing skyscraper. Maybe in 50 years. Oh, and the company that says they are going to build it is a fraud.

Last edited: Jan 18, 2005
17. Jan 19, 2005

### willib

you seem to think that the supply of oil will last forever , it wont .. it is finite , not infinite.. and the sooner that we all realize this the better off we all will be..
and the sooner that we look into alternative energy sources , the better..
we need to design better batteries ..
we need more wind energy production , more and better solar cells .. and less SUV's
and we need to do this now..
thanks for reading , my little rant is over , go about your business.

18. Jan 19, 2005

### Locrian

As a huge proponent of solar and wind energy, whenever I see arguments like this, I cannot help but think to myself

"With friends like these, who needs enemies?"

19. Jan 19, 2005

### brewnog

You are correct, except that Russ does not believe that there is an inexhaustable supply of oil. The thing you don't understand is that for the time being, it's about money. There is plenty of oil left underground for politicians not to have to worry about it, on a short-term basis. Currently, fossil fuels provide the most economically viable source of energy. Not many people would argue that developments have to be made in terms of renewable sources, but these developments ARE being made, and I doubt you're currently running a solar-powered car, or have a wind turbine on your house. Let me guess, you fill up with petrol, and are connected to the grid like everyone else, right?

20. Jan 19, 2005

### Francis M

Jake, I have to agree with Russ and some of the others in this post. Now please don't take some of these peoples posts as an anti-enviornmental rant. It's just that right now right or wrong it's a matter of economics and politics. IT's cheaper to power our vehicles with petrolium. It's also cheaper to power/heat our houses with petrolium. Most people (myself included) can't afford the higher initial upfront expenditure for solar/wind & battery power equipment, and then wait for the economic payoff which they'll never see unless they do number chrunching and comparisons for 5-10 or 20 years down the road. Also as pointed out these things while probably better in the long run don't come without enviornmental detractions, some of which come from the very people that want cleaner energy and build some of this stuff. THere's the beef that there's an impact to the enviornment form manufacturing processes, also the beef about what to do with these cells and batteries when they break or wear out, and also complaints about solar farms and windmills wrecking the view. THere'll also be a higher mainteanence cost involved as the infrastructure for servicing this equipment isn't as established (common) as plumbers and HVAC businesses.
Russ and a few others were simply pointing out to your question earlier that due to current technology and the property of materials it's not economically better to build the type of solar cell you suggested than the ones they build currently. NO one said it couldn't be done, it was just said that at a certain point you're going to spend more money manufacturing/engineering that cell than you'll ever see back efficiancy wise which will make the retail cost higher, which will make whoever buys it have to wait even longer for the system to pay for itself.