Solving the solar cell power conversion dilemma

Jan 24, 2011 By Miranda Marquit feature
A solar cell’s ability to convert sunlight to electric current is limited by the band gaps of the semiconductors from which it is made. For example, semiconductors with wide band gaps respond to shorter wavelengths with higher energies (lower left). A semiconductor with an intermediate band has multiple band gaps and can respond to a range of energies (lower right).

(PhysOrg.com) -- "There is a lot of interest in creating more efficient solar cells that are also simpler than many of the designs common now," Wladek Walukiewicz tells PhysOrg.com. "We think that, through the mixing of certain semiconductors, this is possible."

Walukiewicz has lead a group of scientists working at the Lawrence Berkeley National Laboratory, Rose Street Labs Energy and Sumika Electronic Materials, Inc. They have been studying the properties of GaNAs alloys, and they believe that the characteristics of these semiconductor alloys could lead to more efficient solar cells. Their work appears in : "Engineering the Electronic Band Structure for Multiband Solar Cells."

"In solar cells made of standard , the is produced by electron-hole pairs photo-excited across the separating the conduction and the ," Walukiewicz explains. "Only the photons with the energy larger than the band gap can produce electric current. This creates the solar cell dilemma."

This dilemma is occurs when "a small gap semiconductor absorbs more photons and produces larger current but small voltage whereas a large gap semiconductor produces larger voltage but the current is limited because large fraction of the solar photons is not absorbed," he continues. "However, there is a gap between these bands, with no state. Normally, this doesn't conduct. Exciting a semiconductor with light can help, though."

This dilemma means that in order for a solar cell device to be efficient, it must be complex, incorporating different layers with different gaps so that different portions of the can be absorbed. “Silicon solar cells are only up to20% efficient,” Walukiewicz says. “There are those that are as much as 40% efficient, but they are so complex – and expensive.”

Walukiewicz and his colleagues believe that they have found a way around this. By using GaNAs alloys, the group has created a single material that can absorb multiple portions of the solar spectrum. “Scientists have been mixing semiconductors for years, creating materials with properties tailored for specific applications.” Walukiewicz says. “But they were working with semiconductors that wanted to be mixed. We work with materials that don’t want to be mixed, using special methods to force them.”

At top, a test device of the new multiband solar cell was arranged to block current from the intermediate band; this allowed a wide range of wavelengths found in the solar spectrum to stimulate current that flowed from both conduction and valence bands (electrons and holes, respectively). In a comparison device, at bottom, the current from the intermediate band was not blocked, and it interfered with current from the conduction band, limiting the device’s response.

When this mixing happens, Walukiewicz points out, interesting properties are seen, like the one that allows for an intermediate band of states to be formed in a wide gap semiconductor. The band acts as a stepping stone in the semiconductor band gap. “Our device is quite simple,” he says. “For years, people have been creating efficient solar cells using many semiconductors with different gaps. We have developed a semiconductor built with multiple bands.”

As a result, developed with the GaNAs alloys have the potential to contribute to solving a problem in the world of solar energy. “We don’t know exactly what to expect, but we estimate that a solar cell made as we describe would be more than 40% efficient,” Walukiewicz says. “However, it would be much less complex than what is used now to do the same thing. Right now, it is common for 18 layers of semiconductors to be used to reach that level of efficiency. Our technique could possibly do it with four or five layers.”

Such a device could be much easier to make, and could possibly be scalable for production for a wider market. It would also likely be more cost efficient. “Our objective is to develop it to a point where it would be commercially viable, and even make a difference.”

Explore further: Interfaces within materials can be patterned as a means of controlling the properties of composites

More information: N. López, L.A. Reichertz, K.M. Yu, K. Campman, and W. Walukiewicz, “Engineering the Electronic Band Structure for Multiband Solar Cells,” Physical Review Letters (2011). Available online: link.aps.org/doi/10.1103/PhysRevLett.106.028701

4.6 /5 (25 votes)

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ABSOLUTEKNOWLEDGE
not rated yet Jan 24, 2011
make a prototype

make stock ofeering

lets roll
yoatmon
5 / 5 (1) Jan 24, 2011
Fraunhofer achieved world record of 41.5% with three layers.
rgwalther
1 / 5 (2) Jan 24, 2011
I have to believe that ABSOLUTEKNOWLEDGE, would not include typos. I am curious if the same rules apply as with ABSOLUTEPOWER?
Quantum_Conundrum
2.5 / 5 (2) Jan 24, 2011
If by 40% efficient they mean converting 40% of the solar constant per meter square to electricity, then that would be absolutely incredible.

That's 540watts/m^2.
Skeptic_Heretic
1 / 5 (3) Jan 24, 2011
If by 40% efficient they mean converting 40% of the solar constant per meter square to electricity, then that would be absolutely incredible.

That's 540watts/m^2.

There's no such thing as a solar constant.
Javinator
5 / 5 (3) Jan 24, 2011
I think he's referring to the ~1350W/m^2 that is referred to as Earth's solar constant (1350 * 0.4 = 540). It's the intensity of solar radiation at the edge of the Earth's atmosphere at a right angle.

1350W/m^2 isn't what we receive on the surface of the Earth though. On Earth's surface it's more like 200-400 W/m^2 that we actually receive.

See: http:/home.iprimus.com.au/nielsens/solrad.html
Skeptic_Heretic
5 / 5 (1) Jan 24, 2011
I think he's referring to the ~1350W/m^2 that is referred to as Earth's solar constant (1350 * 0.4 = 540). It's the intensity of solar radiation at the edge of the Earth's atmosphere at a right angle.

1350W/m^2 isn't what we receive on the surface of the Earth though. On Earth's surface it's more like 200-400 W/m^2 that we actually receive.

See: http:/home.iprimus.com.au/nielsens/solrad.html

The sun's output is variable. The use of the word "constant" is the problem.

On a side note: It appears kev is getting into the world of rank feedback, maybe he'll subsequently get into the "reading the article" behavior as well.
Uri
5 / 5 (1) Jan 24, 2011
That's 540watts/m^2.

As other posters have mentioned this assumes the "solar constant" would reach the solar cells. Several Problems: Assuming you have solar cells facing the correct direction at some near optimal angle its actually much worse than this. Javinator's link doesn't take into account the cloud cover, as can be seen by the values being continuous across latitudes. So there will be some reduction to the efficiency. Additionally, though i'm loathe to quote wikipedia, solar efficiency is calculated under standard testing conditions: "STC specifies a temperature of 25 °C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum". I'm fairly certain efficiency would be lower with less than 1000 W/m2 and also when the temp is above 25C, so again worse than the ideal situation. As you approach the areas that have higher average solar insolation, you will also tend to have higher summer temps.
Uri
5 / 5 (1) Jan 24, 2011
Continued

Finally you also must convert DC to AC to make it usable by current electronics (10-20% loss). Larger arrays will likely have higher losses due to increased transmission distances before being converted to AC power. So if you take the ideal of an average of 500 w/m^2 (ie little cloud cover) over a 12 hour period with discrete efficiencies of 80%, 90%, 75% due to the temp of the solar cells / angle of incidence of incoming radiation / 4 hour period. And assume only a 10% loss due to the inverter you get more like ((500 * .8 * .4 * .9) + (500 * .9 * .4 . 9) + (500 * .75 * .4 * .9)) = < 150 watts / m^2 average over a 12 hour day.I know thats a lot of assumptions, and I'm sure a MUCH more accurate model could be described, but not in 2000 characters. Thats still a lot of power available for "free", but it does show how even if we approach maximum efficiency it will still take a lot more coverage than people tend to think.
Uri
not rated yet Jan 24, 2011
((500 * .8 * .4 * .9) + (500 * .9 * .4 . 9) + (500 * .75 * .4 * .9)) = < 150 watts / m^2


Sorry math fail due to the posting system and me not splitting it up better before hand. So for the first 4 hours you average 144 watts/m^2, 162 W/m2 for second 4 hours, and 135 watts/m2 for the last 4 hours assuming 12 hours of sunlight (using my numbers). Its still much more complicated than this as the numbers probably are much closer to 1000 W/m2 during the middle of the day and fall off to quite a bit less in the morning and the evening. Still interesting to do some calcuations yourself, if anyone has a link to a good model, I'd appreciate it.
dirk_bruere
not rated yet Jan 24, 2011
Efficiency is far less important than cost in terms of $/W. Efficiency only matters where area/land is vastly expensive. That is not house roofs nor deserts.
Uri
not rated yet Jan 24, 2011
Efficiency is far less important than cost in terms of $/W. Efficiency only matters where area/land is vastly expensive. That is not house roofs nor deserts.


I will agree to some extent, but when people talk about using solar to power our homes you have to do the math. 920 kilowatt-hours / month / home in the us according to h
ttp://www.eia.doe.gov/ask/electricity_faqs.asp So with solar cells > 2X as efficient as most solar cells being installed today, if you assumed you used half of that during the day you would need 460 KWh / 12 hours / 30 days / 150 Watt hours/m2 ~= 8.5 square meters of solar panels on your roof at an ideal angle / time of year to offset your power usage during the day for every house you lived in during your life. According the the census bureau the average american moves 11.7 times during their lifetime, so even assuming ideal conditions, thats not an insignificant investment to ensure that your domicile is power neutral during the day.
Uri
not rated yet Jan 24, 2011
Oh and as far your comment on cost / watt, you must also realize that the metric is based upon Standard Testing Conditions, i.e. "STC specifies a temperature of 25C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum". If the cost/watt is based upon those conditions you have to consider the fact that its not representative of the real world.
Squeezle42
5 / 5 (1) Jan 24, 2011
There's no such thing as a solar constant.

Call it mean perhaps? :p

This also assumes your intent is to use them on earth, would this not also be of benefit in reducing the cost and weight to get them into space?
Egleton
5 / 5 (1) Jan 25, 2011
Aqueezle42 You are correct. We need to realize O'Neil's dream and become 3 dimensional beings. Whoever colonises L1 will be the door keeper to the cosmos.
But our population is growing exponentially and the magic elixir that has enabled population growth, oil, has peaked. So we are all entering desperate times. Can we keep our minds fixed on the long term solution to exponential growth or will we collapse and die like a cancerous body?
stealthc
not rated yet Jan 27, 2011
God 900kwh/month? per house hold, that is such a waste.....
how many people in that house?
I'll be getting rid of CFL's this month and switching over to led's. I am tired of the waste.
I have unplugged dc transformers, those are going on a breaker board which I can switch on when needed, and switch off when not needed. I am also going to order solar cells for various appliances. I'm getting a dishwasher, but eliminating the stupid stove and a nasty energy waste fridge/freezer for a more efficient design. I am buying a wand vacuum seeing how much of a pain a regular vacuum is (they break too often).

What are you doing to be more neutral? I don't need a nanny state to tell me that I should use energy and resources responsibly --> but I also don't need them offering monopolization opportunities to the corporations on resources either. We've been sold out and shafted by our leaders.
electrodynamic
not rated yet Jan 27, 2011
Why cant the solar cell be made more like a transistor.
You could have an oscillating voltage across the gate changing the gap width, allowing different strength signals through. Changing the rate of oscillation would change the efficiency of the device at different wavelengths.
antialias_physorg
5 / 5 (1) Jan 29, 2011
The solar 'constant' is also the reason why we won't have solely solar powered cars (other than the extremely stripped down contraptions seen in the solar races)

Take the optimum energy the sun delivers per square meter and multiply by the surface area of a car. You get about 5kW (tops) which translates to roughly 7hp under _optimum_ conditions. Your family car isn't going anywhere on that kind of power.
XQZME
1 / 5 (1) Feb 03, 2011
Boy, these solar cells would sure be handy to have right now with all these electrical energy shortages due to these snow storms!